Earthquake triggering

Schuyler Ozbick

The trigger

http://www.eqecat.com/catwatch/virginias-m-5-8-wake-up-call-2011-09-07/

Triggered earthquakes can be seen at distances larger than a regional fault network

They can also occur anytime between seconds and decades after the initial earthquake in the sequence

The trigger

Coulomb failure criterion

Coulomb Stress

Coulomb stress = shear stress – friction coef. (normal stress-pore fluid P)

Fault brought closer to rupturing if shear stress is increased or the effective normal stress is decreased.

Changes in Coulomb stress for a vertical strike-slip fault where green represents no change in stress

Coulomb Stress

1933 Long Beach sequence

1933 Long Beach. M= 6.4 1952 Kern Country California. M= 7.3 1971 San Fernando. M= 6.7 1994 Northridge. M= 6.7

http://content.cdlib.org/ark:/13030/kt8p3014k2/

1933 Long Beach. M= 6.4 1952 Kern Country California. M= 7.3 1971 San Fernando. M= 6.7 1994 Northridge. M= 6.7

1933 Long Beach sequence

http://www.ngdc.noaa.gov/hazardimages/picture/show/109

1933 Long Beach. M= 6.4 1952 Kern Country California. M= 7.3 1971 San Fernando. M= 6.7 1994 Northridge. M= 6.7

1933 Long Beach sequence

http://earthquake.usgs.gov/earthquakes/states/events/1971_02_09_photos.php

1933 Long Beach. M= 6.4 1952 Kern Country California. M= 7.3 1971 San Fernando. M= 6.7 1994 Northridge. M= 6.7

1933 Long Beach sequence

http://www.flickriver.com/places/United+States/California/Los+Angeles/Northridge/search/

Rainbow Mountain sequence July 6, 1954 at south end of Rainbow

Mountain fault August 24, 1954 along Rainbow Mountain

fault December 16, 1954 at Fairview peak December 16, 1954 at Dixie Valley

◦ (4 min 30 sec later) March 29, 1959 south end of Dixie Valley

fault June 23, 1959 in SW Fairview Peak region

Rainbow Mountain sequence

bssa.geoscienceworld.org

April 1992 Joshua Tree quake of M=6.1

June 1992 Landers quake of M=7.3

The Landers quake caused increased seismic activity in ranges from a few meters to beyond 1000 km and triggered many small (M=1-3) events

Landers-Big Bear sequence

Joshua Tree and Landers earthquakes created a new positive Coulomb stress change pattern

The Big Bear earthquake was triggered 3.5 hours after Lander quake.

Big Bear earthquake M=6.3

Landers-Big Bear sequence

After the Big Bear quake, the Coulomb stresses were altered.

Landers-Big Bear sequence

Before Big Bear After Big Bear

April 1992 Joshua Tree quake of M=6.1

June 1992 Landers quake of M=7.3

June 1992 Big Bear quake of M=6.3◦ 3.5 hours after Landers

October 1999 Hector Mine quake of M=7.1

Landers-Big Bear sequence

Does this earthquake belong to a sequence?

Was it triggered by a previous earthquake?

What does the time delay suggest?◦ Redistributed stress postseismatically by relaxation,

rebound and afterslip?

Does it conform to an increased Coulomb stress?◦ Disagreements over stress shadows

Trigger Happy

Coulomb stress and observed aftershocks of Homestead Valley earthquake of 1979 M=5.5

Trigger Happy

Earthquake triggering by static, dynamic, and postseismic stress transfer, Andrew M. Freed, Annu. Rev. Earth Planet. Sci. 2005 33:335-67

Earthquake processes in the Rainbow Mountain-Fairview Peak-Dixie Valley, Nevada, region 1954-1959, Diane I. Doser, Journal of Geophysical Research, vol 91 no. B12 pg. 572-586, Nov 1986

Delayed triggering of 1999 Hector Mine Earthquake by viscoelestic stress transfer, Andrew M. Freed and Jian Lin, Nature vol 411 pg 180-181, May 2001

References