Tremor Monitoring

The HRSN data played an essential role in the discovery of nonvolcanic tremors along the San Andreas Fault (SAF) below Cholame, CA (Nadeau and Dolenc, 2005). The location of the Cholame tremors occupies a critical location between the smaller Parkfield ($\sim$ M6) and much larger Ft. Tejon ($\sim$ M8) rupture zones of the SAF. Because the time-varying nature of tremor activity is believed to reflect time-varying deep deformation and presumably episodes of accelerated stressing of faults, and because an anomalous increase in the rate of Cholame tremor activity preceded the 2004 Parkfield M6 by $\sim$ 21 days, we are continuing to monitor the tremor activity observable by the HRSN to look for anomalous rate changes that may signal an increased likelihood for another large SAF event to the SE. Results of monitoring effort are described further in the "Research" section of this report.

Figure 3.14: Map view (top, left), and across (bottom, left) and along SAF depth sections (top and bottom, right) of double-difference locations resulting from application of the similar event pattern scanning and automated cataloging method using one event from each of the SAFOD HI and SF target sequences as a reference. The magnitudes of the 112 events ranged from 2.2 down to -1.4 Ml. In the across fault sections at the right, inferred circular dimensions of the event ruptures in the SAFOD zone are shown as open circles assuming a 3MPa constant stress-drop model (top) and an increasing stress-drop with decreasing magnitude model inferred from recurrence interval information (Nadeau and Johnson, 1998).