2/3rds chance of a Loma Prieta+ class quake in 30 years

[Rohit Khare]

Still leaves me scratching my head as to whether I should buy quake 
insurance or not. $5K of coverage with a $1K deductible is $150+/yr -- 
more than renter's insurance to begin with.

I'm in a fairly modern apartment building, but I guess the whole point 
is that I couldn't imagine anything happening to *my house*... :-)

Rohit

Quake scientists predict Big One likely by 2032
         Bay Area fault study estimates 62% chance of deadly 6.7 temblor

David Perlman, Chronicle Science Editor
Tuesday, April 22, 2003
URL: http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2003/04/22/QUAKE.TMP

In the most detailed investigation ever made of earthquake hazards 
throughout the Bay Area, scientists warned Monday that at least one 
severely damaging and deadly quake is very likely here within less than 
30 years.

That warning came amid new scientific estimates of the danger levels 
along the region's hazardous seismic faults.

According to the study, there is a 62 percent probability of a major 
quake with a magnitude greater than 6.7 striking the region before the 
year 2032.

There is a more than 80 percent likelihood that a smaller but still 
very damaging temblor of magnitude 6 to 6.6 will strike here during 
that time period, the scientists agreed.

Such a quake in the right place, with a magnitude of 6 or greater, 
could be a total disaster, they said -- particularly if it struck along 
a fault like the Hayward, which runs through the most densely populated 
neighborhoods of the East Bay.

"In fact," said Mary Lou Zoback, former chief of the earthquake hazards 
team at the U.S. Geological Survey in Menlo Park, "when you integrate 
all the estimates over the entire region, a severely damaging quake 
with a magnitude greater than 6 becomes a certainty."

For the first time, more than 100 earthquake experts at the U.S. 
Geological Survey and other public and private agencies joined forces 
this year for the study. They ran new computer programs based on new 
seismic information to calculate losses in life and property that would 
be caused by intense ground shaking from major temblors in the area.

Working with the federal agency's seismologists and geophysicists to 
produce the new estimates have been other scientific teams at the 
state's Office of Emergency Services, the California Geological Survey, 
the Association of Bay Area Governments, and the Federal Emergency 
Management Agency, as well as major universities and private earthquake 
engineering firms.

The purpose of the interagency effort was to once again alert Bay Area 
residents, builders and city officials that new seismic knowledge is 
convincing scientists ever more strongly that somewhere in this 
earth-shaking region a Big One is bound to strike, sooner or later, and 
that only preparedness can minimize the devastation.

Less than four years ago, a smaller working group estimated that the 
30-year probability of a quake with a magnitude of 6.7 or greater 
hitting "somewhere" in the Bay Area was 70 percent. And although the 
new probability for a magnitude 6.7 quake is lowered to 62 percent, 
that decrease doesn't mean the danger is lessened, said geophysicist 
David P. Schwartz of the U.S. Geological Survey, who led the new 
working group.

Estimates compiled by the group depend in varying ways on many factors, 
Schwartz said. Among them are: how rapidly the slabs of the Earth's 
crust are moving throughout the San Andreas Fault zone, where and how 
rapidly some of the smaller faults are creeping in slow motion, and 
even on how much -- if at all -- the abrupt motion of the great 1906 
San Francisco quake provided long-term relief for some of the seismic 
stresses that have been building up underground on the other Bay Area 
faults for centuries.

"Our new data now is much more sophisticated and much more robust," 
Schwartz said, "but our results must still reflect many uncertainties. 
So we have to accept a broad error range, which could be anywhere from 
38 percent to 87 percent, considering many different earthquake 
theories -- which poses a big uncertainty.

"What we do know for sure, however, is that any big earthquake in the 
Bay Area will produce damaging ground motions over broad areas, and at 
substantial distances from the source of the quake."

According to the new report, the most hazardous quake region in the Bay 
Area now is the combined Rodgers Creek and Hayward fault systems, which 
run from north of Santa Rosa to south of Fremont through some of the 
area's most densely populated communities.

The 30-year probability of a large quake along that system alone is 
estimated at 27 percent, and if one does occur on that system, the 
danger from severe shaking is expected to extend westward beneath the 
bay and cause widespread damage to structures built on soft soil along 
the Peninsula and in the pre-1906 bay fill areas of San Francisco's 
waterfront and downtown Financial District, the scientists warn.

Another endangered area lies along the western side of the San Andreas 
Fault in San Mateo, Santa Cruz and Monterey counties. It's an area that 
lies between the infamous San Andreas and the little-known offshore San 
Gregorio Fault. Although the probability of a large quake on the San 
Gregorio is only 10 percent, and on the San Andreas it is 21 percent, 
the combined odds of a magnitude 6.7 quake striking either of them rise 
to 34 percent within 30 years.

The last big earthquake to hit the San Francisco Bay Area was the Loma 
Prieta temblor of 1989. Although it struck in the sparsely settled 
Santa Cruz mountains with a magnitude of 6.9, it made 16,000 buildings 
uninhabitable as far off as San Francisco and the East Bay. A similar 
quake rupturing the entire Hayward Fault is likely to wipe out at least 
150,000 buildings, according to ABAG's current analyses.

With the report issued Monday, the earthquake working group also 
produced a series of "shake maps" showing the intensity of ground 
motion that would be induced by a large quake on any of 34 different 
faults and fault segments in the region.

The maps and other information on the latest probability estimates for 
all significant Bay Area faults are being posted today on a link to the 
U.S. Geological Survey's earthquake site:  http://quake.usgs.gov/