Remote triggering of fault-strength changes on the San Andreas fault at Parkfield

A 20-year seismological record of from the Parkfield area of the San Andreas fault has been used to identify two occasions when long-term changes in fault strength seem to have been induced remotely by large seismic events — the 2004 Sumatra–Andaman and the 1992 Landers earthquakes. In both cases, two changes occurred that are consistent with variations in fault strength: variation in the properties of seismic scatterers with time (reflecting stress-induced migration of fluids) and systematic variation in the characteristics of repeating-earthquake sequences. These findings suggest that the largest earthquakes have a global influence on the strength of the Earth's fault systems. In support of that hypothesis, an unusually large number of M ≥ 8 earthquakes occurred in the three years following the 2004 Sumatra–Andaman earthquake. Changes in fault strength over time can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. Observation of the Parkfield area at the San Andreas fault over the years 1987–2008 now reveals two occasions — the 2004 Sumatra–Andaman earthquake and the 1992 Landers earthquake — where long-term changes in fault strength have probably been induced remotely by large seismic events. Fault strength is a fundamental property of seismogenic zones, and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. Although changes in fault strength have been suggested to explain various phenomena, such as the remote triggering of seismicity1, there has been no means of actually monitoring this important property in situ. Here we argue that ∼20 years of observation (1987–2008) of the Parkfield area at the San Andreas fault have revealed a means of monitoring fault strength. We have identified two occasions where long-term changes in fault strength have been most probably induced remotely by large seismic events, namely the 2004 magnitude (M) 9.1 Sumatra–Andaman earthquake and the earlier 1992 M = 7.3 Landers earthquake. In both cases, the change possessed two manifestations: temporal variations in the properties of seismic scatterers—probably reflecting the stress-induced migration of fluids—and systematic temporal variations in the characteristics of repeating-earthquake sequences that are most consistent with changes in fault strength. In the case of the 1992 Landers earthquake, a period of reduced strength probably triggered the 1993 Parkfield aseismic transient2,3,4,5 as well as the accompanying cluster of four M > 4 earthquakes at Parkfield. The fault-strength changes produced by the distant 2004 Sumatra–Andaman earthquake are especially important, as they suggest that the very largest earthquakes may have a global influence on the strength of the Earth’s fault systems. As such a perturbation would bring many fault zones closer to failure, it should lead to temporal clustering of global seismicity. This hypothesis seems to be supported by the unusually high number of M ≥ 8 earthquakes occurring in the few years following the 2004 Sumatra–Andaman earthquake.