EARTHQUAKES:EARTHQUAKES:WHY? AND HOW?WHY? AND HOW?

EARTHQUAKESEARTHQUAKES

• Caused by plate tectonic stresses

sudden movement or shaking of the Earthsudden movement or shaking of the Earth

• Located at plate boundaries• Resulting in breakage of the Earth’s brittle crust

PLATE TECTONIC STRESSESPLATE TECTONIC STRESSES• Plate boundaries and faults (= cracks where plate sections faults (= cracks where plate sections

are movingare moving in different directions) in different directions) cause friction as plates move

• Plates in a fault zone have STICK-SLIP motion– Periods of no movement (stick)

and fast movement (slip)

– Energy stored as plates stick,

– Energy released as plates slip

EARTHQUAKE DAMAGEEARTHQUAKE DAMAGEMost caused by SURFACE waves (arrive last)

EARTHQUAKE WAVESEARTHQUAKE WAVES• FOCUS FOCUS = place deep within the Earth and along the fault where = place deep within the Earth and along the fault where

rupture occursrupture occurs

• EPICENTEREPICENTER = geographic point = geographic point on surface directly above focus on surface directly above focus

• SEISMIC WAVES produced by the release of energy– move out in circles from the point of rupture (focus)move out in circles from the point of rupture (focus)– 2 types: surface & 2 types: surface & bodybody (travel inside & through earth’s layers) (travel inside & through earth’s layers)

• P waves: back and forth movement of rock; travel thru solid, liquid, gas• S waves: sideways movement of rock; travel thru solids only

Ancient SeismographsAncient Seismographs• Around 132 AD, Chinese scientist Chang

Heng invented the first seismoscope, an instrument that could register the occurrence of an earthquake.

• Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument.

• Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

Modern SeismographsModern Seismographs

• Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

EARTHQUAKE WAVESEARTHQUAKE WAVESSeismographsSeismographs record earthquake waves record earthquake waves

SeismogramsSeismograms show: show:

• Amplitude of seismic waves (how much rock Amplitude of seismic waves (how much rock moves or vibrates) moves or vibrates)

• Distance to the epicenterDistance to the epicenter

• Earthquake directionEarthquake direction

• 3 types of seismic waves show up 3 types of seismic waves show up on seismogramon seismogram

– P waves: shake earth in same direction as wave; travel thru solid, liquid, gas

– S waves: Shake earth sideways to wave direction; travel thru solids only

– Surface waves: circular movement of rock; travel on surface – cause most damage!!

P-Wave(Body Wave)P-Wave(Body Wave)

Primary or compressional (P) waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like water or the liquid layers of the earth.

It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Highest velocity (6 km/sec in the crust)

P-Wave P-Wave

Particle motion

Deformation propagates

Secondary Wave (S Wave) Secondary Wave (S Wave)

Secondary or shear (S) waves The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake.

An S wave is slower than a P wave and can only move through solid rock. (3.6 km/sec in the crust)

This wave moves rock up and down, or side-to-side.

S-WaveS-Wave

Particle motion

Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction. Material returns to its original shape after wave passes.

Deformation propagates

EARTHQUAKE WAVESEARTHQUAKE WAVES

P waves move through solids & liquids

S waves move through solids only!!!

EARTHQUAKE WAVESEARTHQUAKE WAVESBody P waves S waveswaves

AKA

Moves through

Movement of rock

Primary (1st to arrive)

Longitudinal, Compression

all states of matter (solid, liquid, gas)

back and forth movement of rock• push/pull or compression/stretch out• Like slinky down stairs

Vibration is same as the direction of travel

Secondary (2nd to arrive - larger)

Transverse, Shear

Can go through solids only

Move sideways • perpendicular to direction of wave

travel• Like snake

EARTHQUAKE WAVESEARTHQUAKE WAVES

Lets test your understanding!!

Is this a P or an S wave?

P wave!

S Wave

EARTHQUAKEEARTHQUAKE MAGNITUDEMAGNITUDE measures the size of seismic waves measures the size of seismic waves

the energy released by the earthquakethe energy released by the earthquake

Richter scaleRichter scale=measurement of energy released =measurement of energy released based upon wave amplitude (size of vibration)based upon wave amplitude (size of vibration)

• <2 to ~10<2 to ~10

• Amplitude of wave goes up Amplitude of wave goes up by 10 (Logarithmic scale)by 10 (Logarithmic scale)

What you need:What you need:

• Amplitude (size of vibration = wave height)Amplitude (size of vibration = wave height)

• Time between arrival of 1Time between arrival of 1stst P and 1 P and 1stst S waves S waves

HOW TO READ SEISMOGRAMSHOW TO READ SEISMOGRAMS

P & S (body waves) move through earth & arrive first

• P & S waves used to calculate magnitude of earthquake

• Amplitude = height of wave (how much the rock moves; size of vibration)

Seismic Seismic VelocitiesVelocities

Why are seismic waves important? Some things seismic waves are good for include·       Mapping the Interior of the Earth·       Monitoring the Compliance of the Comprehensive Test Ban Treaty·       Detection of Contaminated Aquifers·       Finding Prospective Oil and Natural Gas Locations

IRIS Earth’s Interior Structure Poster – Seismic waves through the Earth

Earth’s interior structure and seismic raypaths that are used to determine the Earth structure.