Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label earthquake. Show all posts
Showing posts with label earthquake. Show all posts

Earthquake Precursors: Signs Before Earthquakes



Earthquake prediction is the ultimate goal of seismologists. Being able to predict when and where an earthquake will occur could save thousands, if not hundreds of thousands, of lives, over the years. Even after decades of study, earthquake forecasting remains notoriously difficult, however. So what are the signs which occur b
efore 
an earthquake – earthquake precursors – and how useful are they?




About the author (who writes this article): Nusrat Kamal Siddiqui is one of the leading Geoscientists from Pakistan. He has a diverse professional career of being a Petroleum Geologist, Hydrologist and Engineering Geologist, both in Pakistan and overseas. He recently published a book " Petroleum Geology, Basin Architecture and Stratigraphy of Pakistan". Click here for further details about the book.


The Precursors

There are some long-term, medium-term and short-term precursors of seismic activity that cause earthquakes.

The long-term precursors are based on statistical studies and the prediction is probabilistic. The medium-term precursors help in predicting the location of an earthquake to a sufficient degree of accuracy. The short-term precursors of seismic events are indicated by changes in geomagnetic field, changes in gravity field, rising of subsurface temperature and rise in ground radioactivity. Agriculture institutions record subsurface temperature at 20, 50 and 100 cm depth as it is useful for monitoring crop growth. In earthquake-prone areas the temperature starts rising about 700-900 days before the event. This readily available data can be of help.

The short-term precursors are more important as they can be observed by a common man, and happen from a few days before the earthquake to just before it happens. With a reducing lag time these are: rise in water in the wells with increased sediments, sudden increase and decrease in river water flow, disturbance in the reception of radio, television, telephones, water fountains on the high grounds, strange behavior of animals, a sudden jump in the number of deliveries in hospitals and malfunctioning of cell phones. These days cell phones are the most handy and common piece of electronic equipment. A general collapse of this system can be noted by masses, and hence could be a very effective means to take timely mitigation measure. It has been found that about 100 to 150 minutes before the earthquake the cell phones start malfunctioning. However, the humans are very careless by nature and there would be only very few who would be observant enough to note the above precursors.  



It is indeed believed that animals exhibit unusual behavior before an earthquake


In the earthquake-prone areas groups of observant and responsible people (including women - they normally haul the water) may be constituted wherein the list of precursors, in local languages, may be distributed and some training imparted. And this exercise may not 
be left to the authorities, for obvious reasons!

Source: Earthquakes are inevitable, Disasters are not– Mitigation, therefore, is better than Prediction by Nusrat K. Siddiqui



Suggested Readings:

1. A systematic compilation of earthquake precursors
2. Earthquakes: prediction, forecasting and mitigation
3. Earthquake Prediction, Control and Mitigation

Defining the “Size” of Earthquakes

Defining the “Size” of Earthquakes 

Some earthquakes shake the ground violently, whereas others can barely be felt. Seismologists have developed two scales to define size in a uniform way, so that they can systematically describe and compare earthquakes. The first scale focuses on the severity of damage at a locality and is called the Mercalli Intensity scale. The second focuses on the amount of ground motion at a specific distance from the epicentre, as measured by a seismometer, and is called the magnitude scale.

How Do We Measure and Locate Earthquakes?

How Do We Measure and Locate Earthquakes?

Most news reports about earthquakes provide information on the size and location of an earthquake. What does this information mean, and how do we obtain it? What’s the difference between a large earthquake and a minor one? How do seismologists locate an epicentre? To answer these questions we must first understand how a seismometer works and how to read the information it provides.

What Causes Earthquakes?

What Causes Earthquakes?

To the causes of earthquakes, Ancient cultures offered a variety of explanations for seismicity (earthquake activity), most of which involved the action or mood of a giant animal or god. Scientific study suggests that seismicity instead occurs for several reasons, including: 
  • the sudden formation of a new fault (a fracture or rupture on which sliding occurs) 
  • sudden slip on an already existing fault
  • a sudden change in the arrangement of atoms in rock  minerals 
  • movement of magma in, or explosion of, a volcano
  • a giant landslide 
  • a meteorite impact
  • an underground nuclear-bomb test
Of these various reasons, faulting related to plate movements is by far the most significant. In other words, where do most earthquakes occur are along faults slip

Earthquake hypocenters and epicentres.
The place within the Earth where rock ruptures and slips, or the place where an explosion occurs, is the hypocenter or focus of the earthquake. Energy radiates from the focus. The point on the surface of the Earth that lies directly above the focus is the epicentre, so maps can portray the position of epicentres (figure above a, b). Since slip on faults causes most earthquakes, we focus our discussion on faults.
How earthquakes happen? Where do most earthquakes occur? Why do earthquakes happen? How do earthquakes happen? Where are earthquakes most likely to occur? Why do earthquakes happen?

Faults in the Crust 

Examples of fault displacement on the San Andreas fault in California.
At first glance, a fault may look simply like a fracture or break that cuts across rock or sediment. But on closer examination, you may be able to see evidence of sliding that occurred on a fault. For example, the rock adjacent to the fault may be broken up into angular fragments or may be pulverized into tiny grains, due to the crushing and grinding that can accompany slip, and the surface of a fault may be polished and grooved as if scratched by a rasp. In some localities, a fault cuts through a distinct marker (a sedimentary bed, an igneous dike, or a fence); where this happens, the end of the marker on one side of the fault is offset relative to the end on the other side. The distance between two ends of the marker, as measured along the fault surface in the direction of slip, is the fault’s displacement (figure above a, b). Many faults are completely underground, and will be visible only if exposed by erosion of overlying rock. But some faults intersect and offset the ground surface, producing a step called a fault scarp (figure below a). The ground surface exposure of a fault is called the fault line or fault trace

The basic types of fault. Fault types are distinguished from one another by the direction of slip relative to the fault surface.
19th-century miners who encountered faults in mine tunnels referred to the rock mass above a sloping fault plane as the hanging wall, because it hung over their heads, and the rock mass below the fault plane as the footwall, because it lay beneath their feet. The miners described the direction in which rock masses slipped on a sloping fault by specifying the direction that the hanging wall moved in relation to the footwall, and we still use these terms today. When the hanging wall slips down the slope of the fault, it’s a normal fault. When the hanging wall slips up the slope, it’s a reverse fault if steep, and a thrust fault if shallowly sloping (figure above a–c). Strike-slip faults are near-vertical planes on which slip occurs parallel to an imaginary horizontal line, called a strike line, on the fault plane no up or down motion takes place on such faults (figure above d).
Faults are found in many locations but don’t panic! Not all of them are likely to be the source of earthquakes. Faults that have moved recently or are likely to move in the near future are called active faults (and if they generate earthquakes, news media sometimes refer to them as “earthquake faults”). Faults that last moved in the distant past and probably won’t move again in the near future are called inactive faults.

