Collaborative Research: High-Resolution Study of Subduction Zone Seismicity and Structure

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Abstract Subduction zones are one of the most important components of the Earth's plate tectonic system. This is reflected by the existence of two major international science initiatives focused on subduction zones: the Subduction Factory (SubFac) and the Seismogenic Zone Experiment (SEIZE), both part of the MARGINS initiative. The SubFac initiative has motivated substantial mineralogical and petrologic work on stability fields of candidate slab constituents (especially hydrated phases) and their relation to intermediate-depth seismicity. One working hypothesis is that the intermediate-depth earthquakes are related to metamorphic dehydration reactions in the subducting slab. Tests of this hypothesis are imperfect for four main reasons - uncertainties in slab thermal structure, slab composition, slab seismic velocity structure, and earthquake locations. The latter impacts earthquake locations and also affects the association between seismic velocities in slab models and candidate mineralogies. This project involves a multi-faceted seismological investigation of subduction zones around the world. The principal project goals are refining knowledge of the distribution of seismic activity within down-going slabs and improving seismic models of their structure. Analysis methods that are being applied in the project include (1) improved waveform alignment techniques, (2) high-precision teleseismic and local earthquake location, (3) associated focal mechanism determination, and (4) double-difference (DD) seismic tomography. Methods 1, 2 and 3 are applied to a broad suite of subduction zones, especially where the presence or absence of a double seismic zone is in question. Methods 1 and 4 are applied to a selected set of subduction zones where adequate local earthquake data are available - Northern Chile, Alaska, Costa Rica, and Taiwan. Previous work indicates that waveform cross-correlation (WCC) time delays verified with the bispectrum waveform alignment method provide better relocation results than those selected with the standard threshold criterion. This improvement is expected for two main reasons. First, unreliable WCC time delay data can be removed using bispectrum checking. Second, the bispectrum verification process can provide more time delay measurements for two close earthquakes and thus more control over their relative positions. Based on previous work with both teleseismic and local events, close to an order of magnitude reduction in relative location scatter can be expected. Refined waveform alignment techniques and a new DD seismic tomography method are applied to local datasets from several subduction zones to sharpen images of seismic velocity structure. All of the proposed target areas have been the subject of one or more previous local earthquake tomography studies, and thus have datasets suitable for DD tomography. Based on initial results applying the DD tomography method to catalog data from northern Honshu, the production of subduction zone seismic images of unprecedented clarity can be anticipated. This project will contribute in a fundamental way to the understanding of subduction zones and related processes. Subduction zones are a fundamental feature of the plate tectonic engine that drives the evolution of the Earth. The structure, state of stress, and evolution of down-going slabs are topics that have received substantial scientific attention yet remain relatively poorly understood. This project will provide improved information on the distribution of seismic activity within down-going slabs along with detailed models of the seismic structure of the slab and overlying mantle wedge, which are critical constraints for geodynamic and petrologic models. In particular, there are several competing hypotheses about the nature of intermediate-depth earthquakes. Through a detailed study of several different subduction zones, seismic evidence for or against these hypotheses will be obtained. This project will take advantage of the new NSF-supported Center for the Integration of Research Teaching and Learning (CIRTL) at UW-Madison in preparing web-based outreach materials based on the research results. Software and data will be shared with other scientists, particularly by providing training on the use of DD tomography.