Abstract
Abstract The mechanical properties of serpentinites are key factors in our understanding of the dynamics of earthquake ruptures in subduction zones, especially intermediate-depth earthquakes. Here, we performed shear rupture experiments on natural antigorite serpentinite, which showed that friction reaches near-zero values during spontaneous dynamic rupture propagation. Rapid coseismic slip (>1 m/s), although it occurs over short distances (<1 mm), induces significant overheating of microscale asperities along the sliding surface, sufficient to produce surface amorphization and likely some melting. Antigorite dehydration occurs in the fault walls, which leaves a partially amorphized material. The water generated potentially contributes to the production of a low-viscosity pressurized melt, explaining the near-zero dynamic friction levels observed in some events. The rapid and dramatic dynamic weakening in serpentinite might be a key process facilitating the propagation of earthquakes at intermediate depths in subduction zones.
Highlights
Serpentinites are formed by hydrothermal alteration of upper mantle ultramafic rocks, and they are found in several key tectonic environments (Hirth and Guillot, 2013)
The events initiated at relatively high shear stress and produced displacements ranging from 30 mm up to 1.03 mm (Fig. 1A; Fig. DR2), which correspond to equivalent moment magnitudes ranging from Mw -3.9 to Mw -2.9
An example of a full stress-time record is given in Figure DR4, which shows that the final stress immediately after rupture remained close to the minimum dynamic stress level, and that the increase to the “long-term” static value occurred over much larger time scales than those of the rupture events
Summary
Serpentinites are formed by hydrothermal alteration of upper mantle ultramafic rocks, and they are found in several key tectonic environments (Hirth and Guillot, 2013). Recent studies have provided evidence that antigorite weakens significantly, with coefficients of friction as low as 0.1 (Hirose and Bystricky, 2007; Kohli et al, 2011; Proctor et al, 2014), when deformed at slip rates comparable to those inferred during earthquake slip (several cm/s and above) Such weakening has commonly been interpreted as the macroscopic manifestation of a process called flash heating (e.g., Rice, 2006), which, in the case of hydrous minerals such as serpentines, arises from the thermal dehydration reaction into talc, olivine, and water at microscale asperities along the frictional interface (Kohli et al, 2011; Proctor et al, 2014)
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