Collaborative Research: Mapping and Understanding Seismic Anisotropy in the Northeast Pacific Ocean

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Understanding the structure of the seafloor and how plate tectonics, which causes plate motion, volcanism, and megathrust earthquakes in subduction zones works, is important for understanding various types of geohazards and what triggers them. This knowledge is critical in order to plan for and understand how to mitigate and/or minimize the impact of disasters, such as those that have occurred in recent years in Thailand and Japan. Global seismic tomography, which uses seismic signals from earthquakes and other earth movements to image Earth's deep structure, has long been the tool of choice to image and understand the structure of Earth's interior and how it deforms. To this end, present tomographic images indicate that the northeast Pacific plate in the area between the continental U.S. and Hawaii is more complex than predicted by present oceanic plate models. Because this apparently atypical area is far from any type of known thermal feature, such as a mid-ocean ridge or a hotspot (i.e., a concentrated upwelling of heat and thus magma from deep inside the Earth like that which formed the Hawaiian Islands) a different mechanism must be acting. Due to the current lack of seismic recording stations in the northeast Pacific, the resolution of crustal and underlying mantle structure in this area is under-resolved and may have resulted in misleading theories of how the Pacific plate has evolved over time. This research addresses this problem by deploying an array of 25 ocean bottom seismometers for 15 months to capture and record seismic signals from earth motions. These data will allow the determination of real structures and will target the part of the Pacific plate that is 40-50 million years old. These new seismic data will allow exploration of the local seismic anisotropy (the nonuniform mineral/crustal structural orientation). Anisotropy forms when mantle minerals align with mantle flow directions or when a plate, such as the Pacific plate, responds to forces tugging along its edges. The analysis of seismic anisotropy therefore allows scientists to explore mantle processes that form, move, and ultimately cause the subduction of Earth's tectonic plates. Broader impacts of the project include graduate student and postdoctoral training in state-of-the-art geophysical at-sea and onshore seismic processing and modeling. Undergraduate students will be involved with special projects. The work complements other international global seismology efforts and involves collaboration with U.K., German, and French scientists. The work broadens participation of underrepresented groups in the sciences by supporting two investigators whose gender is underrepresented in the sciences and who serve as role models for students and other geoscientists. This research provides an integrated analyses of seismic and other geophysical data that will elucidate the structure, heterogeneity, and dynamics of the ocean crust, lithosphere and asthenosphere in a 600~kilometer wide region west of the Moonless Mountain Seamounts on the Pacific plate halfway between the US mainland and Hawaii. Seismic data will be collected using an array of 25 ocean bottom seismometers. Research targets include a regional study, covering of the seismometer array deployment area which permits analysis of surface waves, receiver functions, surface wave azimuthal anisotropy, and shear-wave splitting in the area. Results provide insights into the local seismic structure of 40 to 50 million-year-old Pacific lithosphere and will answer questions about whether this area conforms to predictions from models of normal lithospheric plate cooling or if secondary processes, such as small-scale mantle convection, are impacting plate behavior and crustal/mantle structure. Results will be tested against predictions for a suite of mantle flow conditions and mineral alignment/anisotropy modes. The project will also use the newly collected surface wave data to improve global surface wave dispersion maps, reducing imaging biases in the global dataset that result, in part, from uneven data coverage in the study area. The ocean bottom seismometer array covers an area for which there is inadequate data for the Pacific Array as identified by the global community of seismologists. These data will serve a large global community of seismologists that conduct global tomographic and other studies that examine shallow and deep-Earth heterogeneity and processes that help us better understand how the Earth works and the impacts deep seated mantle-driven processes can have on Earth's surface. The work complements that recently carried out by an international, collaborative, group of US, UK, German, and French scientists in the Atlantic Ocean, which makes possible a comparison of results between both ocean basins. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.