Because subduction has not occurred along the central SAF for several million years, fluids from active subduction are not present, and nonvolcanic tremor activity was not expected along the the central SAF zone.
Recently, however, our detailed inspection of triggered event data from Berkeley's borehole High Resolution Seismic Network (HRSN) at Parkfield, California revealed tremor-like signals originating in the vicinity of Cholame (Figure 13.1) (Nadeau and Dolenc, 2005).
Following the methods of Obara (2002), seismic data recorded continuously with 20 Hz sampling frequency by two stations of the Southern California Seismic Network (SCSN) (Figure 13.1) and with 250 Hz sampling by the HRSN were then used to analyze these events.
Within an 15 km search radius centered 5 km southeast of Cholame (Figure 13.1) and during a 3-year period from 23 December 2000 to 22 December 2003 (i.e., when the M6.5 San Simeon earthquake occurred), 110 tremor events lasting between 4 to 20 minutes were identified. Locations of the tremors indicate that within the search radius, the tremors were confined to an 25 km segment of the SAF and occur at depths of between 20 to 40 km.
The depths, frequency content (generally 1 to 10 Hz), S-wave propagation velocity, and waveform character of the SAF tremors were similar to those of the subduction zone tremors; however, the SAF tremors are less frequent (fewer than 5 events in any 24 hour period), have shorter duration (less than 20 minutes), have smaller peak amplitudes (comparable to M0.5 earthquakes), and release less energy (energy equivalents M1.5).
Our discovery of these nonvolcanic tremors is important for three principal reasons: 1) they occur along a transform plate boundary (i.e., the SAF) in contrast to previous nonvolcanic tremors that occur only in subduction zones, 2) no obvious source for fluid re-charge exists in the area to aid in the genesis of the tremors and 3) the highest level of tremor activity in the region occurs beneath the inferred epicentral region (1) of the moment magnitude (M) 1857 Fort Tejon earthquake whose rupture zone is currently locked.
This segment of the SAF has an estimated earthquake recurrence time of 140 years (+93, -69) (WGCEP, 1995), and it is now over 140 years since the Fort Tejon event. Because stress changes from ETS events may trigger large earthquakes (4), future increases in SAF tremor activity may signal periods of increased probability for the next large earthquake on the segment.
An apparent correlation between tremor and local micro-earthquake rates at Cholame (Nadeau and Dolenc, 2005) also suggests that deep deformation associated with the Cholame tremors (i.e., ETS) may also be stressing the shallower seismogenic zone in this area.
Further evidence for stress-coupling between tremor related deep deformation and shallower SAF earthquake activity is also seen in the correlation between stress release from the nearby 28 September 2004, M6 Parkfield earthquake and the evolution of tremor activity rates in the area (Figure 13.2)
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