Archive for the ‘Earthquakes’ Category

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Lessons from the Carrizo Plain

15 April 2019

Last week I paid my first visit to the Carrizo Plain since 2005. David K. Lynch’s superb Field Guide to the San Andreas Fault says, “Nowhere in California is the San Andreas Fault more dramatically expressed than in the Carrizo Plain, a closed depression between the Temblor Range to the east and the Caliente Range to the west. Water drains in and evaporates leaving the glistening, usually dry Soda Lake. . . . There is little ground cover and the unobstructed views reveal countless tectonic features in all their glory.” That was true in 2005, when I came through in October and didn’t meet another soul. Not true last week — it was the peak of the wildflower season, Soda Lake was a by-god lake, and hundreds of car-driving, selfie-taking visitors were scattered across this wide, remote national monument. (For this post I’m offering some 1000-pixel images, just because.)

Topographically, the Plain is a basin with closed drainage, where all streams, such as they are, lead to Soda Lake. Geologically, the Plain is a sedimentary basin that until just a few million years ago was part of the Great Valley. Then the tectonic plates shifted slightly, the San Andreas fault was squeezed, and on its east side the rocks folded up to form the Temblor Range. Later the whole Plain was raised almost 2000 feet.

Coastal California has been going through rearrangements like this for some time, and Oakland’s younger rocks like the Claremont Shale probably formed in a basin the same way, one that was off the coast. North of the Plain, roadcuts in the Bitterwater Valley expose the kind of rocks being made in the Carrizo basin. They’ve been tilted nearly vertical by forces across the fault, just as their cousins in Oakland have been tilted by squeezing across the Hayward fault.

A place stuck between the Temblor and Caliente Ranges sounds kind of inhospitable, and even though the landscape resembled a gigantic Holi festival, a brisk and parching wind blew the whole time I was there. The Carrizo flowers are as tough as they are beautiful.

You like those purple Phacelias? Here’s a billion of them.

This is the view downvalley toward the San Emigdio Mountains, with the Caliente Range on the right. On the left, the peaks of the Temblor Range are nearly hidden by the lower range of the Elkhorn Hills, which are a large pressure ridge directly along the San Andreas fault. That’s where I drove next.

The most famous, geo-tourist-trappy place in the Elkhorn Hills is at their north end, where the fault has forced Wallace Creek to jog hard to the right. I didn’t go there last week, but this is how it looked from Elkhorn Road in the barren fall of 2005. The creek comes toward you on the right side, turns left behind the frontmost ridge, and cuts through that ridge on the left side. You’re standing on the Pacific plate, moving left about an inch and a quarter per year, and on the other side of that first low ridge is the North America plate.

I’ve shown you the same kind of stream displacement in Oakland, caused by the Hayward fault.

Anyway, down at the south end of the Elkhorn Hills the entire slope is warped by motion on the fault, and the spring vegetation helps bring out the distortion. Every little stream is curled to the left, like grass in a stiff wind. The expression of the tectonics in the landscape is so strong, just looking at this photo makes me clench my teeth. In person, in 3D, it’s even more uncanny.

The great earthquake of 9 January 1857 was centered near here. The ground cracked for some 200 miles. Shaking was felt the entire length of California and into Nevada. In the Carrizo Plain, the ground shifted about 30 feet. After that, the unnamed mountains to the east started being called the Temblor Range, and the San Emigdio Mountains also got their name, honoring the patron saint of earthquakes, at that time.

Ramón Arrowsmith, now at Arizona State University, has studied this region for decades. His 1995 Ph.D. dissertation includes a thorough backgrounder of the sciency side of this mighty, lovely land. But everything he’s doing in California is interesting.

Posted in Earthquakes, Other topics | 2 Comments »

The Hayward earthquake: 1868-2018

15 October 2018

There’ll be lots of press this week about the anniversary of the “Great San Francisco Earthquake” — the original one, 150 years ago on 21 October 1868, caused by a big rip in the Hayward fault just before 8 in the morning. (Also the annual ShakeOut exercise, this year on 18 October at 10:18 am.) Behind the press stories there’s lots of sound info for you, and I’ll put a list of good links at the end of this post. Rather than write another standard thing about the earthquake, I’ll focus on what I’ve been studying lately, which is the problem of earthquake landslides.

Contemporary accounts of the 1868 quake tended to focus on homes wrecked and buildings ruined, but several geological manifestations were widely recorded: the ground cracked open from Oakland all the way to present-day Fremont, some of the cracks spewed muddy water, streams ran high afterward, and many new springs appeared near the crack.

