Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

Nicholas J. van der Elst; Andrew Delorey; David R. Shelly; Paul Johnson

doi:10.1073/pnas.1524316113

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Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

Nicholas J. van der Elst·Andrew Delorey·David R. Shelly·Paul Johnson

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Abstract

Earth tides modulate tremor and low-frequency earthquakes (LFEs) on faults in the vicinity of the brittle−ductile (seismic−aseismic) transition. The response to the tidal stress carries otherwise inaccessible information about fault strength and rheology. Here, we analyze the LFE response to the fortnightly tide, which modulates the amplitude of the daily tidal stress over a 14-d cycle. LFE rate is highest during the waxing fortnightly tide, with LFEs most strongly promoted when the daily stress exceeds the previous peak stress by the widest margin. This pattern implies a threshold failure process, with slip initiated when stress exceeds the local fault strength. Variations in sensitivity to the fortnightly modulation may reflect the degree of stress concentration on LFE-producing brittle asperities embedded within an otherwise aseismic fault.

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Proceedings of the National Academy of Sciences Article

Source-reported bounding extent: 35.5° to 36.5° latitude; -121.25° to -120° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Nicholas J. van der Elst, Andrew Delorey, David R. Shelly, Paul Johnson. 2016-07-18. Fortnightly modulation of San Andreas tremor and low-frequency earthquakes. https://doi.org/10.1073/pnas.1524316113

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