Posted by: walkingthefault | February 24, 2008

Will it Rip Open Like a Burst Zipper?

The traditional picture people have in their minds of great earthquakes is of huge cracks opening in the ground, and cars and people and buildings falling in. Some people even think that California will be demolished and fall off into the Pacific Ocean. That may be the wishful thinking of some in middle America, especially in the “red” States, particularly with Berkeley in mind, but I don’t think it will be quite that bad. Let’s look at some facts.

Firstly, the study of the science of Plate Tectonics tells us that the Pacific Plate, which abuts the North American Plate, is moving inexorably in a north-west direction at a rate of about 3-6cm per year. The boundary between the Pacific Plate and the North American Plate runs through the Bay Area, and manifests itself in massive cracks in the earth such as the San Andreas, the Hayward, and the Calaveras faults. These cracks alternately jam up, and then suddenly give way, so that north-west drift is not a nice smooth ride.

The last big quake on the Hayward Fault occurred 140 years ago in 1868, with an estimated magnitude of 7.0. Most of the actual slippage of the Pacific Plate, about four feet, occurred on the southern half of the fault, around Hayward, hence its name. Digging of trenches across the fault line reveals that the part through Berkeley and to the north remains jammed up solid.

Since roads and streets have been built in the last fifty years or so, fault creep has occurred in the southern part, which may have relieved some of the strain. You can see the deformed curbs, sidewalks, and buildings where they straddle the fault. But there is little evidence of fault creep along the section through Berkeley, except across Memorial Stadium, part of which has moved about thirteen inches since it was built in 1923.

Codornices Park Playing Field

Do you see any evidence of the Hayward Fault, or any fault creep in this photo of the playing field in the southern part of Codornices Park? I don’t, none whatsoever. But it runs right through the center of the photograph. I often try to imagine it after the earthquake has wrought its havoc. Not only will we have lost the ball, but possibly a few of the players as well.

The last big earthquake in the Bay Area, Loma Prieta, occurred on October 17, 1989. 69 people were killed, 3757 were injured, and 12,000 people were left homeless. Most of the damage occurred far from the epicenter, which was in an unpopulated area of the Santa Cruz mountains. The Cypress Structure and Bay Bridge collapses were attributed to construction deficiencies, and the Marina District structural failures to liquefaction of fill, not because they were on, or anywhere near the epicenter or the actual fault line.

But in the Santa Cruz mountains, the ground certainly ripped open like a burst zipper, with a sideways movement of about 1.6 meters. Photo courtesy of USGS.

Loma Prieta Crack

You can see more photos of the zipper effect here.

The previous earthquake comparable to what is likely to happen on the Hayward Fault very soon was the Great San Francisco Earthquake of 1906. Anybody who has not read Simon Winchester’s Book, A Crack in the Edge of the World, should do so at this point, and then take a drive out to Olema in Marin County and look at the evidence. The burst zipper effect can be seen in the fences, roads, and lines of trees displaced by about 8.5 feet. Photo courtesy of USGS.

Fence Near Olema

As with the Loma Prieta earthquake though, and the last Hayward Fault earthquake in 1868, almost nobody lived there at the time. It’s a totally different story for the Hayward Fault today where millions of people live in homes and work in offices built right on top of the fault.

So, to answer the question as to whether the Hayward Fault through Berkeley will rip open like a burst zipper, one has to pose the question, why wouldn’t it? The buildings built on top of it aren’t going to glue it together! The Pacific Plate is moving, like it or not, and the timing points towards any day now. I do not think I am being unnecessarily alarmist. I do think though, that most people who live in the area are being inordinately complacent.

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