Date of Award

8-2018

Author's School

Graduate School of Arts and Sciences

Author's Department

Earth & Planetary Sciences

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

This dissertation utilizes multiple techniques of earthquake location and seismic tomography to investigate deep seismicity in the Tonga flat slab region and the upper mantle structure of the Mariana subduction zone. I study the rupture property and aftershock distributions of the largest earthquake ever recorded in the Tonga flat slab region, defined by scattered seismicity and velocity anomalies. These, along with background seismicity, regional 3-D seismic velocity model and tectonic reconstructions of relict Vitiaz subduction zone, suggest the earthquakes are occurring in the final portion of the slab subducted at the now inactive Vitiaz trench. The associated Coulomb stress change shows many of the aftershocks were dynamically triggered, suggesting fossil slabs contain material that is too warm for earthquake nucleation, but may be near the critical stress susceptible to dynamic triggering.

In order to investigate water cycling at the Mariana subduction zone, I conduct tomographic study of Rayleigh wave to examine velocity and anisotropy anomalies associated with slab hydration. Tomographic results show a low velocity zone within the incoming plate prior to the trench down to about 30±5 km, along with significant trench-parallel anisotropy. The low velocity zone preserves the thickness after the slab is subducted, but the velocity reductions become smaller. An extremely low velocity zone is observed beneath the serpentine seamounts in the outer forearc. These anomalies suggest coexistence of water-filled cracks and mantle serpentinization along the normal faults in the slab before subduction. Water is expelled from the cracks early in subduction, causing a modest increase in the velocity of the subducting mantle, and the water moves upward and causes serpentinization of the outer forearc mantle. The total amount of water carried by hydrous minerals in the Pacific plate at the Mariana trench is estimated from the low velocity zone thickness and associated velocity reduction. If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting, estimates of the global water flux into the sub-arc mantle must be increased by about a factor of three over previous estimates.

I further expand the study of the Mariana subduction zone to the Mariana volcanic arc and backarc. I conduct similar tomographic study of Rayleigh wave to help constrain the melt production and transport beneath volcanic arc and backarc spreading center. Tomographic results show a thick deep low velocity zone (LVZ) beneath the Mariana trough with varying depth. A small size slow velocity anomaly beneath the volcanic arc near the top of the mantle is also imaged. An inclined LVZ is observed west of the arc and connected to the deep LVZ. Tomographic results also show a shallower thin slow velocity layer at the top of the mantle distributed in a narrow channel along the central part of the spreading center, where larger magma supply is suggested. The shallow slow velocity anomaly beneath the volcanic arc and the backarc spreading center may both represent a shallow melt reservoir. However, neither of them is imaged to be directly connected to the deep LVZ, suggesting the melt may be transported through a conduit that is too narrow to be resolved by surface wave tomography.

Language

English (en)

Chair and Committee

Douglas A. Wiens

Committee Members

James A. Conder, Michael J. Krawczynski, Philip Skemer, V. Slava Solomatov

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