Generating Earthquake Energy: Stick-Slip 

What is the relationship between faulting and earthquakes? Earthquakes can happen either when rock breaks and a new fault forms, or when a pre-existing fault suddenly slips again. Let’s look more closely at these two causes. 

A model representing the development of a new fault. Rupturing can generate earthquake-like vibrations.
  • Earthquakes due to fault formation: Imagine that you grip each side of a brick-shaped block of rock with a clamp. Apply an upward push on one of the clamps and a downward push on the other. By doing so, you have applied a “stress” to the rock. (Stress refers to a push, pull, or shear.) At first, the rock bends slightly but doesn't break (figure above a). In fact, if you were to stop applying stress at this stage, the rock would return to its original shape. Geologists refer to such a phenomenon as elastic behaviour the same phenomenon happens when a rubber band returns to its original shape or a bent stick straightens out after you let go. Now repeat the experiment, but bend the rock even more. If you bend the rock far enough, a number of small cracks or breaks start to form. Eventually the cracks connect to one another to form a fracture that cuts across the entire block of rock (figure above b). The instant that this fracture forms, the block breaks in two and the rock on one side suddenly slides past the rock on the other side, and any elastic bending that had built up is released so the rock straightens out or rebounds (figure above c). Because sliding occurs, the fracture has become a fault. A fault can’t slip forever, for friction eventually slows and stops the movement. Friction, defined as the force that resists  sliding on a surface, is caused by the existence of bumps on surfaces these bumps act like tiny anchors and snag on the opposing surface. 
  • Earthquakes due to slip on a pre-existing fault: Once a fault comes into being, it is a scar in the Earth’s crust that can remain weaker than surrounding, intact crust. When stress builds sufficiently, it overcomes friction and the pre-existing fault slips again. This movement takes place before stress becomes great enough to cause new fracturing of surrounding intact rock. Note that after each slip event, friction prevents the fault from slipping again until stress builds again. Geologists refer to such alternation between stress buildup and slip events (earthquakes) as stick-slip  behaviour. 
The breaking of rock that occurs when a fault slips, like the snap of a stick, generates earthquake energy. The concept that earthquakes happen because stresses build up, causing rock adjacent to the fault to bend elastically until slip on the fault occurs is called the  elastic-rebound theory. 
Of note, the major earthquake (or “mainshock”) along a fault may be preceded by smaller ones, called foreshocks, which possibly result from the development of the smaller cracks in the vicinity of what will be the major rupture. Smaller earthquakes, called aftershocks, occur in the days to months following a large earthquake. The largest aftershock tends to be ten times smaller than the mainshock, and most are even smaller. Aftershocks happen because slip during the  mainshock does not leave the fault in a perfectly stable configuration. For example, after the mainshock, irregularities on one side of the fault surface, in their new position, may push into the opposing side and generate new stresses. Such stresses may become large enough to cause a small portion of the fault around the irregularity to slip again, or may trigger slip in a nearby fault.

The Amount of Slip during an Earthquake 

How much of a fault surface slips during an earthquake? The answer depends on the size of the earthquake: the larger the earthquake, the larger the slipped area and the greater the displacement. For example, the major earthquake that hit San Francisco, California, in 1906 ruptured a 430-km-long (measured parallel to the Earth’s surface) by 15-km-deep (measured perpendicular to the Earth’s surface) segment of the San Andreas fault. Thus, the area that slipped was almost 6500 km2. During the 2011 Tohoku earthquake an area 300 km long by 100 km wide (30,000 km2) slipped. 
The amount of slip varies along the length of a fault the maximum observed displacement during the 1906 earthquake was 7 m, in a strike-slip sense. Slip on a thrust fault that caused the 1964 Good Friday earthquake in southern Alaska reached a maximum of 12 m, and the maximum slip during the Tohoku earthquake was over 20 m. Smaller earthquakes, such as the one that hit Northridge, California, in 1994, resulted in only about 0.5-m slip even so, this earthquake toppled homes, ruptured pipelines, and killed 51 people. The smallest-felt earthquakes result from displacements measured in millimetres to centimetres. 
Although the cumulative movement on a fault during a human life span may not amount to much, over geologic time the cumulative movement becomes significant. For example, if earthquakes occurring on a strike-slip fault cause 1 cm of displacement per year, on average, the fault’s movement will yield 10 km of displacement after 1 million years.
Credits: Stephen Marshak (Essentials of Geology)

How do Earthquakes causes damage?

Damages from Earthquakes

An area ravaged by a major earthquake is a heartbreaking sight. The terror and sorrow etched on the faces of survivors mirror the inconceivable destruction. This destruction comes as a result of many processes.

Ground Shaking and Displacement 


An earthquake starts suddenly and may last from a few seconds to a few minutes. Different kinds of earthquake waves cause different kinds of ground motion (figure above). The nature and severity of the shaking at a given location depend on four factors: 
  1. the magnitude of the earthquake, because larger magnitude events release more energy; 
  2. the distance from the focus, because earthquake energy decreases as waves pass through the Earth; 
  3. the nature of the substrate at the location (that is, the character and thickness of different materials beneath the ground surface) because earthquake waves tend to be amplified in weaker substrate; and 
  4. the “frequency” of the earthquake waves (where frequency equals the number of waves that pass a point in a specified interval of time). 


If you’re out in an open field during an earthquake, ground motion alone won’t kill you, you may be knocked off your feet and bounced around a bit, but your body is too flexible to break. Buildings and bridges aren't so lucky (figure above a-d). When earthquake waves pass, they sway, twist back and forth, or lurch up and down, depending on the type of wave motion. As a result, connectors between the frame and facade of a building may separate, so the facade crashes to the ground. The flexing of walls shatters windows and makes roofs collapse. Floors or bridge decks may rise up and slam down on the columns that support them, thereby crushing the columns. Some buildings collapse with their floors piling on top of one another like pancakes in a stack, some crumble into fragments, and some simply tip over. The majority of  earthquake-related deaths and injuries happen when people are hit by debris or are crushed beneath falling walls or roofs. Aftershocks worsen the problem, because they may topple already weakened buildings, trapping rescuers. During earthquakes, roads, rail lines, and pipelines may also buckle and rupture. If a building, fence, road, pipeline, or rail line straddles a fault, slip on the fault can crack the structure and separate it into two pieces.

