Along Strike Variability of the Alaska-Aleutian Subducting Plate from Mode Converted Phases at the Subduction Plate Interface.

Earthquakes and arc volcanism at subduction zones are believed to be directly related to the narrow zone at the top of the subducting plate where sediments and altered oceanic crust are subducted. This complex subduction plate interface can be investigated in high resolution using high frequency (1-10 Hz) mode converted phases (e.g., P-to-S or S-to-P) from local in-slab earthquake signals. Several recent dense broadband seismometer deployments extending across the Alaska-Aleutian subduction zone allow for investigation into the along strike variability of the subducting plate. Preliminary work uses the MOOS seismometer array deployed with ~15 km spacing across the Kenai Peninsula between 2007-2009 and spanned the M9.2 1964 earthquake rupture area. Local earthquake signals recorded by MOOS show many mode-converted signals that arrive between the P- and S-wave arrivals. Two months of data for earthquakes deeper than 50 km were handpicked from the MOOS array, with 251 of the 670 earthquakes showing such mode-converted signals. These converted signals were recorded at most stations within the array; however, their abundance varies significantly between adjacent stations. Low detection at certain stations could be caused by unfavorable subduction geometry, plate interface changes, site effects, or low signal to noise within the P coda. Mode-converted signals arrive at a consistent time relative to P or S arrivals at some stations regardless of the hypocenter location, indicating a conversion point close to the station. However, mode-converted arrival times at other stations are more random. At first glance, the spatial variation in such signals maps high heterogeneity of the plate interface over a distance of a couple of tens of kilometers. We are analyzing the entire two years of MOOS data, the TA and the Alaska Amphibious Community Seismic Experiment, to understand better the heterogeneity and its relationship to earthquake rupture variability.