No abstract

S U M M A R YWe investigate the seismic velocity contrast across the San Andreas fault (SAF) in the Parkfield area using fault zone head waves (FZHW) that propagate along the bimaterial fault interface and direct P waves. We systematically analyse large data sets of near-fault waveforms recorded by several seismic networks over the period 1984-2005. Clear FZHW are observed at many stations on the NE side of the fault in the creeping section of the SAF north of Middle Mountain (MM). This indicates the presence of a sharp bimaterial interface and that the NE side of the fault has lower seismic velocities in that region. The obtained P-wave velocity contrast is about 5-10 per cent north of MM, and it systematically decreases to 0-2 per cent near Gold Hill (GH). The along-strike variations of the velocity contrast are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF near GH, associated with the GH fault, and existing 3-D seismic tomography results. The obtained imaging results offer an explanation for the mixed rupture directions of the M6-type Parkfield earthquakes. The strong velocity contrast around MM is expected to produce a preferred propagation direction to the SE for earthquakes that nucleate near MM (e.g. the 1934 and 1966 Parkfield earthquakes). In contrast, the near-zero velocity contrast and multiple fault branches near GH imply that earthquakes that nucleate near GH (e.g. the 2004 Parkfield earthquake) are not expected to have a preferred propagation direction to the SE, and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities. The observed systematic reduction of the velocity contrast along the SAF from NW of MM to SE of GH provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the Parkfield section and propagate to the SE, and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the NW.

No abstract

[1] We systematically investigate the velocity contrast along the Calaveras fault that ruptured during the 1984 Morgan Hill earthquake using fault zone head waves (FZHW) that refract along the fault interface. We stack waveforms in 353 sets of repeating clusters, and align the peaks or troughs of the direct P waves assuming rightlateral strike-slip focal mechanisms. The obtained velocity contrasts are 2 -3% and 12-14% NW and SE of station CCO, respectively. The FZHW and the fault plane outlined by the relocated seismicity SE of CCO are more complicated than those NW of CCO. The results can be explained by a relatively simple and sharp fault interface in the NW, and a complicated fault structure with a presence of a low-velocity zone in the SE. The along-strike variations in the strength of the velocity contrast are consistent with surface geological mapping and recent 3D tomography studies in this region. Citation: Zhao, P., and Z. Peng (2008), Velocity contrast along the Calaveras fault from analysis of fault zone head waves generated by repeating earthquakes, Geophys.

S U M M A R YWe systematically investigate spatial variations of temporal changes and depth extent of damage zones along the Calaveras fault that ruptured during the 1984 Morgan Hill earthquake by the waveform analysis of 333 sets of repeating earthquakes. We use a sliding window waveform cross-correlation technique to measure traveltime changes in waveforms generated by each repeating cluster. We find clear traveltime delays in the S-and early S-coda waves for events immediately after the Morgan Hill main shock. The amplitudes of the time delays decrease logarithmically with time since the main shock, indicating a time-dependent recovery (healing) process following the abrupt coseismic temporal changes. The largest temporal changes are observed at station CCO that is the closest to the rupture zone of the Morgan Hill main shock. The time delays at this station are larger for clusters in the top 6 km, and decrease systematically at larger depth. In comparison, the time delays observed at other five stations are much smaller, and do not show clear relationship with hypocentral depth. We suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near-surface layers, while the temporal changes at station CCO are likely associated with the damage zone around the Calaveras fault that is well developed in the top few kilometres of the upper crust. Our results are consistent with the inference of a widespread damage and non-linearity in the near-surface layers associated with strong ground motions of nearby large earthquakes, and localized damages and flower-type structures around active faults based on previous studies of fault zone structures and recent 3-D numerical simulations.