Landslides 


The shaking of an earthquake can cause ground on steep slopes or ground underlain by weak sediment to give way. This movement results in a landslide, the tumbling and flow of soil and rock down-slope. Earthquake triggered landslides occur commonly along the coast of  California where expensive homes perch on steep cliffs looking out over the Pacific. When the cliffs collapse, the homes may tumble to the beach below (figure above a and b). Such events lead to the misperception that “California will someday fall into the sea.” Although small portions of the coastline do collapse, the state as a whole remains firmly attached to the continent, despite what  Hollywood  scriptwriters say.

Sediment Liquefaction 

In 1964, an MW 7.5 earthquake struck Niigata, Japan. A portion of the city had been built on land underlain by wet sand. During the ground shaking, foundations of over 15,000 buildings sank into their substrate, causing walls and roofs to crack. Several four-story-high buildings in a newly built apartment complex tipped over (figure above c). The same year, on Good Friday, an MW  9.2 earthquake devastated southern Alaska. In the Turnagain Heights neighbourhood of Anchorage, the event led to catastrophe. The neighbourhood was built on a small terrace of uplifted sediment. The edge of the terrace was a 20-m high escarpment that dropped down to Cook Inlet, a bay of the Pacific Ocean. As the ground shaking began, a layer of wet clay beneath the development turned into mud, and when this happened, the overlying layers of sediment, along with the houses built on top of them, slid seaward. In the process, the layers broke into separate blocks that tilted, turning the landscape into a chaotic jumble, and resulting in the destruction of the neighbourhood (figure above d). 

In 2011, an earthquake in Christchurch, New Zealand, caused sand to erupt and produce small, cone-shaped mounds on the ground surface  (figure above a). The transfer of sand from underground onto the surface led to formation of depressions large enough to  swallow cars (figure above b). All of these examples are manifestations of a phenomenon called sediment liquefaction. During liquefaction, pressure in the water filling the pores between grains in wet sand push the grains apart so that they become surrounded by water and no longer rest against each other. In wet clay, shaking breaks the weak electrostatic charges that hold clay flakes together, so what had been a gel-like, stable mass becomes slippery mud. As the material above the liquefied sediment settles downward, pressure can squeeze the sand upward and out onto the ground surface. The resulting cone-shaped mounds are variously known as sand volcanoes, sand boils, or sand blows. The settling of sedimentary layers down into a liquefied layer can also disrupt bedding and can lead to formation of open fissures of the land surface (figure above c).

Fire 


The shaking during an earthquake can make lamps, stoves, or candles with open flames tip over, and it may break wires or topple power lines, generating sparks. As a consequence, areas already turned to rubble, and even areas not so badly damaged may be consumed by fire. Ruptured gas pipelines and oil tanks feed the flames, sending columns of fire erupting skyward (figure above). Fire fighters might not even be able to reach the fires, because the doors to the fire house won’t open or rubble blocks the streets. Moreover, fire fighters may find themselves without water, for ground shaking and landslides damage water lines. Once a fire starts to spread, it can become an unstoppable inferno. Most of the destruction of the 1906 San Francisco earthquake, in fact, resulted from fire. For three days, the blaze spread through the city until fire fighters contained it by blasting a fire break. By then, 500 blocks of structures had turned to ash, causing 20 times as much financial loss as the shaking itself. When a large earthquake hit Tokyo in 1923, fires set by cooking stoves spread quickly through the wood-and-paper buildings, creating an inferno a “fire storm” that heated the air above the city. As hot air rose, cool air rushed in, creating wind gusts of over 100 mph, which stoked the blaze and incinerated 120,000 people.

Tsunamis 


The azure waters and palm fringed islands of the Indian Ocean’s east coast hide one of the most seismically active plate boundaries on Earth, the Sunda Trench. Along this convergent boundary, the Indian Ocean floor subducts at about 4 cm per year, leading to slip on large thrust faults. Just before 8:00 a.m. on December 26, 2004, the crust above a 1,300-km long by 100-km-wide portion of one of these faults lurched westward by as much as 15 m. The break started at the hypocentre and then propagated north at 2.8 km/s; thus, the rupturing took 9 minutes. This slip triggered a great earthquake (MW 9.3) and pushed the sea floor up by tens of centimetres. The rise of the sea floor, in turn, shoved up the overlying water. Because the area that rose was so broad, the volume of displaced water was immense. As a consequence, a tragedy of an unimaginable extent was about to unfold. Water from above the up-thrust sea floor began moving outward from above the fault zone, a process that generated a series of giant waves travelling at speeds of about 800 km per hour (500  mph) almost the speed of a jet plane (figure above a). Geologists now use the term tsunami for a wave produced by displacement of the sea floor. The displacement can be due to an earthquake, submarine landslide, or volcanic explosion. Tsunami is a Japanese word that translates literally as harbour wave, an apt name because tsunamis can be particularly damaging to harbour towns. In older literature such waves were called “tidal waves,” because when one arrives, water rises as if a tide were coming in, but in fact the waves have nothing to do with daily tidal cycles. Regardless of cause, tsunamis are very different from familiar, wind-driven storm waves. Large wind-driven waves can reach heights of 10 to 30 meters in the open ocean. But even such monsters have wavelengths of only tens of meters, and thus contain a relatively small volume of water. In contrast, although a tsunami in deep water may cause a rise in sea level of at most only a few tens of centimetres a ship crossing one wouldn't even notice tsunamis have wavelengths of tens to hundreds of kilometres and an individual wave can be several kilometres wide, as measured perpendicular to the wave front. Thus, the wave involves a huge volume of water. In simpler terms, we can think of the width of a tsunami, in map view, as being more than 100 times the width of a wind-driven wave. Because of this difference, a storm wave and a tsunami have very different effects when they strike the shore. When a wave approaches the shore, friction between the base of the wave and the sea floor slows the bottom of the wave, so the back of the wave catches up to the front, and the added volume of water builds the wave higher (figure above b). The top of the wave may fall over the front of the wave and cause a breaker. In the case of a wind-driven wave, the breaker may be tall when it washes onto the beach, but because the wave doesn't contain much water, the wave runs out of water and friction slows it to a stop on the beach. Then, gravity causes the water to spill back seaward. In the case of a tsunami, the wave is so wide that, as friction slows the wave, it builds into a “plateau” of water that can be tens of meters high, many kilometres wide, and hundreds of kilometres long. Thus, when a tsunami reaches shore, it contains so much water that it crosses the beach and, if the land is low-lying, just keeps on going, eventually covering a huge area (figure above c). 

