All you need to know about Earthquakes (Nepal Updates)

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What is an earthquake?

• An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. 

• The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

• Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens.

• The largest, main earthquake is called the mainshock. Mainshocks always have after shocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

What causes earthquakes and where do they happen?

• The earth has four major layers: the inner core, outer core, mantle and crust. 

• The crust (Fig.2) and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. 

• Not only that ((Fig. 3) ), but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle piecestectonic plates, and the edges of the plates are called the plate boundaries.

• The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving.

• Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

Why does the earth shake when there is an earthquake?

• While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up.

• When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released.

• The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us! (see P&S Wave inset)

How are earthquakes recorded?

• Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram.

• The seismograph (Fig. 4) has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

How do scientists measure the size of earthquakes?

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface.

So how do they measure an earthquake?

• They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (Fig. 5).

• A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

How can scientists tell where the earthquake happened?

(For more about P waves and S waves: Click Here)

• Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important. You learned how P & S waves each shake the ground in different ways as they travel through it.

• P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was.

• To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right after the lightning, but if you are far away from the lightning, you can count several seconds before you hear the thunder. The further you are from the storm, the longer it will take between the lightning and the thunder.

• P waves are like the lightning, and S waves are like the thunder. The P waves travel faster and shake the ground where you are first. Then the S waves follow and shake the ground also. If you are close to the earthquake, the P and S wave will come one right after the other, but if you are far away, there will be more time between the two. By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. However, they can’t tell in what direction from the seismograph the earthquake was, only how far away it was. If they draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle. But where?

• Scientists then use a method called triangulation to determine exactly where the earthquake was (Fig. 6). It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. 

• If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!

How are quakes classified?

Based on their magnitude, quakes are assigned to a class. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows:

• Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage.
• Major: Magnitude in the rage of 7.0 to 7.9. A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage.
• Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage.
• Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage.
• Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage.
• Minor: Magnitude in the rage of 3.0 to 3.9.
• Micro: Magnitude less than-3.0. Quakes between 2.5 and 3.0 are the smallest generally felt by people.

After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon.

What are the seismic zones of India?

• The Geological Survey of India (GSI.) first published the seismic zoning map of the country in the year 1935.

• With numerous modifications made afterwards, this map was initially based on the amount of damage suffered by the different regions of India because of earthquakes.

• Color coded in different shades of the color red, this map shows the four distinct seismic zones of India. Following are the varied seismic zones of the nation, which are prominently shown in the map:

Zone - II: This is said to be the least active seismic zone
Zone - III: It is included in the moderate seismic zone
Zone - IV: This is considered to be the high seismic zone
Zone - V: It is the highest seismic zone

Can scientists predict earthquakes?

• No, and it is unlikely they will ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.

• First, a statistical study of historical earthquakes in a given region shows how frequent quakes of various magnitudes have been in the past. From those figures, they can guess how likely future quakes will be. For example, if a certain region has been struck by four magnitude 7 quakes in the last 200 years, seismologists would say there is a 50 per cent probability of another such quake occurring in the next 50 years.

• Seismologists can also measure how much stress a certain region of the earth's crust is under, and how quickly that stress is increasing. That knowledge, along with the elapsed time since the last earthquake, helps scientists determine if another earthquake is on the way. This method is far from perfect, though, and requires extremely detailed seismological data, which simply isn't available for most regions.

Is there such a thing as earthquake weather? Can some animals or people tell when an earthquake is about to hit?

• These are two questions that do not yet have definite answers. If weather does affect earthquake occurrence, or if some animals or people can tell when an earthquake is coming, we do not yet understand how it works.

Major Earthquakes (1976-2015 Till now dates):

Date	Location	Magnitude	Estimated number of deaths
April 25,  2015	Nepal (Easten Pokhara & Kathmandu)	7.9	1600+
April 16,  2013	Iran	7.7	40
Sept. 5, 2012	Costa Rica	7.9	Tsunami alert
Aug. 31, 2012	Philippines	7.6	1
Aug. 11, 2012	Northwestern Iran	6.4 and 6.3	306
April 11,  2012	Indian Ocean	8.6	Tsunami alert issued, no major damage
Oct. 23, 2011	Turkey	7.2	603
April 7, 2011	Japan	7.1	3
March 11,  2011	Japan	9.0	Approx. 16,000 (9,000 missing)
Feb. 27, 2010	Chile	8.8	At least 700
Jan. 13, 2010	Haiti	7.0	85,000 to 316,000 (gov't estimate)
Sept. 30, 2009	Near Padang, Indonesia	7.6	1,110
April 6, 2009	Near L'Aquila, Italy	6.3	283
May 12, 2008	Sichuan province, southwest China	7.8	More than 80,000
May 27, 2006	Java, Indonesia	6.3	5,135
Oct. 8, 2005	Pakistan-administered Kashmir	7.6	80,000
March 29,  2005	Indonesia	8.7	1,000
Dec. 26, 2004	Off the coast of Indonesia	9.0	230,000 (includes tsunami victims)
Dec. 26, 2003	Bam, Iran	6.6	26,271
Jan 26, 2001	Gujarat, India	7.7	20,000
June 21, 1990	Iran	7.7	35,000
Dec. 7, 1988	Armenia	6.9	At least 25,000
Sept. 19,  1985	Mexico	8.1	9,500
Sept. 16,  1978	Iran	7.5 to 7.9	15,000
July 28, 1976	China	7.8 to 8.2	242,000
Feb. 4, 1976	Guatemala	7.5	At least 23,000

U also like to see Earthquake Science Explained by USGS: Click Here

Source: usgs.gov, wikipedia.org