STATIC STRESS CHANGES AND THE TRIGGERING OF EARTHQUAKES

To understand whether the 1999 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the g eneral problem of how one earthquake might trigger another. The tenden cy of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally o rientated for failure as a result of the prevailing regional stress fi eld and the stress change caused by the mainshock. We find that the di stribution of aftershocks for the Landers earthquake, as well as for s everal other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulo mb stress on optimally orientated faults rose by more than one-half ba r, and aftershocks are sparse where the Coulomb stress dropped by a si milar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 cen turies. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but dep ends on the fault geometry, sense of slip, and the coefficient of fric tion. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the L anders and Big Bear earthquakes raised the stress along the San Bernar dino segment of the southern San Andreas fault by 2 to 6 bars, hasteni ng the next great earthquake there by about a decade.