Monitoring Nonvolcanic Tremor on the San Andreas Fault

Robert M. Nadeau

Introduction

Nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves (Figure 6.1) may provide important clues to the rheology and processes responsible for the nucleation and seismic cycles of large earthquakes. Previously, 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.

Figure 6.1: Comparison of Cascadia and San Andreas Fault 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 been observed, suggesting that stress changes from ETS events may increase stress and possibly trigger earthquakes in the shallower seismogenic fault zone. No correlation between tremor and deformation changes have yet been observed along the SAF, however, variations in earthquake activity does correlate.
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SAF Tremors

Because subduction has not occurred along the central San Andreas Fault (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, data from Berkeley's borehole High Resolution Seismic Network (HRSN) at Parkfield, California revealed tremor-like signals originating in the vicinity of Cholame, California (Nadeau and Dolenc, 2005). The tremors appear to be confined largely to an  25 km segment of the SAF beneath Cholame 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, of shorter duration, of smaller amplitude, and of lower seismic energy release.

Our discovery of these nonvolcanic tremors is important for three reasons: 1) they occur along a transform rather than a subduction plate boundary zone, 2) no obvious source for fluid re-charge exists in the Cholame area to aid in tremor genesis, and 3) the highest level of tremor activity in the region occurs beneath the inferred epicentral region of the moment magnitude (M) $\sim 7.8$ 1857 Fort Tejon earthquake, whose rupture zone is currently locked.

The Cholame 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, 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 the deep tremor zone and the seismogenic SAF is observed in the correlation between tremor and the 28 September 2004, M6 Parkfield earthquake ($\sim $ 10 km NW of Cholame) (Figure 6.2). Between 1 and 3 months before the Parkfield earthquake, tremor activity was extremely low. Then 20 to 22 days prior to the Parkfield quake, tremor activity showed an anomalously large rate of activity (fore-tremor). This is consistent with the observations of Nadeau and Dolenc (2005), which suggest a relationship between the deep tremor and shallower seismogenic zones in which stress changes in one of these zones induces stress changes in the other with a lag time of a few weeks.

Immediately following the Parkfield mainshock an even larger rate increase in tremor activity occurred, and elevated tremor rates persisted for over 500 days. This elevated rate appears now to have subsided and tremor rates are again low. It is to early to say whether or not the low activity rates now being observed are representative of another precursory quiescent period, but given the critical location of the tremors relative to the Fort Tejon locked zone, and the evidence for coupling of tremor rates and earthquake activity seen so far, we are compelled to continue monitoring the tremors for signs of anomalous activity that may signal an increased likelihood for another large event in the area.

Figure 6.2: Activity rate history of nonvolcanic tremors (green, top) and microearthquakes (red, bottom) detected by the borehole High Resolution Seismic Network (HRSN) at Parkfield, CA. History spans 40 days prior to the San Simeon Earthquake through Sept. 13, 2006 (i.e., 716 days after the Parkfield mainshock). Tremor activity rates were not strongly influenced by the San Simeon event that occurred some 50 km to the west. However, the Parkfield earthquake whose epicenter was about 10 km from the tremor zone had a strong impact. Parkfield aftershocks decayed more rapidly than the tremor activity, suggesting some lag between stress changes in the deep tremor zone relative to that in the seismogenic zone above.
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Acknowledgements

This research was supported by U.S. Geological Survey award no. 06HQGR0167.

References

Nadeau, R.M. and D. Dolenc, Nonvolcanic Tremors Deep Beneath the San Andreas Fault, Science, 307, 389, 2005.

Obara, K., Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan, Science, 296, 1679-1681, 2002.

Rogers, G. and H. Dragert, Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip, Science, 300, 1942-1943, 2003.

Working Group on California Earthquake Probabilities (WGCEP), Seismic hazards in southern California: probable earthquakes, 1994 to 2024, Bull. Seism. Soc. Am., 85, 379-439, 1995.

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