Southern California IntegratedGPS Network (SCIGN)

Kenneth W. Hudnut

U. S. Geological Survey

U.S. – Japan Natural Resources Panel on Earthquake ResearchU. S. Geological Survey, Menlo Park - November 14, 2000

Continuous GPS

• Best tool ever devised for highly accurate, automated, constant monitoring of crustal strain for– long baselines– absolute ref. frame– displacement field– high precision

• SCIGN & other PBO elements require sub-millimeter velocities on the plate boundary scale in order to answer the scientific questions

Faults & Earthquakes

• San Andreas fault zone– North American and Pacific

plate relative motions of 56 mm/yr in a right-lateral sense

• Eastern Californiashear zone– Accomodation of right-

lateral motion inboard of Sierra Nevada block

– Estimated rates of some 8-12 mm/yr (geological & space geodetic)

– Easier to go through than the Big Bend?

SCEC crustal motion map

• Combined EDM,

VLBI, survey-mode

and continuous GPS

rigorously

• Released as a

SCEC product

• Set the bar very

high for the SCIGN

project

Plate tectonic motions• For the past 5

million years, this motion has been very steady at about 5 cm/yr

• From long-base laser strainmeter and geodolite data, steady rates of motion are seen

• Will higher resolution instruments see temporal variation in strain rate? tectonic reconstruction and movie by Tanya Atwater, UCSB

The major objectives of the SCIGN array are:

* To provide regional coverage for estimating earthquake potential throughout Southern California

To identify active blind thrust faults and test models of compressional tectonics in the Los Angeles region

To measure local variations in strain rate that might reveal the mechanical properties of earthquake faults

In the event of an earthquake, to measure permanent crustal deformation not detectable by seismographs, as well as the response of major faults to the regional change in strain

SCIGN project installation: 250 sites by end of 2000

SCIGN station installation

• Each of 5 legs is drilled to 10 meters

• Lowermost 6 meters is anchored to earth by concrete grout

• Uppermost 4 meters is isolated from soil by foam

• Stainless for longevitymovie by John Galetzka, USGS

Analysis comparisonsby King, Hurst, van Domselaar & Langbein

• Time series reprocessed by JPL and SIO; similar -– reference frame

implementation

– processing strategy

• Mean differences for each of 6100+ baselines

• Line length proportional differences at <9 ppb

Hector Mine (Mw7.1)Photo by Paul ‘Kip’ Otis-Diehl,USMC, 29 Palms

Helicopter support by OES and National Guard

Hector Mine eq.: modelled displacement field

Post-seismic deployment

• GPS for precise absolute position changes

• GPS data from

these instruments will also show us afterslip and other post-seismic phenomena

Short-braced monument• Requires

bedrock• Remote

installation is feasible

• Half as expensive as drilled-braced (but not as stable)

• Useful for special post-earthquake network deployments

Post-seismic deformation• Stations near the

earthquake fault continue to move after the earthquake– Less than 20 mm

motion recorded, so we required extremely high precision data

– Too much motion to be explained by aftershocks

– Requires a deep source in the lower crust

– Large scale relaxation phenomenon

– May explain fault interaction between large earthquakes

Inboard shear strain transfer (preliminary & speculative)

• Savage et al. (1993) and Johnson et al. (1994) showed NW-SE pull apart of 8 mm/a in USGS geodolite data prior to 1992 Landers sequence [orange]

• We see similar pattern after Hector Mine, farther to northeast [red]

GPS & telemetry/networking

• Market for GPS boards is driven by Moore’s law (like PC’s) toward faster/better/cheaper, miniaturization, etc.

• Spread spectrum radio and satellite telemetry leading to high bandwidth IP field networking (e.g., TDMA)

• Allows higher sampling rates and more affordable real-time telemetry

SCIGN data acquisition and processing system

New methods: high-resolution topographic mapping and digital photography

• Laser scanning using an airborne platform requires high sampling-rate GPS data during flight to control aircraft position and attitude

• SCIGN stations were operated at 1 and 2 sample per second rates via the radio network

Existing GPS Networks in North America

• International GPS Service (IGS)• So. Calif. Integrated GPS Network

(SCIGN)• Bay Area Regional Deformation

(BARD)• Basin and Range GPS Network

(BARGEN)• Pacific Northwest GPS Array (PANGA)• Eastern Basin and Range &

Yellowstone (EBRY)• Contin. Operating Reference Stations

(CORS)• SuomiNet, FSL, INEGI, WCDA, etc.

Conclusions• Networks of continuously operating GPS stations

in the U. S. and Japan give us a higher resolution method to search for temporal variations in strain with renewed hope of learning about earthquake related processes

• Geodesy is now feasible in a network mode, similar to seismology – it is time to incorporate both, side by side, into modern earthquake monitoring networks – extending our reach to truly broad-band observational capabilities so that we can detect and study a wider range of seismic and aseismic phenomena