Tsunami damage can be catastrophic. The December 2004 waves struck Banda Aceh, a city at the north end of the island of Sumatra, on a beautiful, cloudless day (figure above a). First, the sea receded much farther than anyone had ever seen, exposing large areas of reefs that normally remained submerged even at low tide. People walked out onto the exposed reefs in wonder. But then, with a rumble that grew to a roar, a wall of frothing water began to build in the distance and approach land (figure above b). Puzzled bathers first watched, then ran inland in panic when the threat became clear. As the tsunami approached shore, friction with the sea floor had slowed it to less than 30 km an hour, but it still moved faster than people could run. In places, the wave front reached heights of 15 to 30 m (45 to 100 feet) as it slammed into Banda Aceh (figure above c). The impact of the water ripped boats from their moorings, snapped trees, battered buildings into rubble, and tossed cars and trucks like toys. And the water just kept coming, eventually flooding low-lying land up to 7 km inland (figure above d). It drenched forests and fields with salt water (deadly to plants) and buried fields and streets with up to a meter of sand and mud. When the water level finally returned to normal, a jumble of flotsam, as well as the bodies of unfortunate victims, were dragged out to sea and drifted away. Geologists refer to the tsunami that struck Banda Aceh as a near-field (or local) tsunami, because of its proximity to the earthquake. But the horror of Banda Aceh was merely a preamble to the devastation that would soon visit other stretches of Indian Ocean coast. Far-field (or distant) tsunamis crossed the ocean and struck Sri Lanka 2.5 hours after the earthquake, the coast of India half an hour after that, and the coast of Africa, on the west side of the Indian Ocean, 5.5 hours after the earthquake. In the end, more than 230,000 people died that day. The tsunami that struck Japan soon after the 2011 Tohoku earthquake was vividly captured in high-definition video that was seen throughout the world, generating a new level of international awareness. Though much of the coast was fringed by seawalls, they proved to be a minor impediment to the advance of the wave, which, in places, was 10 to 30 m high when it reached shore. Racing inland the wave erased whole towns, submerging airports and fields. As the wave picked up dirt and debris, it became a viscous slurry, moving with such force that nothing could withstand its impact. 

The devastation of coastal towns was so complete that they looked as though they had been struck by nuclear bombs  (figure above a). But the catastrophe was not over. The wave had also hit a nuclear power plant. Though the plant had withstood ground shaking and had automatically shut down, its radioactive core still needed to be cooled by water in order to remain safe. The tsunami not only destroyed power lines, cutting the plant off from the electrical grid, but it also eliminated backup diesel generators and cut water lines. Thus, cooling pumps stopped functioning. Eventually, water surrounding the heat producing radioactive core of the reactors, as well as the water cooling spent fuel, boiled away. Some of the water separated into hydrogen and oxygen gas, which exploded, and ultimately, the integrity of the nuclear plant was breached so that radioactivity entered the environment (figure above b). Because tsunamis are so dangerous, predicting their arrival can save thousands of lives. A tsunami warning centre in Hawaii keeps track of earthquakes around the Pacific and uses data relayed from tide gauges and sea-floor pressure gauges to determine whether a particular earthquake has generated a tsunami. If observers detect a tsunami, they flash warnings to authorities around the Pacific. 

Disease 

Once the ground shaking and fires have stopped, disease may still threaten lives in an earthquake damaged region. Earthquakes cut water and sewer lines, destroying clean-water supplies and exposing the public to bacteria, and they cut transportation lines, preventing food and medicine from reaching the area. The severity of such problems depends on the ability of emergency services to cope. The lack of sufficient clean water after the 2010 Haiti earthquake led to a cholera epidemic later that year.

Can we predict Earthquakes?

Can seismologists predict earthquakes? 

The answer depends on the time frame of the prediction. With our present understanding of the distribution of seismic zones and the frequency at which earthquakes occur, we can make long-term predictions (on the time scale of decades to centuries). For example, with some certainty, we can say that a major earthquake will rattle Istanbul during the next 100 years, and that a major earthquake probably won’t strike central Canada during the next 10 years. But despite extensive research, seismologists cannot make accurate short-term predictions (on the time scale of hours to weeks or even years). Thus we cannot say, for example, that an earthquake will happen in Montreal at 2:43 P.M. on January 17. In this section, we look at the scientific basis of both long- and short-term predictions and consider the consequences of a prediction. Seismologists refer to studies leading to predictions as seismic-risk, or  seismic-hazard assessment. 

Long-Term Predictions 

A long-term prediction estimates the probability, or likelihood that an earthquake will happen during a specified time range. For example, a seismologist may say, “The probability of a major earthquake occurring in the next 20 years in this state is 20%.” This sentence implies that there’s a 1-in-5 chance that the earthquake will happen before 20 years have passed. Urban planners and civil engineers can use long-term predictions to help create building codes for a region codes requiring stronger, more expensive buildings make sense for regions with greater seismic risk. They may also use predictions to determine whether it is reasonably safe to build vulnerable structures such as nuclear power plants, hospitals, or dams in a given region. Seismologists base long-term earthquake predictions on two pieces of information: the identification of seismic zones and the recurrence interval (the average time between successive events). 

To identify a seismic zone, seismologists produce a map showing the epicentres of earthquakes that have happened  during a set period of time (say, 30 years). Clusters or belts of epicentres define the seismic zone. The basic premise of long-term earthquake prediction can be stated as follows: a region in which there have been many earthquakes in the past will be more likely to experience earthquakes in the future. Seismic zones, therefore, are regions of greater seismic risk. This doesn't mean that a disastrous earthquake can’t happen far from a seismic zone they can and do but the probability that an event will happen in a given time window is less. To determine the recurrence interval for large earthquakes within a given seismic zone, seismologists must determine when large earthquakes happened there in the past. Since the historical record does not provide information far enough back in time, they study geologic evidence for great earthquakes. For example, recognition of a fresh, unweathered fault scarp or trace may indicate that faulting affected an area relatively recently. A trench cut into sedimentary strata near a fault may reveal layers of sand volcanoes and disrupted bedding in the stratigraphic record. Each layer, whose age can be determined by using radiocarbon dating of plant fragments, records the time of an earthquake (figure above). By calculating the number of years between successive events and taking the average, seismologists obtain the recurrence interval. Note that a recurrence interval does not specify the exact number of years between events, only the average number. Since stress builds up over time on a fault, the probability that an earthquake will happen in any given year probably increases as time passes. 


Information on a recurrence interval allows seismologists to refine regional maps illustrating seismic risk (figure above a and b).

