The following statement sounds ridiculous, but bear with the blogger anyway: Assume you could stand on the observation platform on top of Mount Diablo for a few million years. If you looked westwards towards the City and the Pacific Ocean during that time you would see the whole landscape slide by slowly. San Francisco would move to the north, Oakland, the Berkeley Hills and even Walnut Creek, the city right at the foot of Mount Diablo, would move a considerable distance to your right. Now imagine you were to cross the Bay and climb to the top of Mount Tamalpais. Again you camp out there for a million years or so, but during the whole time you look eastwards across the Bay. A similar picture would emerge: Mount Diablo would slide towards the south, the East Bay Hills and all the cities at their foot would also move towards your right.
The cause for the movement is the slipping between the Pacific Plate and its continental counterpart, the North American Plate. They slide past each other horizontally, with the Pacific Plate moving northnorthwest with respect to North America. The direction of the movement depends solely on your point of view. When you stand on Mount Diablo, you are fixed on the North American Plate and only the Pacific Plate seems to slide. However, anchoring yourself on the Farallon Islands, which are on the Pacific Plate, it seems as if the continent would slide by you. But no matter which side you stand on, the other side always slips to your right.
We scientists, of course, are way too impatient to wait for a million years to see the effects of the plates' endless drift. However the same satellite based GPS-System, which helps you navigate around town, enables us to measure the plate motion directly and with high precision. Our GPS instruments are considerably more expensive than the navigation system in your car, but they allow us to pinpoint a location with a precision of less than an inch. In contrast, a typical car nav-system knows its location with a much cruder accuracy of several yards.
By visiting several dozen fixed spots in the Bay Area a few times per year with these precision GPS-tools, or measuring continuously, we can determine the relative positions of these points. The map shows the summary of several years of such measurements. And indeed it turns out, that the various points in the Bay Area move with respect to each another. Look at a black arrows in Livermore or Vacaville and compare them to the arrows at the Farallons or Point Reyes. Each point moves about one inch per year, the East Bay cities to the southwest, while the points in the Pacific move in the opposite direction. How slow is an inch per year? The plate movement in our region has about the same speed which with a healthy fingernail grows. (hra011)
|Offset in curb. (Photo: Horst Rademacher)|
The Hayward Fault underlies some of the most densely populated places in the Bay Area as it runs for approximately 50 miles along the foothills of the East Bay. It is crossed by five major freeways and several water tunnels; BART and other rail lines also intersect it at several locations. The fault is sandwiched between the San Andreas and the Calaveras Faults. It splits off from the Calaveras Fault near Arroyo Agua Caliente Park in the Warm Springs District of Fremont and follows a straight northwesterly line through Hayward, Oakland, and Berkeley to Point Pinole in Richmond, where it disappears under San Pablo Bay.
In contrast to its sister faults, the two flanks of some sections of the Hayward Fault creep past each other in an imperceptible movement of thousandths of an inch per day. Such ultra-slow creep is an earthquake in extreme slow motion which, over months and years, causes cracks in the pavement makes curbs and water lines bend, and results in cracked foundations and offset fence lines.
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For the uninitiated, it takes a little training to find the subtle marks with which the creeping fault expresses itself in the urban jungle of the East Bay. Downtown Hayward, less than half a block east of Mission Boulevard, is full of such signs. The curbs of nearly every cross street are offset by a few inches; parking lots and roads are full of characteristic cracks, which align themselves along the fault in a staggered fashion. To the north, on the campus of Contra Costa College in San Pablo, the fault is also visible through pavement cracks and offset curbs. There, a patio area next to Campus Circle bears an especially interesting mark of the fault's creep. Its bricks, originally laid in a straight and rectangular design, have been moved by the fault and now show a clear curvy pattern. While most of the fault line is covered by urban infrastructure, it also cuts through the Oakland Zoo, where it runs past the otter tank and a bear pen.
Our campus straddles the fault as well. Some of the residence halls near the Greek Theater and the Memorial Stadium at the foot of Strawberry Canyon are built either directly on or just a few yards off the surface trace of the fault. The effects of fault creep in the stadium are clearly visible in the concrete wall above section KK in the south curve, where some sections of the wall have separated from each other by at least four inches.
Learn more about the Hayward Fault by visiting our website. In addition, the USGS has put a virtual tour of the Hayward Fault on Google Earth. (hra010)
Do you know why seismologists dread going to cocktail parties? They hate to say "I don't know!" That, of course, is the only correct answer to the question that inevitably pops up at such parties, when one admits to studying earthquakes: "When is the next big one going to happen?" Well, seismologists really don't know, because earthquake prediction is beyond the reach of any serious scientist. A meaningful prediction has to fulfill three criteria: One has to forecast exactly where, when and with what magnitude an earthquake will strike. Despite considerable research efforts in many countries, nobody anywhere has succeeded in getting all three of those criteria consistently right.
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That, however, does not mean that Earth scientists are oblivious to the risks and dangers earthquakes pose. Among other things, they have developed a method called "probabilistic risk analysis". Although not nearly as precise as a prediction ought to be, it enables seismologists to calculate the chances that an earthquake of a certain size would strike a certain segment of a fault during a specified time window.
