A Global Catalog of Calibrated Earthquake Locations

We present a catalog of calibrated earthquake locations, i.e., hypocenters with minimal bias from unknown Earth structure and for which realistic estimates of absolute location uncertainty are given. Calibrated earthquake locations are determined with a multiple event relocation method (Jordan and Sverdrup, 1981) that has been specialized for this purpose. More than 100 globally-distributed earthquake sequences have been relocated and the analysis of additional clusters or sequences is ongoing. Notable recent earthquake mainshock-aftershock sequences that have been calibrated include 2003 M6.6 Bam, Iran; 2008 M6.0 Wells, Nevada; 2010 M8.8 Maule, Chile; 2011 M7.1 Van, Turkey; 2011 M5.8 Mineral, Virginia; 2011 M5.6 Prague, Oklahoma; 2014 M8.2 Iquique, Chile; 2015 M8.3 Illapel, Chile; and 2015 M7.8 Gorkha, Nepal. Epicentral uncertainties in these studies are typically less than 5 km and, in some cases, better than 1 km (e.g., induced seismicity sequences in the central and eastern United States). Focal depths can usually be determined with uncertainty less than 5 km; otherwise focal depths are assigned from independent observations (e.g., from waveform modeling of moment tensors). Origin times are also calibrated, with typical uncertainties of a few tenths of a second. Location calibration at this level of accuracy requires the use of near-source arrival time readings, an appropriate model for local travel times, weighting of data by empirically-determined uncertainties, and careful removal of outlier readings. These relocation studies are of significant value for the study of individual earthquake sequences, but the broader purpose of this catalog is to provide a set of globally distributed "benchmark" locations that can be used as prior constraints in the development of new regional, national, and global earthquake catalogs, or for validation of advanced Earth models and location techniques. Locations and associated phase data, including metadata descriptions of velocity models and relocation procedures are available through ComCAT (http://earthquake.usgs.gov/earthquakes/search/) using standard USGS search methods.