EARTHQUAKES
Chapter 13
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
Local
rock
strength
Stress
Time
Earthquakes
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
Local
rock
strength
Stress
Time
Earthquakes
When the stress
exceeds the strength
of the rocks…
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
…the fault
slips, causing
an earthquake.
Local
rock
strength
Stress
Time
Earthquakes
When the stress
exceeds the strength
of the rocks…
STRESS BUILDS UNTIL IT
EXCEEDS ROCK STRENGTH
Earthquakes are
the result of
stress that builds
up over time.
Stress gradually builds
as tectonic forces
deform rocks.
…the fault
slips, causing
an earthquake.
The process
repeats again
and again.
Local
rock
strength
Stress
Time
Earthquakes
When the stress
exceeds the strength
of the rocks…
ROCKS DEFORM ELASTICALLY, THEN
REBOUND DURING AN EARTHQUAKE
RUPTURE
TIME 1
A farmer builds a stone wall across a
strike-slip fault.
TIME 2
The relative motion between blocks on
either side of the locked fault causes the
ground and the stone wall to deform.
TIME 3
A new fence is built across the alreadydeformed land.
TIME 4
The rupture displaces the fault, lowering
the stress. The elastic rebound straightens
the rock wall, but the fence exhibits a
reverse curve.
Rocks deform
as strain
develops
Epicenter
Focus
Strike-slip
fault
Stress may build more
quickly—
or less quickly—…
Local rock
strength
Stress
Time
Earthquakes
…and the strength of
the fault may vary,
causing earthquakes
to occur at varying
times and with
varying amounts of
slip.
Focus
0 Seconds
Rupture expands circularly on
fault plane, sending out seismic
waves in all directions.
Fault cracks
at surface
5 Seconds
Rupture continues to expand
as a crack along the fault plane.
Rocks at the surface begin to
rebound from their deformed
state.
Fault crack
extends
10 Seconds
The rupture front progresses
down the fault plane, reducing
the stress.
20 Seconds
Rupture has progressed along
the entire length of the fault.
The earthquake stops.
Spring
Mass
Spring
Mass
The mass is loosely
coupled to Earth.
Earth
moves up
Recording
pen
Earth
moves up
Recording
pen
Upward movement of the Earth causes
downward relative movement of the mass,
and vice versa.
Earth
moves down
Earth
moves down
The pen traces the
differences in motion.
Earth
moves left
Earth
moves right
Earth
moves side
to side
Mass
Hinge
Seismic waves travel through
Earth and over its surface.
Body waves
P waves
S waves
Focus
Surface waves
Mantle
Love waves
(left-right, or back
and forth)
Rayleigh waves
(rolling)
Core
S P
Seismograph
The waves travel at different speeds and
arrive at the seismograph at different times.
Minutes
0
10
20
30
40
50
Surface waves
P
S
P waves are compressional
waves that travel quickly
through rock.
Compression
wave
P waves push and pull
particles in the direction of
their path of travel.
A section of rock expands and then
contracts
Shear-wave
crest
S waves
Shear-wave
crest
S waves push material at
right angles to their path of
travel.
S waves travel at about half
the speed of P waves.
A section of rock shears from a
square to a parallelogram.
Surface waves ripple across
Earth’s surface.
The ground surface moves in a
rolling, elliptical motion.
Wave direction
Rayleigh wave
The ground shakes sideways,
with no vertical motion.
Wave direction
Love wave
Seismic waves arrive at
distant seismographic
stations at different times.
Seismograph
Seismograph
Epicenter
Seismograph
Focus
Time elapsed after start
of earthquake (min)
Because P waves travel
faster than S waves, the
interval between their
travel-time curves
increases with distance.
25 Seismogram
A
By matching the observed
interval to the curves, a
geologist can determine the
distance from the station to
the epicenter.
Seismogram
B
Seismogram
C
20
15
8-minute
interval at
5600 km
S wave
11-minute
interval at
8600 km
P wave
10
5
3-minute
interval at
1500 km
0
2000
4000
6000
8000
10,000
Distance traveled from earthquake epicenter (km)
If the geologist then draws a circle around each
seismographic station,…
Epicenter
B
A
1500 km
C
5600 km
8600 km
…the point at which the circles intersect
is the earthquake’s epicenter.
P
Amplitude
=23 mm
S-wave interval = 24 seconds
…and the time
interval between
the P- and S-waves
to determine the
distance from the
epicenter.
Interval between
S and P waves (s)
Distance (km)
P-wave
S
Richter
magnitude
A geologist
measures the
amplitude of the
largest seismic
wave…
Amplitude
(mm)
P
Amplitude
=23 mm
S-wave interval = 24 seconds
…and the time
interval between
the P- and S-waves
to determine the
distance from the
epicenter.
Interval between
S and P waves (s)
Distance (km)
P-wave
S
Richter
magnitude
A geologist
measures the
amplitude of the
largest seismic
wave…
Amplitude
(mm)
By connecting the
points, the geologist
determines the
Richter magnitude.
Seismographic
stations
Fault
First motion (pull
toward epicenter)
First motion (pull
toward epicenter)
First motion
(push away
from epicenter)
First motion (pull
toward epicenter)
First motion
(push away
from epicenter)
Determining a
left-right or rightleft movement
First motion (pull
toward epicenter)
World seismicity from 1976 to 2002
Mid-ocean ridge (divergence)
Transform fault
(lateral shearing)
Rift valley
(divergence)
Mid-ocean ridge (divergence)
Normal faulting
Transform fault
(lateral shearing)
Rift valley
(divergence)
Mid-ocean ridge (divergence)
Normal faulting
Transform fault
(lateral shearing)
Rift valley
(divergence)
Shallow earthquakes coincide with normal
faulting at divergent boundaries and with
strike-slip faulting at transform boundaries.
Deep-ocean trench (convergence)
Large shallow
earthquakes occur
mainly on thrust faults.
Intermediate- and deepfocus earthquakes occur
in the descending slab.
Southern California fault traces
San Andreas fault
Here, the San Andreas fault is
parallel to plate motion, and the
faulting is right-lateral strike slip.
San Gabriel Mountains
North American
Plate
Pacific
Plate
Los Angeles
Motion of Pacific Plate
relative to motion of
North American Plate
The “Big Bend” causes the Pacific
Plate to compress against the
North American Plate, causing
Southern California earthquakes
(July 1970-June 1995)
Northridge 1994
Magnitude 6.9
Key:
5+
<5
July 1970–June 1995
San Fernando 1971
Magnitude 6.7
Landers 1992
Magnitude 7.3
Tsunami generation
Thrust fault
An earthquake produces
a surge of water that
moves outward as a
tsunami.
A tsunami is only a few
centimeters high in the
deep ocean but can
increase to many meters
Computer simulation of tsunami radiation.
North
America
Epicenter
4 hr 42 min
Hawaii
Computer simulation of tsunami radiation.
North
America
Epicenter
4 hr 42 min
Hawaii
Main tsunami wave reaches Hawaiian Islands
about 4.5 hours after the earthquake.
Tsunami Threat
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