Migration of early aftershocks following the 2004 Parkfield earthquake

Large shallow earthquakes are immediately followed by numerous aftershocks. A significant portion of these events is missing in existing earthquake catalogues, mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks(1-4). However, recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering(2-4), and for tracking postseismic deformation around the rupture zone associated with the mainshock(5-7). Here we use the waveforms of 3,647 relocated earthquakes(8) along the Parkfield section of the San Andreas fault as templates(9,10) to detect missing aftershocks within three days of the 2004 magnitude 6.0 Parkfield earthquake. We identify 11 times more aftershocks than listed in the standard catalogue of the Northern California Seismic Network. We find that the newly detected aftershocks migrate in both along-strike and down-dip directions with logarithmic time since the mainshock, consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip(11,12). The cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days, supporting the view that aftershocks are driven primarily by afterslip(13,14).