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Accelerated Stress Buildup on the Southern San Andreas Fault and Surrounding Regions Caused by Mojave Desert Earthquakes

Andrew M. Freed and Jian Lin (Woods Hole Oceanographic Institution)

Summary

Scientists have hypothesized for decades that one major earthquake can trigger another earthquake on a nearby fault through stress interaction. More recent studies have further suggested that this interaction may be delayed by the slow viscous creeping of rocks in the Earth's lower crust and upper mantle. This is best illustrated by the 1999 magnitude 7.1 Hector Mine earthquake, which occurred only 30 km away from the 1992 magnitude 7.3 Landers quake, but seven and half years later. The delay between these events can be explained by viscous flow consistent with observations of continuous ground deformation following the Landers quake (Freed and Lin, 2001). In our more recent study (Freed and Lin, 2002), we further calculated how the Landers, Hector Mine, and two other earthquakes in the Mojave Desert have changed stresses on the nearby southern San Andreas and adjacent fault systems. We calculated that these earthquakes and continuous viscous creeping at depth are causing a rapid increase of stresses on a section of the San Andreas Fault, called the San Bernardino Mountain segment, that is located only 80 km from Los Angeles (Figure 21.1). The San Bernardino Mountain segment is worthy of special attention because it is capable of producing major earthquakes with magnitude greater than 7. Since the last major earthquake on this segment was over 190 years ago, the fault may be late in its earthquake cycle, and thus the calculated ongoing stress increase on the fault is of added significance. In addition, we calculated that parts of the San Jacinto, Elsinore, and Calico faults are also experiencing accelerated stress buildup. In particular, the Calico fault, which lies just north of the Landers rupture (near Barstow), appears to have the calculated stress patterns and the observed post- Landers aftershock clustering quite similar to the Hector Mine region before the 1999 quake. This makes the Calico fault another candidate for a potential earthquake in the future, where seismic activity should be watched closely.

Figure 21.1: A. Calculated coseismic Coulomb stress changes caused by fault slip associated with the 1992 Joshua Tree (JT), Landers (L), and Big Bear (BB) earthquakes (green lines). Other faults: MS - Mojave segment, SBMS - San Bernardino Mountain segment, and CVS - Coachella Valley segment of San Andreas fault; SJF - San Jacinto fault, EF - Elsinore fault, CF - Calico fault, LF - Lenwood, and BWF - Blackwater fault. Location of future 1999 Hector Mine (HM) earthquake (green dashed line) is also shown. B. Same as A but with addition of stresses associated with 1999 Hector Mine earthquake and postseismic relaxation to the year 2020.
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References

Freed, A.M., and J. Lin, Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer, Nature, 411, 180-183, 2001.

Freed, A.M., and J. Lin, Accelerated stress buildup on the southern San Andreas fault and surrounding regions caused by Mojave Desert earthquakes, Geology, 30, 571-574, 2002.



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