The single remaining natural bridge at Natural Bridges State Beach is better described as a sea arch. Photos by Andrew Alden.

Natural Bridges State Beach is in Santa Cruz, just west of downtown. It's an easy outing and a nice spot, popular for its sheltered beach and butterfly trees. But the official park site says nothing about the rocks, so I must remedy that.

First of all, there's only one natural bridge at Natural Bridges, a sea arch next to a bedrock promontory.

The gap between these two landforms used to be a second arch, and beyond them was a third. People used to drive out on the arches and have picnics. This was around the same time that Yosemite conducted public bear feedings and dumped bonfires over the cliffs every summer evening. Now park systems take preservation more seriously, and the remaining arch is off limits. Even so, the sea will take it down some day, as it did the other two, but without human help this time.

Notice the flat surface of the surrounding land. All of this is the lowest of Santa Cruz's famous marine terraces, carved by the ocean waves some 80,000 years ago when the land lay lower and the sea level was steady. Since that time the sea has fluctuated and the land has risen. Several older terraces lie higher along the coast. The terraces are topped with a thin layer of beach sand and gravel. Today's seacliffs expose the older rock beneath. And with that, let's look at the geologic map of the area (from U.S. Geological Survey Open-File Report 97-489).

Unit Qcl is the lower terrace and Qcu is the higher terraces. Around Soquel Creek are wetland deposits (Qb) and river sediment or alluvium, shown by the lightest color. There are three units of proper rock; from left to right (and oldest to youngest) they're the Santa Margarita Sandstone (Tsm), the Santa Cruz Mudstone (Tsc) and the Purisima Formation (Tp). (We met the Purisima before at Fitzgerald Preserve, San Gregorio Beach and Point Año Nuevo.) Together they date from late Miocene to Pliocene time, about 10 million to 4 million years ago. The reason they crop out in this pattern is apparent in the photo above the map: the rock beds are tilted down to the east, which allowed erosion to expose the older rocks in the west. They got their tilt as this part of California was carried north along the San Andreas fault system.

These facts matter in understanding what you'll see as you approach the bluffs west of the arch and observe the remarkable speckled rock of the Santa Cruz Mudstone.

The underlying Santa Margarita Sandstone is full of organic matter. Beneath it, the Monterey Formation is even more so (it's the foremost of the Bay Area's petroleum source rocks). So for many thousands of years, natural gas and hydrocarbons have been rising through the Santa Cruz Mudstone, both before and after it turned from sediment to stone. They followed a set of millions of parallel cracks, or joints, made by the stresses of moving along the San Andreas fault. After that, chemically active water did the same, depositing iron minerals in the stone. You'll see that the iron mineralization is closely related to the jointing.

In at least two places, you'll find small faults displacing the Santa Cruz Mudstone. These are classified as normal faults, steep faults that drop one side relative to the other. They show nice examples of the upward-splaying pattern called flower structure, a sign that the faulting occurred near the ground surface.

These faults dropped the rocks down on the east side, reinforcing the eastward tilt of the bedding. About a half-mile east of the park, in the bluffs along West Cliff Drive, the top of the Santa Cruz Mudstone comes into view and we can see what all those rising hydrocarbons were doing: feeding a large seafloor ecosystem based on cold seeps. The UC Santa Cruz geology club visited the "toilet bowls" in 2003 and put up pictures, and a UCSC research team described them in detail in the journal Geology in 2001. Such things must exist today in similar places, like the seafloor off Santa Barbara and beneath the Gulf of Mexico.

The gist of the 2008 TV story was that geologists are now able to use special paleoseismic techniques to analyze earthquake faults and determine their seismic history over several thousand years. By noticing patterns in earthquake activity over long periods of time, they can also make predictions about when major events are likely to happen in the future. They determined that a major event of 6.8 or higher happens every 140 years or so on the Hayward Fault. It's been 143 since the last one.

Engineer Khalid Mosalam

A 2003 report by the USGS found that there is a 62% probability of at least one magnitude 6.7 or greater earthquake in the 3-decade interval 2003-2032 within the San Francisco Bay region. With odds like this, I'm grateful that there are people like Khalid Mosalam and his colleagues at the Pacific Earthquake Engineering Research Center's Shaking Table Laboratory who dedicate their careers to learning how to make the built environment that we live in, work in and travel on more safe in an earthquake.

