ALASKA: Relating morphological features of the upper plate slopes to the location and extent of the seismogenic zone: Deep seismic and bathymetric studies of the subduction zone of Alaska and a comparing global study
Final Report Abstract
To better understand earthquake rupture environments and mechanisms in the Kodiak segment of the Alaska subduction zone, we combine seafloor morphology with sub-seafloor images and seismic P-wave velocity structures. We re-processed USGS legacy multichannel seismic (MCS) data with shot- and intra-shot gather interpolation, multiple removal and Kirchhoff depth migration and/or MCS traveltime tomography. The better resolution at greater depth reveals the structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage (1994) wide-angle dataset reveals the P-wave velocity distribution as well as the deep structure of the subducting plate to the ocean crust Moho. The subducting oceanic crust morphology is rough and predominantly covered by thick sediments that reach up to ~3 km thickness at the trench axis. Bathymetry shows two major contrasting upper plate seafloor morphologies: the shallow dipping lower slope consists of trench-parallel ridges that form the accreted frontal prism whereas the steep rough middle and upper slopes are composed of competent older rock with higher seismic velocity. Upper plate deformation introduces complexity above a rough plate interface, whereas the upper plate structure is less disturbed above a smooth plate interface moderated by thick sediment along the plate interface. 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 thrust fault zone marking the backstop splay fault zone (BSFZ). This zone of disturbed structure is the transition between the outer forearc and the frontal prism. It corresponds to the most prominent lateral increase in seismic P-wave velocities, ~25 km landward of the trench axis. Multibeam bathymetry shows the along strike continuity of major thrusts in several MCS-lines that have a > 100 km lateral extent. Our study indicates that the BSFZ located in the Kodiak segment has most likely been activated by co- or post-seismicity associated with the 1964 great earthquake. Southwest of the Kodiak segment, backstop splay fault zones have been identified in the Alaskan-Aleutian subduction zone segments as far as the Unimak segment, forming the transition between the lower slope and the backstop of the inner forearc. The investigation of the Shumagin BSFZ evolved during the studies of the Kodiak segment BSFZ. The potential for a major earthquake in the Shumagin seismic gap, and the tsunami it could generate, was reported in 1971. However, while potentially tsunamigenic splay faults in the adjacent Unimak and Semidi earthquake segments are known, such features along the Shumagin segment were undocumented until recently. Legacy seismic records and also processed seismic data acquired by RV Langseth during the ALEUT project show splay faults separating the frontal prism from the margin framework. A ridge uplifted by the splay fault hanging wall extends along the entire segment. At the plate interface, the splay fault cuts across subducted sediment strata in some images, whereas in others, the plate interface sediment cuts across the fault. Splay fault zones are commonly associated with subducting lower plate relief. During the work on the ALASKA project, a rare, MW 7.9 strike-slip intra-oceanic lithosphere earthquake ruptured the outer rise region of the Kodiak segment in the Gulf of Alaska on 23 January 2018. Based on bathymetric, seismic, magnetic and vertical gravity gradient data and earthquake back-projection we find a complex rupture of a fracture zone system with main and aftershock events along N-S trending oceanic fabric, which ruptured mainly strikeslip and additionally, in normal and oblique slip mechanisms and strike-slip events along E-W oriented fracture zones. To explain the complex faulting behaviour we adopt the classical stress and strain partitioning concept and propose a generalized model for large intraoceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and postprocesses of the nearby great 1964 MW 9.2 megathrust earthquake.
Publications
- (2019) The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin. Geosphere 15 (2) 324–341
Huene, Roland von; Miller, John J.; Krabbenhoeft, Anne
(See online at https://doi.org/10.1130/GES01657.1) - (2021) Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska. Geosphere 17 (2) 409–437
Krabbenhoeft, Anne; Huene, Roland von; Miller, John J.; Klaeschen, Dirk
(See online at https://doi.org/10.1130/GES02275.1) - (2016) Subduction-related structure in the MW 9.2, 1964 megathrust rupture area offshore Kodiak Island, Alaska. In: AGU Fall Meeting 2016, 12.-16.12.2016, San Francisco, USA
Krabbenhoeft, A., von Huene, R., Klaeschen, D., Miller, J. J.
- (2017) 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. In: Subduction Interface Processes Conference, 18.04.-21.04.2017, Barcelona, Spain
Krabbenhoeft, A., von Huene, R., Miller, J. J. und Klaeschen, D.
- (2017) Subduction-related structure at the southwest end (Albatross segment) of the MW 9.2, 1964 megathrust rupture area offshore Kodiak Island, Alaska. In: 77. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 27.03.-30.03.2017, Potsdam, Germany
Krabbenhoeft, A., von Huene, R., Klaeschen, D. und Miller, J. J.
- Strike-slip 23 January 2018 MW 7.9 Gulf of Alaska rare intraplate earthquake: Complex rupture of a fracture zone system, Nature Scientific Reports, 8, 13706, 2018
Krabbenhoeft, A., von Huene, R., Miller, J.J., Lange, D., and Vera F.
(See online at https://doi.org/10.1038/s41598-018-32071-4)
