It’s no secret that Husband and I own property in the Eastern Sierra Nevada mountains, about five miles south of the intersection of highways 108 and 395.  On our property,  cliffs of granite come close to or against the hwy 395 road; this is called the Devil’s Gate, for the muddy grief it gave travelers during the pre-hwy-395 years.  There are springs there. There’s a Spring creek there. Very annoying water, if you’re trying to take a wagon through the Gate in June, and there’s no hwy 395 to help.

Relatively near the gate, there’s an alluvial fan with a creek that runs for a couple of months in the Spring.  We’ve chosen a building spot on the fan, well uphill from the creek, but not so close to the granite wall that it could drop large rocks on us.

Down by hwy 395, there’s a low spot.  It wasn’t there before the construction of hwy 395 in the ’20s — the road builders should have put in a culvert — and we got a county permit to fill it in with the excess excavation material from our home site.  Then someone complained that we were filling in a wetland.

The expert we hired from an independent firm has not yet released her results, but her preliminary finding was that it was probably not a wetland under Federal law, but might well be under California law.

At this point we’re just hoping we don’t have to mitigate the damage already done to the “wetland”.  We have REAL wetlands on our property on the other side of hwy 395, fed by those springs, and we’re as determined as anyone to preserve them.  But this is just a Spring mudhole.

Grunble grumble mutter mutter,

Karen

I have a cousin by marriage — we’ll call her Mary for the sake of this post, not her real name.  She has lots of health issues, though the most dire one is bipolar disorder.  It keeps her from holding down any sort of job.  She’s married to a guy who has troubles of his own, and is often on disability.  Family members say he’s amazingly lazy; I only met him once, at their wedding, so I can’t say.   But they definitely have trouble making the rent AND eating.

I suggested she put up a web page (another relative, who runs a small ISP, will host one for free), explain the situation, and put up a Paypal donate button.  I, and I suspect many of her friends, would be glad to sign up for small Paypal subscriptions.  I got no response.  Perhaps she’s having a “down” episode.

I want to help Mary help herself, but I don’t know how.  She does fantastic things when she’s “up”,  and struggles mightily when she’s “down”.  She does take meds, but they don’t seem to help much.

So I ask you, readers of this fine blog, for suggestions.

Thanks, Karen

In Part 2, we’ve established that the Silver Creek fault is, in fact, the southwestern boundary of the Evergreen basin.  But what’s it doing there, anyhow?  Back sometime around 10 to 15 million years ago, an early Hayward fault formed (an estimated 100 km (62 miles) south of where it is now) with a right stepover in the southeastern most end of it.  Basically that’s where a fault  stops and then starts again some distance away, in this case to the right.  .Now, both the Hayward and the Silver Creek faults are right-lateral strike-slip faults.  This means if you stand facing the fault, and it slips, the ground on the other side of the fault will move right.  When you have a right step in a right-lateral fault system, it tends to pull the ground between the faults apart like so:

And thus creating a pull-apart basin.  So the basin started to be created in the Miocene.  Since then there’s been 175 km (109 miles) of slip on the Hayward Fault, of which 40 km (25 miles) of slip was on the Silver Creek fault itself.

Now, the Hayward fault is a very active fault.  Parts of it are creeping at the surface, but the whole fault system is locked at depth right now and is one of the prime candidates for the next Northern California Big One.  Scary thought, and keeps geologists and emergency managers awake at night.  But what about the Silver Creek fault?

Well, it turns out that about 2 million years ago, give or take a few thousand, the slip that was happening on the Silver Creek fault moved over to the Calaveras fault, which runs east of the Hayward fault.  (See the faults pic in Part 1.)  That doesn’t mean the fault isn’t still moving, but it’s moving at a much slower rate than the Calaveras and Hayward faults, and probably won’t generate any big earthquakes. Probably.

There’s some evidence for a couple of magnitude 6-ish quakes in 1903 down near the south end of the Silver Creek fault, but they’re not very well located, and it could very well be that they were produced by the Calaveras or another fault.  Because there is another fault, one that I haven’t talked at all about yet, and which is completely buried and only inferred.  But there’s some pretty decent inference for inferring it.  I’ll talk about it in Part 4.

Meanwhile, those of you who live in California, there are fault systems in both the northern and southern parts of the state primed to go off and produce the Next Big One.  Plan for a few days of food, water, no electricity, etc.  Expect cell phone systems to be down.  Expect land lines to be overloaded.  Expect hospitals to be backed up.  But, as Douglas Adams says, don’t panic.

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

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.

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

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.

References:

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.

I live in California.  Our official State Rock is serpentine.  The only problem with that is that serpentine is a mineral, not a rock.   Rock consisting mostly of serpentine is called serpentinite.  AAAAGGGGHHHH!  (Jumps up and down in frustration.)  Admittedly, where serpentinite occurs, it’s really, really, full of serpentine.  But still…mutter grumble grumble mutter. Continue reading “My state’s official rock is a mineral!” →

UPDATE:  One commenter wanted to know what I do with obsidian beads.  So I’ve added a photo of one of my beaded necklaces at the end of the post.  It’ll be up for sale at http://www.etsy.com/shop/gemmyjoy in a week or two.  /UPDATE

Rocks are my friends.  I especially like the ones I can pick up, look over in my hand ( maybe with a handlens) and say “this is cool!” If it’s something that somebody can make beads out of, so much the better — I’m a beader.  Obsidian qualifies.

Obsidian is an extremely felsic (feldspar/quartz-rich) lava with a chemical composition similar to rhyolite. Think silica-rich.  Really, really, really silica-rich.  But while rhyolite congeals into microscopic crystals, obsidian doesn’t, and has a glassy composition.  For a long time, the “accepted wisdom” was that obsidian cooled too quickly to crystallize.  Anyone who’s ever seen an obsidian dome next to a rhyolite flow (there’s one at Medicine Lake, California) can tell that explanation is a bunch of hooey.  I haven’t kept up with the literature in recent years about new theories for obsidian formation, so if anyone can point me to a paper in the comments I’d be grateful.

There are obsidian domes all over the eastern and northeastern parts of California, my home state.  Most of these are associated with what are still considered active (quiescent) volcanoes.  The biggest obsidian dome I’ve seen, though, is the Big Obsidian Dome at Newberry Caldera in Oregon.

So I’ll start off with some Newberry pics: Continue reading “Obsidian” →

I last visited Newberry Crater, Oregon, and it’s flank cinder cone Lava Butte, in the summer of 2003.  Husband and I met up with his parents in a campground near Bend, and introduced them to volcanoes.  Newberry Crater is interesting to potter about — especially its Big Obsidian Flow — but it has such fascinating underlying geology (hint: it isn’t a classic subduction zone volcano) that it deserves a blog post of it’s own.  Soon.  For now, I want to talk about Lava Butte, a classic cinder cone. Continue reading “Lava Butte, Oregon” →