Tectonic geodesy took a great leap forward when we published the first coseismic interferogram on the cover of Nature in the summer of 1993 [Massonnet et al., 1993]. Twelve years later, in 2005, interferometry on synthetic aperture radar images (INSAR) has become a widely used and widely accepted geophysical technique for measuring topography and deformation of the Earth’s lithosphere and cryosphere. Confronted with conventional models, these INSAR measurements have significantly advanced the study of earthquakes, volcanos, landslides, subsidence and glaciers.
The Global Positioning System can achieve sub-centimeter estimates of relative position with a relatively inexpensive and lightweight instrument for less than “10 kg, 10 Watts, and 10 $K”. Since the most precise solutions involve post-processing data from multiple instruments, it typically requires several days between acquisition and estimate. The constellation of satellites came into use gradually beginning in 1985 and becoming fully operational in 1992. Data from this early period are typically more difficult to analyze and may yield less precise results than more recent surveys. For reviews of geophysical applications, see Dixon , Hager et al., , and Segall and Davis . For earthquake studies, GPS networks tend to operate in one of two end-member modes: Continuous operation of permanently installed, widely-spaced antennas (CGPS), or intermittent occupation of densely-spaced benchmarks in “campaign” mode. The former offers good temporal resolution (1 measurement/30 seconds = 33 mHz) but poor spatial resolution (> 100 km between stations), while the latter offers poor temporal resolution (1 measurement/year = 32 nHz) and good spatial resolution (~10 km between stations). This trade-off between temporal and spatial resolution creates a difficult decision in the face of limited resources. Although a compromise “hybrid” strategy could rotate expensive receivers on a roughly monthly basis through several fixed monuments, this approach has yet to be deployed, apparently because it requires more manpower than permanent installations.
Figure 1. Left: Map of Izmit region showing GPS sites (4-character, named sites are continuous stations operating before and after the main shock; two additional continuous stations used in this study are located off the map at 40.61°N, 27.59°E, and 40.97°N, 27.96°E) and observed (including 95% confidence ellipses) and modeled (yellow arrows) horizontal coseismic displacements relative to a station in Ankara, Turkey (ANKR, located at 39.89°N, 32.76°E). The five segment fault model used to investigate slip distribution the Izmit earthquake epicenter and focal mechanism from the Harvard CMT Catalog (http://www.seismology.harvard.edu/CMTsearch.html), and pre-earthquake seismicity (http://quake.geo.berkeley.edu/cnss) are also shown. Light lines are mapped or inferred faults [Barka, 1997]. MV = Mudurnu Valley fault. Right: Map of observed postseismic GPS station displacements (black arrows) relative to ANKR (located at 39.89°N, 32.76°E) during the first 75 days following the earthquake. Error ellipses indicate 95% confidence intervals. Modeled station displacements (yellow arrows) were computed with the slip distributed dislocation model shown in Fig. 3C. Station names (four-character ID) indicate continuously operating sites installed within 48 hours following the main shock. Red dots indicate aftershocks of the first 30 days. The blue dotted line indicates the fault geometry used in the postseismic model inversions (note that the fault is extended to the east of the coseismic fault model to include the Duzce segment). The "beach ball" shows the location and focal mechanism of the MW 7.2, 12 November 1999, Düzce earthquake. From Reilinger et al. .