The Spectrum of Fault Slip Behaviors and the Mechanics of Slow Earthquakes

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Earthquakes represent a sudden release of elastic energy that is stored in the rocks adjacent to tectonic faults, driving catastrophic failure. In normal (fast) earthquakes the rupture zone expands at a few kilometers per second, and fault slip rates reach 1 to 10 meters per second. However, tectonic faults also fail in slow earthquakes with rupture durations of months or more and fault slip speeds of a of small fraction of an inch per second or less. Recent work shows that tectonic faults fail in spectrum of slip modes that includes slow earthquakes, normal (fast) earthquake, and many other forms of slip. These slow modes of slip can transfer stress to the fast earthquake zone and potentially trigger damaging, normal earthquakes. However, we know very little about the mechanics of slow earthquakes. This research effort will address questions such as: what determines the rupture speed of slow earthquakes? A central element of this work is carefully conducted laboratory studies of repetitive, slow stick-slip events with continuous measurement of acoustic emission, elastic wave speed and amplitude, and fault zone friction behavior. Hypotheses to be tested include: 1) does slow slip failure represents prematurely arrested normal (fast) slip for a range of materials and 2) can the same fault zone can host slow and fast slip behaviors.Slow earthquakes are one form of transient fault slip that may load seismogenic portions of fault zones and abet damaging earthquakes. The origin of slow earthquakes and related forms of transient fault slip is poorly understood. The project will take a systematic approach to investigate: 1) the underlying processes of slow slip, with focus on the frictional mechanisms and continuum coupling that may explain the spectrum of fault slip behaviors in nature, 2) microstructural studies of the laboratory samples to assess how shear localization and strain distribution compare between normal (fast) and slow stick-slip, and 3) acoustic measurement of fault zone elastic properties, with particular focus on precursors to failure. The proposed study will extend the analogy between frictional stick-slip and normal (fast) earthquakes to include slow slip events. The results will provide insight on the mechanics of slow earthquakes and other forms of quasi-dynamic fault slip. All data and results obtained will be published in peer-reviewed journals and made available via public websites.