Hi All. The 2.3Md quake north of Parkfield today (09/17/2003) is an interesting quake in that it is almost a pure thrust quake. There are two thrust faults in this area just to the east of the San Andreas Fault. They are the Table Mountain Fault and the Gold Hill Fault.

Not very much is known about these two faults as well as some others located east and south of Parkfield. The Parkfield-Cholame Segment of the San Andreas Fault is west of the southern Diablo Range and north of the northern Tremblor Range in central California. The active main trace of the fault is characterized by a 1-km stepover in Cholame Valley and a 5 degree left bend in the fault trace at Middle Mountain in the northern part of the area. The fault creeps to the northwest and is locked to the southeast, which is where the 1857 quake occurred.

The area of today’s quake is a mix of volcanic and sedimentary rocks. The Gold Hill Exotic Block (where I feel the quake occurred) is composed of fine to coarse-grained hornblende quartz gabbro. These rocks are similar in age and composition to the hornblende gabbro of Eagle Rest Peak 150 km to the southeast and the hornblende quartz gabbro of Logan 220 km to the northwest. The unit is fault bounded by the Jack Ranch Thrust Fault on the northeast and south and by the San Andreas Fault on the southwest.

By the way. Petra and I were told there is a caldera just to the north of Table Mountain. This is from a rhyolite volcano that self-destructed millions of years ago. I have no information about the caldera, or that it is indeed a caldera. However I have no reason not to believe the source of the information. I’m sure that any self respecting geologist could confirm this in a heartbeat, but as I’m not a geologist, self respecting or otherwise I can’t confirm this. I do know that there are a lot of indications of past volcanic activity in the area most particularly in the Lang Canyon area. Take Care…Don in creepy town