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Tidal triggering of LFEs near Parkfield, CA

THOMAS, A.T., UC-Berkeley, Berkeley,CA,USA,; BURGMANN, R.B., UC-Berkeley, Berkeley,CA,USA,; SHELLY, D.R., USGS, Menlo Park, CA, USA,

Studies of nonvolcanic tremor (NVT) in Japan, Cascadia, and Parkfield, CA have established the significant impact small stress perturbations, such as the solid earth and ocean tides, have on NVT generation. Similar results irrespective of tectonic environment suggest that extremely high pore fluid pressures are required to produce NVT. Here we analyze the influence of the solid earth and ocean tides on a catalog of ~500,000 low frequency earthquakes (LFE) distributed along a 150km section of the San Andreas Fault centered at Parkfield [Shelly et al. this meeting]. LFEs comprising the tremor signal are grouped into families based on waveform similarity and precisely located using waveform cross-correlation. Analogous to repeating earthquakes, LFE families are thought to represent deformation on the same patch of fault. While the locations of repeating earthquakes are assumed to be coincident with the location of asperities in the fault zone, NVT occur below the seismogenic zone, where fault zones behave ductilely. Here we explore the sensitivity of each of these LFE families to the tidally induced shear and normal stresses on the SAF. Preliminary results show spatially smooth variations in both shear and normal stress sensitivity that could potentially be explained by along-fault variations in pore pressure. Assuming that the individual families are fault patches on which the pore pressures are locally high (i.e. near-lithostatic), we quantify the effective normal stress at each LFE location using rate and state friction and LFE rate differences during times when the tides are promoting (or retarding) failure. In addition to mapping pore pressures in the deep fault zone, we further explore spatiotemporal variations in triggering behavior to determine how static stress changes influence modulation, if propagating slip pulses also experience tidal triggering, and if families with quasi-periodic recurrence behaviors are more or less sensitive to tidal effects.


Last Modified: 2011 Aug 10