DID YOU WRITE THIS IN YOUR NOTES?
- energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond.
- 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!
8. How are earthquakes recorded?
8. Earthquakes are recorded by instruments called seismographs.
- The recording they make is called a seismogram.
- 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.
- 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.
9. 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 .
- 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.
10. 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.
- The first to arrive at the seismograph: Primary Waves or 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.
- P waves travel faster and shake the ground where you are first.
- 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?
Know this Vocabulary for Earthquakes: Faults, Seismic waves, S Waves, P waves, Surface Waves, Focus, Epicenter, Tension, Shearing, Compression, Reverse fault, Normal Fault , Strike-slip Fault, Hanging Wall, Footwall
credits to: https://earthquake.usgs.gov/learn/kids/eqscience.php