[GPS Fault Slip Sensors]{.smallcaps}

## K. W. Hudnut, G. J. Anderson, A. Aspiotes, N. E. King, R. A. Moffitt, K. F. Stark

U.S. Geological Survey - SCIGN

525 South Wilson Avenue, Pasadena, California 91106 USA

(e-mail: hudnut@usgs.gov)

## [Introduction]{.smallcaps}

GPS can be used in real-time for potentially important emergency
response applications as well as many scientific research uses, although
scientific uses typically do not require real-time results. Here, we
present a new concept to include GPS fault slip sensors into earthquake
early warning systems, as an augmentation to seismic inertial sensors.
We have successfully tested this GPS fault slip sensor in a prototype
deployment on the San Andreas fault as of 15 November 2002.

Of course, the potential for issuing a warning of imminent strong
shaking was initially proposed by J. D. Cooper in 1868. The idea has
been further developed more recently (e.g., *Heaton*, 1985; *Ellsworth &
Heaton*, 1994), and seismic systems for this have now been demonstrated
and implemented, for example, the Urgent Earthquake Detection and Alarm
System (UrEDAS) system in Japan (*Nakamura and Tucker*, 1988). Some
limitations in the robustness and trustworthiness of the systems, as
well as costs, may explain why these arrays are not more widespread. To
our knowledge, before now, only seismic sensors have been previously
considered in designing solutions to this problem. We will describe, and
also show test results from, the first deployment of a GPS-based
augmentation to earthquake alert systems. GPS can be used to detect
surface slip on a fault in real-time. Thus, adding GPS can improve
response time and robustness of the system, and can decrease the chance
that a small event (without surface rupture) is somehow interpreted by
the seismic sensors as being a large event.

![](media/image1.jpeg){width="5.875196850393701in"
height="4.493700787401575in"}Our system will robustly detect slip on a
fault, in real-time, if slip exceeds a few centimeters. Where two SCIGN
stations straddle the San Andreas fault near Gorman, the fault is not
known to creep, so detected displacement would be a clear indication
that a large event has begun. Slip at the ground surface would likely
occur within seconds after a large event has begun, conceivably well
before the seismic array data alone can yet be relied upon to assess
whether the event is large or small. With this, and several other
specific cases where we are deploying this instrumentation, we feel that
GPS data acquired and processed in real-time can significantly add to
earthquake alert systems. Along with real-time use of GPS for monitoring
large engineered structures, the potential for providing earthquake
information rapidly after future earthquakes, from a variety of
real-time GPS systems, is considerable. The GPS slip sensor concept
relies upon high sampling-rate data, acquired and processed in
real-time. Preferably, the data would be from a braced array that is
located close to, and spanning, the main strands of an active fault.
Such arrays are not likely to be included in projects that are intended
to study longer wavelength deformation of the crust, so may require
special \'zipper\' deployments along hazardous active faults.
Observations from such near-field GPS and broad-band seismic sensor
arrays, associated with future large earthquakes, could well provide
unique and vitally important data for gaining insight into aspects of
the rupture process and earthquake source physics in general.

[Methods]{.smallcaps}

Normally, we telemeter data from SCIGN stations once every 4 hours by
dial-up telephone line service, or by dial-up through a spread-spectrum
radio network. We have upgraded SCIGN standard telephone line telemetry
to two stations on either side of the San Andreas fault near Gorman,
California. In this case, we made the upgrade to real-time telemetry by
using Frame Relay, through device servers to convert the serial protocol
data to TCP/IP protocol before streaming the data back through the
TriNet seismic telemetry system. Frame Relay service is generally
available outside of the Los Angeles metropolitan region, for remote
sites, for approximately \$200 (US) per month. Figure 2 shows how we do
an upgrade from our standard telephone line telemetry system to
incorporate and make use of the digital service line (DSL). DSL is
generally available in the Los Angeles metropolitan region for
approximately \$50 (US) per month.

Once the data have been received at the USGS in Pasadena, our main
office, we split the real-time data stream and provide these data to
multiple computers. We then are able to process the data in real-time,
on each computer separately, using one of the several commercially
available software packages. Currently two of these are being evaluated
as part of no-cost trial demonstration agreement with each of the
software vendors. In the future we will also concurrently be testing
three or four commercially available real-time GPS processing codes. We
then compare the results of the different systems. Our intent is to take
real-time ascii output of station position changes from each commercial
software package, and then combine those results to produce a single
real-time output for use in providing earthquake response information to
the public and to other government agencies for their use in emergency
operations, etc. This system is only working partly at this time, but
the concept is illustrated in Figure 3. At the APEC Symposium we will
report on the current status and progress on ongoing development of this
system.

![](media/image2.jpeg){width="6.05625in" height="4.835416666666666in"}

![](media/image3.jpeg){width="6.25in" height="4.752777777777778in"}

[References]{.smallcaps}

Ellsworth, William L., and Thomas H. Heaton (April, 1994) Real-Time
Analysis of Earthquakes: Early Warning Systems and Rapid Damage
Assessment, *Sensors*.

Heaton, T. H. (1985) A Model for a Seismic Computerized Alert Network,
*Science* 228, 987-990.

Nakamura, Y., and B. E. Tucker (1988) Japan\'s Earthquake Warning
System: Should it Be Imported To California?, *California Geology*, pp.
33-40.