Short-Term Predictions 

Short-term predictions, specifying that an earthquake will happen on a given date or within a time window of days to years, are not and may never be reliable. Seismologists have considered, and discounted as unreliable, many supposed bases for short-term prediction. For example, a swarm of fore-shocks may indicate that rock is beginning to crack in advance of a main-shock, but such swarms can be identified only in hindsight. Precise surveys show that the surface of the ground may warp slightly prior to an earthquake, but no one can determine how much warping will take place before an earthquake will happen. Prediction studies focused on measuring changes in water levels in wells, radon gas in spring water, electrical signals emitted by minerals, or agitation of animals have met with similar skepticism. The concept of a short-term prediction should not be confused with the concept of an earthquake early warning system. An early warning system works as follows. When an earthquake happens, the seismic waves it produces start travelling through the Earth. Seismic stations closer to the epicentre may detect an earthquake before the seismic waves have had time to reach populated areas farther from the epicentre. The instant that seismic stations detect the earthquake, a computer approximates the epicentre location, then sends a signal to a control centre, which automatically sends out emergency signals to areas that might be affected. The signals shut down gas pipelines, trains, nuclear reactors, power lines, and other vulnerable infrastructure. The signal also sets off sirens and alerts broadcasters to send out warnings on radio, TV, and cell-phone networks to warn people that an earthquake is about to begin. Unless the focus is directly under the city, the warning may precede the arrival of the first earthquake waves by several seconds, not a lot of time, but hopefully enough to prevent some infrastructure damage and perhaps enough for people to seek a safer location.

Where and Why Do Earthquakes Occur?

Earthquakes occurring place


How earthquakes happen? Where do most earthquakes occur? Why do earthquakes happen? How do earthquakes happen? Where are earthquakes most likely to occur? Why do earthquakes happen? Where in the world do most earthquakes occur?
Earthquakes do not take place everywhere on the globe. By plotting the distribution of earthquake epicentres on a map, seismologists find where do most earthquakes occur, but not all, earthquakes occur in fairly narrow seismic belts, or seismic zones. Most seismic belts correspond to plate boundaries, and where most earthquakes occur within these belts are called plate-boundary earthquakes. Earthquakes that occur away from plate boundaries are called intra-plate earthquakes (figure above). Eighty percent of the earthquake energy released on Earth comes from the plate-boundary earthquakes in the belts surrounding the Pacific Ocean. 
Earthquakes do not occur at random depths in the Earth. Seismologists distinguish three classes of earthquakes based on focus depth: shallow-focus earthquakes occur in the top 60 km of the Earth, intermediate-focus earthquakes take place between 60 and 300 km, and deep-focus earthquakes occur down to a depth of about 660 km. Earthquakes cannot happen still deeper in the Earth, because rock at great depth cannot rupture or change in a manner that produces shock waves. In this section, we look at the characteristics of earthquakes in various geologic settings and learn why earthquakes take place where they do. 

Plate-Boundary Earthquakes 

As we've noted, the most earthquakes occur at faults along plate boundaries, for the relative motion between plates causes slip on faults. We find different kinds of faulting at different types of plate boundaries.

Divergent plate-boundary seismicity


At divergent plate boundaries (mid-ocean ridges), two oceanic plates form and move apart. Divergent boundaries are broken into spreading segments linked by transform faults. Therefore, two kinds of faults develop at divergent boundaries. Along spreading segments, stretching generates normal faults, whereas along transform faults strike-slip displacement occurs (figure above). 

Transform plate-boundary seismicity


At transform plate boundaries, where one plate slides past another without the production or consumption of oceanic lithosphere, most faulting results in strike-slip motion. The majority of transform faults in the world link segments of oceanic ridges, but a few, such as the San Andreas fault of California, the Alpine fault of New Zealand, and the Anatolian faults in Turkey, cut through continental lithosphere or volcanic arcs. All transform-fault earthquakes have a shallow focus, so the larger ones on land can cause disaster. The 2010 earthquake in Haiti is a tragic example of such an earthquake (Box 8.1). As another example of a transform-fault earthquake, consider the slip of the San Andreas fault near San Francisco in 1906 (a in figure above). In the wake of the gold rush, San Francisco was a booming city with broad streets and numerous large buildings. But it was built on the transform boundary along which the Pacific Plate moves north at an average rate of 6 cm per year, relative to North America. Because of the stick-slip behaviour of the fault, this movement doesn't occur smoothly but happens in jerks, each of which causes an earthquake. At 5:12 A.M. on April 18, the fault near San Francisco slipped by as much as 7 m, and earthquake waves slammed into the city. Witnesses watched in horror as the street undulated like ocean waves. Buildings swayed and banged together, laundry lines stretched and snapped, church bells rang, and then towers, facades, and houses toppled. Judging from the damage, seismologists estimate that the largest shock would have registered a seismic moment magnitude of 7.9. Most building collapse took place down-town. Fire followed soon after, consuming huge areas of the city, for most buildings were made of wood (b in figure above). In the end, about five hundred people died, and a quarter of a million were left homeless. The San Francisco earthquake has not been the only one to strike along the San Andreas and nearby related faults. Over a dozen major earthquakes have happened on these faults during the past two centuries, including the 1857 magnitude 7.7 earthquake just east of Los Angeles, and the 1989 MW 7.1 Loma Prieta earthquake, which occurred 100 km south of San Francisco but nevertheless shut down a World Series game and caused the collapse of a double-decker freeway (c in figure above).

Convergent plate-boundary seismicity


Convergent plate boundaries are complicated regions at which several different kinds of earthquakes take place. Shallow-focus earthquakes occur in both the sub-ducting plate and the overriding  plate. Specifically, as the down-going plate begins to  subduct, it scrapes along the base of the overriding plate. Large thrust faults develop along the contact between the down-going and overriding plates, and shear on these faults can produce disastrous, shallow earthquakes. In some cases, the push applied by the down-going plate compresses the overriding plate and triggers shallow earthquakes in the overriding  plate. In contrast to other types of plate boundaries, convergent plate boundaries also host intermediate-focus and deep focus earthquakes. These occur in the down-going slab as it sinks into the mantle, defining the sloping band of seismicity called a Wadati-Benioff zone, after the seismologists who first recognised it (a in figure above). These earthquakes occur partly in response to stresses caused by shear between the down-going lithosphere plate and surrounding asthenosphere, and partly by the pull of the sinking deeper part of the plate on the shallower part. Why can intermediate- and deep-focus earthquakes of a Wadati-Benioff zone take place? Shouldn't the rock of a subducted plate at these depths be too warm and soft to break seismically? Seismologists have determined that rock is such a good insulator that the interior of a plate actually remains cool enough to break seismically, even down to a depth of about 300 km. And they found that at the extreme pressures developed in deeply subducted lithosphere, certain minerals can suddenly collapse to form new, denser minerals, a process that could generate an earthquake. Earthquakes in southern Alaska, eastern Japan, the western coast of South America, the coast of Oregon and Washington, and along island arcs in the western Pacific serve as examples of convergent-boundary earthquakes. Some of these earthquakes are large, and occur near populated areas, so they can be devastating. 
Notable examples include the 1960 MW 9.5 earthquake in Chile, the largest earthquake on record; the 1964 MW 9.2 Good Friday earthquake near Anchorage, Alaska; the 1995 MW 6.9 earthquake in Kobe, Japan, which devastated the city (b & c in figure above); the 2004 MW 9.3 Sumatra earthquake, which triggered the giant Indian Ocean waves that killed 230,000 people; the 2010 MW 8.8 Chilean earthquake; and the 2011 MW 9.0 Tohoku earthquake, which also triggered a tsunami.

