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SACRAMENTO – Data
collected by the California Department
of Conservation during the September 28
magnitude 6.0 Parkfield earthquake has
yielded some surprising results that
ultimately may help scientists and
engineers improve public safety.
Acting State
Geologist Michael Reichle, head of DOC’s
California Geological Survey, said:
“We’re getting some very interesting
data, a change in the typical patterns
we see in earthquakes. A lot of people
in the scientific community are going to
be excited about this information.”
Only a small portion
of the data collected from 44 CGS and 10
United States Geological Survey stations
scattered around the Monterey County
village – instruments that have been
waiting since 1982 for the San Andreas
fault to rumble -- has been analyzed.
“So far, the data
shows that there’s more complexity in
the near-fault area than we ever could
have guessed,” said Anthony Shakal,
Supervising Geologist for CGS’ Strong
Motion Instrumentation Program. “This
means that with careful analysis, we can
improve our understanding of ground
shaking near the fault, which in turn
could lead to changes in the building
codes and engineering design practices.”
Three interesting
results have been noted so far.
First, the earthquake
ruptured to the northwest, stopping at
Middle Mountain, rather than to the
southeast, as the last two earthquakes
of similar magnitude in that area had
done.
“We anticipated the
earthquake, which is why we had all
those instruments there for all those
years,” Shakal said. “We knew it would
come; we just had to be patient. But the
direction it took, for some reason, was
different than expected, and that’s
something that will have to be studied
closely.”
Second, the data
shows that the shaking beyond 10
kilometers (6.25 miles) from the fault
was actually less than predicted by
formulas currently used to develop
ground motions for engineering designs.
CGS also observed less-than-expected
shaking at distance in the San Simeon
earthquake last December and in the 1992
Landers event.
If this pattern holds
up, it may mean designs can be less
stringent at some distances, saving
money without reducing safety. That’s
the importance of actual strong shaking
measurements, Shakal noted: In this case
they may show that there’s a greater
margin of safety in the range beyond
about 20 miles from an earthquake than
was previously believed.
Finally, and most
significantly, there was an anomaly in
the measured peak acceleration or
movement.
Shaking occurred at
about .3 G – about a third the force of
gravity – in Parkfield, which is about
six miles northwest of the epicenter and
within a half-mile of the main trace of
the San Andreas. The local schoolhouse
moved back and forth about seven inches
with the ground, but retained its
structural integrity, according to Moh
Huang, a civil engineer with CGS.
Meanwhile, both
northwest and southeast of the village
of 18 people, stations measured shaking
that was three times as intense as in
Parkfield.
“There seems to be no
simple way to explain this in terms of
the mechanics of the earthquake,” Shakal
said. “Perhaps there’s some geologic
effect at work here, such as the soil
formation or deep geology underneath the
town.”
Shakal pointed out
that there was a similar but reversed
phenomenon during the magnitude 6.7
Northridge earthquake in 1994: Shaking
measured at a hill in the city of
Tarzana was much higher than that in
surrounding areas closer to the
epicenter.
“Generally, there’s a
huge difference in ground shaking
amplitude depending on whether you’re in
front or behind the direction of the
rupture,” said Shakal, noting that the
phenomenon is sometimes compared to the
well known Doppler effect. “It’s sort of
like the difference between looking down
the barrel of a gun and being behind it.
“But in Parkfield,
there was tremendous ground shaking both
in front of and behind the direction of
the rupture. And the shaking was
significantly lower than in nearby
areas. Obviously, we need to continue to
expand our knowledge of how these things
work in order to be able to plan and
design for this kind of thing in future
earthquakes.”
Among the other
things the California Geological Survey
hopes to learn from the recent Parkfield
quake is the speed at which the fault
rupture spread. Seismologists from the
Strong Motion Instrumentation Program
are digitizing and analyzing the film
records taken from the Parkfield array
to calculate both the ground velocity
and displacement (movement) at each
station. The recorded data will be used
in the computer models to determine the
fault rupture velocity. The rupture
velocity is important in understanding
and predicting strong motion.
The presence of the
instrument array – a line of
accelerographs along the fault and three
lines extending perpendicular to the
fault (picture a backwards E) – will
allow CGS scientists to gather an
unprecedented amount of information.
Only one strong motion instrument that
recorded the Loma Prieta earthquake of
1989 was near the fault, compared to the
54 at Parkfield.
The entire Parkfield
area is a basically an earthquake
laboratory for not only the California
Geological Survey, but also the USGS and
other scientific and educational
entities. The self-proclaimed
“Earthquake Capital of the World,”
Parkfield has experienced earthquakes in
the magnitude 5.5-6 range every couple
of decades going back to 1857. However,
last month’s temblor was a late arrival;
the previous quake in that magnitude
range occurred in 1966.
“Those instruments
have been faithfully waiting for this
earthquake,” Shakal said. “Because of
careful planning and anticipation, this
is the best-measured earthquake rupture
process ever. We’ll be able to see
details of the rupture that we’ve never
seen before and begin to get an
understanding of the variability of
ground motions near a fault.”
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