Only in more distant places like the San Mateo Peninsula did people mention rockfalls and landslides — which tells me those were the most widespread forms of damage that day. Landslides were everywhere, even in places where buildings weren’t bothered. And though our buildings are even stronger today, the landscape is weaker than ever.

Landslides were everywhere in the East Bay hills in 1868 because that’s always what happens. No one made mention of them because they were unremarkable. The hills were empty countryside, a mix of spent rangeland and razed redwood groves. What matter was a landslide in that waste? But nowadays . . . when the next large Hayward fault earthquake comes, landslides will cause widespread and costly misery — in the high bedrock hills as well as the low gravel hills of the Fan and along the fault.

We don’t see the scars of the 1868 slides today because after a few decades, they fade out. Here’s a small slump in Sibley Regional Volcanic Preserve that I photographed in 2005, 2009 and 2017.

I think in another 12 years it will be pretty subtle.

The high hills owe their basic shape to earthquakes and landslides. Their sides are steep slopes, swept clean and straight as the Hayward fault raises the heights by about a millimeter a year, one earthquake at a time. So they’re always primed to slide.

USGS landslide researcher David Keefer estimated in a 1984 paper that earthquakes as small as magnitude 4 are large enough to cause rockfalls. A magnitude 6.8 event, the size of the 1868 quake, would cause up to 1000 rockfalls in the affected area.

And up to 1000 slumps like the one at Sibley.

And up to 1000 debris slides, what people usually call mudslides. The majority of these will start along roadcuts, which inherently destabilize hillslopes . . .

. . . and near hillside homes, which do the same.

The city will be overwhelmed. The roads will take months to clear, years to fix.

All these photos show rainfall landslides, or sites where one could happen. They’re the usual kind. They happen where the flow of water, both on the ground and below it, unbalances the slope and makes it fail. They especially tend to form on the sides and floors of valleys, where the slopes converge.

Earthquake landslides, though, tend to start on ridgetops and promontories, landforms that focus seismic energy toward their tops. They also aren’t confined to the wet season. Sites that aren’t prone to rainy-season landslides may instead be the preferred targets of earthquake slides. If we’re lucky during the next big quake, as we were in 1868, the ground will be dry. If we aren’t, and the ground is waterlogged, well, heaven help us because we’ll get both kinds of landslide at once.

This danger is built in with the tectonically active setting that makes Oakland so beautiful. We have to prevent what we can, and cope with what we can’t.

Here’s that list of Hayward fault resources. Actually, the whole list is on one page, from the U.S. Geological Survey:

earthquake.usgs.gov/earthquakes/events/1868calif

In that list I want to single out “What to Expect in a Big Urban Earthquake,” a phone-friendly “geonarrative” aimed squarely at people who live in cities — that’s us.

Posted in Earthquakes, Landslides | 1 Comment »

News from the HayWired fault

16 April 2018


The ruins of Morse & Heslop’s mill, Haywards, after the 1868 earthquake (Bancroft Library image)

This week the media will mark San Francisco Earthquake Day, 18 April, as they always do but with an extra message for 2018 — this year will be the 150th anniversary of the original “Big One” in the Bay area, the Hayward earthquake of 21 October 1868. The U.S. Geological Survey and a host of partner agencies and organizations will roll out the next piece of a master plan that will guide the response to future large East Bay earthquake on our very own Hayward fault, usually called “a ticking seismic time bomb” by the intrepid researchers who get in front of cameras and audiences.

That first piece, Volume 1 of a planned series, is a scenario called HayWired: a description of a typical magnitude-7 earthquake, modeled after the 1868 quake, presented in as complete detail as we know how, with a special focus on its probable effects today in our highly electronic state. It’s online now at the USGS site.

I wrote about this project here last July, so you can read that post for more background. After that you may enjoy the USGS’s “geonarrative” about HayWired. It’s awesome, but only on a touchscreen device.

Today, though, I wanted to provide some details from the original quake. In 1868 a committee was convened to create a report on the event, but it never finished a report, so whatever work they did was lost. We only know as much as we do because after the San Francisco earthquake of 1906, the commission set up under UC Berkeley’s Andrew Lawson to investigate it decided to add a chapter on previous earthquakes. There were enough survivors of 1868 at the time to record quite a bit of detail. So here are some tidbits from the famous Lawson Report of 1908 about the Hayward quake of 1868.

The great shock happened about 7:50 a.m. on 21 October 1868, and ground ruptures were recorded, with 3 feet of offset in some places, from Mills College down to Warm Springs. North of San Leandro, though, “The county was then unsettled, and the information consisted of reports of cow-boys riding the range.” From Oakland we have these details:

  • “A house near old Blair Park, in the present Piedmont district of Oakland, was badly damaged.”
  • “Pans of milk and tubs of water emptied out almost in a moment, trees whipt about like straws; many houses twisted 5 or 6 inches out of square, particularly those on brick foundations.”
  • “In Brooklyn, as in Oakland, many chimneys were broken off at the roofs.”
  • “The bed of San Leandro Creek, which had been dry for several months, became filled with a stream of water 6 feet wide and a foot deep.”

