Remotely triggered seismicity in Yunnan, southwestern China, following the 2004 M w 9.3 Sumatra earthquake

Following the 2004 M w 9.3 Sumatra earthquake, seismicity rate increased sharply over a wide area of up to ∼2700 km away in Yunnan province, southwestern China. Raised seismicity lasts for ∼14 days. During this period, more than 800 earthquakes with magnitude between 1.5 and 5.1 occurred. This is perhaps the most impressive example of remotely triggered seismicity yet observed. Major events were clustered at several sites that exhibit complex fault geometries, such as step overs and junctures. We use statistic approaches including the β statistics to examine the statistical significance of the seismic rate increases associated with the Sumatra main shock and conclude that there is a reasonable probability that the raised seismicity was remotely triggered by the Sumatra earthquake. Both rapid onset of dynamic triggering (the very first event is a M 4.6 earthquake occurred during the passage of the Rayleigh wave from the Sumatra earthquake) and delayed response (activated a few hours to a few days after the Sumatra earthquake) are well established. The triggered activities show more earthquake swarm‐like characteristics as indicated by the epidemic‐type aftershock sequence modeling results (large percentage of random components and less magnitude dependence in Omori law type self‐triggering). Multiple sources of evidence, including intensive hydrothermal activities, and low velocity and high V p /V s zones in the lower to middle crust suggests that magma/mantle‐generated fluids have a role in the region. High fluid pressure in branched fault zones weakened the faults, making them sensitive to external disturbances and leading to fluid‐driven seismicity.