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Interactions between the Landers and Hector Mine, California, Earthquakes from Space Geodesy, Boundary Element Modeling, and Time-Dependent Friction

The Geophysical Institute
University of Alaska–Fairbanks
P.O. Box 757320
Fairbanks, Alaska, 99775-7320
(E.J.P.)

Department of Earth and Planetary Science
Berkeley Seismological Laboratory
307 McCone Hall
University of California–Berkeley
Berkeley, California 94720
(R.B.)

The M_w 7.1 Hector Mine earthquake happened 7 years after the 1992 M_w 7.3 Landers earthquake, within a system of right-lateral strike slip faults comprising the Eastern California Shear Zone. Because estimates of recurrence intervals of M 7 earthquakes on these faults range anywhere from 1500 to 50,000 years, the close temporal spacing of these two earthquakes suggests that there is some interaction between the two. Current models of simple Coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause-and-effect relationship. The interaction between the computed normal and right-lateral shear stress reductions induced by the Landers earthquake at the Hector Mine hypocenter is unclear.

We use a combination of space geodesy, boundary element modeling, and computer modeling of time-dependent fault friction to investigate the interaction between the two earthquakes. We compare stress changes on the Hector Mine rupture plane induced by the Landers earthquake with the detailed slip distribution inferred using a combination of GPS and InSAR data. The slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events. For each earthquake the azimuth of the remote maximum compressive stress was approximately 17° ± 6°, and the magnitude of the shear stress drop was 8 ± 1 MPa on the Landers rupture and 10 ± 2 MPa on the Hector Mine rupture. Using a simple spring-and-slider model as a proxy for the rate-and-state frictional response of the Hector Mine faults to Landers earthquake-imposed stress steps, we find that, while a decrease in normal stress at the Hector Mine hypocenter would, by itself, have caused the Hector Mine earthquake to nucleate immediately, the simultaneous decrease in shear stress may have caused a delay in the peak shear loading. Consideration of an acceptable range of rate-and-state friction parameters, secular strain rates, and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years.

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