(PT) Measuring Earthquakes Lesson
Measuring Earthquakes
Every day, worldwide, there are about 8,000 earthquakes. The sudden release of energy causes seismic waves – and it's the seismic waves that make the earth shake. In order to figure out where an earthquake was centered, you need to know where that earthquake began. We know earthquakes bein in rock below the surface. Most earthquakes start in the Lithosphere. The focus is the point benearth Earth's surface where rock that is under stress breaks, triggering an earthquake. The point on the surface directly above the focus is called the epicenter.
Seismic Waves
As we said, it's seismic waves that make the earth shake.
There are three categories of seismic waves:
- P-waves
- S-waves
- Surface waves
An earthquake sends out two types of waves from its focus: P waves and S waves. When these waves reach Earth's surface at the epicenter, surface waves develop. The instrument that measures seismic wave vibrations is called a seismograph.
How are earthquakes recorded?
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. 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. 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.
The Focus and the Epicenter
Epicenter
The epicenter of an earthquake is determined by triangulation. This means that seismic data is needed from at least three different locations, and where this data intersects tells us the epicenter. When an earthquake occurs, it is recorded on numerous seismographs located in different directions. The seismograms at these locations show when the first seismic waves, the P waves, arrive and then when the next waves, the S waves, arrive. Knowing how fast each of these waves travel, scientists can calculate how far away the epicenter was from each seismograph. What they don't know is the precise direction the waves came from–the direction of the epicenter. Scientists then must use a map. Around each of three seismograph locations, a circle is drawn on the map with a radius that equals the known distance to the epicenter. These three circles intersect at a single point. This point is the location of the earthquake's epicenter. The spot directly above the focus (on top of the ground or ocean) is called the epicenter of the earthquake.
Focus
The underground spot where the rock breaks is called the focus of the earthquake.
It is usually found at depths less than 70 km.
Shallow focus earthquakes start above 70km, while deep focus earthquakes begin below 300km.
Seismologist
A seismologist is a scientist who studies earthquakes.
Richter Scale
The Richter Scale is used to determine the magnitude (size) of an earthquake.
Mercalli Scale
The Mercalli scale is used to rate earthquakes by their intensity. An earthquake's intensity is the strength of ground motion in a specific location. The Mercalli scale includes 12 steps to describe how earthquakes affect people, buildings, and the land surface. This scale is not as accurate and a given earthquake could be rated differently when using the scale to measure because different areas may receive different amounts of damage.
Earthquake Intensity |
Earthquake Effects |
---|---|
I-II |
Almost unnoticeable |
III-IV |
People notice vibrations like those from a passing truck. Unstable objects disturbed. |
V-VI |
Dishes and windows rattle. Books knocked off shelves. Slight damage. |
VII-VIII |
People run outdoors. Moderate to heavy damage. |
IX-X |
Buildings jolted off foundations or destroyed. Cracks appear in ground and landslides occur. |
XI-XII |
Severe damage. Wide cracks appear in ground. Waves seen on ground surface. |
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