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Slow Earthquakes, Preseismic Velocity Changes, and the Origin of Slow Frictional Stick-Slip

Science  13 Sep 2013:
Vol. 341, Issue 6151, pp. 1229-1232
DOI: 10.1126/science.1239577

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Slow Stick-Slip

While the character of slow earthquakes has been approximated for some time, precise slip histories and the underlying mechanisms have remained elusive. Kaproth and Marone (p. 1229; published online 15 August) have made laboratory observations of repetitive, slow stick-slip in fault-zone materials and developed a mechanical explanation for how earthquake-like dynamic slip nucleation could start and then arrest so as to produce slow slip. As preseismic slip is a precursor to rupture, temporal variations in elastic wave speeds should be monitored in regions of high seismic hazard.

Abstract

Earthquakes normally occur as frictional stick-slip instabilities, resulting in catastrophic failure and seismic rupture. Tectonic faults also fail in slow earthquakes with rupture durations of months or more, yet their origin is poorly understood. Here, we present laboratory observations of repetitive, slow stick-slip in serpentinite fault zones and mechanical evidence for their origin. We document a transition from unstable to stable frictional behavior with increasing slip velocity, providing a mechanism to limit the speed of slow earthquakes. We also document reduction of P-wave speed within the active shear zone before stick-slip events. If similar mechanisms operate in nature, our results suggest that higher-resolution studies of elastic properties in tectonic fault zones may aid in the search for reliable earthquake precursors.

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