Poster #012, Earthquake Geology

The source of shallow slow earthquakes: Insights from field observations and experiments on an exhumed subduction mélange

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Poster Presentation

2020 SCEC Annual Meeting, Poster #012, SCEC Contribution #10528
Field constraints on the mechanics behind slow earthquake phenomena exist downdip of the seismogenic zone. However, interpretations based on observations from the shallow subduction interface, which may also host slow earthquake phenomena, are largely absent. We examine a subduction mélange exhumed from conditions representing the source of shallow slow earthquake phenomena (the Mugi mélange, Japan). The mélange consists of a shale matrix containing rigid blocks, including basalt which is altered along the margins. Cataclasite-bearing faults attest to localized faulting along the altered margins of basaltic blocks, concurrent with distributed shear in the shale matrix. These cataclasite-b...earing faults link individual blocks. Microstructures show mutually crosscutting tensile and shear veins, consistent with failure having occurred at, or near, lithostatic pore fluid pressures. Shear experiments (in the triaxial sawcut configuration) were performed on altered basalt and the shale matrix from the mélange at in-situ conditions of deformation (Pc = 120 MPa, T = 150 °C) for two pore fluid factors (0.36, 0.7). The altered basalt exhibits velocity-weakening behavior while the shale matrix exhibits velocity-strengthening behavior. The shale is frictionally weaker than the altered basalt. We model the stress concentrations around the altered margins of basaltic blocks during distributed shear in the shale, and show that frictional failure of the altered basalt is predicted to occur at lower imposed strain rates than frictional failure of the shale, favoring fault development along block margins. Calculations of critical nucleation lengths for the blocks show they would fail dynamically at hydrostatic pore fluid pressures, producing microearthquakes. At near lithostatic pore fluid pressures, block lengths are below the critical nucleation length for dynamic failure and may produce slow earthquake phenomena (i.e. low-frequency earthquakes). Linking faults through the shale may facilitate larger magnitude slow earthquake phenomena (i.e. very low-frequency earthquakes). Mixing of velocity-weakening blocks into a viscously flowing, velocity-strengthening matrix may serve as a common mechanism for slow earthquake phenomena updip and downdip of the seismogenic zone.
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