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.
The Earth’s inner core was discovered by Inge Lehmann in 1936. Although it constitutes only ~0.7% of the Earth’s volume, the inner core plays a very important role in maintaining the magnetic field, which is believed to be generated by a convective dynamo operating in the Earth’s fluid outer core. The convection is thought to be driven by both thermal and compositional buoyancy sources provided by the growth of the iron-rich inner core, as latent heat and light elements are released when iron solidifies to the inner core as the Earth slowly cools. Although it is well known that the entire Earth’s core was formed through a process known as density stratification, the formation and evolution of the inner core, however, is poorly understood. For example, it is not known whether the inner core experienced continuous or episodic growth. Studying depth varying seismic structure of the inner core thus holds the key to understand the formation and evolution of the inner core. We use the difference in travel times between seismic waves reflected at the underside of the inner core boundary and those traversing the inner core to constrain the seismic anisotropy. We calculated traveltime residuals for waves generated by two deep earthquakes that occurred in Indonesia and Argentina respectively, recorded by seismic arrays in Venezuela and China. The travel-time residuals are systematically larger, by about 1.8 s, for waves that travel roughly along the equatorial plane of the inner core (Indonesia–Venezuela) than for those travelling in a direction at an angle of ~28° to the equatorial plane (from Argentina to China). The difference in travel times is arguably most sensitive to the structure near the center of the Earth, and thus provides evidence for deep layering within the inner core (Figure 1). Earthquakes are caused by sudden fails along faults, either because of the build-up of stress or because of the weakening of the fault. Fault strength is a fundamental property of seismogenic zones, and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. Although changes in fault strength have been suggested to explain various phenomena, such as the remote triggering of seismicity, there has been no means of actually monitoring this important property in situ. Using seismic waveform data of repeating sources, we found that temporal changes in seismic scattering properties along the San Andreas Fault provide a valuable means of monitoring fault strength and subsurface stress. We have identified two occasions where long-term changes in fault strength have been most probably induced remotely by large seismic events, namely the 2004 magnitude (M) 9.1 Sumatra–Andaman earthquake and the earlier 1992 M7.3 Landers earthquake. In both cases, the change possessed two manifestations: temporal variations in the properties of seismic scatterers—probably reflecting the stress-induced migration of fluids—and systematic temporal variations in the characteristics of repeating-earthquake sequences that are most consistent with changes in fault strength. In the case of the 1992 Landers earthquake, a period of reduced strength probably triggered the 1993 Parkfield aseismic transient as well as the accompanying cluster of four M.4 earthquakes at Parkfield. The fault-strength changes produced by the distant 2004 Sumatra–Andaman earthquake are especially important, as they suggest that the very largest earthquakes may have a global influence on the strength of the Earth’s fault systems. As such a perturbation would bring many fault zones closer to failure, it should lead to temporal clustering of global seismicity. This hypothesis seems to be supported by the unusually high number of M≥8 earthquakes occurring in the few years following the 2004 Sumatra–Andaman earthquake.