Northern California seismologists have repeatedly analyzed the seismic risk for the Bay Area (see map). They currently predict, that there is almost a "two out of three chance, that one or more earthquakes of magnitude 6.7 or larger will strike in the Bay Area in the next 30 years". What does that mean in layman's terms? If you continue to live here for 30 years, you will almost certainly experience an earthquake as strong as the Loma Prieta quake, which struck the Bay Area in the fall of 1989, killing 63 people.
Seismologists have even calculated detailed probabilities for the various earthquake faults in the Bay Area
(October 7, 2008). These calculations take into account the current slip rates along these faults and how often earthquake occurred there in prehistoric times. It turns out, that the lowest chances (3-4 percent over the next 30 years) are in the region around Livermore, Mt. Diablo and Concord. There is a one in ten chance, that a strong earthquake will happen along the San Gregorio Fault between Monterey and Pacifica. The segment of the San Andreas Fault in our region which slipped during the 1906 San Francisco Earthquake has a one in five chance of going off again during the next three decades. The highest probability, however, is along the Hayward Fault in the East Bay and along its northern extension, the Rodgers Creek Fault. Read more about the riskiest fault in the Bay Area in our next blog. (hra009)
|Faults in the Bay Area (USGS and PG&E)|
The slipping and sliding between the Pacific Plate and its continental counterpart, the North American Plate, not only generate most of the seismicity in California (see blog October 3, 2008), they are also the cause for the earthquakes in the Bay Area. While in most of the State the boundary between the two plates is defined solely by the San Andreas Fault, the situation is different in our region. Here the movement of the two plates is not confined to one fault alone. For reasons that are not yet fully understood by Earth scientists, the tectonic slip in the Bay Area is spread over several fault lines which run roughly parallel to each other in a 50 mile wide corridor of seismic danger.
The main strand is of course the San Andreas Fault itself, running under the coastal hills west of San Jose and along the spine of the Peninsula. In Daly City it dips into the ocean, only to appear again at Stinson Beach, carving out the elongated cigar shaped valley of Tomales Bay further to the north. Since the conquest of "Alta California" by Spain in the late 18th century, most of the strong quakes in our region have occured along the San Andreas Fault, such as the Great San Francisco earthquake of 1906 and the Loma Prieta Earthquake of 1989.
The Calaveras Fault branches off from the San Andreas just south of Hollister and then turns north, with the potential to wreak havoc among the towns and cities along the 680 freeway all the way to Walnut Creek and Concord. The Hayward Fault is sandwiched between the two other fault lines. It runs for approximately 50 miles along the foothills of the East Bay. Other significant faults are the Greenville Fault in the Livermore Valley, the offshore San Gregorio Fault which runs through Half Moon Bay, and the Rodgers Creek Fault between San Pablo Bay and Healdsburg.
This widening of the boundary zone between the two plates not only spreads the seismicity over a wide area. It is also responsible for the unique landscape in the Bay Area. Because the three major fault lines take up the plate movement and essentially split the tectonic sliding almost equally amongst themselves, most topographical features in the Bay Area follow the northwesterly trend defined by the plate boundary. Road builders, for instance, conveniently used the valleys formed by the three faults to lay out part of the highway network, like the 280 Freeway on the Peninsula, Highway 13 in Piedmont and Montclair and the 680 corridor in the East Bay. Not only the valleys but also the crestlines of the various hills in the Bay Area follow this northwesterly trend. (hra008)
|Figure 1: Seismicity and Faults in California (USGS)|
In the same way as earthquakes are neither evenly nor randomly distributed throughout the world (see blog September 29, 2008), California also has a few earthquake zones as well as vast areas which are essentially void of all Earth's tremors. There are actually distinct bands and clusters of seismicity in our state which can be clearly spotted on a map of earthquakes.
The most famous zone of all is, of course, the San Andreas Fault. It snakes almost all the way through the Golden State, from the Salton Sea in the Imperial Valley in the south, to Cape Mendocino in the north. At a first glance it looks as if earthquake foci line up along this zone like pearls on a string. But looking a little bit more closely one finds that the San Andreas Fault is not just one clear thin line but a zone of tectonic movement, which can be up to several dozen miles wide. Sometimes the zone consists of several faults, which parallel each other.
The picture gets murkier in the Los Angeles Basin. The reason is that the crust under LA is cracking along dozens of short fault segments, many of them not yet even named by seismologists. The ultimate cause for the earthquakes along the San Andreas system and in the LA basin is the sliding of the Pacific Plate against the North American Plate with a velocity of about 2.5 inches per year.
Where the San Andreas Fault ends in the north, the seismicity fans out into the Pacific Ocean like an elephant's trunk to the west of Cape Mendocino. There the "Mendocino Fracture Zone" takes over the steady slide of the tectonic plates.
Other clear bands of seismicity occur along the Garlock Fault in Southern California's Transverse Ranges, along its continuation through the Owens Valley, and further north along the steep eastern flank of the Sierra Nevada. The earthquakes there are not caused only by the sliding of the plates, but are also a consequence of the slow lifting of the Sierra Nevada, which has occured during the last several million years.
And last but not least, there is a third class of earthquakes in California. These temblors can be found in clusters around the Geysers along the Sonoma and Lake County border, around Mammoth Lakes east of Yosemite, and to a smaller extent around the Coso Field near Ridgecrest. The causes of these mostly small quakes are remnants of volcanic and geothermal activity, like the restless Long Valley caldera next to Mammoth, which blew up in a gigantic volcanic eruption 760,000 years ago. (hra007)