The Science of Earthquakes
The Science of Earthquakes
Originally written by Lisa Wald for “The Green Frog News”
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 aftershocks 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. (figure 2) The crust 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. (figure 3) Not only that, but these puzzle pieces keep slowly   moving around, sliding past one another and bumping into each other. We call   these puzzle pieces tectonic 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. (figure 4) The   seismograph 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 (figure 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?
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 (figure 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!
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.
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.