Nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves (Figure 13.1) may provide important clues to the rheology and processes responsible for the nucleation and seismic cycles of large earthquakes. Prior to our research, nonvolcanic tremors had only been observed in subduction zones (i.e., thrust fault plate boundaries) (e.g., Obara, 2002; Rogers and Dragert, 2003), where fluids from subduction processes were believed to play an important role in generating these tremors.
In Cascadia, a significant correlation between subduction zone tremor activity and subseismogenic zone (i.e. beneath the upper 15 km of Earth's crust where earthquakes occur) slow slip events (referred to as episodic tremor and slip (ETS, Rogers and Dragert, 2003)) has also been observed, suggesting that stress changes from ETS events increase stress and possibly trigger earthquakes in the shallower seismogenic fault zone.
Berkeley Seismological Laboratory
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