Hi All. The creep at the SJB data still continues at an accelerated pace, but is starting to slow down. The creep is associated with the San Andreas Fault. It should also be noted that there was a change in tensor strain at almost the same time as the increase in creep started.

This started me to thinking about the Loma Prieta quake and the 1906 quake and where the next quake might occur on the San Andreas Fault. The area that is quiet at this time is the section of the fault between Portola Valley and the area just south of the entrance to the bay.

The San Andreas Fault trends northwesterly and extends more than 800 miles from the Gulf of California to Cape Mendocino north of San Francisco. It has been the source of many large earthquakes including an 1838 earthquake located on the peninsula south of San Francisco (magnitude in excess of 7.0), an 1865 earthquake northeast of Santa Cruz, and the famous 1906 San Francisco Earthquake with magnitude 8.3.

The Loma Prieta Earthquake essentially repeats the 1865 event and is the first major rupture along the San Andreas since 1906. The length of fault rupture generally extended from the southern end of the 1906 break and thus relieved strain, which had accumulated since before that time.

The recent rupture is unusual for earthquakes on the San Andreas Fault. Past earthquakes have been the result of horizontal slip of the ground on the two sides of the fault, with the southwest side moving northwest relative to the northeast side. The recent rupture of the Loma Prieta Earthquake includes not only this horizontal component of slip, but also a significant thrusting of the southwest side up and over the northeast side. The fault has steep dip to the southwest in this area. The portion of the Pacific plate including Santa Cruz and Watsonville moved 6.5 feet north and 4.6 feet up relative to the portion of the North American plate.

Another unusual feature of this earthquake was the lack of primary surface fault rupture. The focal depth has been placed at 11 miles. This is unusually deep, as typical California earthquake focal depths are 4 to 6 miles. The combination of rugged topography, thick soil, and forest cover could also make surface breaks less noticeable. The State Commission report of the 1906 earthquake described very similar surface rupture characteristics along the Santa Cruz Mountains portion of the San Andreas Fault. The impressive cracks shown in the media were nothing more then superficial features caused by the strong shaking. The largest such crack was located near the intersection of Summit Road and Highway 17 in Santa Cruz County, and was 650 yards long, 2.5 feet wide, and showed 2.5 feet of left-lateral offset at one point. Numerous ground failures occurred over an area 60 miles long by 25 miles wide, stretching from San Gregorio to Hollister, as well as additional ground failure occurring in a narrow strip along the coast from Santa Cruz all the way up to San Francisco.

Sitting here and waiting for what’s next is sort of like waiting for the next shoe to drop. Take Care…Don in creepy town.

• Loma Prieta Quake