Sorry for the long hiatus, everyone, but life sort of caught up with me for a few months. To attempt to make amends, I’ll tell you a geologic mystery story that was just solved — provisionally — a few years ago. It regards a fault that crosses my home base, the Santa Clara Valley. It’s called the Silver Creek fault, after a creek that roughly runs along some of its length, though the rocks it cuts aren’t the type to provide silver ore.
Let’s start with some regional perspective. The Santa Clara Valley is at the south end of San Francisco Bay in California. Anyone even slightly familiar with the local geology knows that the San Francisco Bay Area has all sorts of faults. Most of them, or at least most of the ones geologists worry about, are the result of two tectonic plates sliding along past each other: the Pacific plate, to the southwest, and the Sierra Nevada-Great Valley microplate, to the northeast, are moving right-laterally relative to one another. That is, if you stand on either plate and look at the other one, it appears to be moving to the right.
The map above, from Simpson and others (2004), shows the major faults in the south and central parts of the San Francisco Bay Area. Ignore the color codes; they’re only important for the original document. Also, these are fault zones; they often have little strands and cross-faults and are generally messy things. Ground breaks reluctantly and makes messes when it does.
Finally, these are just the major strike-slip fault zones. The rocks to either side of these faults primarily slide past each fault. But there’s always some motion of the rocks either pulling apart from each other or pushing up one side over the other, and these motions are called dip-slip. Dip-slip, it turns out, will be important to the story.
The map above also shows the Silver Creek Fault. But, drawn in 2004, it postdates at least part of our mystery. Here’s a 2005 map from another USGS paper (Wentworth and Tinsley, 2005) that shows the visible part of the Silver Creek fault in a continuous line and the inferred part in a dashed line:
The southeastern part of the Silver Creek fault is visible in the rocks along the southeastern end of the valley. The northwestern part of the Silver Creek fault is inferred because it is covered in valley alluvium. From the surface, there’s no trace of the fault. Which brings us to the mystery questions: so how did the scientists figure out where the trace really is? What kind of an effect does the fault have on the valley? And is it going to kick off a nasty earthquake that will devastate the Santa Clara Valley?
Stay tuned for Part 2.
Simpson, R.W., Graymer, R.C., Jachens, D.A., Ponce, C.M., Wentworth, C.W., 2004, Cross-Sections and Maps Showing Double-Difference Relocated Earthquakes from 1984-2000 along the Hayward and Calaveras Faults, California, U.S. Geological Survey Open-File Report 2004-1083, http://pubs.usgs.gov/of/2004/1083, accessed 4/7/2013.
Wentworth, C.W., and Tinsley, J.C., 2005, Geologic Setting, Stratigraphy, and Detailed Velocity Structure of the Coyote Creek Borehole, Santa Clara Valley, California, in Asten, M.W., and Boore, D.M., eds., Blind comparisons of shear-wave velocities at closely spaced sites in San Jose, California: U.S. Geological Survey Open-File Report 2005-1169, http://pubs.usgs.gov/of/2005/1169/of2005-1169.pdf, accessed 4/7/2013.