Anomalous Early Aftershock Decay Rate of the 2004 Mw6.0 Parkfield, California, Earthquake

Zhigang Peng, John E. Vidale, & Heidi Houston

Published September 2006, SCEC Contribution #1028

We analyze the seismicity rate immediately after the 2004 Mw6.0 Parkfield, California, earthquake from near-source seismograms. By scrutinizing high-frequency signals, we can distinguish mainshock coda from early aftershocks occurring as soon as 30 s after the mainshock. We find, as expected, that a significant fraction of aftershocks in the first few hours after the main shock are missing in the Northern California Seismic Network catalog. We observe a steady rate of aftershocks in the first 130 s, followed by a power-law decay of aftershock activity. Thus, there appears to be a distinct early stage of aftershock activity that does not fit the Omori's law with a constant p value, a phenomenon that we refer to as Early Aftershock Deficiency (EAD). Our observation suggests that mainshock rupture and aftershocks are distinct processes, not described by a single Omori's law. Several physical models of aftershocks can explain the EAD.

Citation
Peng, Z., Vidale, J. E., & Houston, H. (2006). Anomalous Early Aftershock Decay Rate of the 2004 Mw6.0 Parkfield, California, Earthquake. Geophysical Research Letters, 33, L17307. doi: 10.1029/2006GL026744.