EPS 101 Spring 2003 – Adrian Brearley – Lectures 24-26

Earthquakes (Chapter 16)

Key concepts

Know:

1. What an earthquake is and what types of seismic waves are produced during an earthquake.
2. How earthquakes are located and measured
3. How this information can tell us more about the earth.

Earthquake – a shaking or vibration of the ground.

Elastic rebound theory – theory that explains why earthquake occurs – caused by the sudden release of elastic energy stored up in rocks as a result of deformation. Usually occurs because a locked fault plane ruptures and causes a movement along the fault.

Slip – amount of displacement along a fault plan after movement along the fault.

Focus – the point on the fault plane at which slip initiates or starts.

Epicenter – the geographic point on the Earth’s surface directly above the focus of the earthquake.

Seismic waves – intense vibrations that travel outwards through the earth from the earthquake focus. Close to the epicenter can cause a violent shaking of the ground.

Seismograph – highly sensitive instrument used for the detection of earthquakes.

Seismic waves

P-waves – primary waves – the first seismic waves to arrive at a seismograph, because they have the highest velocities. Also termed compressional waves, because they travel through solids as a series of compressions and expansions. The motion of particles in a compressional wave is along the direction that the wave is moving. P waves can travel through solid, liquids or gases, like sound waves. Travel at about 5 km/s

S-waves – secondary waves – second group of seismic waves to arrive at the seismograph. Travel at half the speed of P-waves. S-waves are shear waves, because they push material at right angles to their path of travel. Can only travel through solids.

Surface waves – seismic waves that only travel at or close to the surface of the earth.

Size of an Earthquake

Richter magnitude – measure of the intensity of an earthquake based on the size (amplitude) of ground movement caused by the earthquake. Based on a logarithmic scale, i.e. each increment of 1 on the Richter magnitude means the ground movements were 10 times greater..

Moment magnitude – measure of the intensity of an earthquake prefered by seismologists. It is a more direct measure of the energy released by an earthquake at the focus and is the product of the slip on the fault plane, the area of the fault break and the rigidity of stiffness of the rock.

Other effects associated with earthquakes

Tsunami - a tidal wave produced by earthquakes on the sea floor. Can reach speeds of 800 km per hour with waves 20 meters high when they reach the coast.