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Matthew A. d'Alessio, David Schmidt, Roland Bürgmann, and
the UC Berkeley Active Tectonics Group
If you need to take measurements in the middle of a busy
intersection, or in a stadium that houses thousands of voracious
football fans, or even at a freeway offramp in the middle of
rush-hour, you need an urban geodesist. And the Hayward fault, which
passes through one of the most densely populated strips in
California, is a place where urban geodesy is king.
Geodesy, the science of surveying and measuring the earth's surface,
is one of the many tools used by earthquake scientists to understand
the behavior of faults. This paper describes the Berkeley
Seismological Laboratory's ongoing effort to use geodesy to monitor
the movement of the Hayward fault, as well as some of the challenges
introduced by doing science in a densely populated urban region.
The Hayward fault is unique because it not only produces large
earthquakes like the estimated M7 1868 earthquake, but it also slowly
slips along in a process called aseismic creep. In recent years, the
Berkeley Seismological Laboratory has begun utilizing measurements
with the Global Positioning System (GPS) to monitor creep along the
Hayward fault. This work will provide an unprecedented spatial
resolution of GPS measurements about a creeping fault and should
allow us to determine where the fault is creeping, how fast it is
creeping, and most importantly, how deep the creep extends. Since
aseismic creep relieves stress along the fault, identifying locked
patches at depth can give us clues to determine where large
earthquakes might eventually nucleate and how big they could be.
In a GPS ``campaign'', we use research-grade GPS equipment to
determine the location of a point on the ground to a precision of
less than 5 mm. In other words, we can determine locations on earth
to within an area half the size of a dime. The next year, we repeat
the campaign and return to the exact same location to determine the
position again. Because the Hayward fault is slowly creeping along,
the spot will have moved; with our high precision positioning, we
hope to determine exactly where and how fast. The precision and
reliability of our measurements depend on several factors that are
all more difficult in the urban setting. See the table on the
following page for more details.
As part of the Berkeley Seismological Laboratory's ongoing effort to
monitor active deformation along the Hayward fault, we determine
positions of over fifty benchmarks within ten kilometers of the fault
(Figure 25.1). Of these fifty monuments, about two
thirds of them are in logistically difficult urban settings while the
remainder are in the scenic rolling hills that flank the fault. The
first surveys of most benchmarks were made in 1998, and our group has
been returning each summer to take new measurements.
The Hayward fault and urban areas of the East Bay. Each
triangle shows the location of a benchmark surveyed annually since
In the last year, the Active Tectonics group purchased seven new,
state of the art, geodetic grade GPS receivers. The Trimble 5700
receivers are smaller, lighter, and draw less power than receivers
used in the past, making them ideal for both urban geodesy and more
remote GPS sites that can only be accessed by hiking. Initial trials
using the new receivers show that they produce good quality data that
is consistent with the receivers used in past campaigns. The 2002
campaign, which is currently underway, promises to yield excellent
estimates of the velocities over the last four years.
Results from our test site at UC Berkeley's Memorial Stadium appear
in Figure 25.2. We should note that in order to
take measurements at this site, three of the stations must be set up
on staircases inside the stadium, meaning that the operator must make
sure that the tripod is perfectly level even though the legs are on a
completely uneven surface. Further, the tripod at station STAA must
carefully straddle a railing along one stairway. Despite these
difficult conditions, our calculations of the fault-parallel velocity
across the stadium from these observations is
- in very close agreement with the creep rate determined
from offset curbs along nearby Dwight Way of and
. We expect results from the rest of our
2002 campaign (currently underway) to yield equally reliable
Velocities for 1998 - 2002 of four benchmark in UC
Berkeley's Memorial Stadium, shown relative to station STAC. The
Hayward fault cuts down the middle of the stadium. These observations
show that the Hayward fault creeps at a rate of about
The locations of faults are from a recent
paleoseismological report by Geomatrix Consultants, 2001.
The more densely populated the region, the more valuable seismic
hazard assessment becomes. Unfortunately, the more densely populated
the region, the more difficult it is to collect reliable data. This
is the challenge to the urban geodesist.
The following table describes conditions that contribute to high
quality estimates of position during a GPS campaign, and why it is
challenging to achieve these conditions in the urban environment:
||If the location we survey is on unstable
ground, the movement we record may not be related to fault motion at
||Near-surface rocks in urban areas are
often artificial fill which can be less stable.
||We must position our instruments in exactly
the same location each time with less than a millimeter of tolerance
- a job that requires a fair amount of patience, skill, and
||Even with proper safety gear, try
achieving millimeter-level precision in the middle of a busy
||GPS receivers determine position by recording
broadcasts from satellites orbiting overhead. Overhead obstructions
can therefore block or distort the signal.
||Tall buildings, fences, trees, buses, etc.
|Long Duration Measurements
||We must collect data for at least 6 to 72
hours continuously. During that time, the instrument must not be
disturbed and cannot move even a millimeter.
||An operator must guard the instrument
from both theft and disturbance during the entire time. In busy
settings with passing cars, the vibrations can often move the
instrument slightly. When placed on asphalt, the instrument can sink
into the pavement on hot days.
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