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.
Highlights
Some of the world’s largest earthquakes occur in Alaska, such as the great 1964 MW 9.2 earthquake (Fig. 1)
We propose that a significant condition promoting large orthogonal intra-oceanic strike-slip earthquakes near subduction zones may be the oblique alignment of fracture zones (FZs) systems and crustal fabric ~30–35° to the trench axis (Fig. 5)
The 23 January 2018 MW 7.9 strike-slip oceanic lithosphere earthquake is a rare example of an oceanic intraplate strike-slip rupture
Summary
Some of the world’s largest earthquakes occur in Alaska, such as the great 1964 MW 9.2 earthquake (Fig. 1). Two major Pacific Plate N-S trending strike-slip earthquakes (Fig. 2, upper right; MW 7.8/7.7 in 1987/1988) ruptured a composite length of ~250 km in the central Gulf of Alaska[3] This region has hosted a spatially persistent cluster of diffuse seismicity since [4,5,6] where complex N-S aftershock patterns dominate and ENE – WSW trending aftershock clusters reactivated fracture zones (FZs)[4]. The other region of previous intra-oceanic earthquakes near a subduction megathrust is located offshore northern Sumatra and includes the largest recorded strike-slip intraplate great earthquakes (MW 8.6 & 8.2 of 2012; e.g.15) seaward of coseismic peak slip that occurred during the great 2004 Sumatra megathrust earthquake This earthquake sequence most likely triggered by the MW 9.2 2004 earthquake (e.g.16), ruptured a network of en échelon faults orthogonal and parallel to the 90°E Ridge[15]
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