I'd included about a minute of video from the Shaking Table Lab in the 2008 piece but I always regretted that I wasn't able to show more of this facility. So when we started putting together an entire episode focused around the theme of earthquakes, I thought a short segment about the Shaking Table would be perfect for this show.

Substation equipment getting shaken up on the table

When we were there shooting in 2008, they were testing some electrical substation switches which were interesting but definitely not as dramatic as some of the other structures they build and shake in three axes, often until collapse. The generous engineers at PEER were able to provide us some videos of other structures they tested including a two story house, a masonry wall and a bridge pier support. The 20' x 20' table is one of the largest in the world to be able to move in three directions (translation and rotation) so, according to Mosalam, it's an extremely important piece of equipment at UC Berkeley and has contributed to important research that will result in people being safer when the next 'big one' hits.

^{Sutro Tower and the gnarled chert of the Marin Headlandsterrane are parts of a splendid spectacle at San Francisco's Corona Heights. Photos by Andrew Alden.}

San Francisco is "America's Favorite City" for many great reasons. Geologists have their own list of reasons to love it, many of which are Franciscan—by which we mean the peculiar association of rocks found up and down the Coast Ranges. Corona Heights is a small hill west of the Castro district that features some of these rocks, but one of its main attractions is a world-class slickenside.

You get to Corona Heights by walking about a mile west from the 16th/Mission BART station, or by taking the F line up Market Street to the Noe or Sanchez stops. Here's the hill as seen from 16th Street. You can't miss it.

Corona Heights is a hardworking hill that formerly yielded clay and chert. Years of quarrying have accentuated its rugged appearance and left splendid exposures of the bedrock. Along the way, if you keep an eye on appearances of old concrete (doesn't everybody?), you may spot the old chert aggregate around the neighborhood.

Corona Heights, like Bernal Heights to the southeast, is an island of resistant chert that is part of the dismembered Marin Headlands terrane. It houses a fine playground and the Randall Museum. It has a dedicated group doing habitat restoration. But we're here to see the slickenside.

A slickenside is the polished surface that is created along a fault as the rocks rub against each other. They are common in faulted rocks, but only in patches the size of your hand. (I showed you some in the Calera Limestone.) Larger ones are rarer. Quarrying at Corona Heights uncovered one the size of a big front yard, one of the world's largest exposures. To see it, start at Castro and 15th, go uphill on 15th and turn left on Beaver Street. Again, you can't miss it.

The site is also a playground, named for Sidney Peixotto. If you continue up 15th Street instead, you'll see the exposure this way.

But the real joy of the thing comes at close hand, where you can feel the mirror-smooth surface that only a hard stone like chert can provide.

Surfaces like this are of scientific interest, although the data base is still scant. UC Santa Cruz researcher James Kirkpatrick has surveyed this site by lidar. He and others hope that close analysis can tell us more about how faults behave in detail. That's an advanced topic; the rest of us can collect the geocache at this spot.

Afterward is a good time to climb to the top of the hill and take in one of the city's best views. On a clear day you can see Mount Diablo (click for a 1000-pixel version).

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_{The Hayward fault crosses Oakland's Temescal Regional Park. Photo courtesy Andrew Alden}

Earthquakes will never be as predictable as the weather, I think it's safe to say, but if you get to know your nearest faults then earthquakes will be less of a surprise. When our local faults rupture, you will be less likely to panic and more likely to get through the event unscathed. So when I urge you to friend a fault, it's not to infect you with a scientific hobby but to bring you a lasting practical benefit.

A fault is a crack that has had movement along it. Geologists find them everywhere, and geologic maps are festooned with them. Almost all of them are inactive, though, and they're generally obscure even to practiced eyes. You can see what I mean on the U.S. Geological Survey's zoomable geologic map of the Bay Area.

_{US Geological Survey image}

Here's a part of it showing the area around Fremont. We can't be expected to worry about every heavy line, can we? Thankfully, no; and not the colors and symbols either. That's geology stuff: bedrock and contacts between stratigraphic units. But the two heaviest lines are important. On the left is the Hayward Fault and on the right is the Calaveras fault, both capable of major shocks.

_{US Geological Survey image}

Another USGS map called the Quaternary map focuses on just the active faults—well, the sort-of active ones that have moved during the Quaternary Period. You need to know only two things about the Quaternary: it's pronounced "qua-TERN-ary" and for our purposes it includes the last 1.8 million years of geologic time. Now a fault that hasn't moved in a million years, like a volcano that hasn't erupted in a million years, is not much of a threat. The color codes on the faults match the time of the latest fault movement, angry red being historic time (namely, 1868).

