Subduction Zone Interface Structure beneath Kodiak Island, Alaska: Constraints from Receiver Functions Across a Spatially Dense Node Array

Session: Recent Development in Ultra-Dense Seismic Arrays With Nodes and Distributed Acoustic Sensing (DAS) [Poster]
Type: Poster
Date: 4/30/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Kodiak Island is an exposed part of an accretionary complex along the Alaska-Aleutian subduction zone that formed as the Pacific plate subducted below the North American plate. There is no consensus regarding the accretion mechanism in this region, but underplating by duplex accretion has been used to explain its formation. Although low frequency earthquakes on the western Alaska-Aleutian subduction zone are not well documented, tremors have been recorded beneath Kodiak island. Unconsolidated sediment thickness and fluid distribution along the subduction zone interface are some of the factors thought to affect slip stability. Here we use Ps receiver functions to identify potential low seismic velocity anomalies and estimate Vp/Vs within the oceanic crust to constrain metamorphism and fluid pressure. The study will allow us to test the hypothesis: Two distinct layers will compose the plate interface beneath Kodiak Island, a shallower LVL related to sediment subduction and an oceanic layer that has higher velocity than the sediment but lower velocity than the underlying oceanic mantle. In May 2019, we deployed 398 autonomous three-component 5-Hz Fairfield Nodal Zland geophones on Kodiak Island as part of the Alaska Amphibious Community Seismic Experiment. The dense lines consisted of nodes at approximately 200-m spacing recording continuously for about 30 days adjacent to the ~40 km-long Chiniak Highway system across northeastern Kodiak Island. A total of 86 events with an MW > 5.0 were recorded during that time period with 12 events having an MW > 6.0 and an epicentral distance between 30° and 90°. Preliminary analysis of the raw waveforms from a few selected earthquakes reveal a clear incident P-wave that is identifiable across the array. When the P-arrivals are aligned, other coherent phases, such as PcP, are clearly observed throughout the array. Strong arrivals early in the P coda with different slowness than the incident P wave suggest that the dense array will yield a high-resolution image of small-scale features such as the subduction zone interface structure.

Presenting Author: Evans A. Onyango

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