UseIT Tutorial # 3 Earthquakes in the Southern California Fault System Tom Jordan June 16, 2011.

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