In Part 1 I gave you the setting of the Silver Creek fault. Here I’ll talk about how scientists at the U.S. Geological Survey figured out where the buried part of the fault runs.
The earliest (I think) tool these scientists used to analyze the geology of the Santa Clara Valley was gravimetry. Gravimeters, developed in the ’70s-’80s time frame, measure gravity to such a fine degree that they can be used to determine what’s underground. Valleys are usually covered, and to some extent filled in, with alluvium that local streams have eroded out of the surrounding hills/mountains. They can get very filled-in, because over time what’s loose-ish near the surface compacts under the weight of the overlying alluvium and subsides. This makes more room for more alluvium! But still, even the compacted alluvium is not as dense as the surrounding mountain rocks. A gravimetry survey can distinguish rock from alluvium — and the deeper the alluvium, the lower the microgravity. First, here’s the Google Earth map of the modern Santa Clara Valley:
And here’s the microgravity map of the Santa Clara Valley:
By golly, there are TWO valleys under some of that alluvium! The one on the northeast, with all the deep, deep blue areas, is called the Evergreen Basin. And while we now know that it is bounded to the west by the Silver Creek fault, that wasn’t known when the gravimetry survey was done, it was just suspected. (Sorry about all the “after” photos, but these guys like to publish when they’re certain they know what they’re talking about.)
The next clue, which I used to have a pic of, but can’t find, is about hydrology. The Santa Clara valley, before it was Silicon Valley, was an ideal place to grow fruit trees. The whole valley was filled with agricultural activity. And since it doesn’t rain in coastal California in the summer, they pumped groundwater to water those trees. A lot of groundwater. When towns and small cities started to spring up, and grew, and grew, they pumped more and more groundwater. There wasn’t that much to pump. Land started subsiding. In downtown San Jose, it subsided as much as 16 feet in some places. Obviously, this couldn’t last, and the valley now gets its water from the rivers that drain the Sierra Nevada mountains and cross California’s Central Valley. There’s still pumping going on, though, and percolation ponds to counteract it; that’s just how the water is managed. So every summer there’s a couple of centimeters, give or take, of recoverable subsidence. Except it STOPS, to the east, at an invisible barrier. There’s this annual subsidence in San José… but it abruptly stops at the boundary of the Evergreen Basin. Any geologist worth her hammer would be yelling, “fault!” Faults often form hydrologic barriers. So that’s the next piece of evidence for the Silver Creek Fault.
To really determine whether the barrier is a fault, the U.S.G.S. decided to run a seismic reflection profile. To do this, they run a line of sensors designed to detect seismic reflections. Then, using a truck with a BIG weight in the back, they smack the ground really hard. This makes a seismic disturbance that is reflected back from the different layers of alluvium and rock, to give an idea of where layers are beneath the surface. What makes these layers? They’re simply layers of slightly different composition of alluvium — sand vs. clay, for instance — or in rock, changes in rock type or rock density. In valleys like the Santa Clara where all the alluvium has been deposited by streams, layers naturally vary in density and composition, as streams move around, have floods, create graded banks, and carry on like this for thousands of years.
Now, this wasn’t the first, nor the last, seismic reflection profile that has been done in the Santa Clara Valley, but what makes it unique is that it was done through downtown San José, against the backdrop of lots of other seismic disturbances: big trucks, construction, trains, etc. But the smart geophysicists were able to filter out most of that, and produce this profile:
All the squiggly, mostly horizontal lines are reflections from various layers. Just to make sure you can’t miss it, they’ve marked the Silver Creek fault in red;but if you look closely, you can see it in the profile. To the left of the line is the “noise” that comes back from solid, uniform-composition rock; to the right is the multitude of little lines that represent alluvial layers. “Franciscan Basement” refers to rocks of a group named “Franciscan”. You can see there’s no nice, gradual, left edge to the Evergreen Basement; the transition is very abrupt, and clearly indicates a fault. Bingo! The Silver Creek Fault forms the western boundary of the Evergreen Basin.
Great. So there’s proof that the Santa Clara Valley — Silicon Valley — has its very own fault, a less than reassuring thought to the people who live and work here. So what kind of fault is it? Has it moved a lot in the past, and will it do something nasty any day now? Stay tuned for part 3.
Wentworth, C.M., Williams, R.A., Jachens, R.C., Graymer, R.W., Stephenson, W.J., 2010, The Quaternary Silver Creek Fault Beneath the Santa Clara Valley, California, U.S. Geological Survey Open File Report 2010-1010, http://pubs.usgs.gov/of/2010/1010/, accessed 4/9/2013