Using the latest satellite technology, scientists have found
evidence that the northern segment of the dangerously unstable
Hayward Fault is steadily slipping and is far less likely to generate
a major earthquake than researchers had thought.
The conclusion offers scant comfort, however, for people living or
working around the densely populated East Bay fault. Earthquake
experts continue to warn that a major temblor on any of the Bay
Area's faults is bound to shake the land everywhere -- with
casualties possibly in the thousands and billions of dollars in
A research team headed by Roland Burgmann of the University of
California at Berkeley has found that the rocks six miles deep within
the northern segment of the Hayward Fault are constantly slipping on
both sides of the fault in a motion seismologists call ``aseismic
Repeated clusters of tiny, imperceptible ``microearthquakes''
detected beneath Berkeley add more evidence that the two sides of the
fault's northern segment are not ``locked'' deep underground. Rather,
they slip past each other, and that steady slip releases seismic
strain that builds up on the fault so the strain is unlikely to snap
suddenly in a single violent quake, Burgmann and his colleagues find.
The scientists are publishing a report in the journal Science today
that describes in detail the evidence they have gathered from radar
observations by two European Space Agency satellites and by the U.S.
Global Positioning System, a network of 24 satellites in orbits
12,000 miles high.
The Hayward Fault cleaves the Bay Area for more than 60 miles,
stretching from San Pablo Bay in the north to southern Fremont near
the Santa Clara County line. Scientists divide the fault into two
segments whose seismic histories differ significantly.
The southern segment between Oakland and Fremont was last hit by a
major quake in 1868. But recent trenching on a golf course in El
Cerrito shows that the last big quake occurred on the northern
segment much earlier -- sometime between the mid-1600s and the
arrival of Spanish colonists in 1776.
Puzzled by the northern segment's long absence of large quakes,
Burgmann used a new technique called ``synthetic aperture radar
interferometry'' to gather data from the European satellites as they
passed overhead in 1992, and again five years later. The data
detected evidence of extremely subtle changes in the earth's surface
between the two sets of observations.
Those changes are measured in millimeters -- only a few fractions of
an inch -- but with the aid of complex mathematical computations they
provide powerful evidence of changes occurring deep within the earth.
The extraordinarily precise measurements, he said, could well be
used to understand the behavior of the region's other major faults
and to reveal portions of faults long considered seismically
dangerous that may be less likely to produce major quakes.
Burgmann's satellite radar observations, combined with data from the
Global Positioning System and the record of ``microearthquakes,''
show that the two sides of the Hayward Fault zone, down to a depth of
seven miles, are slipping past each other at slightly more than two-
tenths of an inch a year along a 12-mile stretch of the fault's
NORTHERN FAULT NOT LOCKED
That steady motion, however small, establishes that the northern
fault segment is not ``locked'' deep underground as it is on the
southern segment, and therefore unlikely to pose a threat.
``However, other hazards -- from the southern Hayward Fault, the San
Andreas, and other nearby faults -- leave the need to build
reinforced homes and the need to be prepared just as high as
before,'' Burgmann said.
Scores of small earthquakes constantly rattle the Bay Area, and the
slow motion of ``aseismic creep'' can easily be seen in offset
curbstones and cracks in the pavements of many East Bay communities
along the Hayward Fault -- and also, as science students like to note
--in the cracked walls of the University of California's Memorial
Stadium in Berkeley.
Early in its research, Burgmann's team provided data to the
earthquake hazard working group headed by geophysicist David Schwartz
of the U.S. Geological Survey. Last year, Burgmann's data caused the
group to lower the probability of a major quake striking on the
Hayward Fault's northern segment from 28 percent to 16 percent within
the next 30 years.
But there has always been a mystery about the northernmost end of
the Hayward Fault, in part because scientists have not yet traced it
down beneath the bottom of San Pablo Bay.
Just on the other side of the bay, the Rodgers Creek Fault runs
northward into Sonoma County as far as Santa Rosa. Many earthquake
researchers believe that the two faults must be linked, and last
year's working group set the probability at 32 percent for a major
devastating quake on the two faults together -- the highest threat
for any fault region in the study.
PART OF SAN ANDREAS
All the faults that lace the Bay Area are, in fact, smaller branches
of the great San Andreas Fault zone, which runs for more than 600
miles from Point Delgada on the far Northern California coast all the
way down to the southern San Bernardino region.
The San Andreas marks the edges of two great moving slabs of the
earth's crust where the Pacific plate far beneath the ocean has been
grinding slowly northward against the North American continental
plate for millions of years, distorting the crust and generating
seismic hazards on land that never cease.
Burgmann's team included scientists from the Lawrence Berkeley
National Laboratory, NASA's Jet Propulsion Laboratory in Pasadena and
NORTHERN HAYWARD FAULT POSSIBLY LESS DANGEROUS
UC Berkeley have found that rocks deep within the northern segment
of the Hayward Fault are in a constant motion called ``aseismic
creep.'' Because the two sides of the north-ern segment of the
fault are not ``locked,'' their creeping motion eases the seismic
strain on the fault so it cannot build up and snap suddenly
in a single violent quake.
E-mail David Perlman at email@example.com.