There are two good ways to visit our local faults. One is visiting them in parks (see the list below), and the other is the freelance approach of tracking them through the neighborhood. For that, the best tool is the State of California's Alquist-Priolo Earthquake Zone Maps, which have just been placed online by the California Geological Survey. Mandated by the Alquist-Priolo Act of 1972, these maps display the locations of faults that have ruptured during Holocene time, which is geologese for the last 11,000 years. Unlike Quaternary faults, activity on these Holocene faults is a pretty sure thing. And the maps display the detailed fault traces as mapped by geologists, superimposed on a high-quality topographic map.

With these, you can drive or stroll an area and assess the land for yourself. In places like downtown Hayward, the signs are plain and plentiful. In many others, you'll wonder what the heck those geologists were seeing. (The answer is that they were looking at historical aerial photos and data, finding subtle clues on the ground, and doing a lot of connecting dots.) You'll have a head-scratching good time, and you won't see your landscape the same way again.

Visit the San Andreas fault:
Fort Ross—Take Fort Ross Road east about 0.5 mile and spot the painted line across the road; an interpretive trail is nearby.
Olema—Take the Earthquake Trail near the Bear Valley Visitor Center in Point Reyes National Seashore.
Los Trancos Ridge—This ridgetop park above Palo Alto has an earthquake trail along the fault.
Sanborn County Park—South of Cupertino in the Santa Cruz Mountains is this park with the 2.5-mile San Andreas trail along the fault trace.
San Juan Bautista—The fault runs just yards east of the mission here.

Visit the Hayward fault:
Berkeley—Tour the fault in "Bear territory" including the infamous football stadium built across the fault in 1923.
Oakland—Lake Temescal park displays the fault in several places.
Hayward—The historic downtown and old City Hall straddle the fault, and signs of steady (aseismic) creep are abundant here.

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So we know – or should know – the seismic risks of living in one of the most vibrant, diverse places in the U.S. Short of leaving the region, what can we do?

Well, one of the most illuminating things about working on this story for me was learning a bit about retrofitting one’s home to make it withstand the lateral and vertical forces that accompany a strong earthquake. In short, you need to build shear walls – made of reinforced plywood and shear transfer ties – and bolt them to the walls in the foundation of your house. Suprisingly, there are no official codes as to what constitutes a proper seismic retrofit of a residential unit in California, nor is there a dearth of licensed contractors who will offer quotes and purport to retrofit your home but without any standards in place, homeowners are often at a loss to evaluate the quality of the retrofit which can easily exceed ten thousand dollars, depending on the size of the home and its location. Still, homeowners can avail themselves of a few retrofit resources online, such as Plan Set A, a guideline for retrofitting one's home that has been approved by building departments of several Bay Area municipalities such as Oakland and Hayward. Also on the Association of Bay Area Government's web site is a set of schematics illustrating shear wall construction. If you are interested in retrofitting your home, you should get quotes from several contractors, consult your city's building department to inquire about permits and possibly consult a structural engineer to perform a building analysis on your home.

If you're like me, though, and don’t own a home but want to prepare for "the big one," it's imperative to get an earthquake survival kit. The <red Cross sells earthquake survival kits but why not make your own, provided that it has water, first aid supplies, a flashlight, food rations and other essentials for you to survive 72 hours while waiting for emergency help. If you want to make your own kit, try the USGS, the city and county of San Francisco, or helpful suggestions from the San Francisco Chronicle and LA Times.

Living in earthquake country, it pays to be vigilant. I applaud the 1868 Hayward Earthquake Alliance, a consortium of agencies that are raising awareness of the risk posed by the Hayward fault with a series of events aimed at educating the public about the importance of preparedness, including a city-wide drill in San Francisco on October 21st, the 140th anniversary of the 1868 Hayward earthquake. We may not be able to predict when exactly the next earthquake on the Hayward fault may occur but we can start planning today to mitigate its effects.

For those who aren't familiar with the Hayward fault, check out our this link to the USGS Google Earth tour over the fault.

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Quest Educational Resources

 Print Guide - Mt. Diablo State Park

 Mt. Diablo State Park KML file

 Tips to get the kids in your life out into nature

 Designing an Exploration on Google Maps

Additional Links

• Mount Diablo Interpretive Association
• Ron's Galls – Wasp Galls on California Oaks
• Exploratorium: Faultline: Cookie Subduction

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

Additional Links

• Link