Characterizing Sub-seafloor Seismic Structure of the Alaska Peninsula
Along the Alaska-Aleutian Subduction Zone
Abstract
A shallow sub-seafloor seismic model that includes well-determined
seismic velocities and clarifies sediment-crust discontinuities is
needed to characterize the physical properties of marine sediments and
the oceanic crust and to serve as a reference for deeper seismic
modeling endeavors. This study estimates the seismic structure of marine
sediments and the shallow oceanic crust of the Alaska-Aleutian
subduction zone at the Alaska Peninsula, using data from the Alaska
Amphibious Community Seismic Experiment (AACSE). We measure seafloor
compliance and Ps converted wave delays from AACSE ocean-bottom
seismometers (OBS) and seafloor pressure data and interpret these
measurements using a joint Bayesian Monte Carlo inversion to produce a
sub-seafloor S-wave velocity model beneath each available OBS station.
The sediment thickness across the array varies considerably, ranging
from about 50 m to 2.80 km, with the thickest sediment located in the
accretionary wedge. Lithological composition plays an important role in
shaping the seismic properties of seafloor sediment. Deep-sea deposits
on the incoming plate, which contain biogenic materials, tend to have
reduced S-wave velocities, contrasting with the clay-rich sediments in
the forearc and accretionary wedge. A difference in S-wave velocities is
observed for upper oceanic crust formed at fast-rate (Shumagin) and
intermediate-rate (Semidi) spreading centers. The reduced S-wave
velocities in the Semidi crust may be caused by increased faulting and a
less mafic composition, related to a previous period of
intermediate-rate spreading.