Earthquakes due to Rifting and Collision

Continental rifts


The stretching of crust at continental rifts generates normal faults (figure above). Active rifts today include the East African Rift, the Basin and Range Province (mostly in Nevada, Utah, and Arizona), and the Rio Grande Rift (in New Mexico). In all these places, shallow earthquakes occur, similar in nature to the earthquakes at mid-ocean ridges. But in contrast to mid-ocean ridges, these seismic zones can be located under populated areas, and thus cause major damage.

Collision zones

Two continents collide when the oceanic lithosphere that once separated them has been completely subducted. Such collisions produce great mountain ranges such as the Alpine-Himalayan chain and caused the catastrophic 2005 earthquake in Pakistan. Though a variety of earthquakes happen in collision zones, the most common result from movement on thrust faults (see figure above). The magnitude 9.0 earthquake that occurred in Lisbon, Portugal, on All Saint’s Day, November 1, 1755, serves as an example of a collision-related thrust event. The event happened when stresses arising from the northward push of Africa against Europe caused a thrust fault beneath the Atlantic Ocean floor west of Lisbon to slip. The resulting ground shaking toppled 85% of the city’s buildings. Fires set by overturned stoves then consumed much of the wreckage. Uplift of the sea floor by the thrust movement also produced a tsunami that inundated the coast and washed away Lisbon’s harbour.  
Not only did the catastrophe destroy irreplaceable structures (including the library that housed all the records of Portuguese exploration) and countless Renaissance artworks, but it led the intelligentsia of that time to question long-held beliefs about philosophy. Influential works by Voltaire (1694–1778) address the philosophical implications of the event.

Intraplate Earthquakes

Some earthquakes occur in the interiors of plates and are not associated with plate boundaries, active rifts, or collision zones (Divergent plate-boundary seismicity figure). These intraplate earthquakes account for only about 5% of the earthquake energy released in a year. Almost all have a shallow focus. What causes intraplate earthquakes? Most seismologists favour the idea that force applied to the boundary of a plate can cause the interior of the plate to break suddenly at weak, pre-existing fault zones, some of which may have formed initially during the Precambrian. In Europe, a number of intraplate earthquakes have been recorded. For example, an earthquake with a magnitude of 4.8 hit central England, near Birmingham, in 2002. In North America, intraplate earthquakes occur in the vicinities of New Madrid, Missouri; Charleston, South Carolina; eastern Tennessee; and Montreal. 

A magnitude 7.3 earthquake occurred near Charleston in 1886, ringing church bells up and down the coast and vibrating buildings as far away as Chicago. In Charleston itself, over 90% of the buildings were damaged, and sixty people died. In 2011, a magnitude 5.9 earthquake struck central Virginia, abruptly reminding residents of the eastern United States that the region is not immune to seismicity. The tremor was felt from the Carolinas to New England. The largest intraplate earthquakes to affect the United States happened in the early 19th century, near New Madrid, which lies near the Mississippi River in southernmost Missouri. During the winter of 1811–12, three magnitude  7 to 7.4 earthquakes struck the region. Displacement of the ground surface temporarily reversed the flow of the Mississippi River and toppled cabins (a in figure above). The earthquakes resulted from slip on faults that underlie the Mississippi Valley (b in figure above). Both St. Louis, Missouri, and Memphis, Tennessee, lie close to the epicenter, so major earthquakes in the area now could be disastrous.
Credits: Stephen Marshak (Essentials of Geology)

Ground actually open up and move!


The Fitness of Geology



"I do not know of any fields in which professionals enjoy their work more than geologists do. Perhaps this is due to the uniqueness of work in the geological sciences. What other science requires the use of both the mind and body?" ——John Wakabayashi
Science is a labor of the mind and will, but some of its specialties call upon the muscles as well. Perhaps the foremost of those is geology. I believe geologists are the fittest of scientists and, aside from one or two specific hazards, the most likely to have long and productive lives.
Meeting Chair John Wakabayashi talks about the geology along Panoche Road

Physical Fitness

The geologist's fieldwork is the kind of freeform, nonrepetitive workout that gyms can't offer:
  • For legs and feet there's lots of walking, kneeling, crouching and standing on tiptoe at outcrops.
  • For core strength and cardiovascular fitness there's carrying a pack while scrambling over boulders, slogging down streambeds and clambering around quarries. There's reaching high overhead for that elusive mineral pocket and lying down at full length to get the closest view of an exposure.
  • For the upper body there's turning and lifting rocks, as well as the free-weight exercise known as using a rock hammer. There's drilling core, holding maps in the breeze, pushing aside brush.
  • Lastly, geologists talk using their hands and arms vigorously. This is especially true of structural geologists. There's a joke that the way to shut geologists up is to tie their arms down.
Carrying specimens back to the vehicle combines all of these. And every minute of the day is different.
A day in the field leaves one pleasantly tired and ready for a mild muscle relaxant.

Mental Fitness

At every step geologists are also using their eyes and minds. Geology seems particularly to call upon multiple intelligences as well as the method of multiple working hypotheses.
Fieldwork is motion with a point—it is constant observation. They say that the best geologist is the one who has seen the most rocks. Seeing rocks is an intense mental workout in four dimensions, three in space and one in time. The minerals, textures, fabric, colors and fossils all must be noted and assessed. The regional setting and neighboring rocks bear on the immediate scene as well.
When rocks are brought back to the laboratory, all the field evidence must be kept in mind there too—and when the field site is revisited, the lab results may change what to look at. Indeed, they may change what is being seen.
The ability to plan complex undertakings and change plans on the fly is an essential geologic skill. Whether it's equipment breakdowns, injury or illness in the team, bad weather, the threat of wild animals, or salvaging a failed expedition, mental fitness is what prevails. The exemplar in this respect is one-armed geologist John Wesley Powell, who led the first expedition down the Grand Canyon in 1869 without a map and emerged a hero.
Mental exercise is known to help prevent or forestall dementias related to age, such as Alzheimer's disease. Playing chess works, but the geologist's open-ended kind of mental exercise is better suited to our native brain. Geologists who teach or give presentations—that is, most of them—get the same stimulation year round. And in a field that many can excel in, but none can master, the challenge of geology never fades.

