Earthquakes and the Bay
The earthquake that hit Japan was incredibly big. Its magnitude was 10 times the size of the recent Christchurch quake in New Zealand. The ensuing tsunami and the ongoing nuclear situation make this one of the most serious natural disasters to hit the developed world in recent memory.
As one of many people who live atop an active tectonic fault, I have paid a close attention to seismic events. Many people have looked at the Japanese quake and began to worry that we are perhaps tempting fate, living on such an unstable and violent part of the world's crust.
However, having talked to some Geologists and read widely on the subject, perhaps I can offer some thoughts that may placate some of these fears.
San Francisco is built at the top of a peninsula close to the San Andreas fault. The San Andreas fault runs over 800 miles through California, dropping into the ocean just south of the city near Pacifica.
The most important thing to know about the San Andreas fault is that it is a transverse fault, that is, it is the join between two tectonic plates that are moving perpendicular to one another. Earthquakes on this type of fault are caused when one plate 'catches' on the other, allowing force to build up until a weak point along the fault slips and releases the tension along the fault.
This is quite a simplified view of the dynamics that go on along a fault though. Faults are not the nice clean cracks that are often featured in pictures as they cut through the landscape. Millennia of quakes and movement mean that the land around the fault is fractured and deformed. In fact many sub faults run through the Bay Area, notably the Hayward Fault that runs up the East Bay through the Berkeley Hills.
Close to the fault, the rocks are often severely cracked - the area that was at the epicenter of the 1906 earthquake at Mussel Rock is so deformed that the fault is a wide mess of boulders and rocks.
This matters because the strength of an earthquake in this situation is limited by the strength of the rock that the fault runs through - the force builds up through small rocks, and can only remain until the rocks crack. Another factor that goes into the magnitude of a quake is the length of the rupture along the fault.
Because of this, an earthquake in the Bay Area could never reach the magnitudes we've seen in Japan. In fact the 1906 earthquake was close to the theorised maximum for this area - it is very unlikely that we would see significantly more than a magnitude 8 in the Bay Area.
The 1906 quake hit very close to San Francisco, with the epicenter only a few miles from the city. Even still, it was the ensuing fire that caused the most destruction. The building codes that we have devised, and the engineering techniques we have developed since then mean that San Francisco is well prepared for 'The Big One'.
The Hayward fault is actually far more of a worry, as it travels through so much urban area. Even still, a large quake here would have far less of a toll than the Japanese quakes.
Interestingly, counter to my initial instincts, the BART tunnel is far safer in the event of a quake than the bridges. It is extremely difficult to dampen the vibrations of a quake along a large span such as the Golden Gate bridge, especially as the frequency of the quake could be very similar to the resonant frequency of the bridge.
So the Bay Area is fairly safe. But let me counter any fears I may have allayed with a warning.
At the North of California, the San Andreas fault, having turned inland, becomes the Cascadia fault. This fault stretches north to Vancouver island below Oregon and Washington. Unlike the San Andreas fault, the Cascadia fault is a subduction zone; this means that the oceanic plate, instead of rubbing adjacent to the North American Plate, dives underneath it, scraping with it any rocks and seawater that they may contain.
Subduction zones are incredibly violent - when the water that is sucked down is superheated by the magma, it can cause explosive volcanoes such as Mount St Helens.
Less well known, however, is the ability of subduction zones to produce truly massive earthquakes. In 1960 an earthquake measuring 9.5 on the Richter scale rocked Chile - the largest recorded quake. Including this, nine of the ten largest quakes in the last 100 years were in subduction zones. The Japan quake was in a subduction zone.
Seattle and Portland lie above the Cascadia fault and both could be vulnerable to an earthquake as large or larger than the recent Japan quake. Tsunamis would also be generated by a quake here. The Pacific Northwest is considered unready for such a quake - many buildings are not earthquake safe as the dangers of the fault are not well known. "The geology and numerical models predict tsunamis could reach as high as 80 to 100 feet in Oregon, which is similar to the tsunami that struck Sumatra," quotes http://www.oregonlive.com/news/index.ssf/2009/04/big_earthquake_coming_sooner_t.html OregonLive.com
So some concern then for residents of the Pacific Northwest. With the Japanese quake still in the news, hopefully people will be motivated to prepare for such a disaster - people should have at least 3 days of food and water stocked in their houses.