GROUND TRUTH LOCATION USING A SYNERGY BETWEEN INSAR AND SEISMIC METHODS

The objective of this study is to utilize Synthetic Aperture Radar Interferomtery (InSAR) to estimate accurate hypocenter locations for shallow and moderate-sized earthquakes in the Middle East and Eastern Asia. We have acquired different levels of Digital Terrain Elevation Data (DTED) from National Imagery and Mapping Agency for this region to improve the InSAR processing. Once surface deformation is identified in the InSAR signal and associated with an earthquake, the hypocenter location is estimated from a combination of geodetic and seismic modeling to provide accurate locations for regional velocity model calibration. One primary difficulty in the analysis of InSAR is the uncertainty in hypocenter location with the increasing earthquake size because of the increase of fault area with seismic moment. Another difficulty is the trade-off between fault area and fault slip and between focal mechanism and location because InSAR only measures the 3-components of surface deformation along a line of sight (e.g., Lohman et al., 2002). Saikia et al. (2002) combined InSAR and seismic data modeling to estimate the location of the 1999/04/30 Mw 4.8 earthquake in Southern Iran. The focal depth is 4 km, which was estimated from modeling InSAR data. When we compared the Lg-waves at station RAYN between this event and a nearby 1998/11/13 earthquake (depth=9 km), the Lg-wave characteristics were similar indicating that both are shallow depth earthquakes. We performed a gridsearch and inversion of the regional long-period waveforms and high-frequency P-waves to reaffirm this earthquake’s shallow depth and help constrain the focal mechanism estimated from InSAR modeling. Using the accurate hypocenter location estimated from InSAR and seismic waveform modeling (probably now GT1), we obtained empirical SSSCs at regional stations between (0-30°) and relocated 11 other earthquakes (4.4 < mb < 5.7) that occurred near this event. The SSSC significantly reduced the area of the error ellipse for these earthquakes. In this study, we used InSAR data to examine additional events in Southern Iran. We examined the 1998/10/18 (mb 4.2), 1997/05/05 (mb 4.8), 1997/09/18 (mb 4.7), and 1999/04/28 (mb 4.8) earthquakes. The satellite radar data available for these events spanned a time interval that included several events making the association of the observed deformation field with each earthquake difficult. For example, radar data for events of 1997/05/05 and 1997/09/18 were obtained from track 478 /frame 3069, which spanned a time interval between April 1996 and April 1999. During this time interval, several events occurred within 50-80 km of the identified SAR signal locations. However, based on InSAR analysis, most of these events did not contribute to the radar signal as they were too small (mb<4.5). Broadband waveforms from station RAYN were compiled for all events occurring during each time interval of the radar data and investigated for their high-frequency Lg-wave characteristics. We also modeled the teleseismic and regional P-waves and long-period waveforms. The following are a list of conclusions: 1). processing and modeling of InSAR data identifies four shallow earthquakes, 2). within the limits of resolution, seismic depths determined from the modeling of the long-period seismic waveform and far-regional broadband P waveforms are consistent with the depths determined by modeling the InSAR ground deformation, 3). the ISC depths are often poorly estimated, and 4). locations of small seismic events improve, when the SSSCs estimated using the location of events determined from the synergy of InSAR and seismic are applied during location. 25th Seismic Research Review Nuclear Explosion Monitoring: Building the Knowledge Base