Imagine a busy intersection with a three-way stop sign at rush hour. It takes only a few irresponsible drivers who don't want to wait their turn and traffic jams and accidents are likely to occur. Something very similar is happening every day in the Earth's crust under Cape Mendocino. There upheaval is caused not by impatient drivers but by tectonic plates, which are crashing into each other with unstoppable momentum. This tectonic three-way intersection is called the Mendocino Triple Junction. It is one of the few places in the world where three of the gigantic plates which continuously drift on the Earth's mantle meet.
Along the length of most of California, the San Andreas Fault defines the boundary between the Pacific Plate to the west and the North American Plate to the east. Along the fault line, the Pacific Plate slides horizontally in a northnorthwesterly direction with respect to North America (see blog October 17, 2008).
Under Cape Mendocino, this trip comes to an abrupt halt, because another plate gets in the way. North of the Cape the off-shore Gorda Plate, a small cousin of the Pacific Plate, glides under the North American Plate in what is called a subduction zone. The boundary line between the Pacific Plate and the Gorda Plate is a fault similiar to the San Andreas, called variably the Mendocino Transform or the Mendocino Fracture Zone. It goes from Cape Mendocino for several hundred miles due west into the Pacific Ocean. Along this fault, the Gorda Plate slides horizontally to the east with respect to the Pacific Plate with a speed of about 2 inches per year.
Because of the different directions and the various rates of movement of the three plates, the area under Cape Mendocino and a region immediately off-shore has to absorb a lot of mechanical strain. That makes this area one of the most seismically active regions in the state. During the last three decades alone, two strong earthquakes with magnitudes above 7 have occured there, one in 1980 and the other one in 1994. Every year at least 80 temblors with magnitudes over three are recorded in this region, like the earthquake duo which shook the area early on Sunday morning.
Another tectonically important triple junction is the Afar Triangle, where the Golf of Aden and the Red Sea meet the northern end of the East African Rift Valley. The Galapagos Islands are also located on a triple junction. Under the islands made famous by Darwin's research, three oceanic plates meet: Pacific, Nazca and Cocos. There is however a major difference between the Galapagos Triple Junction and the three-way intersection under Cape Mendocino. While under Northern California the plates crash into one another, the three plates under Galapaos are moving away from each other. (hra013)
It was a balmy morning reminiscent of Indian summer 140 years ago today. Many of the 260,000 people who lived in the Bay Area in the late 1860's were already busily engaging in their day's activities. Their hustle however came to an abrupt halt, when at seven minutes before 8 am, the Earth under a 900 square mile region, from Gilroy and Santa Cruz in the south to Santa Rosa in the north, began to shake violently. The Hayward Fault had gotten loose and shook off its tectonic stress in an earthquake with a magnitude of almost 7.
|Alamada County Courthouse in San Leandro (photo courtesy of Bancroft Library)|
When the shaking was over, nearly every building in Hayward was extensively damaged or completely wrecked, while in San Leandro, the second floor of the Alameda County courthouse collapsed and many other buildings were destroyed. The most famous victim of the quake's fury was Mission San Jose, at that time almost 100 years old. Its adobe walls crumbled and the church was left in ruins.
While Hayward and San Leandro bore the brunt of the seismic shaking, major damage also occured on the other side of the Bay. Indeed, the destruction in San Francisco was so large that the temblor became known as the "Great San Francisco Earthquake." This honorary title was removed four decades later, when a bigger earthquake came along - the disaster on April 18, 1906, when most of the City went up in flames.
During the Hayward quake 140 years ago today, the ground cracked in a straight line that could be traced for nearly 20 miles from San Leandro south to Arroyo Agua Caliente. And unlike the modest cracks in the Hayward parking lots today, this was a huge gash along which the flanks of the fault slipped past each other about six feet.
|Damage to flour mill (drawing courtesy of Bancroft Library)|
Were a similar earthquake to occur today, the consequences in damage and human life would be immensely greater and far more serious than those of Hurricane Katrina. The "1868 Hayward Earthquake Alliance," a private-public consortium of 123 organizations raising awareness about the imminent danger lurking under our feet, has put together studies and scenarios by various researchers and modelers about the potential earthquake's frightening aftermaths: The immediate commercial and residential economic losses, including damage to private buildings and their contents would surpass $120 billion. Thousands of people might be unemployed due to damage to commercial and industrial buidlings, and at least 90,000 residential units would be destroyed, displacing almost 220,000 residents. Major transportation infrastructure, such as Interstates 580 and 880 and the Caldecott Tunnel, are likely to be unusable for months. BART and the major rail lines will be affected and might not run for weeks. Water tunnels and pipelines supplying drinking water from the Sierra to the Bay Area are likely to break and leave residential and commercial spigots dry for days or weeks.
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.
|Click here to view larger image as PDF|
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.
|View larger image as PDF (12 MB )|
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)