Ongoing Activity

Nadeau and Dolenc (2005) found that changes in tremor and micro-earthquake rates at Cholame appeared to correlate. This suggests that deep deformation associated with the Cholame tremors (i.e., ETS) may also be stressing the shallower seismogenic zone in this area. Subsequent monitoring of the Cholame tremors has revealed further evidence for such stress-coupling. Of particular note have been the rate changes associated with the 22 December 2003, M6.5 San Simeon, CA and the 28 September 2004, M6 Parkfield, CA earthquakes (epicenters $\sim$ 50 km west and $\sim$ 10 km NW of Cholame, respectively) that have now been observed (Fig. 2.14).

Between 1 and 3 months before the Parkfield earthquake, tremor activity was relatively low, near pre-San Simeon levels. The activity then spiked between 20 and 22 days prior to the Parkfield mainshock. The relationship of this fore-tremor (FT) to the Parkfield mainshock is suggestive of coupling between deep stress changes associated with the tremors and stress changes in the shallower seismogenic zone leading to the Parkfield M6.0 mainshock.

More profound, however, has been the large and long lasting increase in overall tremor rates following the Parkfield event. Immediately following the the mainshock, tremor rates increased to unprecedented levels that persisted for several days. For several weeks following this period tremor rates remained extremely high but decayed rapidly (similar to the decay of aftershocks in the region). Then, $\sim$ 80 days after the mainshock, tremor rates appear to have entered into a new state where overall rates decay much more slowly and where the dominant pattern of activity exhibits a pattern of multi-scale quasi-periodic variation (i.e., with periodicities of $\sim$ 75 and 330 days). This pattern has persisted up to the time of this report, and it is not yet clear whether the rate behavior reflects solely the response of the tremor source region to stress from the Parkfield mainshock or if mainshock stresses have activated other tremor related processes (e.g., fluid migration or transient deformation).

In any case, the pattern of tremor rate behavior relative to the San Simeon and Parkfield events supports the argument that nearby moderate magnitude earthquakes can stimulate deep NVT activity and that such events may have a significant impact on the long-term evolution of NVT activity. In addition, because 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, future increases in SAF tremor activity may signal periods of more rapid stress change and an increased probability for the next large earthquake on the Cholame segment.

Figure 2.14: Activity rate history of nonvolcanic tremors detected by the borehole High Resolution Seismic Network (HRSN) at Parkfield (PF), CA. Histories span 2240 days (3 years 65 days prior to the PF mainshock to 2 years 349 days after the event). The number of minutes of tremor activity for each day is computed, yielding a time series of activity rates with sampling interval of 1 day. The time series is then smoothed with boxcars of 4 different window lengths (panel A, 7.5 days; B, 15 days; C, 30 days and D, 60 days) stepped at daily intervals with values plotted at the center of each window. This is done to help illustrate the multi-scale periodicity of the data. Black lines spanning all four panels are times of the 2003 and 2004 San Simeon and PF earthquakes. Black lines in panel 1 are times of the 2002 Denali M8.1 and 2004 Sumatra M9.3 earthquakes. Triggering of tremor activity from these global events appears to be relatively insignificant compared to triggering related to the San Simeon and PF events. FT refers to apparent fore-tremor event preceding the PF mainshock. Small vertical dashes (Panel C) are 75 day intervals approximating periodicity on this scale following the PF mainshock. Short horizontal lines (panel D) show the approximate 330 day rate pulses.