ABSTRACT

About 75% of all earthquakes occur in the upper 60 km of the Earth. The remaining events, known as intermediate and deep earthquakes, take place over a depth range of 60 to 700 km and are focused within lithospheric oceanic slabs descending into the mantle at convergent plate boundaries. The distribution of these events has provided unique and direct evidence of mantle convection, the driving force behind plate tectonics that shapes the surface of the Earth. Though the information derived from deep earthquakes have been essential for understanding the Earth's dynamic system as a whole, the physical mechanisms causing these events are still a mystery. In contrast to their shallow counterparts, deep earthquakes occur at depths where high temperatures and pressures should inhibit seismic brittle failure. Several mechanisms have been proposed to explain their occurrence, though differentiating between them has been difficult partly due to resolution limitations in both seismic velocity models, which are critical in constraining the geometry and internal physical properties of subducting slabs, and earthquake source models, which characterize the spatial and temporal evolution of source regions during seismic failure. The goal of this study is to improve seismic velocity structure and earthquake source imaging resolution in the Japan, Kuril, and Izu-Bonin regions, which host a significant number of deep earthquakes. The improved seismic images will clarify the spatial relationships between earthquake source properties and the internal structure of subducting slabs. These relationships will provide a new set of fundamental constraints for evaluating the viability of proposed deep earthquake source mechanisms. Through this project, undergraduate students will have opportunities to work on the proposed research activities, K-12 outreach events will be organized to encourage girls to pursue STEM field careers, and public lectures will be given on the work to adults who participate in lifelong learning programs.

It is still unclear where deep-focus earthquakes nucleate and propagate within a slab, and as a result, details of the Earth's dynamic inner workings in the lower half of the upper mantle are still missing. Addressing this issue critically depends on accurate high-resolution images of both the slab internal structure and deep-focus earthquake source properties. Previous seismic image resolution and accuracy at depths below 300 km were limited from sparse data coverage and theoretical approximations used in traditional seismic tomography. Classical ray-theory based tomography images indicate that deep-focus hypocenters coincide with the highest wavespeed anomalies within the slab, traditionally viewed as the slab's cold core, where phase transformational faulting, involving the breakdown of metastable olivine, is considered as a likely cause of deep-focus earthquakes. However, with an unprecedented seismic data set in East Asia aided by the advanced full waveform tomography technique, the new images of the Japan, Kuril, and Izu-Bonin slabs (EARA2014) show that deep-focus earthquakes consistently occur near the top of high wavespeed regions, possibly indicating that these events occur near the top of the subducting slab. This intriguing observation motivated this proposal to further explore and resolve the fine-scale wavespeed variations and earthquake source properties in these slabs using high frequency full waveform information. The central hypothesis of this project is that deep-focus earthquakes nucleate and propagate along the top of the slab, where oceanic crust and a hydrous serpentine layer are located, i.e. away from the slab's cold core. In order to test this hypothesis, the following three specific goals will be pursued: (1) obtain a slab structural model with improved spatial resolution from the existing model EARA2014 using higher frequency seismic waveforms; (2) relocate deep-focus hypocenters and image deep-focus earthquake rupture propagation with the aid of the new tomographic slab model; (3) establish spatial relationships between the slab internal structure and deep-focus earthquake locations and rupture properties. If the central hypothesis of this project is supported by the proposed work, then there will be a paradigm shift in terms of our understanding of the nature of deep-focus earthquakes, and consequently, mechanisms other than phase transformational faulting need to be considered.

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