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Jian Lin and Ross Stein recently investigated the key features of thrust earthquake triggering, inhibition, clustering, and interaction with strike-slip earthquakes (Lin & Stein, JGR, 2004). Whereas slip on surface-cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress above the source fault. Short thrust ruptures are efficient at triggering earthquakes of similar size on adjacent thrust faults. They calculate that the 1983 Mw=6.7 Coalinga earthquake brought the subsequent 1983 Mw=6.0 Nuñez and 1985 Mw=6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to failure. Subduction zone ruptures are calculated to promote normal faulting in the outer rise, and to promote thrust faulting on the periphery of the seismic rupture and its downdip extension. They examine stress changes imparted by the 1960 Mw=9.5 and 1995 Mw=8.1 Chile earthquakes. Calculated Coulomb stress increases of 2-20 bars correspond closely to sites of aftershocks and postseismic slip. Furthermore, they argue that slip on major strike-slip systems modulates the stress acting on nearby thrust and strike-slip faults. They calculate that the 1857 Mw=7.9 Fort Tejon earthquake on the San Andreas and subsequent interseismic slip brought the Coalinga fault ~1 bar closer to failure, but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw=7.3 Kern County shock, but inhibited slip on the left-lateral Garlock fault, which has not ruptured since 1857.

Figure 12. Calculated stress changes for two great Chilean subduction earthquakes. (a) Coulomb stress changes associated with the 22 May 1960 Mw = 9.5 Chile earthquake, using the variable slip planar model of Barrientos and Ward [1990], which assumes a uniform rake equal to the plate convergence vector on a 20º-dipping plane, smoothed to 50 x 50 km patches. Stress changes are superimposed on aftershocks from Cifuentes [1989] (International Seismological Centre and International Seismological Service catalogs, 21 May 1961 to 31 December 1983, M≥5.8). Small red stars are 18 August 1974 Ms = 7.1 and 10 May 1975 Ms = 7.8 shocks, and large black stars are the main 1960 subevents. About 75% of the aftershocks lie in regions of calculated Coulomb stress increase. Stress is calculated on dipping planes encompassing the rupture surface and projected onto ground surface. Because this calculation is on the rupture surface, shear stress changes dominate. (b) Cross section of stress changes along W-E in (a), with corresponding aftershocks along the profile from Cifuentes [1989] (red circles) and Kadinsky-Cade [1985] (black circles). (c) Coulomb stress changes associated with the 30 July 1995 Mw = 8.1 Antofagasta, Chile, earthquake, based on the variable slip model of Klotz et al. [1999], smoothed to 25 X 25 km patches (the model of Ihmle´ and Ruegg [1997] yielded similar patterns of stress changes with minor differences in detail). Large white and gray stars show the NEIC epicenter and the Harvard centroid moment tensor (CMT), respectively, while small circles show 1995 aftershocks located by tomographic inversion by Husen et al. [2000] from a 90-day seismometer deployment. Mw≥6.0 aftershocks during the first 3.3 years following the main shock are shown as red stars. The site of postseismic slip inferred from geodetic observations by Chlieh et al. (submitted manuscript, 2003) is shown as the dotted contour. Stress is calculated on dipping planes encompassing the rupture surface and projected onto ground surface. (d) Cross section of stress changes with 1995 aftershocks from Husen et al. [2000].