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Dr. Zhigang Peng
Summary Sentence: A seminar by Dr. Jenny Nakai, School of Earth and Atmospheric Sciences
Full Summary: No summary paragraph submitted.
J. NakaiNear Trench Evidence for Fluids from 3D Seismic Attenuation, Northern Hikurangi Subduction Margin, North Island, New Zealand
The Hikurangi subduction margin east of the North Island, New Zealand is the site of frequent shallow slow slip events. The causes behind the slow-slip at shallow depths close to the oceanic trench are unknown, but the release of fluids along the plate interface is suspected.
We image seismic attenuation near the Hikurangi trench offshore New Zealand, using ocean bottom and land-based seismometers, revealing low Qp and Qs (measures of energy loss) above a recurring shallow slow-slip event and within the subducting Hikurangi Plateau. Seismic attenuation, energy loss of seismic waves, can be used to detect high temperatures, melt, the presence of fluids, and fractures.
We use local earthquake P- and S-waves from 180 earthquakes to invert for t*, and subsequently invert for Qp and Qs, offshore the North Island directly above the area of slow slip. We image Qp and Qs to ~25 km depth, increase resolution of previously identified coastal low Q, and find a new region of even higher attenuation directly above the shallow slow slip event in 2014-2015, beneath the offshore array. This highest attenuation is downdip of a subducting seamount, and is spatially correlated with a high seismic reflectivity zone and Vp/Vs>1.85, all of which provide evidence for the presence of fluids.
The Qp and Qs is low at the trench and in the subducting plate, suggesting that seismic wave scattering due to faults, fractures, and the inherent heterogeneous composition of the Hikurangi Plateau, a large igneous province, plays a role in seismic attenuation.
