The Mystery Fault, Part 2


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:

SCValleyToday

And here’s the microgravity map of the Santa Clara Valley:

gravity

 

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:

reflectionProfile

 

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.

References:

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

 

 

 

Comments

  1. rowanvt says

    D: That fault looks to be less than 2 miles from where I work! nooooooooooooooooooo!

  2. lyle says

    Actually gravimeters were developed in the 1920s and were first used to find salt domes on the gulf coast in the search for oil. I recall using one and measuring the change in gravity between the basement and the 4th floor of the building in which the geology department was located in 1971. Yes they became digital and much faster to take a reading in the 1980s but have been around a lot longer. It should be noted that these were relative gravimeters, and in the time frame you talk about absolute gravimeters came about. A relative gravimeter measures the change in gravity relative to some location, while an absolute one measures an object dropping in a vacuum.

  3. Karen Locke says

    rq, one of my classmates was on the crew that did that particular seismic profile, and here’s his description of it. Basically you lay out a row of sensors about a city block long give or take, untangle the wires that run between them as you go, hook them up to a laptop with some special data-gathering software, and have your big weight truck come along and do its THUMP. Then you shift down to the next block and repeat. I gather it annoyed the heck out of the neighbors.

    What you get, with the raw data, is return times of reflections. To translate that into distances, there’s this complicated method that I don’t completely understand (I’m not a geophysicist) where you account for the speed that seismic waves travel through various media, look for signs of street noise and filter them out manually, and ultimately come up with a depth profile. I’d like to know how it’s done, too, and when I was in school I pestered the geophysicist on our faculty to give a seminar, but he never did it. But within the geophysics world it’s a very well-understood technique, and it’s done all the time in oil exploration.