Hi All. This is dated material going back to 1994 and because of that I had to use an archive copy so pictures and graphs are not available.

Not sure as to which thrust faults Paul Segall is talking about, but believe one of them may be the Monte Vista thrust fault, as there is some indication that it moved during the Loma Prieta quake. A complex zone of southwest-dipping thrust faults is present along the western margin of Santa Clara Valley, California. The segment of range-front faults extending from Los Gatos through Los Altos Hills occupies a desirable area for continued residential development. Local faults include the Berrocal, Shannon, Monte Vista, and Altamont. Some homes are using the upside of the Monte Vista thrust fault in their landscaping scene. In other words they are sitting on top of it.

The other thrust faults east of the Santa Cruz Mountains are located on the eastern side of the Santa Clara Valley. These faults include the Piercy, Coyote Creek, Silver Creek, San Jose, Evergreen, Quimby, Berryessa, Crosley, and Warm Springs faults that underlie the communities of eastern Santa Clara Valley, including San Jose and Milpitas. It is known as the East Valley thrust system.

“Stream terrace profiling and geomorphic map features suggest that faults within the East Valley thrust system may experience repeated, minor offset and likely only rupture in secondary response to large earthquakes on the nearby Hayward and Calaveras faults and, thus, may not be fully independent seismic sources. Integration of geomorphic and structural information is helping to define the style of strain transfer at the southern end of the Hayward fault and to quantify the magnitude and rate of shortening on the East Valley thrust system.”

To me that sounds a little like a double barrel shotgun. If the first barrel doesn’t get you the second one might.

“Using data gathered during the quake and in studies performed over the past five years, geophysicists have learned that the Bay Area is riddled with a much more complex web of earthquake faults than previously understood. While experts probably will be debating for years the detailed interpretation of recent studies, geophysicists from Stanford and elsewhere have made several key findings:”

“The ¹89 quake actually may not have occurred on the San Andreas, but rather on a subsidiary fault. Its motion included substantial lifting and sinking rather than the pure horizontal sliding motion normally seen on strike-slip faults like the San Andreas. Beroza analyzed seismic waves generated during the quake, and Stanford colleague Paul Segall made accurate measurements of the change in position of certain ³benchmarks² on the earth¹s surface to show that much of the motion during the Loma Prieta quake was vertical motion typical of reverse or ³thrust² faults.
³We are realizing that reverse [thrust] faults are very common in California - many of our most damaging earthquakes are not on major strike-slip faults, but on subsidiary faults,² Segall said. The 1994 Northridge earthquake in Los Angeles was such a quake - it occurred on a ³blind² thrust fault whose existence was previously unknown.”

“Segall¹s data show that stress transfer from the Loma Prieta quake activated other thrust faults known to exist to the east of the Santa Cruz Mountains. The increased slip of these faults has caused the peak of Loma Prieta to rise about 1-1/2 inches since the quake, regaining about one-third of the height it lost due to horizontal ground motions during the quake itself.
Uneven patterns of destruction observed after Loma Prieta helped point to the existence of previously unknown fault structures in the Bay Area. Damage to the Cypress elevated freeway structure in Oakland was so much greater than expected at that distance from the epicenter that it led some scientists to speculate that seismic energy was reflected off horizontal rock layers or faults deep in the earth¹s crust.”

Sure hope you don’t get bored reading all of this. I’m having one of my sleepless nights and this is my way of getting sleepy. Doesn’t work most of the time, but neither does anything else. Take Care…Don in creepy town

http://66.102.7.104/search?q=cache:L75VLkwTNGsJ:www.stanford.edu/dept/news/pr/94/941011Arc4099.html+loma+prieta+quake+stress+transfer&hl=en

http://66.102.7.104/search?q=cache:ha0SSih5Sc0J:www.stanford.edu/dept/news/pr/91/911217Arc1012.html+loma+prieta+quake+stress+transfer&hl=en