Part One

Elastic Rebound Theory

• The elastic rebound theory is an explanation for how energy is spread during earthquakes.

• As plates shift on opposite sides of a fault are subjected to force, they accumulate energy and slowly deform until their internal strength is exceeded.

• At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks try to snap back to their original undeformed shape.

Measuring Earthquakes

• A seismogram is a record of the seismic waves from an earthquake. A seismograph or seismometer is the measuring instrument that creates the seismogram. Almost all seismometers are based on the principle of inertia: a suspended mass tends to remain still when the ground moves. The relative motion between the suspended mass and the ground will then be a measure of the ground’s motion.

Two

P-waves: Primary wavesCompressional (Longitudinal) wavesPass through solids and liquids

S-waves: Secondary wavesShear (Transverse) waves~ 2/3 as fast as P-wavesPass through solids but not liquids

Body Waves

Raleigh waves: Retrograde elliptical motion

Love waves: Polarised in plain of Earth's surface

. . . these waves cause trouble for Engineers!

Surface Waves

Raleigh

Love

World Wide Standard Seismograph Network (WWSSN) set up to monitor Nuclear Test Ban treaty.

WWSSN drum seismogram

1964 Alaska earthquakeMagnitude 9.2

Largest measured earthquake in Alaska

Damaged school1964 Alaska earthquake

Analogue WWSSN instruments are being replaced by modern digital instruments in an expanded Global Seismic Network

(GSN)

Modern digital seismometers

Locating an Epicentre

• By looking at the seismograms from different recording stations we can find out the epicentre of the earthquake. The signals arrive first at the closest station and last at the one furthest away. The time difference between the P- and S-waves tells us the distance the earthquake is from the seismograph. By measuring this at three stations we can work out where the epicentre is.

Locating an Epicentre

Locating an earthquake

Liquefaction – San Francisco 1989

Flooding (dam break)