Relative Earthquake locations Using Surface Waves in Continental Regions

Accurate and precise hypocenter locations are an essential observation needed for seismic event characterization and analysis. Traditional location estimation methods that utilize seismic arrival times provide relatively imprecise results in remote regions, where there is a lack of numerous seismic stations near the events. Recent work has made great progress in improving the relative hypocentral precision using arrival times measured using cross correlations of body and surface waveforms. In addition, InSAR data can provide accurate centroid locations of select shallow seismic events in relatively limited numbers. In this work we use intermediate-period (30-80s) regional and teleseismic Rayleigh and Love waves to estimate relative locations of several hundred moderately-sized seismic events in regions around Iran’s Zagros Mountains that occurred between 1990 and 2016. The approach has been used to effectively relocate seismic events along remote oceanic transform faults. Extending this method to continental regions provides the opportunity to compare the surface-wave based relative locations with a suite of InSAR centroid location estimates. In comparison with oceanic applications, use of surface-wave time shifts in the Zagros region presents new challenges, including more variation in faulting orientation, depth, and differences in intermediate-period dispersion caused by the structure of the continental crust. Simple assumptions about similarities in source geometry and inter-earthquake wave slowness (inverse of speed) are less applicable. We explore these challenges using earthquakes in regions around the Zagros Mountains of Iran, with an emphasis on the Fin and Qeshm Island earthquake sequences. Our ultimate goal is to adapt this approach to work with continental regions, and if possible to account for variations in source depth and faulting geometry to refine the relative locations and better characterize this region of high seismic activity and hazard.