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

Migration of Early Aftershocks Following the Mw6.0 2004 Parkfield Earthquake Newly detected aftershocks showing migrations after the 2004 Mw6.0 Parkfield earthquake. (a) Map of the Parkfield section of the San Andreas Fault (SAF; white lines). The newly detected aftershocks are color-coded by their occurrence times (in logarithmic scales) since the Parkfield mainshock (green star). (b) Mean cross- correlation (CC) functions for the template event 20040928233349. The black dots are positive detections above the threshold (red dashed line) and the red dot corresponds to the detected M2.56 event at ~140 s after the mainshock. (c) The histogram of the mean CC functions. (d) A comparison of the template waveforms (red) and the continuous waveforms (gray) for each component of 11 stations. Waveforms shown in green and blue colors correspond to other two events that occurred nearby. The arrows mark the origin times of the three events.
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.
</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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