^{The carbonate crust called tufa coats a high boulder over the Salton Sea, testifying that the Colorado River fed mighty Lake Cahuilla there in the geologically recent past. Photo by Andrew Alden.}

Geologic research is a remote and dusty undertaking that can look tedious to most people. But if it bears on subjects we fear, like earthquakes, even painstaking background projects can be blown out of proportion. A recent example from Southern California went from an intriguing journal article to "we're all gonna die" TV stories in the space of a day. Maybe it will help you spot the next time this happens in the Bay Area.

The article, published in the prestigious Nature Geoscience on June 26, presented the fruits of years of heat and tedium on the foul waters of the Salton Sea, in southernmost California. The Salton Sea occupies a sinking basin, the Salton Trough, that would rather be part of the Sea of Cortez except that the Colorado River has built a delta that dams it dry. As the river wanders over its delta, draining sometimes south and sometimes north, it periodically creates a geologically temporary lake in the Salton basin that has filled and dried out many times in the last few million years. (A short video by the University of Redlands illustrates the process.)

The repeating lake is known as Lake Cahuilla, and the Salton Sea is today's version of it. The Salton Sea is actually a terrible blunder, formed in 1905 when an irrigation canal tapping the Colorado River burst its banks and flooded a huge area of farmland in the dry bed of Lake Cahuilla. The breach was eventually fixed before the reborn lake could drown everything from Mexicali to Indio, but floods continued to be a problem until Hoover Dam was built upstream 30 years later.

The Salton Sea covers up the southern end of the San Andreas fault, and that's where our science story begins. A research team from Scripps Institution of Oceanography crisscrossed the drowned land in a small boat with a state-of-the-art CHIRP sonar system. They mapped a swarm of short faults running from the end of the San Andreas toward the end of the Imperial fault lying to the south—a tectonic configuration called a stepover. Here's the figure showing how it all fits together.

_{Tectonic setting of the Salton Trough and Salton Sea. SJF is San Jacinto fault, SSAF is southern San Andreas fault and IF is Imperial fault. Green grid is ship tracks in the dashed box; awz (acoustic wipeout zone) marks disturbed sediments. (Scripps)}

From their sonar database the researchers extracted a decent record of recent ruptures on some of these stepover faults. My figure below shows, schematically, how the sediment record is built as downdrops on the fault combine with sediment being deposited on top.

The researchers ran this process backward to reconstruct the history of faulting for a few thousand years into the past.

What made things interesting was how this record of stepover quakes meshed with records of lake floods and San Andreas mega-quakes. The correlation is imperfect, but intriguing: every time the lake was reborn in a flood, the stepover faults gave way, and half of the recent mega-quakes coincided with stepover quakes. To connect the dots further, the researchers said that "at least one of the last three dry-basin floods coincides with an earthquake on the SSAF (southern San Andreas fault)." In their cautious words, "We propose that loading by (Lake Cahuilla) may have induced failure on faults beneath the Salton Sea that, in turn, has the potential to trigger an earthquake rupture on the SSAF."

That's all ingenious science (seismic oceanography, in the desert!), but not news. To get news takes a press officer with a nose for a hook, and the hook is that the SSAF hasn't broken in some 300 years when we might (given the recent record) expect it to happen more often. Has our failure to let Lake Cahuilla be reborn held off triggering the next Big One? Well, has it??

That question is something for geologists to wave their arms about at meetings and parties, not hold out to the world as a sure thing. But between the paper, whose abstract is world-readable, and the press release from Scripps we can see the story begin to morph. It's a scattershot press release, meant to serve any news angle, that immediately confuses today's managed Salton Sea with wild prehistoric Lake Cahuilla. It includes a great hook, a quote about the "overdue" SSAF likening it to "a woman who is 15 months pregnant." The quote is only part of the background to the science, just a tangential part of the paper itself, but irresistible to the troutlike mind of the news reporter.

First out the chute on June 26 was the San Diego Union-Tribune's website, where Gary Robbins immediately removed all scientific uncertainty in his first sentence: "The Salton Sea east of San Diego is a deceptively dangerous backwater, hiding faults that repeatedly produce powerful earthquakes that jolt all of Southern California, says a new study by the Scripps Institution of Oceanography."

