CAREER: Seismic Imaging of the Earth's Mid-Mantle, the Deep Inner Core and Stress Transients

William Marsh Rice University, Houston TX

Investigators

Abstract

This proposal seeks to make significant progress in our basic understanding of deep processes that are related to the formation and evolution of the Earth's inner core and mantle heterogeneities. Another successful outcome of the proposed work could also constitute a major step towards monitoring subsurface stress transients that accompany and perhaps precede seismic activity. This proposal will promote seismology studies for undergraduates and raise public awareness and readiness for large earthquakes and tsunamis. The Earth's mantle and core are characterized by thermal and chemical heterogeneities at all length scales. Seismology provides a powerful means of exploring these heterogeneities. The PI intends to take advantage of recent developments in passive seismic observations and imaging techniques to map out seismic heterogeneities. This will enhance our understanding of deep Earth processes that are related to formation and evolution of the Earth's interior. The research will focus on the structure of the middle mantle and the innermost part of the inner core, as they are relatively less well studied than the rest of the Earth's interior. Yet, they are very important for understanding mantle convective circulation as well as the formation and evolution of the core. Travel time tomography has been the most efficient tool to image 3D velocity variations in the mantle. Many tomographic images reveal a mantle that can be in general divided into three domains. A relatively homogeneous middle mantle is sandwiched by two strong heterogeneous layers, the uppermost and lowermost mantle. Using converted waves from deep earthquakes several studies, including the PI's, have found the existence of mid-mantle reflectors associated with subduction zones. In general seismic reflectors in the mantle result from abrupt changes in composition, mineral phase, anisotropic structure, and or partial melt accumulation. Their distribution closely reflects the compositional, thermal and dynamic state of the mantle, providing critical complementary information to tomography. I plan to apply and reformulate existing imaging techniques, such as Kirchhoff migration and generalized radon transform methods to SS reflections, P to S, and S to P conversion data to improve images of Earth's middle mantle. In addition to studying the mantle structure, I will also research the nature of the innermost inner core. Compared to the top ~400 km of the inner core, the deep part of the inner core is less well known because of the inapplicability of the reference phase method commonly used in inner core studies. I have found a new reference phase, PKIIKP, which is observable at antipodal distances and can be used to study seismic structure in the center of the Earth. I propose to extend the search for PKIIKP to all the available array data. In particular I will start to analyze array data of deep earthquakes occurring in South America recorded by regional networks of the China Earthquake Administration. Another focus of my research involves understanding the time-varying stress field at seismogenic depths. This is perhaps the single most crucial parameter for understanding the earthquake triggering process. Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. It is well known from laboratory experiments that seismic velocities vary with the level of the applied stress. In principle, this dependence constitutes a stress meter, provided that the induced velocity changes can be measured precisely and continuously. In collaborating with scientists from Carnegie Institution of Washington (CIW) and Lawrence Berkeley National Laboratory (LBNL), I have conducted several continuous active source cross-well experiments to measure in situ seismic velocity changes along fixed baselines at Earth's surface and seismogenic depths. In either case we have demonstrated that stress changes such as variations in barometric pressure are detectable. Especially at the SAFOD drill site (San Andreas Fault Observatory at Depth) we observed co-seismic velocity changes from two earthquakes and preseismic velocity changes that might be related to pre-rupture dilatancy. In order to verify these observations, I propose to conduct a series of controlled source experiments at SAFOD and other segments of the San Andreas Fault. I also plan to develop time-lapse seismic imaging (4D) techniques for the detection of seismic and magmatic crustal stress changes. In terms of education, I propose four major activities: (1) utilize an on-campus seismograph to promote students' appreciation to seismology and Earth science; (2) promote seismology in local community colleges and displaying modern seismograph at local science museum to raise public awareness and readiness for large earthquakes and tsunamis; (3) develop a new introductory geophysics course "An Introduction of Plate tectonics, Earthquakes and Volcanoes" for major and non-major undergraduates, (4) provide research activities for undergraduate students.