Hi Cathryn - while you are waiting for a real scientist to chime in, I can help some. Like you, I recall hearing that Loma Prieta occurred on some fault other than the SAF, early on. I happen to have the Seismological Society of America's Special Edition on the Loma Prieta quake in my library. It was published two years after the quake, by which time much initial analysis had been done. At the time of publication, the authors of the introductory article (Thomas C. Hanks and Helmut Krawinkler) considered that the SAF as the source of the quake was "an arguable proposition". It seems, in my readings since, that it is now accepted to have occurred on the SAF. If you want to pursue the controversy further, you might search for Segall and Lisowski's 1990 paper. In the Special Issue, Prentice and Schwarz provide the chief argument against the SAF being responsible. One big problem is that, if it was on the SAF, the analysis of the quake suggests it should have re-ruptured the portion of the fault in the Santa Cruz Mountains which ruptured in 1906 (according to Prentice and Schwarz). But this quake did not do so. They provide other arguments, too.

The quake was, as you noted, right lateral strike-slip with a significant reverse slip component. The vertical motion was in the sense of lifting the Santa Cruz Mountains on the SW (coast) side of the fault, yet was counter to what would be expected to explain the high portion of the Santa Cruz Mountains to the NE side of the fault. This is also part of the argument against the quake being on the SAF. I am sure someone with more knowledge and time than I have right now could explain it more fully.

If I read the information correctly, the vertical component of slip was not unexpected, and is similar to the creation of the Transverse Ranges in Southern California resulting from the SAF's adjacent Big Bend.

One interesting sentence in the aforementioned introductory article bears on the well-known supposed prediction of the Loma Prieta quake. It states that the component of reverse slip mentioned above, "while not unexpected in this terrane of crustal shortening and vertical tectonics, has raised questions about the legitimacy of the forecast for rupture of the southern Santa Cruz Mountain segment of the San Andreas Fault, and, indeed, whether the Loma Prieta earthquake actually ruptured the San Andreas Fault at all.

Michael Williams
Arroyo Grande, CA USA