Strike-slip 23 January 2018 MW 7.9 Gulf of Alaska rare intraplate earthquake: Complex rupture of a fracture zone system

Abstract

Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 MW 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic 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 post-processes of the nearby great 1964 MW 9.2 megathrust earthquake.

The 2018 Mw 7.9 Offshore Kodiak, Alaska, Earthquake: An Unusual Outer Rise Strike‐Slip Earthquake

“…Although the fault parameters increase gradually, the overall misfit decreases rapidly ( Figures S6 and S7). Taking the rupture path determined by back-projection investigation (Krabbenhoeft et al, 2018;Lay et al, 2018;Ruppert et al, 2018) and aftershock distribution (Lay et al, 2018;Ruppert et al, 2018;Zhao et al, 2018) into account, we still need to add another fault segment (F5(λ) = 260°, F5(γ) ≥ −25°, and F5(γ) ≤ 35°) to update the optimal faulting geometry. Figures S8 and S9 show that the five-fault model produces the smallest misfit between predictions and observations.…”

“…To understand the seismogenic structures of the 2018 Kodiak earthquake, several analyses have been performed on the earthquake rupture properties. Among them, most slip models showed a complex rupture pattern involving several quasi-orthogonal strike-slip fault planes (Krabbenhoeft et al, 2018;Lay et al, 2018;Ruppert et al, 2018;Zhao et al, 2018). Krabbenhoeft et al (2018) performed short-period back-projections and examined aftershocks, inferring, but not quantifying complex faulting, initially northward, then eastward.…”

Coseismic Rupture Geometry and Slip Rupture Process During the 2018 Mw 7.1 Anchorage, South‐Central Alaska Earthquake: Intraplate Normal Faulting by Slab Tear Constrained by Geodetic and Teleseismic Data

“…The Anchorage earthquake sustains the results of seismic image from previous results (Eberhart‐Phillips et al, ; Kim et al, ). On the other hand, earthquakes in subduction zones can be divided into intracrustal thrust event and intraoceanic strike‐slip/normal event (Krabbenhoeft et al, ). This Anchorage earthquake determined by our slip model indicates it occurred in the Yakutat oceanic crust with normal faulting, consistent with the knowledge of stress accumulation in subduction zone.…”

“…Schematic map of subduction structure and its seismic behavior beneath the south‐central Alaska (after Krabbenhoeft et al, ; Kim et al, ).…”

Resolving Complicated Faulting Process Using Multi‐Point‐Source Representation: Iterative Inversion Algorithm Improvement and Application to Recent Complex Earthquakes

“…As for the great 2012 Indo‐Australia intraplate earthquake, the aftershock distribution of the 2018 Alaska event is diffuse around the source area (Figure a), indicating that several nearly orthogonal faults ruptured during the mainshock. Detailed rupture analysis has been performed using GPS observations in Alaska, teleseismic BP images, aftershock locations, tsunami recordings, and inversion of teleseismic body waves, indicating a complicated rupture process on several conjugate and en échelon faults (e.g.,Krabbenhoeft et al, ; Lay et al, ; Ruppert et al, ; Wen et al, ). Four fault planes were used by Lay et al () to fit teleseismic body‐waves, GPS displacements, and tsunami observations, including two right‐lateral and two left‐lateral faults.…”