Chapter 5.2 Earthquakes and seismic waves

An earthquake is the shaking that results from the sudden movement of rock along a fault.

The focus is the area beneath 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 carry energy from an earthquake away from the focus

Types of seismic waves P waves (primary waves) - The first waves to arrive. P waves compress and expand the ground like an accordion. They can move through solids and liquids.

S waves (secondary waves) – The second wave to arrive. They move side to side and up and down. S waves can only move through solids.

Surface waves – Result when P and S waves reach the surface Surface waves – Result when P and S waves reach the surface. They move more slowly than P an S waves, but produce severe ground movement.

Measuring Earthquakes Mercalli scale – rates earthquakes according to their intensity, or strength at a given place. The same earthquake can have different Mercalli ratings at different locations because the intensity is different at each location.

Richter scale – assigns a magnitude number to an earthquake based on the size of the seismic waves. - Seismic waves are measured using a seismograph, which is an instrument that records and measures seismic waves.

Moment Magnitude scale – a rating system that estimates the total energy released by an earthquake.

The effects of an earthquake increase with magnitude

Locating the Epicenter Geologists use seismic waves to locate an earthquakes epicenter. Seismic waves arrive at different times. The farther away an earthquake the greater the time between the arrival of P waves and S waves.

Scientists use data from 3 different seismograph stations to determine the location of the epicenter