Hi All. The M=3.4 quake appears to have occurred on the Pinole fault. The “first motion mechanism” shows the quake to be a thrust quake with some right lateral movement.

The Pinole fault may be a potential linking structure between the active Hayward and Rodgers Creek faults, thereby significantly increasing the potential rupture length and, hence, the maximum earthquake which may be generated by these faults.

Previous studies (1980) have indicated that the onshore part of the Pinole fault is inactive. More recent geophysical survey results now calls into question this assumed inactivity. Age-dating of sediments offset in San Pablo Bay shows that the Pinole fault has a recurrence interval of about 900 years; a recurrence rate that is about a factor-of-five smaller than either the Hayward or the Rodgers Creek faults.

Based on the displacement of latest Pleistocene and Holocene deposits, a minimum slip rate of 0.3 to 1.25 mm/yr is calculated for the Pinole fault. An estimated rupture length of 9 to 19 km indicates that the Pinole fault is capable of generating an earthquake of Mw 6.2 to 6.6. Some seismologists think this may be the fault responsible for the 1898 M=6.5 Mare Island quake. Trenching across the southern end of the Rodgers Creek fault has never shown any rupture. If this is correct then this means that the last time there was a major quake on the Rodgers Creek fault was between 1730 and 1760 with a reoccurrence rate of 250 years. Of course it could also mean that they didn’t dig in the right place.

Today’s quake could be a foreshock, but the only way of knowing that for sure is if a larger quake occurs. There has been plenty of time for stress to build in the area and it is only a matter of time before the next major quake occurs and it will occur. It has to occur simply because it has in the past. Don in creepy town

http://quake.wr.usgs.gov/research/deformation/modeling/papers/2003/Parsons_StepoverBSSA.pdf

• Hayward, Rodgers Creek, Pinole Fault