Tuesday, May 09, 2006

Earthquake Risk

RESkeptic asked me to post the following on his behalf (Blogger is, as usual, making doing it himself difficult); it can be considered a companion post to this one:
Here's a very interesting map I found on USGS. I think this serves well to "burst the bubble" about the assumption that damage from big quakes are localized to a specific area. What I've written below could serve as the writeup for a new article.

When we look back to the earthquake of 1906, we tend to think of the damage and loss of life in San Francisco. However, a quake of similar magnitude (7.8) could produce far more damage today than we may realize. Consider this USGS map, which estimates shaking intensity and damage to Marin and Sonoma counties. To my surprise, areas of Sonoma county such as Santa Rosa, Sebastopol, and Bodega Bay could be heavily damaged by another San Andreas quake centered near SF. Given what could happen to these areas, are all residential and commercial structures adequately prepared for a big earthquake? That would be something to look into if you're considering buying a home in these areas.

While Marin and Sonoma counties have historically fewer quakes than the corridor from San Jose to Monterey, it's important to note how Marin's local geology could magnify earthquake damage. As documented earlier, liquefaction poses a risk in some areas. Additionally, local geology and wet weather could heighten the risk to homes in our coastal hills. If you've driven highway 1 recently into Stinson Beach, you cannot miss the sliding caused by bedrock instability helped along by strong winter rains. While we normally consider bedrock to be stable, hillsides containing serpentine can be an exception to this rule. Since serpentine is a very slick fault line rock, hillsides containing both serpentine and ground water can give way--especially in an earthquake. This is a hidden risk present around Mt. Tam, the headlands and Tiburon. I've even seen new homes in "level" tracts containing serpentine that sustained continued, expensive damage due to settling. Given the complex geology and possible hazards in Marin, I think that anyone shopping for a home should be aware of their risks, even hiring a specialist to assess these risks before purchasing. If the 1989 Loma Prieta earthquake is any indication, it can be very expensive and sometimes impossible to fix your home after an earthquake.

Here is the source page for this map, along with several others for the 1906 earthquake.

3 Comments:

Blogger cajun100 said...

The Association of Bay Area Governments (search ABAG) in Oakland maintains a variety of similar maps for ground shaking and for liquifaction, and the detail is somewhat better. In fact, you'll be even more wary of many locales after looking at the maps (i.e., Santa Rosa, Rohnert Park, and many parts of Marin County). These are "generalized" of course, but still indicate a picture that should be of concern when locating or investing.

May 10, 2006, 4:07:00 PM  
Blogger marin_explorer said...

Thanks for posting that while I was away. Since everyone has focused on SF lately, I wanted to show how there could be a lot more at risk than "The City".

May 11, 2006, 11:50:00 PM  
Blogger Marinite said...

Thanks for posting that while I was away.

Oh, man! I'm sorry. It didn't even occur to me that this post occurred while you were away.

As some consolation -- this topic and that pic are getting a lot of hits so there is quite a bit of interest.

May 12, 2006, 9:58:00 AM  

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