Nonvolcanic tremor activity along the Parkfield-Cholame section of the San Andreas fault (SAF) in central California has been recorded since 2001 (Nadeau and Dolenc, 2005; Nadeau and Guilhem, 2009). In addition to ambient tremors, triggered tremors associated with the surface waves of large teleseisms have been recognized (Gomberg et al., 2008; Peng et al., 2009). However, no tremors triggered by regional earthquakes have been observed within the region and in other tremor regions. Rubinstein et al. (2009) explained the absence of tremor triggering from regional quakes in Cascadia by the similar frequency bands (i.e. 1-15 Hz) and waveform characteristics of the P and S coda waves from regional events. However Brodsky and Prejean (2005) showed that large earthquakes at regional and teleseismic distances have triggered microearthquakes and that the large-amplitude long-period surface waves appeared to favor the triggering more than shorter period waves of similar amplitude. To effectively separate the potential triggered tremors from the regional coda waves, we propose to examine higher frequency bands (i.e. above 15 Hz).
By systematically searching the high-frequency (i.e. 15 Hz) filtered seismograms (250 samples per second) of 99 M5+ earthquakes occurring between July 2001 and April 2010 and distributed between 100 and 1200 km from the broadband seismic station PKD located at Parkfield, we visually identified four earthquakes that triggered tremors in central California: the 15 June 2005 M7.2 Mendocino, 04 January 2006 M6.6 Baja California (BC), 03 August 2009 M6.9 BC, and 04 April 2010 M7.2 BC earthquakes. The triggered tremors constitute consecutive bursts of energy that are phase-correlated with the passing of the surface waves (Figure 2.46).
We found that between 3 and 15 Hz, which corresponds to the typical frequency band for nonvolcanic tremors, the data mainly show the emergent P and S waves without clear indication of triggered tremors. However, tremors are best observed between 15 and 30 Hz for the 2006 and 2010 earthquakes and between 25 and 40 Hz for the 2005 and 2009 events (Figure 2.46a).
We adapted the envelope-based location algorithm already used for ambient tremors to locate the four triggered tremors. Their sources, with the exception of the 2009 event, are found on or close to the main SAF segment, within the region where ambient and teleseismically triggered tremors have been detected. However, the 2009 BC earthquake appears to occur about 25 km NE of the fault.
Most of tremor signals occur in phase with the surface waves, seeming to indicate a causal relationship between occurrence of tremors and surface waves. We measured the peak ground velocities (PGV) of the surface waves of the 99 earthquakes as well as of the teleseisms studied by Peng et al. (2009) recorded on the transverse and vertical components of the PKD station (Figure 2.47a), and we found that the events that did trigger tremors in central California have among the largest PGVs. This is emphasized after filtering the data between 30 and 200 sec (Figure 2.47b).
If we consider a PGV threshold of 0.01 cm/s to separate triggering and non-triggering earthquakes at both regional and teleseismic distances and use a nominal surface wave velocity of 3.5 km/s and the elastic modulus of 35 GPa at tremor depth we found a corresponding dynamic stress of about 1 kPa (Figure 2.47b).
We show that tremors are not bandpass limited to 1-15 Hz but can have significant energy at higher frequencies. By searching seismic records above 15 Hz we found four triggered tremors correlating with the passage of the surface waves. High-frequency content is also observed for teleseismically triggered tremors. Finally, our calculated dynamic stress change of 1 kPa is in agreement with other findings, indicating that it is important to continue detecting and analyzing the tremor activity in central California in order to better define the fault mechanics and interactions in the deep crust.
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Gomberg, J., J. L. Rubinstein, Z. Peng, K.C. Creager, J. E. Vidale, and P. Bodin, Widespread triggering of nonvolcanic tremor in California, Science, 319, 173, 2008.
Guilhem, A., Z. Peng, and R. M. Nadeau, High-frequency identification of non-volcanic tremor triggered by regional earthquakes, Geophys. Res. Lett., 37, in press, 2010.
Nadeau, R. M., and D. Dolenc, Nonvolcanic tremors deep beneath the San Andreas Fault, Science, 307, 389, 2005.
Nadeau, R. M., and A. Guilhem, Nonvolcanic tremor and the 2003 San Simeon and 2004 Parkfield, California earthquakes, Science, 325, 191-193, 2009.
Peng, Z., J. E. Vidale, K. C. Creager, A. G. Wech, R. M. Nadeau, and K.C. Creager, Remote triggering of tremor along the San Andreas Fault in central California, J. Geophys. Res., 114, B00A06, 2009.
Rubinstein, J. L., J. Gomberg, J. E. Vidale, A. G. Wech, H. Kao, K. C. Creager, and G. Rogers, Seismic wave triggering of nonvolcanic tremor, episodic tremor and slip, and earthquakes on Vancouver Island, J. Geophys. Res., 114, B00A01, 2009.
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