Hi All. Recently, analysis of stress-induced borehole breakouts in petroleum wells along the Carrizo plain segment of the San Andreas fault in southern California indicate that the angle between the maximum horizontal compressive stress and the San Andreas increases from about 25-45° near the fault to 65-85° at distances greater than 20 km (Castillo and Hickman, 1995). Although the significance of this observation for fault strength is somewhat ambiguous owing to scatter in the data and the lack of information on the horizontal differential stress magnitudes adjacent to the fault (c.f., Zoback and Roller, 1979), these observations might suggest that the Carrizo plain section of the San Andreas fault is able to support higher levels of shear stress as compared to other weaker segments of the fault.

What does this mean? Does this mean that a major quake is more probable along the San Andreas fault in the Carrizo Plain? If so then when will that quake occur? There may be some hints as to the most probable time it could occur, but they are just that. Hints and not very well substantiated ones at that.

Since 1993, two 3-component borehole strainmeters, on opposite sides of the fault at Parkfield, are recording slower contraction and slower extension, respectively, according to Ross Gwyther and Michael Gladwin of CSIRO, Queensland, Australia, long-term collaborators at Parkfield. Three-fault crossing, near fault-parallel baselines measured using the two-color laser Electronic Distance Meter (EDM) are lengthening or shortening more rapidly since early 1993. Faster relative slip is also indicated by creepmeters spanning the fault within the two-color EDM network. Roeloffs said the strainmeter, creep and EDM data are all consistent with accelerated relative motion across a portion of the San Andreas fault, but the data cannot determine the location of the source area.

"The acceleration of strain rate is small, 0.1 to 0.5 parts per billion per year," Roeloffs said, "and can only be detected with years of data from sensitive instruments." The observations have been controversial because such slow changes in deformation rate could be affected by non-tectonic variations in precipitation and groundwater levels. Between 1991 and 1993, groundwater levels began to rise in response to increased annual rainfall.

A strain rate change before the 1989 Loma Prieta, Calif. earthquake was reported in 1991 by Dr. Gladwin and his colleagues, but the lack of other instrumentation makes it difficult to conclude that it was actually an earthquake precursor. A "slow earthquake" on the San Andreas fault near San Juan Bautista that was reported in 1996 by Dr. Alan Linde of the Carnegie Institution of Washington demonstrates that strain rate variations are not always immediately followed by large seismic events. While not a major quake we had a 5.4Ml in August 1998 in the area of San Juan Bautista. There was also a report of acceleration in creep before the 1984 Morgan Hill quake although I have been unable to find any reference for that report.

Does this mean that a large to major quake will occur every time there is acceleration in creep and or strain along a strike slip fault, or does it only occur on certain faults and only on certain segments of the fault? Take Care…Don in creepy town