BURSTS AND WAVES: THE RESPONSE OF NON-VOLCANIC TREMOR TO EXTERNAL STRESSING

Numerous studies have shown that very small stresses can influence significantly the generation of non-volcanic tremor. Efforts to explain the timing of episodic non-volcanic tremor and slow-slip through outside forces began quite early in the study of the topic. Recently, Nakata et al 2008, Rubinstein et al, 2008; and Shelly et al. 2007, have shown that during ETS events, tremor is strongly influenced by the tides, its amplitude rising and falling at the tidal periods of 12.4 and 24 hours. Long period ground motion from large earthquakes at teleseismic distances has also been shown to influence the generation of tremor [e.g., Miyazawa and Mori, 2006; Rubinstein et al., 2007; and Gomberg et al., 2008]. Teleseismic events have been shown to trigger tremor in a number of different locations, including California, Japan, Taiwan, and Cascadia.

Here, I review the predominant ways in which tremor and slow-slip are believed to be affected by outside stresses and the approximate amplitudes of these stresses. Given these excitation mechanisms and the assumption that tremor is the product of frictional slip on the plate interface I assess the implications of the small stress perturbations for candidate weakening mechanisms that might influence the generation of tremor. While it is commonly assumed that very low effective pressures are needed to produce this triggering behavior, they are not. It is also possible that tremor is only triggered when a region is in a critical or near-critical state, as was the case in Central Vancouver Island at the time of the 1999 Oaxaca earthquake.