Slow Slip in Depth

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Geology The largest earthquakes occur at subduction zones, which accommodate the convergence of two tectonic plates. Thanks to dense GPS and seismic networks along these zones, we have a more detailed, and now complicated, view of how this convergence occurs: Many subduction zones exhibit a phenomenon termed “slow slip.” Gomberg et al. provide an overview of this process in the Cascadia Subduction Zone, where it has been heavily studied. Here, the GPS data show that the westward motion of the North American plate reverses regularly over periods ranging from days to several weeks. Unusual small earthquake tremors at depths of 40 to 50 km along the subduction zone, a bit deeper than the area expected for major quakes, increase dramatically at this time and explain the lag in plate motion. Nearly 40 such events have been studied in the Cascadia subduction zone, and they also vary from north to south. The current model posits that the slow slip is related to fluid release by metamorphic reactions in the subduction zone, but implies that the plate interface at depth is weak and critically stressed. Thus understanding and monitoring slow slip may be critical for hazard assessment here and elsewhere. Geol. Soc. Am. Bull. 122 , 963 (2010).