Earthquake triggering by seismic waves following the Landers and Hector Mine earthquakes

The proximity and similarity of the 1992, magnitude 7.3 Landers and 1999, m agnitude 7.1 Hector Mine earthquakes in California permit testing of earthq uake triggering hypotheses not previously possible. The Hector Mine earthqu ake confirmed inferences that transient, oscillatory 'dynamic' deformations radiated as seismic waves can trigger seismicity rate increases, as propos ed for the Landers earthquake(1-6). Here we quantify the spatial and tempor al patterns of the seismicity rate changes(7). The seismicity rate increase was to the north for the Landers earthquake and primarily to the south for the Hector Mine earthquake. We suggest that rupture directivity results in elevated dynamic deformations north and south of the Landers and Hector Mi ne faults, respectively, as evident in the asymmetry of the recorded seismi c velocity fields. Both dynamic and static stress changes seem important fo r triggering in the near field with dynamic stress changes dominating at gr eater distances. Peak seismic velocities recorded for each earthquake sugge st the existence of, and place bounds on, dynamic triggering thresholds. Th ese thresholds vary from a few tenths to a few MPa in most places, depend o n local conditions, and exceed inferred static thresholds by more than an o rder of magnitude. At some sites, the onset of triggering was delayed until after the dynamic deformations subsided. Physical mechanisms consistent wi th all these observations may be similar to those that give rise to liquefa ction or cyclic fatigue.