Download CAUSES OF EARTHQUAKES Ch. 3, pp. 75

CAUSES OF EARTHQUAKES
• How do earthquakes happen?
• What kinds of seismic waves are
produced by 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 theory
Turkey, 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
A rock
Apply a force
(stress)
Deformation
(strain)
Release the force
(elastic rebound)
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
http://civilx.unm.edu/laboratories_ss/mechmat/
bendwoodfracture.jpg
Brittle materials store elastic strain energy
but break if a threshold stress is exceeded
Elastic materials store and
release elastic energy
(bounce back)
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
One idea about how faults work at depth
rock formerly in the lower
crust, now exposed next to a
highway in western
Massachusetts
plate 1
plate 2
elastic upper and
middle crust
ductile lower crust (for continents)
Fig 3.2
uppermost crust fault stuck
between
earthquakes
ductile creeping fault
zone at depth - no
earthquakes
shear zones can merge
at depth and many
ductile layers and zones
can be present
Elastic Rebound Theory
Fig. 3.3
Elastic stresses
build up as rock
deforms slowly
over time
Rupture occurs when elastic stresses
exceed what the fault can bear
(friction).
Rocks along fault spring back to
undeformed state (“elastic rebound”)
What happens along the fault during
an earthquake?
1994 M = 7.4 Landers,
California earthquake
!!!!! epicenter
hypocenter
Maximum slip is not usually at the hypocenter
Rupture propagates away from the hypocentre at
about 2-3 km / sec (slower near surface)
Rupture begins at the hypocenter and travels
away (!unilateral! = one-way, "bilateral! = both
ways)
Rupture is usually discontinuous (or absent) at
the surface, but likely continuous (smooth) at
depth
Peyrat et al., 2000
Animation of the Landers earthquake rupture
SEISMIC WAVES
Plummer, Canadian First
Edition, Ch. 3 pp. 76-77
from the USGS Earthquake
Hazards Team website
First, Two Questions:
Seismology
• Earthquake seismologists
– Describe, explain, assess, predict(?) earthquakes
• What is seismology?
• What do seismologists do?
• 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
Two Kinds of Seismic Waves
P (Primary) wave: particles move
in line with wave direction
body wave: travels through the inside
the Earth
surface wave: travels along the surface
of the Earth
body wave
Body Waves
S (secondary) wave: particles move
perpendicular 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
Rayleigh wave: particles move
elliptically as in ocean waves
surface wave
Love wave: particles move
perpendicular to wave direction,
and horizontal
surface wave
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)