1685 Bulletin of the Seismological Society of America, 91, 6, pp. 1685–1693, December 2001 A Physical Basis for Time Clustering of Large Earthquakes by Jean Che ´ry, Se ´bastien Merkel, and Ste ´phane Bouissou* Abstract We develop a theory that links stress interaction between earthquakes and the occurrence of temporal clustering. Coseismic static stress change in the vi- cinity (50 km) of large earthquakes suggests that perturbations of 0.1 to 1 bars may affect the occurrence of other earthquakes. At larger distances, interactions also seem to exist: four M 8 earthquakes have occurred in Mongolia on distant faults (400 km) during the last century. Also, paleoseismic observations documenting much longer time periods display a time clustering of major events. We demonstrate with simple mechanical concepts that postseismic stress relaxation magnifies the coseismic stress change and has a major effect on fault interaction during the seismic cycle. In the simple case where two distant faults are coupled, the probabilistic occurrence of triggered earthquakes may increase dramatically due to long range postseismic coupling. Introduction Since large earthquakes have been identified as an elas- tic rebound of the crust around a fault (Reid, 1910), the prediction of time and location of large earthquakes has been a major aim of the earth science community. Unfortunately, numerous searches for geophysical precursors and extensive modeling of earthquake mechanics have been unsuccessful in predicting earthquake occurrence (Geller et al., 1997). In contrast to volcanic eruptions, which occur at very specific and identified sites often with clear precursors (i.e., Dvorak and Dzurizin, 1997), active faults are spread over large areas and seem to generate earthquakes in an unpredictable manner. Even in a tectonic zone where detailed fault maps exist, we always face a triple uncertainty about the next earthquake: its location, its time, and its size. We study in the following sections of this article the problem of earth- quake timing by simulating earthquake occurrence on two distant faults. We assume in this model that secular stress accumulation on the faults and earthquake stress transfer through the crust control the seismic cycle. We specifically model the postseismic stress change and show that it is the main factor responsible for earthquake time clustering. The article is divided into five sections: (1) We first summarize observations of earthquake-induced seismicity and earth- quake time clustering. (2) We then present the principle of earthquake interaction and estimate the magnitude of stress drop during a large earthquake. (3) We describe a physical model of the seismic cycle which incorporates local stress drop on a fault and distant stress change between faults. (4) We present a numerical device able to simulate earthquake *Present address: Geosciences Azur, Sophia Antopolis, Valbonne, France. series on two coupled faults. (5) Finally, we perform a para- metric study that shows under which conditions earthquake time clustering takes place. Earthquake Stress Change and Earthquake Time Clustering Two classes of observations support the idea that a me- chanical coupling exists between distant earthquakes. First, local and distant seismicity evolve after a large earthquake. This change happens immediately after an earthquake and may last for decades. Second, large earthquakes are some- times clustered in time, suggesting that fault failure on one fault may affect earthquake occurrence on another fault. A large earthquake often induces microseismicity and other earthquakes during the hours, days, or months follow- ing the mainshock (Omori law); (see Scholz, 1990). This effect is generally explained by an elastic stress change in the crust, which leads to earlier failure at other potential rupture zones. With the precise knowledge of fault geometry and coseismic slip distribution, a stress change can be esti- mated for any part of the crust (e.g., Chinnery, 1963; Okada, 1992). A positive shear stress change Ds on a fault plane should advance the time of the next earthquake on this fault, and is known as the clock advance. In the case where the normal stress change is neglected, the clock advance is sim- ply given by Δ Δ t = / τσ ɺ , where Ds is the shear stress change and ɺ σ the rate of secular stress accumulation onto the fault. A large earthquake on a fault system may also inhibit seismicity. A clear example is provided by the 1906 San Fransisco earthquake, which was followed by the absence of moderate and large earthquakes in Northern California