Variation of Attenuation along the SW Okinawa Trough: Effect of the Eurasian Lithosphere
Date Issued
2010
Date
2010
Author(s)
Ko, Yen-Ting
Abstract
Seismic imaging of wave speed and attenuation is one of the most effective probes to reveal the structures of and the geodynamic processes in the earth’s interior. Physical parameters like velocity and attenuation complement each other in interpreting mantle properties, with the latter sensitive to temperature and water content most and insensitive to compositions and errors in event times. However, both the source spectrum and the attenuation of the medium dictate waveforms. The challenge facing us is to isolate the influence of corner frequency fc from the determination of t* or Q. In this study, we present a new method, the cluster event method (CEM), to determine both fc and Q in a robust way and apply it to depict the Q variations in the mantle wedge along the axis of the westernmost Okinawa trough (OT). The CEM features sharing of Q among neighboring paths and allowing single fc for each event, yielding a formulation solved best with the optimization algorithm of simulated annealing.
The two ocean-bottom seismometers deployed in the OT joined the BATS in northern Taiwan to form 3 profile groups that facilitates an observation of how the mantle wedge changes as approaching the edge of the subduction zone. Our results reveal a dramatic increase in Q value, from 60 beneath the moderately rifting OT to about 1000 beneath northeast Taiwan where the backarc rifting is incipient. A quantitative estimation indicates that ~400 C cooling is required to explain the increase. Several mutually-interrelated causes are pursued, and the most significant contributors is the 200-250 C cooling due to the weakening of the OT toward Taiwan. Other thermal factors are derived from the contact between the Eurasian lithosphere and the slab and may contribute only 50 C. The total thermal effect amounts to 250-300 C.
In this thesis we propose that water plays an important role to raise the Q value. The retarded subduction by the resistance along the Eurasia-slab boundary cuts short the water supply from the slab, decreasing the amount of water in the mantle wedge. A factor of 10 depletion of water in the mantle wedge can fill the 100-150 C deficit left from the inferred thermal budget alone. The temperature-water scenario integrates regional tectonics, mantle dynamics, and seismology into a self-consistent working model that not only sheds light on the evolution of the OT but also helps direct new research effort to a global characterization of the plate edge dynamics.
Subjects
seismic attenuation
mantle wedge
Okinawa trough
water content
plate edge
Type
thesis
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