Persistence of the Parkfield seismicity patterns through the 2004 mainshock

The pattern of seismicity on the Parkfield section of the San Andreas fault in the 35 years prior to the Sept. 28 2004, M6 quake defines distinctive features that include sub-horizontal streaks of seismicity, an abundance of repeating earthquakes, persistent clusters of earthquakes, and large regions devoid of microearthquakes (Waldhauser et al., GRL, 2004). The 2004 M 6 Parkfield earthquake nucleated about 12 km southeast of the town of Parkfield and generated more than 1700 aftershocks in the three weeks following the mainshock, including a M 5 event on both October 29 and 30. Only little seismic activity occurred in the immediate vicinity of the hypocenter prior to the mainshock, with the closest notable cluster of microearthquakes at about 4 km distance. Preliminary results from double-difference aftershock relocations show that the pre-2004 seismicity pattern continues in the aftershock distribution of the 2004 mainshock. The densest concentration of aftershocks occurs in the middle of the rupture along a 12 km long streak between 4 and 6 km depth that can also be recognized in the preceding seismicity. Few reliably located aftershocks or preshocks occur at shallower depths, suggesting that it marks a transition in the stability field of the fault from velocity weakening to velocity strengthening behavior. The two M 5 aftershocks locate at the NW end of the aftershock zone, within the 'deep streak' of background seismicity beneath Middle Mountain at about 11 km depth described by Waldhauser et al. (GRL, 2004). The 'deep streak' bounds the bottom of a prominent aseismic area that contains the 1934 and 1966 Parkfield earthquake hypocenters. This 'hole' is bounded above by a second streak. Both streaks are defined by microseismicity that persist over the 35 year observational period 1969-2004. They also contain events of up to M 5, and waveform comparisons with earthquakes from the Wood-Anderson period (1931 onward) show that most have recurred one or more times since the 1934 Parkfield earthquake. In particular, the M 5.0 aftershock of 09/29 17:10 appears to be a repeat of a M 4.8 event that occurred on Nov 14, 1993, which has 3 earlier known occurrences in 1934 (M 4.9), 1939 (M 5.2) and 1956 (M 4.9). Waveform comparisons of all post-1984 earthquakes in the NCSN catalog and M larger than 3.8 earthquakes from the Wood-Anderson era is now in progress. However, the evidence collected to date suggests that the patterns of seismicity at Parkfield persists through not only the 2004 earthquake but the 1934 and 1966 earthquakes as well. Either local geometric or mechanical properties of the fault would appear to be required to allow the source zones of repeating earthquakes to survive the broad scale redistribution of stress in the 1934, 1966 and 2004 mainshocks.