Social Fitness

The day is long gone when geology was a solo occupation. Well-run teams do better science than individuals. The best geologist must not only see the most rocks, but must work well with the most people. The best geologist is effective in email as well as in the field: a glance at the authorship of journal articles shows that collaborators may be anywhere in the world.
A geologist may deal with suspicious landowners to gain access to their property, not to mention strangers in the woods and petty officials in foreign countries. All of these possibilities call for good social skills. And after every interaction, geologists have another story to tell each other.

Hazards of the Field

Physical, mental and social fitness go far in the field, but some special hazards wait there too. Bears and wolves and other large animals make firearms a necessity in many areas. Falls, cuts and sprains require skill with first aid. There are hazardous plants to watch out for, too. Although technology makes hunger, thirst and getting lost less likely than in the old days, a passing familiarity with wilderness survival is good to have.
Then there's the sun. Long hours in the sun, especially at high altitude, make skin cancer an occupational hazard for geologists.
Finally, it's essential to be a good driver, and it's helpful to be handy with tools. Every geologist has at least one story about a car, a boat or a plane




.

Earth Story - Ring Of Fire


Scientists Predict 9.2 EARTHQUAKE TSUNAMI to hit west coast soon


Formation of Tsunami (3d Simulation)


Indian plate is pushing on Eurasia (earthquake)


Yet another incident evident of the Indian plate movement towards North into the Eurasia. Another earthquake struck Pakistan few minutes back (10 August,2015) . Magnitude calculation shows was 6.7 on the Richter scale while it shake most of the Pakistan. The focal point is to be in the Eskasem, Badakhshan, Afghanistan-Tajikistan border. It epicentre was 200 kilometres below surface while the surface waves was propagating about 15 seconds. 
This simply is the evident of the Indian plate movement while in the past couple of days, being too active. As one earthquake struck today while 7 is the count for the last past week. Its count in a month goes to 16 while 150 in year time. Crustal deformation is still active, mountains being getting higher like 7 millimetres annually. 
If the movement continues with this speed its far enough but what about some higher rate?. Probably it will go on a higher rate as to see the count of earthquake in this very active zone where Afghanistan, China, Pakistan, India, Nepal and Tajikistan lies. Faults being activated with intervals, different epicentres and will continue to do so. As there are many major faults in this area with to many minor faults, each the resulting force Indian plate due on the Eurasia. 
So as now the movement of the Indian plate is northward, will it drift the the southern parts toward North?. Surely will but the feel is not so huge as it only measures to be centimetres and not like metres. As both are large masses no one can push aside each other so it keeps mountain ranges building, old ones keep higher and higher while, the new deformation zones keep on their deformation because the force is to accommodate and later folds transform into faults and gives more earthquakes.

Indonesia Earthquake 2015



The Indonesian province of Papua had experienced a powerful earthquake of magnitude 7.0 recorded by the U.S Geological Survey. The earthquake just struck off on 27/7/2015 at 06.41 a.m. local time where the epicentre was 250 kilometres west of the provincial capital Jayapura and was at the depth of 52 kilometres. As the area is largely vegetated and mountainous region, there was no tsunami warning issued after the Earthquake. The Australian government and Hawaii tsunami monitoring system has declared the no tsunami threat. In March 2012 a 6.2-magnitude earthquake struck the region without any tsunami warning issued or loss or reports of damage. One of Irian Jaya’s worst earthquakes was in 1981, which killed at least 305 people and displaced more than a 1,000 villagers in the district of Kurina Areal, near the border of Papua New Guinea. 
Quakes in this region with strong magnitudes are common. In May of last year a 5.0 magnitude Indonesia quake erupted just after 8:40 pm local time tonight. This quake was forty miles below sea level. USGS tells news that this region is filled with fault lines. “At its northern and southern terminations, subduction at the Manila Trench is interrupted by arc-continent collision, between the northern Philippine arc and the Eurasian continental margin at Taiwan and between the Sulu-Borneo Block and Luzon at the island of Mindoro.” They add “The Philippine fault, which extends over 1,200 km within the Philippine arc, is seismically active. The fault has been associated with major historical earthquakes, including the destructive M7.6 Luzon earthquake of 1990 (Yoshida and Abe, 1992). A number of other active intra-arc fault systems are associated with high seismic activity, including the Cotabato Fault and the Verde Passage-Sibuyan Sea Fault (Galgana et al., 2007).”

An official of the Indonesia's Meteorology and Geophysics Agency said the quake was felt across the province and that the strongest hit area was Sarmi, a town on the northern coast of the island, but there were no immediate reports of injuries or damage. But it did cause panicked residents to ran out of their homes, said Sutopo Purwon Nugroho, a spokesman for the National Disaster Mitigation Agency. Indonesia is prone to earthquakes due to its location on the so-called Pacific "Ring of Fire." A massive earthquake off Sumatra island in 2004 triggered a tsunami that killed 230,000 people in a dozen countries, mostly in Indonesia's Aceh province.

What are earthquakes and what causes them?


Earthquake

An earthquake is the brittle, sudden failure of the earth's crust or mantle. Earthquakes are caused by several factors however the common element is that stress builds in rocks until the yield strength of the rock is exceeded, at which point rupture occurs. The relative movement between the major tectonic plates is responsible for the stress build-up that causes the vast majority of earthquakes.

To understand the earthquake process some simple physical quantities should be defined first.


Stress is the force per unit area.
Elastic deformation: A material changes shape when stressed but after the stress is removed, it returns to its original shape. The bonds between the molecules and atoms of an elastically behaving material, when stressed, stretch and bend, but retain memory of their original configuration. Once the stress is released, the stored energy is released and the material returns to its original shape examples are rubber band or super ball and rocks
Brittle deformation: Rupture occurs in response when stress that is exerted on the material exceeds that materials yield strength example are glass, ceramic and rocks
Plastic deformation: Flow occurs in response to stress and material does not return to original shape after applied stress is removed. Just like a Play Dough which can be shaped as liked. Some rocks (rock salt or halite) and other evaporites flow when subjected to stress.

How does elastic rocks break?. 


It can be best understood by the following example of a plywood. When the strip of plywood is subjected to a bending force, it deforms elastically at first. If the bending force is released, the wood returns to its original unbent shape. However, if the bending exceeds the yield strength of the weakest part of the wood, that part will rupture. Once the rupture occurs, we all hear the cracking noise - this represents the propagation of acoustic energy through the air due to the rupture. In other words, the rupture releases energy into the surrounding medium and this energy spreads away from the point of rupture. The farther that one is located from the point of the rupture, the softer the rupture noise is because that finite amount of energy released by the cracking wood is being spread over a larger volume as it moves away from the source.

