The effects of faulting environment on the seismic source: Triggering, source parameter relationships, and energy considerations.

摘要

Here I investigate the influence of faulting environment on various properties of the seismic source, including triggering, and source parameter relationships. I address three fundamental questions: (1) What is the importance of tectonic regime for earthquake triggering? (2) Can some unconventional seismic signals be explained using simple sources in combination with a complicated travel path through a heterogeneous medium with a complicated velocity structure? (3) Does fault maturity influence the dynamics of faulting? I find that tectonic regime and fault maturity do influence earthquake nucleation and source parameters. In addition, I find that faulting environment complicates seismic signals and that accounting for path effects in complicated faulting environments is key in understanding earthquake source physics.;First, I observe a lack of triggering in Japan. Two geophysical factors differentiate Japan from other environments where triggering has been observed, namely, a lack of widespread onshore tectonic extension, and a higher seismicity rate. The lack of triggering suggests that one or both of these factors inhibit triggering in Japan. Previous incidences of remote triggering occur primarily in locally extensional or transtensional tectonic environments, suggesting that a lack of triggering is consistent with the largely compressional tectonic regime [Hill, 2008]. A triggering mechanism of fracture unclogging is consistent with the observation of no triggering the in the presence of high ambient seismicity. The commonly invoked mechanisms of rate-state frictional evolution and sub-critical crack growth are both inconsistent in the presence of high seismicity.;Second, I observe that unusual waveforms of the volcanic hybrid events associated with the Mount St. Helens eruption are easily explained using a simple stick-slip source in combination with a low rupture velocity and a complicated wave propagation path through a layered, heterogeneous medium. Such a complicated velocity structure causes multiple reflections, refractions, and protracts the earthquake waveform. The seismic moment/corner frequency relationship of the hybrid earthquakes follows the theoretical relationship expected for a shear displacement on a 2-dimensional fault. There is no reason to expect moment and corner frequency to scale in such a relationship for a more complicated source with a volumetric component, and therefore, no reason to infer the presence of sub-surface fluids.;Finally, I observe that earthquakes located on the San Andreas fault near Parkfield have a constant duration over a catalog magnitude range of 1.4-3.7. Earthquakes located on secondary faults with far less cumulative displacement in the same area obey the common relationship of seismic moment increasing as the cube of the duration. The constant duration over the range magnitudes suggests that the variation in magnitude is due entirely to variation in average slip on the fault plane. The observations are consistent with a faulting model of a locked asperity in a creeping fault which slips a different amount each time it ruptures.