Later that day a Los Angeles TV station rewrote Robbins's story under the headline "Mega Quake Around the Corner?" with the lead sentence, "Man's interference with Colorado River floods that used to regularly flow to the Salton Sea may have 'stopped the clock' on a regular series of big earthquakes, setting the stage for a mega quake that could wreck Southern California, scientists said Sunday." The accidental flood of 1905 has become "interference" with regular flows to the Salton Sea, and the reasoning has turned upside-down: If we had let the Imperial Valley drown, Southern California's "Big One" might already have happened and been a few tenths of a magnitude unit smaller. How anyone wins under that scenario is not explained.

After a few days, more and better journalists weighed in. Some of them had actually studied the Nature paper (as I have). Many, including Discovery News's Tim Wall, apparently did not. USA Today's Dan Vergano did well. So did Chris Clarke on KCET's website. Charles Q. Choi did fresh reporting, as usual, for OurAmazingPlanet.com. Science News's Devin Powell wins my prize for his succinct, factual and well-written account. As you peruse the items collected by Google News under a search on "salton earthquake," look for those items that get the details right—specifically the fact that the Salton Sea arose by mismanagement, not on purpose.

The Bay Area has excellent media when it comes to our own earthquakes, with seasoned staff and good support from academic and government experts. But still—if scary news comes up, look around the science mediasphere and wait a few days before deciding what you think. Even better, track down the news at its source whenever you can. Learn who to trust.

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^{Pebble Beach is on the coast just north of Bean Hollow State Beach. Come for the pebbles, stay for the rocks.}

It was a weekday the last time I visited Pebble Beach, on the San Mateo coast just south of Pescadero, and several busloads of children came and went. They flocked to the pebbles, flung them high, and were off in their buses 15 minutes later leaving the beach to me. I did the same here when I was a kid, and you can too—the pebbles are wonderful—but I can show you a better time today.

Pebble Beach (also called Bean Hollow Beach North) is a great place to look at the deep sea floor without getting your feet wet. The rocks exposed there are scrubbed clean by surf and nicely tipped up on edge for easy viewing, and they present many instructive mysteries that students actually pay professors and buy textbooks to learn.

But first, about the pebbles. You get to the beach on Route 1 going south from San Gregorio Beach, which skirts the coast on flat land that is largely planted to artichokes and ranchettes. This tableland is a marine terrace or wave-cut platform. See the setting in the photo below, from the California Coastal Records Project.

The San Mateo coast is famous for these terraces, especially down around Santa Cruz. They are land that was cut flat by the sea during the Pleistocene epoch, then lifted by tectonic activity. The terraces consist of beach sand and offshore sediment from tens of thousands of years ago, and most of the pebbles of Pebble Beach are recycled from that Pleistocene gravel.

Now let's look at the geologic map of the area (from USGS Open-File Report 98-137):

Pebble Beach is right where the symbol that says "63" is. The areas marked "Qmt" are the marine terraces. If you examine the red contours, which show elevation at 50-meter intervals, you'll see that the terraces occupy at least three different levels.

The rocks on the beach, revealed as the terrace is eroded away, are mapped in green and belong to the Pigeon Point Formation, a thick pile of former deep-sea sediments dating from the Late Cretaceous epoch, some time between 100 and 65 million years ago. That "63" symbol means that the beds of rock, originally horizontal, are tilted to an angle of 63 degrees. And here they are, exposed again just as the sea left them in the Pleistocene.

The surface between the steeply dipping rocks and the horizontally bedded terrace deposits is called an angular unconformity. In this case it represents a gap in time of at least 65 million years.

The next five photos are cross-section views, shot looking directly down that 63-degree angle at the rocks.

Here's where fine-grained siltstone, light-colored sandstone and cobblestone conglomerate are stacked. There's another angular unconformity between the siltstone and sandstone. And look at the wavy top surface of the sandstone—the conglomerate clearly arrived in a rush, pushing into the sandstone while it was still loose sand. This was a very active environment, an offshore basin where large amounts of coarse sediment washed down periodically in underwater landslides. In fact, it was rather like offshore California today. The many small stones embedded in these rocks were eroded free to become the gravel in the marine terrace. So the pebbles of Pebble Beach have been recycled two times, and maybe more.