Fault breaks which results in earthquake

So how does earthquake occurs in the crust?

In the earth's crust and in particular, within the fault zones that accommodate the motion between the rigid interiors of plates, the crust deforms elastically between earthquakes. The faults have geometric irregularities (bends) that prevent the crust on either side of the fault from slipping smoothly (creeping) in response to the steady state motion of the plates on either side of the fault. Because friction prevents steady state slip along a fault, rocks near the fault deform elastically in response to plate motion far from the fault. Once the amount of elastically stored energy exceeds the strength of the weakest area of rocks along a fault, that patch of the fault ruptures. At the point of rupture, rocks on either side of the fault slip to their new location and in the process, release lots of stored energy that propagates away from the point of rupture. A small rupture in one area of a fault can place a sudden strain on a nearby, more strongly locked section of the fault and cause that section of the fault to rupture, too. Thus, one earthquake can trigger another. Faults often have bends; the rocks on the fault face can have different frictional and elastic properties; fluids may lubricate the fault; and other nearby faults may change the local stresses.

Once an earthquake has occurred along a section of a fault, much of the stress on those rocks is relieved. However, since steady state plate motion is still occurring, stress immediately begins to build again, leading to the earthquake cycle in which repeat earthquakes occur along sections of a fault. The frequency and strength of earthquakes along a given fault depends on how quickly the stress builds, how weakly or strongly the fault is locked in a particular region, and interactions with other nearby faults that are also responding to the stress build up. This makes it difficult to to model the earthquake cycle.

Rupture and propagation of seismic energy

To understand the propagation of seismic waves one can demonstrate it by throwing a stone into a pond. Well ripples on the pond carries energy away from the point of impact. Some of the energy is also carried down into the pond as sound, which we could hear if we were beneath water when the stone was thrown. In a similar fashion, during an earthquake rupture, two broad categories of seismic waves are generated.
  • Body waves, which carry seismic energy through the interior of the earth
  • Surface waves, which carry seismic energy along the surface.
  • Body waves can be further sub-divided as follows:
  • P wave (primary) is Compressional. Particles displaced in direction of energy propagation
  • S wave (secondary) is Shearing. Particles displaced perpendicular to direction of energy propagation.
Surface waves, which cause the earth's surface to roll as they pass by are often responsible for the majority of earthquake damage. Surface wave amplitudes can reach several meters, meaning that during a large earthquake, one end of your house could be in the trough of a surface wave several meters beneath the other end of your house which could be surfing on the crest of a surface wave. Surface waves travel slowly often take several minutes or longer to travel tens of miles. Body waves arrive within seconds but aren't as likely to cause major shaking.

Why are seismic waves useful?

Seismic waves are useful for locating earthquakes, determining the amount of energy that was released, and determining what type of fault slip occurred. Seismologists routinely exploit this information using a global network of seismographs that continuously feed their readings into several analysis centres. Earthquake locations (epicentres) and magnitudes are typically available less than an hour after an earthquake. 

To find the location, three things required to completely describe the location of an earthquake 
  • Its latitude 
  • Its longitude 
  • Its depth. 
These three together describe the earthquake focal point, which is the point within the earth where an earthquake started to rupture a fault. The point on the earth's surface directly above the focus is called the epicentre.
Magnitude: The magnitude of an earthquake measures the total amount of ground shaking produced near the epicentre. There are many scales to measure the magnitude of an earthquake but the most used one is Richter scale Richter magnitudes vary from 1 to about 9, with 1 being very small and 9 being enormous. In general, an increase of 1 point in the magnitude represents a 10X increase in the amount of ground motion and a 31X increase in the amount of energy release.
Intensity: An alternative way to measure the size of an earthquake is by its effect on humans and surface features such as buildings. This technique has shortcomings because it depends on the often subjective observations of individuals. However, for earthquakes that occurred before regular instrumental recording made it possibly to routinely estimate earthquake magnitudes, estimates of intensity are the only way to locate epicentres and determine how large the earthquake was. 

Earthquake Risk factors

  • Fault movement: direct breakage of structure built on fault trace.
  • Ground Shaking: ground vibration caused by seismic waves travelling away from focus.
  • Landslide: ground shaking can induce failure of weak slopes.
  • Liquefaction: ground shaking of wet soil can induce creep of soil.
  • Tsunami: disturbance of sea floor causing seismic sea wave.
  • Fire: rupture of gas lines etc.

Earthquakes

Rocks stores energy and release which results is earthquakes. Stress is a force which is exerted on an object and in Earth scenario these forces are tectonic forces. Tectonic stress is developed by the movement of plates. Every object has elastic limit where it can come back to its original position, however in the case of rocks when stress is applied it deforms according to the force direction. When stress is released it bears the shape of deformation and do not comes back to its original position, this is called plastic deformation. By this deformation earthquakes does not occur. In other condition where deformation is elastic, so every object has its limit beyond which it ruptures. Under elastic deformation beyond limit it ruptures brittle and the elastic energy stored by rocks is released and surrounding rocks elastically comes back to its original position and this creates vibrations and travel through crust which are earthquakes. Earthquakes are also produced when already present fault block slip along each other.

Earthquake waves

Waves travel through Earth's surface and reflect back to surface or refract through interior. Waves that travel through the rocks are called seismic waves and seismic waves can be produced both by earthquakes and explosions. Study of earthquakes and getting evidence of Earth's interior based on seismic waves. Earthquake produces different types of seismic waves,body waves generated from the rupture point which is called focus. The Earth's surface above the focus point is called epicenter. Body waves carry energy from the focal point of the earthquake and then from the epicenter surface waves initiates. Surface waves are just like waves on the surface of water.

Body waves

Body waves are of two types which travel through the Earth. P waves and S waves. P wave is also called compressional wave because of its nature as an elastic wave causes alternate compression and expansion of the rock. P waves travel with a speed of 4-7 km in Earth's crust and about 8 km per second in the uppermost part of the mantle. P waves are primary waves because they are first to reach the surface as its velocity is huge. 
The second type is S waves which a shear waves and it moves just like water waves. S waves motion can be illustrated when you tie a rope and on the other end jerk it up and down. S waves is slow wave that travel at speed of 3-4 km per second in the crust. 
P waves can travel in the molten rocks but S waves can only pass through solid rocks because of the less bounding between liquid and gases.

Surface waves

Surface waves travel much slowly than the body waves and there are two types of surface waves. Rayleigh wave which motion is similar to that of ocean wave, up and down. the other is Love wave and its motion is side to side (left right).