Measuring Earthquakes Chapter 8.2 Measuring Earthquakes

Seismographs are instruments that record earthquake waves. When waves from an earthquake move a seismograph a weight stays in place but the earth vibrations. Current seismograms amplify and electronically record ground motion.

Earthquake Waves There are 2 types of seismic waves – surface waves and body waves

Surface Waves These are waves that travel along the outer layer of the Earth. These waves cause the ground and anything resting on the ground to move. Surface waves move like ocean waves – up and down and side to side. These are the most destructive waves.

Body waves These waves travel through the Earth’s interior. They are either P waves or S waves. P waves are push-pull waves, they are also called compression waves. S waves shake the particles at right angles to their direction of travel. A seismogram shows all 3 types of waves. The waves usually arrive in this order: P waves, S waves, and then surface waves.

Locating an Earthquake The distance between P and S waves help indicate how far away the earthquake was.

Earthquake Direction To find the direction of the earthquake they take reading from several stations and see where they overlap. That gives the direction.

Earthquake zones Many earthquakes occur on the edge of the Pacific Ocean. Another area is around the Mediterranean Sea.

Measuring Earthquakes There are 2 different measurements for earthquakes – intensity and magnitude. Intensity is the amount of shaking at a given location based on the amount of damage done. Magnitude is the measure of the size of seismic waves or the amount of energy released at the source of the earthquake.

Richter Scale The Richter scale is based on the amplitude of the largest seismic wave. This is an outdated way to measure earthquakes.

Moment Magnitude This measures the amount of displacement along where the slippage occurred (how much the Earth moved). Based on the (surface area of the fault) x (average displacement along the fault) x (rigidity of rock)

Modified Mercalli Scale This is another scale that measures the intensity in terms of the earthquakes intensity at different locations.