Migration of Early Aftershocks Following the Mw6.0 2004 Parkfield Earthquake

Migration of Early Aftershocks Following the Mw6.0 2004 Parkfield Earthquake Early aftershocks of the 2004 Parkfield earthquake recorded by the SAFOD Pilot Hole station. (a) The raw vertical-component seismogram recorded at the SAFOD Pilot Hole station PH001 within the first 1000 s after the 2004 Mw6.0 Parkfield earthquake. (b) The corresponding spectrogram of (a) showing the mainshock and numerous early aftershocks. The think horizontal line at 40 Hz marks the corner of the high-pass filter, and the thin bands around 60 and 184 Hz mark the continuous electronic noise. (c) The envelope functions of the median-averaged spectrogram above 40 Hz. The vertical blue and red lines mark the origin times of 16 and 130 aftershocks listed in the NCSN catalog and detected by our technique, respectively.
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A large shallow earthquake is immediately followed by numerous aftershocks with a significant portion missing in existing earthquake catalogs, mainly due to masking of the mainshock coda and overlapping arrivals [e.g., Peng et al., 2006]. Recovering these missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering, and tracking post-seismic deformation around the mainshock rupture zone. We use waveforms of relocated events along the Parkfield section of the San Andreas Fault (SAF) as templates, and scan through continuous waveforms for 3 days around the 2004 Mw6.0 Parkfield earthquake to detect missing aftershocks [Peng and Zhao, 2009]. We identify 11 times more aftershocks than reported in the standard Northern California Seismic Network (NCSN) catalog. The newly detected aftershocks show clear migration in both along-strike and down-dip directions with logarithmic time since the mainshock, consistent with the numerical simulations on expansions of aftershocks caused by propagating afterslip. The cumulative number of early aftershocks increases linearly with postseismic deformation in the first 2 days, supporting the view that aftershocks are driven primarily by afterslip.
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References
</p><p>Peng, Z., J. E. Vidale, and H. Houston (2006), Anomalous early aftershock decay rates of the 2004 M6 Parkfield earthquake, Geophys. Res. Lett., 33, L17307, doi:10.1029/2006GL026744.
</p><p>Peng, Z., and P. Zhao (2009), Migration of early aftershocks following the 2004 Parkfield earthquake, Nature Geosci., 2, 877-881, doi:10.1038/ ngeo697.
</p><p>Acknowledgements: This work was supported by the USGS NEHRP program G09AP00114.</p>

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