Stress triggering, Fred Pollitz

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F. F. Pollitz,: Stress triggering of the 1999 Hector Mine earthquake by transient deformation following the 1992 Landers earthquak, Bull. Seis. Soc. Amer.,submitted 2001.; [Printable article (1.47Mb)]

Stress triggering of the Hector Mine earthquake

U.S. Geological Survey, Menlo Park, California

Triggered earthquakes can occur many years after a primary earthquake and at variable distance from it. The secondary earthquake can occur in a region which had only very small static stress change, as is suspected to be the case for the 1944 Tonankai/1946 Nankaido (primary) to 1995 Kobe (secondary) earthquakes, and the 1957 Aleutian/1964 Prince William Sound earthquakes (primary) to 1970-1994 western North America earthquake progression (secondary).

Most earthquake pairs which exhibit a substantial time delay, however, involve earthquakes in fairly close priximity to one another, such as the August 1999 Izmit, Turkey (primary) to November 1999 Duzce, Turkey (secondary) earthquakes. A very interesting earthquake pair involves the 1992 Landers earthquake and 1999 Hector Mine earthquakes in the Mojave Desert (Figure 1). These two earthquakes occurred only seven years apart in a region where repeat times are thousands of years.

Figure 1. Active faults in the western-central Mojave Desert and epicenters of M > 5 earthquakes from April, 1992 to August, 2000.
The surface ruptures of the 1992 Landers and 1999 Hector Mine earthquakes are shown as heavy red and orange lines, respectively.

A strong candidate for time-dependent stress transfer between the two events is viscoelastic relaxation of the lower crust and mantle following the 1992 Landers earthquake (Figure 2). This process involves relaxation of coseismic shear stresses in the ductile volume beneath the fault governed by two viscosity values. Using a viscoelastic model derived by Pollitz and others (2000) we have derived the pattern of change in Coulomb failure stress related to the Landers earthquake, both the static stress change and the cumulative postseismic stress change over the intervening 7 years (Figure 3).

Figure 2. Viscoelastic stratification of the central Mojave Desert. Viscosities are constrained by the post-Landers study of Pollitz et al. (2000).

Figure 3. Snapshots of change in Coulomb failure stress at depth 7 km (top) just after the 1992 Landers earthquake and (bottom) just before the 1999 Hector Mine earthquake, assuming an effective coefficient of friction of 0.7.

The difference between the two images arises from the postseismic stress change from viscoelastic relaxation of the lower crust and mantle accumulated during the 7.3 years between the two earthquakes. Representative fault surfaces for the Landers, Big Bear, and Hector Mine earthquakes are indicated by white lines, and the Hector Mine nucleation point by a red circle. Epicenters of earthquakes with magnitude > 3.0 and depth 2-12 km are superimposed, covering the time periods (top) 07/01/92-12/31/92 and (bottom) 10/15/97-10/15/99.

The regional stress changes clearly evolve from 1992 to 1999, stressing some regions more (brown and red) and others less (blue). The stress change on the impending Hector Mine rupture surface, shown in a depth section (Figure 4), illustrate that while the Hector Mine epicenter (red circle) was located in a neutrally-encouraged region at the time of the Landers earthquake, its stress increased by about 0.7 bars by the time of the Hector Mine earthquake. Such a stress increase is known to be associated with static stress triggering, suggesting that the viscoelastic relaxation mechanism is important for delayed triggering.

Figure 4. Depth profile of change in Coulomb failure stress derived from (top) the Landers static stress change and (bottom) the static stress change plus the total postseismic stress change at the time of the Hector Mine earthquake. The stress changes are calculated on potentially failing vertical faults striking N29W and passing through the Hector Mine nucleation point (indicated by a black star), with effective coefficient of friction 0.7.

For further details see the paper submitted to the BSSA Special Issue on the Hector Mine earthquake:

F. F. Pollitz,: Stress triggering of the 1999 Hector Mine earthquake by transient deformation following the 1992 Landers earthquak, Bull. Seis. Soc. Amer.,submitted 2001.