D., M. Rossi, et al. (1993). "The displacement field of the Landers
earthquake mapped by radar interferometry." Nature 364: 138-142. PDF.
Geodetic data, obtained by ground- or space-based techniques, can be used to infer the distribution of slip on a fault that has ruptured in an earthquake. Although most geodetic techniques require a surveyed network to be in place before the earthquake satellite images, when collected at regular intervals, can capture co-seismic displacements without advance knowledge of the earthquake’s location. Synthetic aperture radar (SAR) interferometry, first introduced in 1974 for topographic mapping can also be used to detect changes in the ground surface, by removing the signal from the topography. Here we use SAR interferometry to caputure the movements produced by the 1992 earthquake in Landers, California. We construct an interferogram by combining topographic information with SAR images obtained by the ERS-1 satellite before and after the earthquake. The observed changes in range from the ground surface to the satellite agree well with the slip measured in the field, with the displacements measured by surveying, and with the results of an elastic dislocation model. As a geodetic tool, the SAR interferogram provides a denser spatial sampling (100 m per pixel) than surveying methods and a better precisiοn (~3 cm) than previous space imaging techniques.
Massonnet, D., K. L. Feigl, et al. (1994). "Radar
interferometric mapping of deformation in the year after the Landers
earthquake." Nature 369: 227–230. PDF
Although the 1992 Landers, California earthquake sequence occurred in an area well sampled by geodetic networks, the postseismic deformation in the months following the earthquake has been measured at only 15 geodetic stations. Another shortcoming in the geodetic coverage occurs west of the primary rupture, where the existing geodetic observations suggest, but cannot resolve, sympathetic slip on secondary faults. Such measurements, which are needed to place the Landers earthquake sequence in the context of a recurring seismic cycle in California, can be obtained with the dense spatial coverage provided by satellite radar interferometry. Here we present radar maps of the surface deformation field which reveal features that would otherwise have been poorly sampled, particularly if the earthquake had occurred in a less accessible area. We see triggered slip at the level of several centimeters as far as 100 km from the primary rupture and can resolve the geodetic signal of at least one small (magnitude ~5) aftershock. The amount of postseismic deformation following the main shock is less than a decimeter at the surface and is consistent with an exponential decay time of several months.
Feigl, K. L. and C. H. Thurber (2009). "A method for
modelling radar interferograms without phase unwrapping: application to
the M 5 Fawnskin, California earthquake of 1992 December 4."
Geophysical Journal International 176(2): 491-504. PDF
Interferometric analysis of synthetic aperture radar images (InSAR) measures the phase shifts between two images acquired at two distinct times. These ambiguous 'wrapped' phase values range from -1/2 to +1/2 cycles. The standard approach interprets the phase values in terms of the change in distance between the ground and the radar instrument by resolving the integer ambiguities in a process known as 'unwrapping'. To avoid unwrapping, we have developed, validated and applied a new method for modelling the wrapped phase data directly. The method defines a cost function in terms of wrapped phase to measure the misfit between the observed and modelled values of phase. By minimizing the cost function with a simulated annealing algorithm, the method estimates parameters in a non-linear model. Since the wrapped phase residuals are compatible with a von Mises distribution, several parametric statistical tests can be used to evaluate the fit of the model to the data. The method, named General Inversion for Phase Technique (GIPhT), can handle noisy, wrapped phase data. Applying GIPhT to two interferograms in the area of Fawnskin, California, we estimate a set of model parameters describing a magnitude 5 aftershock of the 1992 Landers earthquake. The resulting simulation fits the data well. The phase final residuals have a circular mean deviation less than 0.15 cycles per datum. Sampling the final residuals, we find the circular standard deviation of a phase measurement to be approximately 0.2 cycle, corresponding to 6 mm in range.