The increasing availability of large numbers of high-quality digital records from the global seismic networks has made possible a variety of new ways to study earthquakes and deep Earth structure. By analyzing hundreds or thousands of seismograms, it is often possible to resolve new features in the data, or to perform more comprehensive analyses of problems that were previously addressed on smaller scales. This project will continue analyses of global seismic data at U.C. San Diego to examine a variety of geophysical issues. These include: (1) Study of seismic discontinuities in the upper mantle using reflected and converted seismic phases. Resolving details of mantle discontinuity properties is important, both for modeling of mantle composition and for understanding the effect that the discontinuities can have on mantle convection. (2) Imaging earthquake rupture using back-projection methods. These methods have the potential to provide near-real-time images of the rupture extent of large earthquakes, which will help in planning disaster relief operations. (3) Analyses of earthquake spectra to resolve directivity and stress drop. This will help determine whether large, damaging earthquakes are simply "scaled up" versions of smaller earthquakes, in which case strong ground motions could be predicted for future large quakes by studying records of the much more numerous smaller earthquakes that are detected instrumentally. (4) Study and modeling of high-frequency waves scattered from small-scale mantle and core structure. This will provide information on compositional variations in the mantle at much finer scales than are possible from other seismic methods and will provide constraints on geochemical models of the mantle. (5) Development and application of waveform cross-correlation and cluster analysis techniques to long-period waveforms. This provides a practical way to process the large volumes of seismic data that are currently available and will lead to improved models of three-dimensional seismic velocity variations. (6) Observing and modeling lateral variations of seismic attenuation in the mantle, which will provide valuable constraints on mantle temperatures and chemistry. This project supports the educational program at U.C. San Diego by providing funds for graduate and postdoctoral students. This research will lead to improved models of Earth structure and earthquake rupture processes, which will be of interest to the tectonics, geodynamics and mineral physics communities. Results will be widely disseminated through publications, conference presentations, and material provided to education and outreach programs.
NSF grant EAR-0849322 was used to further develop and apply numerical methods to model seismic wave propagation on all spatial scales, from the entire globe to hydrocarbon fields, and to develop and implement the "adjoint" method to address seismological inverse problems, e.g., to "image" Earth's interior or the earthquake rupture process. We have recently successfully performed our first "adjoint tomography" of southern California. This improved model is actively being used for assessments of seismic hazard in this earthquake-prone part of the US. The grant has resulted in more than 20 publications to date, and has partially supported the education and training of six graduate students (Min Chen, now a postdoc at MIT; Vala Hjorleifsdottir, now an assistant professor at UNAM; Qinya Liu, now an assistant professor at the University of Toronto; Carl Tape, now an assistant professor at the University of Alaska, Fairbanks; Yang Luo, 4th year Princeton graduate student; Hejun Zhu, 3rd year Princeton graduate student) and three postdoctoral fellows (Alessia Maggi, now a Physicien Adjoint at the University of Strasbourg; Tarje Nissen-Meyer, now an Oberassistent at ETH, Zurich; Daniel Peter, Princeton University). In the context of education and outreach, we have developed a near real-time system for the simulation of global earthquakes. Prompted by a trigger from the Global Centroid Moment Tensor Project (http://globalCMT.org), the system automatically calculates earthquake animations and synthetic seismograms at more than 1800 seismographic stations, which are made available via the internet and IRIS (http://iris.edu).
