Improved Ground Truth in Southern Asia Using In-Country Data, Analyst Waveform Review, and Advanced Algorithms

This research has the goal of developing in-country data sets that can be used to improve ground-based monitoring capabilities in southern Asia, in particular the region bounded by 20-44 deg N and 41-67 deg E, by providing information needed to develop and test more accurate travel-time models for seismic phases that propagate in the crust and upper mantle. We have also incorporated phase picks from an experienced analyst who reviewed waveforms of particular interest for specific events. These in-country arrival times and analyst-reviewed picks have been associated with known earthquakes reported by international agencies, combined with existing bulletin readings, and relocated using the Engdahl et al. 1998 EHB methodology. Using in-country data, we have formed new events, mostly at lower magnitudes that were not previously included in standard global earthquake catalogs. This has resulted in a catalog of earthquakes in the region for the period 1923-2008 for events larger than about magnitude 2.5. Catalog events larger than about magnitude 4.0 have been highly reviewed. Events at lower magnitudes have been relocated with a standard procedure similar to the EHB procedure, but not all have been systematically reviewed. We have performed hypocentroidal decomposition HDC calibration analyses on 27 earthquake clusters, containing 989 events, in the region. Of these, three clusters could not be calibrated at all. Twenty-two clusters contain at least one event with a calibrated location that meets GT5 criteria, a total of 549 GT5 events. We present a summary of the results of these calibration studies in the form of absolute travel-time information derived from the calibrated clusters, showing distance-dependence of travel times of different phases from different source regions. We also present summaries of empirical reading error determinations and of travel-time variability for different phases.