Influence on subduction boundary segmentation at the southwest end (Albatross segment) of the Mw 9.2, 1964 Kodiak Island, and the northeast end (Semidi segment) of the Mw 8.3 megathrust rupture areas offshore Alaska

• Main Authors: Krabbenhoeft, Anne, von Huene, Roland, Miller, John J., Klaeschen, Dirk
• Format: Conference Object
• Language: English
• Published: 2017
• Subjects: Kodiak; Alaska
• Online Access: https://oceanrep.geomar.de/id/eprint/37759/; https://oceanrep.geomar.de/id/eprint/37759/1/FB4_GDY_akrabbenhoeft_etal_SIP17_Alaska.jpg

Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture propagation, with a focus on barriers that limit rupture, i.e. the southwest end of the Albatross segment at the boundary with the 1938 Semidi Mw 8.3 earthquake segment, we relate multibeam seafloor morphology with sub-seafloor images and seismic P-wave velocity structure. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration and/or MCS traveltime tomography. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage wide-angle dataset reveals the P-wave velocity and the deep structure of the subducting plate to depths of the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by thick sediment cover that reaches ~3 km in the Albatross and ~1.5-2 km thickness in the Semidi segment at the trench axis. In both segments, bathymetry shows two major contrasting upper plate morphologies: trench-parallel ridges form the accreted prism of the shallow dipping lower slope whereas the steep rough middle and upper slopes are composed of competent older rock. Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a splay fault zone (SFZ) marking the boundary between the margin framework and the frontal prism. This SFZ corresponds to the most prominent lateral increase in seismic P-wave velocities, ~25 km landward of the trench axis. Major differences in the Albatross/Semidi segments are 1.) origin of subducting sediment (Surveyor vs. Kodiak fan) 2.) geometry of the subducting plate (gentle vs. steep dip) beneath the lower and middle slopes.