Annual modulation of triggered seismicity following the 1992 Landers earthquake in California

The mechanism responsible for the triggering of earthquakes remains one of the least-understood aspects of the earthquake process. The magnitude-7.3 L anders, California earthquake of 28 June 1992 was followed for several week s by triggered seismic activity over a large area, encompassing much of the western United States(1). Here we show that this triggered seismicity mark ed the beginning of a five-year trend, consisting of an elevated microearth quake rate that was modulated by an annual cycle, decaying with time. The a nnual cycle is mainly associated with several hydrothermal or volcanic regi ons where short-term triggering was also observed. These data indicate that the Landers earthquake produced long-term physical changes in these areas, and that an environmental source of stress-plausibly barometric pressure-m ight be responsible for the annual variation.