Triggered earthquakes and tremor: Their characteristics and physical mechanisms
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Dynamic triggering of earthquakes is investigated and compared among three geothermal/volcanic regions of California, and dynamic triggering of tremor is investigated and compared among several strike-slip faults in the Western Hemisphere. The characteristics of the triggering waves - amplitude, frequency, and incidence of the triggering waves on faults – are explored. Results indicate that for earthquake triggering, peak dynamic stress (amplitude) appears to be the primary controlling factor. On the other hand, tremor triggering is controlled by amplitude, frequency, and incidence of the seismic waves on the fault. In addition, earthquake triggering primarily occurs in geothermal regions with extensional tectonics, while tremor triggering occurs predominately in compressional or transpressional tectonic environments. A geothermal region’s susceptibility to triggering by external stressing is found to be proportional to its background activity rate. However, tremor triggering appears to occur more often than earthquake triggering. The observation that tremor is triggered more often than earthquakes could be a reflection of a fault being weaker at depth (where tremor occurs) than in the shallower crust (where earthquakes occur). That is, tremor may be more easily triggered due to a fault having low effective stress, pressurized fluids, and/or a perpetual state of near-criticality at depths where tremor occurs. Potential research directions that further explore dynamic triggering of earthquakes and tremor are discussed.