17 IV 2003 - 22:39 - terra tremens

Because I will probably not be awake tomorrow morning to tell you about it:

At 5:12 AM, local time, on the 18th of April, 1906, an earthquake measuring approximately 7.9 on the Richter scale (seismographs hadn't been invented yet, so all figures are really just estimates based on ground slippage and damage) struck Northern California. (For those of you keeping score from the other side of the Atlantic, that was 1:12 PM, UTC.) The earthquake of 1906 is known as the 'San Francisco' earthquake, even though its epicenter was really closer to the northern suburb of Santa Rosa, because this was the biggest city in the affected area, Los Angeles at that point being just a town with too little water and San Jose being an expanse of orchards. It was, of course, not only the earthquake that did the damage in this city, but also the fires that raged for three days aftewards. To this day, due to the simple fact that wood does relatively well in earthquakes but is also inflammable, San Francisco remains one of the most fire-prone 'big cities' in the United States. Of course, in 1906 we were completely dependent on the water pipes that carried water up from the Peninsula (where, incidentally, the two main reservoirs lie directly in the fault trace and thus are two long lines of water) -- the city's fire departments didn't have the equipment that pumps water from the Bay, and so their only weapon was dynamite. Much of the downtown area burned.

I mention this because tomorrow, at 5:12 AM, earthquake survivors will congregate at Lotta's Fountain downtown for a ceremony to commemorate the earthquake. Said ceremony culminates in the song 'San Francisco' from Beach Blanket Babylon, another long-running San Francisco institution. The number of survivors gets smaller every year...

Something that's interesting about this particular earthquake is that a really, really long stretch of the fault slipped (about 300 miles). For those of you who don't know, the San Andreas system is basically a strike-slip fault, meaning it basically makes the motion you make when rubbing your hands to warm yourself (although in one direction only; Nature is not quite that perverse). Friction means that the rocks sort of stick, and when they jump free, voila! -- an earthquake. A much smaller section of fault slipped in the more recent 1989 Loma Prieta earthquake (Loma Prieta is the name of a hill out near Santa Cruz, a good way south -- this earthquake's magnitude was 6.9) -- and the fact that this southern section of the fault slipped meant that stress was increased on adjacent segments of the fault, among them the segments to the north that underlie the suburbs and part of the City of San Francisco.

One doesn't think about these things when one lives here, however, as to think about them constantly would be to go mad from fear and live out in Kansas (where one would soon learn to fear tornadoes), and instead lives in a pleasant state of denial, at least until these big anniversaries (October 17, 1989 is the only one for which there is *any* fuss) swing around. At which point I at least remember (or look up some facts), speculate, and shudder.

For your reading pleasure (and an excellent refresher for me, as I thought I had outgrown my interest in seismology):

This is a list of links compiled by the Museum of the City of San Francisco. Links include eyewitness accounts, photographs of the damage, and casualty lists. The map of the epicenter no longer works.

This is a link to the United States Geological Survey's page on the 1906 earthquake. (The USGS has its offices in Menlo Park, another suburb of the City). The 'When Will It Happen Again?' link is sobering, especially when they start giving probabilities for each local fault segment's slip.

A related link is this one, also by the USGS and one you can find off the main page, but the bottom map is one that illustrates the fault segment slip lengths for major earthquakes in the Bay Area. The top one will give you some idea of the concept of 'seismic gap,' which is to say a pause in earthquake activity. Such a gap usually gets filled; the concept was developed by a Japanese geologist who was studying the history of the Kanto plain and realized that, to judge from historical records, there had been no seismic activity in the Tokyo-Yokohama area for quite some time. Too long a time. He began making warnings in -- 1911? I think it was, or something quite early -- in the tens, at any rate, but was ignored. Then the great Kanto Earthquake of 1923 occurred, and the seismic gap was closed. [/digression]

This is a USGS report that includes some of the icons you've already seen and also an interesting set of photos at the top that basically make the point that we stand to lose a lot if a major earthquake should strike again (although City Hall is on big rubber washers and should just roll back and forth in the next earthquake, many buildings, especially in Chinatown, are made of unreinforced masonry, which generally crumbles to bits in earthquakes with great attendant loss of life.) I own a copy of the magazine they mention towards the end and, if I can find it, will scan some of the images showing the soil types that underlie the cities of the Bay Area (hint: you want to be on bedrock like Sausalito and Candlestick Park, not landfill like the Marina) and the fault systems of the region.

This link is to a site run by St. Louis University and is interesting for its thumbnailed photographs of damage. As a brief note, the Hayward fault is shown, but if you continue it up into the north, basically following the existing line and the dark green valley you can see, that is the outline of the Rodgers Creek Fault. Some believe that an earthquake on the Hayward fault has the possibility of triggering an earthquake on the Rodgers Creek Fault. Santa Rosa, as I mentioned before, was almost completely leveled, and you may find the photographs of earthquake damage in the other towns of interest as well. Hollister, a fruit-growing town somewhere around San Juan Bautista, was also completely leveled; one of the more horrifying stories from the earthquake was the collapse of Agnews Mental Hospital in the South Bay, which killed almost all the inmates and doctors and nurses. This makes me nervous when I think about the progress of seismic retrofit programs on the hospitals in the area today.

(Remember, too, that many hospitals in the East Bay stand directly on the Hayward fault -- the fault is a yard-line in Memorial Stadium -- and that if a large earthquake were to occur on that fault, in the middle of a school day, the results for both hospitals and UC Berkeley would very likely be disastrous. See Marc Reisner's book 'A Dangerous Place: California's Unsettling Fate' for a better examination and dramatization of this possibility than I am producing, or ever could produce.)

Finally for information about the earthquake, I give you this link, UC Berkeley's seismology department itself, which answers some frequently asked questions and provides more than a few good links.

If you should feel unsatisfied after this collection of links, there is a collection of eyewitness accounts for various California earthquakes (including the powerful but relatively without effect Fort Tejon earthquake -- very few people lived in the state at the time) on the Internet here.

We now return you to our normal programming.