Hi All. Many times I find information by looking for it, but on occasions it’s handed to me, so to speak. The person I spoke to at the Berkeley Seismological Labs gave me a link to go to saying that I could and would find a lot of information there. I have the link on my desktop, but hadn’t gone there yet as I felt it was to early for any information about the San Simeon quake. Boy was I wrong.

There was an event that occurred just before the main event. You can’t see it on any of the seismographs as the signature of the main event obscures any events that occurred just before it. Several people in the Cambria area have reported "shaking" or "rumbling" around 10:40 AM on Dec 22, prior to the M6.5. Analysis of the seismic data indicates that this is most likely a sonic boom, and not an earthquake.

See the link. This figure shows a portion of an "electronic helicorder" for the period 10AM-11AM 12/22 PST from station PCB (near Cambria) that shows a small signal at 2003/12/22 10:38 PST. The impulse-response signal at 18:46 is a daily calibration signal. A blow up of the trace at that time shows the detail of the signal.

Triggered seismicity has been observed in the Geysers region north of San Francisco. This region often produces triggered earthquakes after large events in California.

The San Simeon mainshock also triggered a small amount of right-lateral creep on the San Andreas Fault near Parkfield. At most creep meters, this creep occurred as a step of 1mm at the time of the mainshock, although at station X461 the creep continued at a diminishing rate for about 24 hours. More than 24 hours after the mainshock, very rapid (~2 mm/day) creep started at CRR1, and still continues. The creep may have been triggered by the passing seismic waves, or from permanent stress changes.

Creep was also triggered at the San Juan Bautista creep sites. However at site XMR1 an increase in creep can be seen before the quake. Creep at this site still continues at present time.

An increase in creep isn’t uncommon, as it has been seen with other large quakes. The Loma Prieta quake caused an increase in creep on the Calaveras Fault in the Hollister area. They have been measuring horizontal slip at two sites on the Calaveras fault in the Hollister area for 16 years. Slip at both sites has been episodic with intervals of relatively rapid right slip typically lasting a couple months or less alternating with longer periods of time when little net slip occurs. The LPEQ occurred during an interval of slower movement that had persisted for about a year at Site 4. The earthquake apparently triggered up to 14 mm of right slip at Seventh Street.

Slip at Site 6 along Wright Road just 2.3 km northwest of Site 4 is also episodic. The LPEQ occurred during an interval of slower movement that had persisted for about a year at Wright Road (similar to the situation at Seventh Street). The earthquake apparently triggered up to 12 mm of right slip. The overall slip rate at Wright Road is several mm/yr faster than the rate at nearby Seventh Street. Either the creep rate decreases significantly from Wright Road southeast to Seventh Street or undetected surface movement is occurring outside the 89.7 m-long survey line at Seventh Street.

I have a feeling seismologist are going to be looking at the San Simeon quake for sometime to come. Take Care…Don in creepy town

Reference

THEODOLITE MEASUREMENTS OF CREEP RATES ON SAN FRANCISCO BAY REGION FAULTS

• CISN