Integrated satellite interferometry in Southern California

Yehuda Bock, & Simon Williams

Published 1997, SCEC Contribution #357

Geodetic studies of crustal deformation and earthquakes in California, which began with the analysis of repeated triangulation measurements after the 1906 San Francisco earthquake, have been revolutionized by new satellite interferometric techniques in space geodesy and remote sensing. Today, space geodetic monitoring of crustal deformation at regional scales relies on a combination of continuous Global Positioning System regional arrays (CGPS), such as the Southern California Integrated GPS Network (SCIGN), and less frequent field OPS surveys. A global continuous GPS array, the International GPS Service for Geodynamics (IGS), provides precise satellite ephemerides and a well-defined terrestrial reference frame. Data from CGPS arrays can also be used to map tropospheric water vapor and ionospheric disturbances. Today there is much interest in the use of satellite-based synthetic aperture radar (SAR) for monitoring crustal deformation. Radar phase differences for satellite images acquired before and after the 1992 Landers earthquake in southern California provided, for the first time, a spectacular contour map of coseismic deformation [e.g., Massonnet et al., 1994]. However, while this technique called interferometric SAR, or INSAR for short, provides dense spatial resolution, its temporal resolution is inferior to that of CGPS, and its measurements are less accurate. In Figure 1, the Landers earthquake coseismic displacements that were estimated at four Permanent GPS Geodetic Array (PGGA) site locations [Bock et al., 1993] are compared with the interferogram determined by Massonnet et al. [1994]. The GPS analysis yields estimates of absolute coseismic displacement with millimeter accuracy more than 150 km from the surface rupture zone, but this technique could not compete with the spatial coverage provided by INSAR even if hundreds of GPS sites were used. On the other hand, interferometric fringes are restricted to a smaller region about the Landers rupture zone than the coseismic displacements observed by the PGGA. This is a consequence of having assumed zero deformation at the outer edges of the interferogram (to reduce satellite orbit error), when clearly this is not the case. Furthermore, the nearly instantaneous satellite images are more affected by atmospheric refraction errors than the continuous GPS measurements are. Developing a way to harness CGPS and INSAR to function effectively as a single geodetic instrument could provide a highly detailed and accurate picture of crustal deformation. Fortunately, the two techniques are quite complementary as summarized in Table 1. For example, one of the great difficulties in high precision GPS measurements is monumentation. How do you attach the GPS antenna to the ground in a stable way? INSAR, although it measures the least stable part of the ground (the surface), could potentially overcome this problem because of the spatial averaging inherent in this technique (each value being the average over a 30 m x 30 m square for ERS 2 imagery, for example). On the other hand, it seems doubtful, without some additional source of geodetic control, that INSAR could make stable measurements over periods of years and contribute to measurements of interseismic deformation that are currently considered to be at the extreme limit of the technique’s resolution. The southern California GPS array provides good temporal resolution (daily) for crustal deformation monitoring but, with approximately 45 sites, the array is lacking in spatial resolution (Figure 2). Expansion of the array to 250 stations [Prescott, 1996], which should be completed in 1998, will help resolve this problem. An expanded SCIGN could be used, however, in a more strategic way than presently envisaged by taking into account the strengths of both CGPS and INSAR, allowing us to expand the spatial extent of the network from the Los Angeles region to all of southern California without sacrificing SCIGNs original objectives. Furthermore, the proposed network design guarantees that any large earthquake that occurs in southern California during the lifetime of the project will be adequately sampled to observe possible postseismic deformation signals of long- wavelength due, for example, to redistribution of strain across neighboring locked faults or to viscous relaxation at depth.

Citation
Bock, Y., & Williams, S. (1997). Integrated satellite interferometry in Southern California. Eos, Transactions American Geophysical Union, 78(29), 293. doi: 10.1029/97EO00192.