Complex Faulting and Triggered Rupture During the 2018 M_W7.9 Offshore Kodiak, Alaska, Earthquake

Abstract

We combine aftershock relocations, source mechanisms, teleseismic P wave backprojection, and Global Positioning System data inversion to constrain complex faulting geometry of the 2018 MW 7.9 offshore Kodiak earthquake. Relocated aftershocks delineate several N‐S trends including a prominent 110‐km‐long segment, as well as broad NE‐SW trends. Global Positioning System modeling and backprojection indicate that the NE‐SW trending left‐lateral strike‐slip segments released most energy dominating far‐field crustal deformation and radiated wavefield. Backprojection infers fast E‐to‐W rupture propagations superimposed on a slower S‐to‐N migration. We propose a five‐segment model of the rupture that was partially driven by dynamic triggering.

The 2018 MW 7.9 Gulf of Alaska Earthquake: Multiple Fault Rupture in the Pacific Plate

“…There is likely slip in the region of diffused aftershocks and backprojection features northwest of the hypocenter, but this is not resolved by the current modeling. Ruppert et al (2018) have a fifth fault trending northward in this area, but negligible slip was put on such a fault orientation in our inversions.…”

“…The images in Figure 2 share some features in common with the routine backprojections performed by Incorporated Research Institutions for Seismology (http://ds.iris.edu/spud/backprojection/16325047) and have some key similarities to each other; later loci of radiation tend to plot north of the hypocenter, with concentrations trending both northwest and northeast. The backprojection for Europe is very similar to the one shown in Ruppert et al (2018), although they use a hypocenter further south than the NEIC location. Even allowing for complex interference that obscures the precise subevent locations, the images do not suggest rupture on any dominant plane, and the aftershock distribution extends both southward and westward to areas where no high-frequency radiation is imaged.…”

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

“…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.…”

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.…”

“…In order to match the tsunami waveforms, some rotation of the N‐S faults relative to the seismicity was required. Ruppert et al () include a minor fifth fault. The relatively compact nature of the source results in the ambiguity of the faulting geometry, and there is uncertainty in the degree to which aftershock lineations represent regions of large coseismic slip.…”