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)
Earthquakes occur everywhere - so, at least, it seems. Temblors happen on all continents and beneath the deep oceans. They shake the world's highest mountains, the Himalayas, and the Earth's deepest valley, the Dead Sea. Even from under the ice caps of both polar regions, seismometers regularly record rumblings in the Earth's crust. But a more detailed look reveals that the distribution of earthquake foci in the world is by no means random. And neither are they evenly or regularly spaced. Instead, when plotted on a world map, earthquake locations look like narrow bands winding through the continents and oceans (see map). What are these zones and why are most earthquakes foci concentrated there?
Simply put, temblors happen when rock breaks under force. Inside the Earth, the most important of such rock crushing forces is the "tectonic stress." It is exerted on the Earth's crust by the movement of the giant, rigid plates, which float on a subterreanean sea of hot and plastic rock called the asthenosphere. There are about twelve huge and another dozen smaller plates. Where ever such plates crush into or slide past each other during their respective drifts on the Earth's surface, the collision is able to break the rock, thus causing earthquakes. In principle, the effects of such plate collisions are similar to a car wreck where two automobiles hit each other, albeit on a much larger scale.
The bands of earthquake foci in the map reflect these collision zones of the tectonic plates. In fact, they very clearly mark the boundaries of the plates. Look for instance at North America. The underlying plate is much bigger than the continent itself. It stretches from Iceland in the East all the way to the most far flung Aleutian islands in the West and reaches from Alaska to the Caribbean and beyond to the Azores, the island archipelago in middle of the Atlantic Ocean.
But earthquakes happen not only where plates collide. They also occur where two plates move away from each other in the so called "spreading zones." One of these zones is the Mid-Atlantic Ridge where Europe moves away from North America at the rate of about one inch per year. You will find such ridges in every major ocean basin. In fact, there are many more miles of plate boundaries under the oceans than on land. As a consequence, the number of submarine earthquakes is also larger than the number of quakes on land. (hra006)
Everybody has probably done it while frolicking at the beach: Stand with both feet on the wet sand and move your body quickly up and down several times without lifting your feet. After a short while the ground gives way. You sink a few inches into the sand and you might even lose your balance. Well, you just simulated one of the most dangerous effects seismic waves can have on buildings: Soil liquefaction.
How can soil, which is hard enough for you to walk on, lose its strength and stiffness just because it is shaken a little bit? The secret is the water in the soil. Liquefaction occurs only in soils in which the space between individual sand particles is completely filled with water. Such soils are called "water saturated". The water exerts pressure on the particles, which in turn determines how tightly they are packed together. Before an earthquake, the water pressure is low and static. During the dynamic shaking, however, the water pressure can increase so much that the particles can move freely past each other. Once that happens, the soil loses its strength and becomes a gooey, slippery liquid.
"Sand boils" are a relatively harmless consequence of such liquefaction; they look like small mud volcanoes (Figure 1). The overpressure inside the soil causes the sand to squirt out like lava from a volcanic crater. However, when soil liquefaction occurs under a building, it may sink into the soil, like your feet did during your experiment at the beach. The building might even tip over.
The first time seismologists fully recognized the devastating effects of soil liquefaction was in 1964. During an earthquake under Niigata in Japan several apartment buildings sank into the ground and tipped over, because the water saturated soil on which they were built liquefied (Figure 2). The Bay Area is by no means immune to liquefaction, because many buildings in low lying areas are built on soils saturated with water from the Bay. Liquefaction can occur in all areas shaded brown and yellow in the map in Figure 3. The Association of Bay Area Governments (ABAG) has published a booklet with detailed information about the hazards of liquefaction in our region.(hra005)
|Figure 3: Map of liquefaction susceptibility in the San Francisco Bay Area (courtesy of USGS). (Click to view larger image.)|