Nonvolcanic Tremor Activity in the Parkfield-Cholame region of California and the 2003 M6.5 San Simeon and 2004 M6.5 Parkfield Earthquakes

We investigate the possible relationship between nonvolcanic tremor (NVT) activity and fault zone processes responsible for the generation of earthquakes. NVTs correlate with deep slow slip events along subduction zones in Japan and Cascadia and recently it has been observed that tidal stresses as well as dynamic stress changes produced by the passage of surface waves from large teleseisms are capable of inducing NVT activity. These observations suggest that tremor rate changes are closely related to and even stimulated by stress changes in deep fault zones. The computed values of tidal and dynamic stress changes also appear to be small, on the order of a few kilopascals, indicating that NVTs are more sensitive to stress changes than triggered earthquakes. We present a multi-year analysis of the NVT activity since August 2001 along the San Andreas Fault near Cholame, California detected and located using envelope cross-correlation methods. During this period two strong earthquakes occurred in proximity of the tremor-generating region: the 2003 M6.5 San Simeon and 2004 M6.0 Parkfield earthquakes. We qualitatively and quantitatively show that since these two events, long- term background NVT activity rates have increased. Our calculations of static Coulomb stress changes induced by the two earthquakes show that the amplitude of the static stress changes is small (below a few tens of kilopascals) but that they are enough to activate tremor activity and that the degree of stress change transmitted into the tremor zone correlates with the degree of tremor activation. Finally we show that a new pattern in the NVT activity has also emerged following the 2004 Parkfield event with the occurrence of quasi- periodic tremor episodes that have persisted well into 2008. Our results suggest that NVT activity can provide clues to variations in fault zone stress levels at tremor depth.