how do earthquakes happen? • what kinds of seismic waves ...€¦ · earthquakes uppermost crust...

• How do earthquakes happen? • What kinds of seismic waves are

produced by earthquakes?

CAUSES OF EARTHQUAKES

Ch. 3, pp. 75-76, Plummer Canadian First Edition

What is an earthquake?• Sudden release of elastic energy (snap)• Induced by stress buildup• Released when stress > strength of

fault• Energy released as

– seismic waves– displacement along faults– heat and other energy

• Elastic rebound theoryTurkey, 1999

• Elastic deformation– relatively small forces– small displacements– materials shape restored when force is removed– energy can pass as waves

• Plastic (or ductile) deformation– like putty – materials respond to forces by changing shape– no storage of energy

• Brittle deformation – material stores energy put in by the force. They accumulate stress.

– at some point the material will break– catastrophic release of energy– repeated breaking of pre-existing weak surfaces

(faults)

Cold rock is brittle (for example, ocean lithosphere and most of continental lithosphere)

Hot rock is ductile (for example, the asthenosphere and some parts of continental plates)

Both lithosphere and asthenosphere act elastic if stress change is sudden and small (e.g., seismic waves)

In ductile materials, elastic stress does not build up and earthquakes don’t happen

Elastic materials store and release elastic energy

If there is too much strain, the rock will 1) deform or flow (plastic or ductile deformation) 2) break, or slip along an existing fault (brittle deformation) - earthquake! - rapid release of elastic strain energy, some in the form of vibrations or waves

Deformation(strain)

Apply a force(stress)

Release the force(elastic rebound)

A rock

Elastic materials store and release elastic energy

(bounce back)

http://civilx.unm.edu/laboratories_ss/mechmat/bendwoodfracture.jpg

Brittle materials store elastic strain energy but break if a threshold stress is exceeded

Brittle materials containing faults: the fault will slip if a threshold stress is exceeded

Plastic or ductile materials flow and do not really hold elastic strain energy

rock formerly in the lower crust, now exposed next to a highway in western Massachusetts

One idea about how faults work at depth

ductile creeping fault zone at depth - no earthquakes

uppermost crust -fault stuck between earthquakes

shear zones can merge at depth and many ductile layers and zones can be present

ductile lower crust (for continents)

plate 1

plate 2

elastic upper and middle crust

Fig 3.2

Rupture occurs when elastic stresses exceed what the fault can bear (friction).

Elastic stressesbuild up as rockdeforms slowly over time Rocks along fault spring back to

undeformed state (“elastic rebound”)

Elastic Rebound Theory

Fig. 3.3

What happens along the fault during an earthquake?

1994 M = 7.4 Landers, California earthquake

epicenter

Peyrat et al., 2000

hypocenter

!!!!!

Maximum slip is not usually at the hypocenter

Rupture begins at the hypocenter and travels away (’unilateral’ = one-way, ‘bilateral’ = both ways)

Rupture propagates away from the hypocentre at about 2-3 km / sec (slower near surface)

Rupture is usually discontinuous (or absent) at the surface, but likely continuous (smooth) at depth

Animation of the Landers earthquake rupture

from the USGS Earthquake Hazards Team website

SEISMIC WAVESPlummer, Canadian First Edition, Ch. 3 pp. 76-77

• What is seismology?

• What do seismologists do?

First, Two Questions:

Seismology• Earthquake seismologists

– Describe, explain, assess, predict(?) earthquakes• Global seismologists

– Map deep Earth structure using seismic waves generated by earthquakes

– Goal is to understand whole Earth dynamics• Exploration seismologists

– Make their own earthquakes (explosives, etc.)– Search for valuable resources (water, oil, gas)– Image much smaller regions than global

seismologists

surface wave: travels along the surface of the Earth

body wave: travels through the inside the Earth

Two Kinds of Seismic Waves

body wave

P (Primary) wave: particles movein line with wave direction

S (secondary) wave: particles moveperpendicular to wave direction body wave

• P Waves (Primary, or Compressional) - change in volume of the material - particle motions are parallel to the direction the wave travels - the wave spreads out in all directions from the earthquake in 3D (spherical spreading) - fastest seismic wave• S Waves (Shear, or Secondary) - change in shape of the material - particle motions are perpendicular to the direction the wave travels - spherical spreading - slower than P wave

Body Waves

surface wave

Rayleigh wave: particles moveelliptically as in ocean waves

surface wave

Love wave: particles moveperpendicular to wave direction,and horizontal

Surface waves• Love waves:

– like S-waves with no vertical displacement– Ground moves side-to-side horizontally– destroy buildings, major seismic hazard– circular speading from a point (2D), like a pebble in a

pond• Rayleigh waves:

– like ocean rolling waves with vertical and horizontal displacement

– elliptical particle motion– circular spreading from a point (2D)