Here are some more signs of vigorous activity, including load marks from heavy gravel landing on soft silt:

Rip-up clasts are chunks of fine-grained seafloor that were swept up by strong sand flows:

Drag-folded laminations show how overriding sand flows deformed the soft silt beneath them:

And here are some tiny thin veins of bitumen, ancient organic matter that may once have been a mat of vegetation:

The final attraction of Pebble Beach is the honeycomb weathering, often called tafoni, that forms when salt air attacks clean sandstone:

Too bad the schoolkids can't stay all afternoon! But you can.

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Updated: On March 11, 2011, a massive 8.8 magnitude earthquake hit the Pacific Ocean nearby Northeastern Japan. The quake, and a tsunami that followed, caused massive damage and loss of life. The news put quake prone California on alert. While many of us would rather not think about the possibility of another major quake, we are surrounded by active faults. One East Bay fault has scientists especially concerned.

It's been called the most dangerous fault in the U.S. The Hayward Fault runs 40 miles, from San Pablo Bay to Fremont, through some of the most densely populated areas in the country. Every 140 years for the past two thousand the Hayward Fault has jolted the East Bay. Geologists have figured out the regular history of these quakes by carbon dating trenches along the fault. A lesser known cousin of the San Andreas the Hayward fault is a creeper. Basically, it moves, slowly, along the surface but deep inside… it's locked until tension builds up and and it slips. It appears that it is time for the fault to slip again. The last major earthquake on the Hayward fault was 1868. Scientists believe that the temblor registered 7.0 in magnitude. Hayward and San Leandro were devastated. But if the quake were to happen today, it would be a much different story.

I met Mary Lou Zoback out at the Fremont Bart station, which sits right on top of the Hayward Fault. She pointed out cracks in the parking lot from the creeping fault. Zoback is a geophysicist who worked 28 years at the U.S. Geological Survey and who has done catastrophe modeling of risky residential buildings. Her company estimates that a 6.8 quake, or bigger, on the Hayward Fault could cause a disaster on par with Hurricane Katrina, causing 168 billion dollars in damage and leaving at least 200,000 homeless.

A number of public buildings in the east bay are undergoing retrofitting to make them more structurally sound. Area hospitals have until 2013 to meet seismic safety standards. There is a state inventory of public schools prone to collapse in a major quake, but no such list exists for private schools. And retrofitting standards for risky residences are confusing. I talked with Jim Cook, of Bay Area Retrofit. He says existing codes are unclear and there really is no specific licensing for seismic home retrofitters. Cook has been fighting local governments for years to improve seismic safety standards.

Homeowners can have their home evaluated but what if you are a renter? Many apartments and condos can collapse in earthquakes because they have parking or open commercial space on the first floor making this story weak or "soft." According to the Association of Bay Area Governments Earthquake and Hazards Program, soft-story apartment buildings were responsible for about two-thirds of the 46,000 uninhabitable housing units in the 1991 Northridge earthquake. In the Bay Area, unreinforced masonry (older buildings constructed of brick, stone or cement blocks) continues to be a threat.

The thought of a big earthquake is scary enough, never mind the chaos that can happen in the aftermath. But the damage from a large earthquake has repercussions that can last for a very long time. We can still see the scars from the 1989 Loma Prieta earthquake. Downtown Santa Cruz is not yet fully rebuilt and retrofitting continues on the Bay Bridge. We can prevent a lot of damage up front by shoring up our buildings and creating a family disaster plan and an earthquake kit. The Hayward Earthquake Alliance has put together some really helpful information on how to prepare for a major quake.

Listen to the Hayward Fault Radio Report and view the recent QUEST TV segment on the fault online.

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Quest Educational Resources

 Print Guide - San Andreas Fault Trail

 San Andreas Fault Trail KML file

 Tips to get the kids in your life out into nature

 Designing an Exploration on Google Maps

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