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Impearls: Earthquakes III

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Earthdate 2005-10-15

Earthquakes III

USGS: earthquake distribution in the U.S. map, 1989

Historical earthquake distribution across the United States
(click on image for larger scale)
 
 

The magnitude 7.6 earthquake (on the Richter scale) which occurred a week ago on October 8, 2005 (earthdate 2005-10-08) in north Pakistan has, it now appears, killed some 40,000 people — may they rest in peace — and maimed many more.  Contrariwise, the magnitude 7.0 quake on October 17, 1989 (1989-10-17) in the Santa Cruz Mountains — the “Loma Prieta,” sometimes called the San Francisco “World Series” Earthquake — killed a mere 66 (though their lives of course are mourned as well), 42 of whom died from an elevated highway which collapsed (the “Cypress Structure”).

The magnitude of this discrepancy has led some people to conclude that the United States has something of a special dispensation or protection with regard to earthquake deaths, usually attributed secularly to our wise earthquake-resistant building codes.  However, this is basically false for several reasons.  I've written about this before, here and here, but in the context of an earthquake of this size and scope, it's worth repeating some of these truths.

First, and simplest to remedy, many states don't yet have adequate building codes, and the map above makes clear that earthquakes, even large ones (the red dots on the map above are earthquakes of a magnitude equal to or greater than than the Loma Prieta quake), occur all over the United States.  As I wrote before, when as a Californian I see all those beautiful brick buildings covering large portions of the eastern half of this country, I think: deathtrap!

Secondly, and more importantly, the basic reason why California and the rest of the nation are seemingly immune to high fatalities and destruction in severe earthquakes isn't because the state's (and country's) cities are (yet) able to survive such shocks more or less intact, but rather this record is an artifact of the fact that California, for example — for all it's being the most populous state in the Union, with a population of more than 35 million — is a very large state which is still for the most part largely empty of human beings.  More than three quarters of the population lives within the three largest conurbations — San Diego; Los Angeles-Long Beach-Anaheim; and San Francisco-Oakland-San Jose.  When earthquakes occur at something like random locations along the various faults which reach through the long state, usually there are relatively few people living nearby — as the Loma Prieta quake illustrates, occurring more than 60 miles from San Francisco, though somewhat closer to San Jose.  Most quakes ordinarily happen even further from populated areas.

However, “ordinarily” ain't necessarily is, as the 1906 San Francisco earthquake disastrously revealed, exhibiting a magnitude of 8.3, more than an order of magnitude greater than Loma Prieta (a magnitude change of +1 on the Richter scale involves 10 times as much earth shaking and some 30 times greater energy release).  It's these magnitude 8.x monsters that California fears — the looming “Big One.”

Not only are the California building codes inadequate to deflect earthquakes of this 8.x magnitude (we don't even have the experience of how various structures respond to earthquakes of this size to design an adequate response at this point), but even for lesser quakes like Loma Prieta (7.x), where the building codes, when they apply, are arguably passable, for political reasons these codes are never made retroactive.  It isn't so much new buildings that are at risk.  Individual wood frame dwellings generally do quite well in earthquakes; and when they fail, they generally do so in a way which doesn't massacre the inhabitants.  Nor was a pane of glass lost out of all the skyscrapers in San Franciso.  No, it's generally older structures that are the problem.

While I sympathize with the plight of property-owners who would have a tremendous burden upgrading those structures without some significant help, the fact remains that there still exist some tens of thousands of unreinforced masonry or concrete structures in California — more or less the same kind of fragile dwellings and buildings which shattered into ruin last week in Pakistan, and similar to the “Cypress Structure” elevated freeway which collapsed, killing the bulk of those who died in the Loma Prieta quake — except that unlike that freeway, these structures extend in three dimensions rather than one, and are full of people.

Thus an earthquake of roughly the same magnitude as the one which hit last week in Pakistan — still small compared with the “Big One” — but were such a quake to occur beneath the center of one of California's cities (for instance, downtown Hayward on the Hayward fault in the San Francisco Bay Area), it would kill far more people and do much greater damage than Loma Prieta.  Here's what the U.S. Geological Survey has predicted in this regard: 1

An earthquake of magnitude 7.5 on the eastern Bay Area's Hayward fault […], for example, is likely to be far more destructive than the Loma Prieta event.  An “Earthquake Planning Scenario” developed by the California Division of Mines and Geology […] and “An Assessment of the Consequences and Preparations for a Catastrophic California Earthquake” by the Federal Emergency Management Agency […] anticipate the following effects:

  • Deaths: 1,500 - 4,500
  • Injuries: 45,000 - 135,000
  • Damage: More than $40 billion
  • One or more hospitals will be destroyed
  • All four bridges to the East Bay will probably be closed for hours to days
  • Access to and travel within the East Bay will be difficult and limited to emergency traffic.
  • Only San Jose International Airport may be available for large aircraft.
  • The damage in San Francisco is likely to be severe — the Embarcadero area is as close to the Hayward fault as it is to the San Andreas fault.

In the aftermath of such a disaster, it's clear that many people will scream and shout that “we weren't told!”  Well, as I've pointed out before and reiterate now, we have been informed and warned.  Sixteen years have passed since Loma Prieta, and the above forecast.

Moreover, no doubt at that moment we'll hear “suggestions” like were heard after Loma Prieta — that California simply be evacuated.  Well, California wasn't and isn't going to be evacuated; any more than New Orleans (the most recent target of such proposals) — the nexus where all the river traffic moving up and down the center of America meets the ocean-going freighters traveling out to sea — is going to be razed to the ground and forgotten.  Among its other virtues, as I've written before, with regard to earthquakes California is the country's great earthquake laboratory, where we all gain the experience needed to cope and live with these great spasms in the earth.
 
 

Reference

1 Peter L. Ward and Robert A. Page, “The Loma Prieta Earthquake of October 17, 1989,” U.S. Geological Survey publication, November 1989; pp. 7 and 16 (map).



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