earthquake basics (Part 1)

(1)

Earthquake Basics

(Part 1)

(2)

Earthquake Basics

• Earthquakes occur because of a sudden

release of

stored energy when the rocks

_{stored energy}

exceed the elastic limit.

• This energy has built up over long periods

of time as a result of tectonic forces within

the earth.

• Most earthquakes take place along

faults

in

the upper 40 kilometers of the earth's

(3)

(4)

(5)

(6)

Earthquakes

• The vibration of Earth produced by the rapid

release of stored energy.

• Focus -

Focus -

place underground where earthquake originates

• Epicenter -

Epicenter -

Spot on the surface immediately above the

focus

• Fault -

Fault -

Plane where movement occurs

(7)

Elastic Rebound Theory

• Theory that explains why earthquakes occur:

1. Ground moves causing rocks to deform

2. Rocks break  Earthquake 3. Rock’s shape goes back to

original shape, but in different positions

• Time 1_{Time 1} – A farmer builds a stone wall across a fault a few years after the last movement occurred

(8)

– Time 3_{Time 3} – Before the next movement, – Before the next movement, a new fence is built across the

a new fence is built across the

already-deformed land. When the

stress exceeds the strength of the

fault (

fault (elastic limitelastic limit), movement ), movement

begins at the focus. The movement

quickly occurs across the fault (an

earthquake).

– Time 4Time 4 – The rupture displaces the – The rupture displaces the fault, decreasing the stress and the

fault, decreasing the stress and the

elastic rebound restores the blocks

to their pre-stressed state. The

rebound straightens the rock wall,

but wooden fence exhibits a reverse

curve.

(9)

(10)

Seismic Waves

• Seismic waves

– Are the energy released during an

earthquake that travel through the Earth

• Two Types:

–

Body waves

_{Body waves}

• Travel through the earth's interior

–

Surface Waves

_{Surface Waves}

(11)

S-Wave (secondary)

• vertical shearing motion

perpendicular to direction

of wave propagation

“Body Wave” Motions

P-Wave (primary)

(12)

Body Wave Motions

P-wave (primary)

(13)

Seismic P (compressional) and S (shear) wave

propagation. Both are

body waves

which travel

(14)

Compressional Wave (P-Wave) Animation

Deformation propagates. Particle motion consists of alternating

compression and dilation

(15)

Shear Wave (S-Wave) Animation

Deformation propagates. Particle motion consists of alternating

transverse motion

. Particle motion is perpendicular to the

(16)

Rayleigh Waves

• complex,

orbital,

• “water wave” type of motion

Surface Wave Motions

“

Love” Waves

(17)

Love Wave Animation

Deformation propagates. Particle motion consists of alternating

transverse motion

(18)

Rayleigh Wave Animation

Deformation propagates. Particle motion consists of

elliptical

motions in the vertical plane

and parallel to the direction of

(19)

Rayleigh and Love wave propagation. Both are

surface waves. Raleigh waves travel in the direction

of wave propagation, Love waves travel

(20)

Travel of Seismic Waves

• P and S waves travel through Earth’s interior



(mantle and core: info. on density and composition)

• Surfaces waves (Love, Rayleigh) travel through the crust



info about the crust

(21)

P, S, and Surface

Wave

Motion

P - Waves

- are “primary waves”. They travel with a velocity that depends on the elastic properties of the rock through which they travel. P-waves are the same thing as sound waves. They move through the material by

compressing it. The velocity of the P-wave depends on how easily the

material can be compressed, how rigid the material is, and the density of the material.

_S-Waves

(22)

P, S, and Surface

Wave

Motion

Surface Waves

- Surface waves differ from body waves in that they

do not travel through the Earth, but instead travel along paths

nearly parallel to the surface of the Earth. Surface waves behave

like S-waves in that they cause up and down and side to side

(23)

• The difference in the speed of travel of the two waves has to do with what has to happen to the medium (the ground or the earth) to transmit the energy of the seismic wave.

•The P wave is a longitudinal wave or a compression wave. The force is applied in the direction that the wave is traveling. The ground must move in that direction. Ground, or earth , is pretty incompressible, so the energy is transferred pretty quickly.

•In the S wave, the medium is displaced in a transverse (up and down - compared to the line of travel) way, and the medium must shearshear or "move away" from the material right next to it to cause the shear and transmit the wave. This takes more time, and this is why the S wave moves more slowly than the P wave in seismic events.

Why do p-waves travel faster than s-waves?

(24)

Recording Earthquakes on Seismograms

vertical ground motion

(25)

Detecting and measuring seismic waves

Seismometers

:

• The paper roll moves with the ground motion

(26)

Seismographs

HOW SEISMOGRAPHS WORK

(27)

Tells you:

1) How far away the earthquake occurred, based on the

time difference between p and s –wave arrivals

2) Magnitude of ground motion, based on the amplitude

of the surface waves

(28)

Richter Earthquake Magnitude Scale

• Richter scale (M = -2 to infinity); earthquakes range

from ~ 2 to ~ 9.5. Largest earthquake ever recorded

Chile, 1960, M = 9.5

• Based on maximum amplitude of seismogram recording.

(29)

Richter Earthquake Magnitude Scale

• Richter scale (M = -2 to infinity); earthquakes range

from ~ 2 to ~ 9.5. Largest earthquake ever recorded

Chile, 1960, M = 9.5

• Based on maximum amplitude of seismogram recording.

• Each whole number increase in magnitude corresponds

to a release of 32 more energy.

(30)

• Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfoldtenfold increase in measured amplitude.

• As an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 32 times more energy than the amount associated with the preceding whole number value.