At Haywards (the name of Hayward at the time) and neighboring towns, “nearly every house was thrown off its foundations.” Dozens of aftershocks were recorded in Haywards during the first 12 hours.

R.C. Vose of Roberts’ Landing wrote, “Our house broke in three pieces, each part falling outward. A boiler of hot water was on the stove, and with the first deafening jolt, the hot water came my way, giving me a bath I have never forgotten.”

In Fremont, Tyson’s Lagoon, the body of water next to the BART station, drained dry and remained dry for three years.

In San Jose, situated as it is in a large sedimentary basin, damage to brick buildings was universal.

12 aftershocks felt in San Francisco during the first day. Damage was most severe in the “made land” (artificial fill) in the former Yerba Buena Cove. Out at the Cliff House “the shock, however, did no damage, not even upsetting the glassware in the bar.” Ships at sea felt strong vibrations, like running aground or the anchor chains running out.

The earthquake was felt in Chico and many Sierra foothill towns, even in Carson, Nevada. Damaging shocks were reported throughout Sonoma County, as well as in Stockton and Sacramento.

All of this will happen again the next time a major quake strikes the Hayward fault. However, today there are hundreds of times as many people living in the affected areas. Back then, about 30 people died. Do the math and pay attention this week.

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Earthquake apps and post-quake observations

16 October 2017

Seems like disaster is in the air every October. This year the catastrophe is wildfire in the North Bay, bringing up memories of our own turn in the line of fire this very week in 1991.

Yesterday the local paper published an article by Seung Lee that explicitly linked the October fires of 2017 to the Oakland Earthquake of 20XX, not here yet but sure to come.

Lee pointed to some promising smartphone apps that could save us lots of anguish and maybe even lives. Of course, MyShake came first. It’s an Android app (IOS version pending) that turns your phone into a crude but effective, networked seismograph-plus-earthquake-alarm. I’m watching MyShake closely and will let you know when iPhones can participate.

Lee mentioned several apps to help with communications, offering flexibility in the face of degraded cell service and bringing more superpowers to your smartphone. They include Zello, which turns phones into crude but effective walkie-talkies; FireChat, which enables phones to network without internet or cell service by using peer-to-peer technology; and NextRadio, which turns last-generation phones into FM radios. Needless to say, a portable charger belongs in your purse and go-bag — fully charged.

But I’m also writing to point out some possibilities for us to help science after a large East Bay earthquake, once you’ve taken care of yourself and those nearby. Lots of geologists will show up, doing different things. Some will be inspecting damage as consultants. Some will be there doing science on behalf of state and federal agencies. Professors will come with their students, teaching them real-life lessons in disaster response and collaboration with other scientists. You can give them a hand during the aftermath. And some more things you can do of your own initiative.

If you see ruptures in the ground, collect some data. Measure their offsets, photograph them with date and time stamps, and insert objects for scale like a coin or lens cover or your own hand. Do these things quickly before city crews come in to fix the damage. Document offsets in buildings, too. Repeat these observations as the days go on, because afterslip — continuing fault motion once the shaking is over — is a new and lively topic among earthquake scientists.

Ambitious amateurs can practice the structure-from-motion (SfM) technique, by which a series of photos taken from all sides can be turned into accurate three-dimensional models. This yielded dramatically good images after the 2014 Napa earthquake. If I were a maker type, I’d do this. Any practitioners out there?

Monitor your local streams. As earthquakes rattle the ground, they shake down the material of the hills as surely as you’d shake down a jar of coffee beans. When this happens, groundwater gets pushed aside, and a sudden rush of water fills the streams for a while. The Napa earthquake of 24 August 2014 did this all over the North Bay, which I posted about at the time, and Oakland was affected, too. An Eastmont Hills homeowner whose property has a tiny backyard stream valley, dry most of the year, told me that he saw water rise in it within hours of the Napa quake, and the stream ran high for about two weeks.

The Oakland Earthquake of 20XX will be a Katrina-sized event, much worse than the Loma Prieta quake (another October surprise that happened 28 years ago tomorrow). The more we’re aware, the more we prepare, the less likely that our local Katrina will be our Katrina catastrophe. And let’s hope it picks a different month to strike.

In housekeeping news, I’ve pulled earthquake-related posts into their own category, mostly separate from posts about the Hayward fault.

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