UseIT Tutorial # 3 Earthquakes in the Southern California Fault System Tom Jordan June 16, 2011
Dec 23, 2015
UseIT Tutorial # 3 Earthquakes in the
Southern California Fault SystemTom Jordan
June 16, 2011
Three Types of Plate Boundary
Transform Faultlateral motion
Spreading Centerdivergent motion
Subduction Zoneconvergent motion
Present-Day Mosaic of Plates
Active Faulting in CaliforniaActive 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 Significant Earthquakes in Southern California during the 20California during the 20thth Century Century
Los Angeles Region
San Andreas fault
Los Angeles Region
San Andreas fault
Sierra Madre faultWhittier
fault
Newport-Inglewood fault
Palos Verdes fault
Hollywood-Santa Monica-Malibu Coast fault
Santa Ynez fault
Raymond fault
Los Angeles Region
Puente Hills “Blind” Thrust Fault
What causes earthquakes?
Sudden slip on a fault that has reached its breaking strength
(“tectonic” earthquake)
19061906M 7.9M 7.9
18571857M 7.9M 7.9 16801680
M 7.7M 7.7
On average, large On average, large earthquakes recur on the earthquakes recur on the San Andreas fault about San Andreas fault about
every 100-150 yearsevery 100-150 years
Earthquakes on the San Andreas Fault
Pacific plate Pacific plate motion relative to motion relative to the North the North American plateAmerican plate
50 mm/yr50 mm/yr
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
San AndreasFault
Fence built across San Andreas faults near Bolinas, California, was offset by 3 m
1906 San Francisco Earthquake1906 San Francisco Earthquake
Strike-slipfault
TIME 1A farmer builds a stone wall across a strike-slip fault.
ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE
Rocks deformas straindevelops
Strike-slipfault
ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE
TIME 1A farmer builds a stone wall across a strike-slip fault.
TIME 2The relative motion between blocks on either side of the locked fault causes the ground and the stone wall to deform.
Rocks deformas straindevelops
Strike-slipfault
ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE
Focus
Epicenter
TIME 1A farmer builds a stone wall across a strike-slip fault.
TIME 2The relative motion between blocks on either side of the locked fault causes the ground and the stone wall to deform.
TIME 3A new fence is built across the already-deformed land.
Rocks deformas straindevelops
Strike-slipfault
ROCKS DEFORM ELASTICALLY, THEN REBOUND DURING AN EARTHQUAKE RUPTURE
TIME 1A farmer builds a stone wall across a strike-slip fault.
TIME 2The relative motion between blocks on either side of the locked fault causes the ground and the stone wall to deform.
Focus
Epicenter
TIME 3A new fence is built across the already-deformed land.
TIME 4The rupture displaces the fault, lowering the stress. The elastic rebound straightens the rock wall, but the fence exhibits a reverse curve.
faultdisplacement
time
Yieldstress
time
stress
Reid’s (1910) Elastic Rebound TheoryReid’s (1910) Elastic Rebound Theory
Basestress
faulttrace
Map view
Recurrence Interval
Reid’s (1910) Elastic Rebound TheoryReid’s (1910) Elastic Rebound Theory
What happens during the earthquake?
~ 150 years
Focus0 SecondsRupture expands circularly on fault plane, sending out seismic waves in all directions.
5 SecondsRupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state.
10 SecondsThe rupture front progresses down the fault plane, reducing the stress.
20 SecondsRupture has progressed alongthe entire length of the fault.The earthquake stops.
Fault cracksat surface
Fault crackextends
Rupture expansion Rupture expansion during a large (M7) during a large (M7) earthquakeearthquake
Foreshocks and AftershocksForeshocks and Aftershocks
Aftershocks of 27 Aftershocks of 27 Feb 2010 Chile Feb 2010 Chile
Earthquake (M8.8)Earthquake (M8.8)
500 km
Two Ways to Measure Two Ways to Measure Earthquake SizeEarthquake Size
• MagnitudeMagnitude– Measures the size of the rupture on a Measures the size of the rupture on a
fault (e.g., on the San Andreas fault)fault (e.g., on the San Andreas fault)
• IntensityIntensity– Measures the size of the ground Measures the size of the ground
shaking at a particular site (e.g., here in shaking at a particular site (e.g., here in this classroom)this classroom)
Earthquake MagnitudeEarthquake Magnitude
For each increase of 1 unit in magnitude:For each increase of 1 unit in magnitude:– Energy increases by a factor of 33Energy increases by a factor of 33– Fault area increases by a factor of 10Fault area increases by a factor of 10– Fault slip increases by a factor of 3.3Fault slip increases by a factor of 3.3
30 km x 20 km = 600 km2
Aftershocks of 27 Aftershocks of 27 Feb 2010 Chile Feb 2010 Chile
Earthquake (M8.8)Earthquake (M8.8)
500 km
600 km x 100 km = 60,000 km2
Earthquake MagnitudeEarthquake Magnitude
For each increase of 1 unit in magnitude:For each increase of 1 unit in magnitude:– Energy increases by a factor of 33Energy increases by a factor of 33– Fault area increases by a factor of 10Fault area increases by a factor of 10– Fault slip increases by a factor of 3.3Fault 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 Intensity50
0 km
500
km
M 7.6M 7.9
End Tutorial #3