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Migration of Early Aftershocks Following the Mw6.0 2004
Parkfield Earthquake
Zhigang Peng (Georgia Institute of Technology), Peng Zhao (Georgia Institute of Technology)
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.
References
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.
Peng, Z., and P. Zhao (2009), Migration of early aftershocks following the 2004 Parkfield earthquake, Nature Geosci., 2, 877-881, doi:10.1038/
ngeo697.
Acknowledgements: This work was supported by the USGS NEHRP program G09AP00114.
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 crosscorrelation
(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.
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.
2010 IRIS Core Programs Proposal | Volume II | Earthquake Source Studies | II-73