Comprehensive high-precision relocation of global seismicty
DESCRIPTION: Hundreds of thousands of earthquakes are detected and recorded each year by regional and global seismic networks. Seismic network operators routinely analyze their seismograms to estimate various parameters, including time, location, and magnitude of each new earthquake, adding to a rapidly growing archive of digital seismic data. Catalogs of earthquake locations are a fundamental source of data in seismology, and in the Earth sciences in general. Yet these individual locations have had notoriously low spatial resolution, which limits their potential to address fundamental questions concerning the structure and composition of the Earth?s interior, the nature of earthquakes, and the seismic hazards they impose on our society and the built environment. The main goal of this project is to significantly improve the location precision of millions of earthquakes on a global scale by applying higher-order earthquake location methods that take advantage of the increasing quantity and quality of seismic archives and the availability of affordable computing power. This project will focus on using the wealth of high-precision data to better understand, in a comparative way, the seismogenic structure and processes that control subduction zones worldwide. Results from this project are expected to find many applications in the Earth Sciences, and to have social impact in the areas of natural hazards and seismic verification of the Comprehensive Nuclear-Test Ban Treaty (CTBT). The tremendous growth in global seismic archives over the last several decades, the continuous fall in computing costs, and advances in earthquake relocation methods provide a unique opportunity to substantially improve the spatial resolution of standard global earthquake catalogs. The locations of hypocenters in these catalogs, typically estimated one event at a time from phase arrival times observed at global seismic stations, have notoriously low accuracy that hamper studies in a wide range of research areas, not only in Seismology, but in the Geosciences in general. This continuing project builds on previous work that developed high-efficiency waveform cross-correlation and double-difference (DD) algorithms to relocate more than 3 million earthquakes listed in the combined bulletins of the International Seismological Centre (ISC) and NEIC?s Earthquake Data Report, spanning the years 1964-2012. These tools increase the precision of the earthquake locations by harnessing new procedures that can cope with the computational load of this massive-scale undertaking. Cross-correlation based phase delay times are measured on billions of pairs of waveforms, and efficient teleseismic double-difference algorithms invert these data in combination with reported phase arrival times. Initial results using this approach already reveal characteristic seismicity patterns in time and space, and provide new data that help gain insight into seismogenic processes at unprecedented resolution. This project will focus on the fine-scale structure of Wadati-Benioff zone seismicity at subduction zones and the nature of narrow double-seismic zones, and analyze the relocated seismicity associated with recent great subduction earthquakes to better understand their structural and kinematic behavior. A global DD catalog will likely find many applications in the Earth Sciences, and may help improve our understanding of the physical processes controlling earthquakes, our ability to image the structure and composition of the Earth?s interior, and our capability to estimate the hazards imposed by large earthquakes.