Posted by: walkingthefault | March 3, 2008

Some Clarifications & Additional Information

A few people have given me some feedback and comments on my recent posts, which I have paraphrased and then addressed, as follows:

You talk about the possibility of the Hayward Fault through Berkeley “ripping open like a burst zipper”. Is that where the epicenter will be?

The short answer is, not necessarily. In a big earthquake there are three potential problem areas where the most damage can occur, and they may not all be in the same place.

  1. At or around the epicenter, which is the point on the earth’s surface above the hypocenter where the earthquake originates.
  2. Where the fault actually rips open (the zipper effect).
  3. Wherever there is extreme vulnerability due to ground conditions and/or poor construction.

So, where do you want to be when the earthquake happens? Preferably as far away as possible from all of the above.

In the 1989 Loma Prieta Earthquake, the epicenter occurred in the Santa Cruz Mountains, far from inhabited areas. That’s near where the fault ripped open too. But the greatest damage occurred in San Francisco’s Marina District, 60 miles away, due to the liquefaction of the soil and construction deficiencies, and on Oakland’s Cypress Structure, 65 miles away, due to inadequate freeway construction.

In the Great 1906 San Francisco Earthquake, the epicenter occurred two miles offshore near Mussell Rock. Where the fault ripped open though was near Olema, forty miles to the north, “where the fault zone is half a mile wide, and filled with soft soils, crumbling limestones, rotting granites, and generally pulverized rocks” (Simon Winchester). But as we well know, the greatest damage occurred in San Francisco itself, due mainly to the ensuing fires and lack of water

In your Scary Picture chapter you show that the fault line passes right under the Tots’ Playground in Codornices Park. How do you know it will be exactly at that spot? Couldn’t it be a hundred feet to the east or west?

Yes, absolutely it could. But really, what difference does it make? The Hayward Fault is actually a “complex zone of deformation which can span hundreds of feet in width”, the leftover evidence of many earthquakes over thousands of years. Various university teams, together with the USGS have dug trenches at various places across the fault in order to pinpoint its location.

If the experts say the red line is in that spot, I am inclined to believe them. However, I don’t believe that even they know if the red line is exactly where the zipper will necessarily rip.

Another issue may be with the differing relative accuracies of the source material for the fault line before it was superimposed on the Google Earth imagery. I have seen many places where, it the line were moved a bit, it would fit the topography better.

For example, look at the location of the fault line here at the back of the Claremont Hotel. The location of that trench (in blue) is too close to the back of the hotel. In fact, it’s almost under it.

Claremont Hotel Rear Google

If you take a look at this photo of the same scene as taken from the stick pin vantage point. the fault line is likely a bit to the left, closer to the bank. 

Claremont Hotel Rear

That’s little consolation for the guests though if the fault rips open there. The hotel’s going to fall down anyway, and then catch fire.

We already had the Loma Prieta Earthquake in 1989. Didn’t that relieve the tension, making another big earthquake unlikely for a very long while?

Not on the Hayward Fault it didn’t! The branch of the San Andreas Fault in the Santa Cruz mountains is about eighty miles from the Hayward Fault in Berkeley. In fact, there is recently discovered evidence that a slippage on one fault may actually INCREASE the strain on another one parallel to it.

For example, scientists postulate that the 5.6 earthquake on the Calaveras Fault last October near San Jose may have increased the strain on the Hayward Fault farther north.

Take a look at this map and try to imagine the Pacific Plate twisting slowly around anti-clockwise in a north-west direction. Think what happens each time one of those faults breaks. You can easily see that the pressure on the next fault over could just as easily increase as decrease. Map courtesy of USGS.

 Bay Area Faults

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Responses

  1. Is it just natural motion for the Pacific Plate to move in a roughly counter-clockwise motion? Likewise for the North American Plate–is it moving in a natural clockwise direction? I hope they do whatever earthquake safety construction they can to the Claremont. It seems, though, that fate may come knocking, for it already dodged a bullet with the big fire. Interesting point about the effect of nearby fault slippage on other faults.


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