UseIT Tutorial # 3
Earthquakes in the
Southern California Fault
System
Tom Jordan
June 16, 2011
Three Types of Plate Boundary
Transform Fault
lateral motion
Spreading Center
divergent motion
Subduction Zone
convergent motion
Present-Day Mosaic of Plates
Active Faulting in California
Direction of
Pacific Plate
motion
San Andreas Fault System
San Andreas System
Pacific - North America Plate Boundary
Pacific - North America Plate Boundary
Pacific - North America Plate Boundary
Significant Earthquakes in Southern
California during the 20th Century
Los Angeles Region
Los Angeles Region
San Andreas
fault
Los Angeles Region
Hollywood-Santa Monica-Malibu Coast
fault
Palos Verdes
fault
Santa Ynez
fault
Newport-Inglewood
Whittier
fault
fault
Raymond
fault
San Andreas
fault
Sierra Madre
fault
Puente Hills “Blind” Thrust Fault
What causes earthquakes?
Sudden slip on a fault that has
reached its breaking strength
(“tectonic” earthquake)
Earthquakes on the
San Andreas Fault
On average, large
earthquakes recur on the
San Andreas fault about
every 100-150 years
1906
M 7.9
Pacific plate
motion relative to
the North
American plate
50 mm/yr
1857
M 7.9
1680
M 7.7
San Andreas Fault
Offset by 130 m in 3700
years
130 m / 3700 yr = 35 m/kyr
5 m of slip per eqk implies
~ 7 eqk/kyr or, on average,
~ 1 eqk every 140 yr
1906 San Francisco Earthquake
Fence built across San Andreas faults near Bolinas,
California, was offset by 3 m
ROCKS DEFORM ELASTICALLY, THEN
REBOUND DURING AN EARTHQUAKE
RUPTURE
TIME 1
A farmer builds a stone wall across a
strike-slip fault.
Strike-slip
fault
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.
Rocks deform
as strain
develops
Strike-slip
fault
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.
Rocks deform
as strain
develops
TIME 3
A new fence is built across the alreadydeformed land.
Epicenter
Focus
Strike-slip
fault
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.
Rocks deform
as strain
develops
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.
Epicenter
Focus
Strike-slip
fault
Reid’s (1910) Elastic Rebound Theory
fault
displacement
fault
trace

time
stress
Recurrence Interval
Yield
stress
Map view
Base
stress
time
Reid’s (1910) Elastic Rebound Theory
~ 150 years
What happens
during the
earthquake?
Focus
0 Seconds
Rupture expands circularly on
fault plane, sending out seismic
waves in all directions.
Fault cracks
at surface
Rupture expansion
during a large (M7)
earthquake
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.
Foreshocks and Aftershocks
Aftershocks of 27
Feb 2010 Chile
Earthquake (M8.8)
500 km
Two Ways to Measure
Earthquake Size
• Magnitude
– Measures the size of the rupture on a
fault (e.g., on the San Andreas fault)
• Intensity
– Measures the size of the ground
shaking at a particular site (e.g., here in
this classroom)
Earthquake Magnitude
For each increase of 1 unit in magnitude:
– Energy increases by a factor of 33
– Fault area increases by a factor of 10
– Fault slip increases by a factor of 3.3
30 km x 20 km = 600 km2
Aftershocks of 27
Feb 2010 Chile
Earthquake (M8.8)
500 km
600 km x 100 km = 60,000 km2
Earthquake Magnitude
For each increase of 1 unit in magnitude:
– Energy increases by a factor of 33
– Fault area increases by a factor of 10
– Fault slip increases by a factor of 3.3
Frequency-Magnitude Statistics
An increase of one magnitude unit corresponds to an order of
magnitude decrease in the number of earthquakes.
Shaking Intensity
Shaking Intensity
Isoseismic (“equal
shaking”) map for the
Northridge
earthquake of
January 17, 1994
(M 6.7)
Shaking Intensity
Isoseismic (“equal
shaking”) map for the
great San Francisco
earthquake of April
18, 1906
(M7.8)
Shaking Intensity
M 7.6
500 km
500 km
M 7.9
End Tutorial #3