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 $\sim $ 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.

Figure 13.1: Thirty-four well-located tremors (small circles) along the northern Cholame Segment (CS) of the SAF. Triangles and squares are seismic stations of the HRSN and SCSN, respectively. Large circle is $\sim $ 15-km-radius search zone. Star is the epicenter of the 28 September 2004, M6 Parkfield earthquake. (Inset, top) Horizontal component seismograms (3 to 8 Hz band-pass filtered) of two tremor events (ev1 and ev2) recorded by the stations with gray symbols. (Left) Approximate rupture zone of the 22 December 2003, M6.5 San Simeon Earthquake that occurred $\sim $ 50 km to the west of the tremor zone. (Bottom inset) The location of Cholame, California.
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