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

Abstract

A major (M W 7.9) intraplate earthquake ruptured the Pacific plate seaward of the Alaska subduction zone near Kodiak Island on 23 January 2018. The aftershock seismicity is diffuse, with both NNW-and ENE-trending distributions, while long-period point source moment tensors have near-horizontal compressional and tensional principal strain axes and significant non-double-couple components. Backprojections from three large-aperture networks indicate sources of short-period radiation not aligned with the best double-couple fault planes. A suite of finite-fault rupture models with one to four faults was considered, and a four-fault model, dominated by right-lateral slip on an SSE trending, westward-dipping fault, is compatible with most seismic, GPS, and tsunami data. However, the precise geometry, timing, and slip distribution of the complex set of faults is not well resolved. The sequence appears to be the result of intraplate stresses influenced by slab pull, the 1964 Alaska earthquake, and collision of the Yakutat terrane in northeastern Alaska.Plain Language Summary On 23 January 2018 a very large earthquake, with magnitude 7.8, ruptured in the Pacific plate southwest of the Alaskan subduction zone. There are multiple indications of complex faulting for this event: The point source moment tensor is not consistent with a single fault rupture; the aftershock distribution is diffuse, with nearly orthogonal trends in seismicity; the aftershock mechanisms are diverse; backprojections of short-period seismic waves show complex patterns of high-frequency energy release not on a single plane; and teleseismic waveforms are complex. Inversions of the teleseismic signals for a variety of models with from one to four different faults being allowed provide slip models that are used to predict regional GPS observations from Alaska along with deepwater tsunami recordings from seafloor pressure sensors at Deep-ocean Assessment and Reporting of Tsunamis (DART) stations. The primary rupture occurred on a fault trending SSE, dipping to the west, and several nearly perpendicular faults appear to have ruptured as well, but the limited spatial extent of the rupture makes it difficult to resolve the details of the faulting.

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

“…Based on the above studies, we find that the most slip in the GPS-based models concentrates on two WSW-trending sinistral strike-slip faults (Ruppert et al, 2018;Zhao et al, 2018). However, the slip distribution from teleseismic body waves favors a majority of slip on the SSE-trending dextral strike-slip fault (Lay et al, 2018). In addition, the models do not agree well with the distribution of the high-frequency radiators, indicating that the accuracy of the slip distribution still needs to be improved.…”

“…To better constrain the absolute fault location and overall rupture length, we model the deepwater tsunami observations with good azimuthal coverage (Lay et al, 2018). Model predictions are compared with recordings from five DART stations (Figure 1b).…”

“…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.…”

“…Krabbenhoeft et al (2018) performed short-period back-projections and examined aftershocks, inferring, but not quantifying complex faulting, initially northward, then eastward. Lay et al (2018) carried out short-period back-projections to evaluate the likely faulting geometry and the rupture velocity, inverted broadband P and SH data, and predicted GPS displacements and tsunami recordings, iterating to improve overall fits. Their preferred model involves four faults, with a maximum peak slip of 15.6 m on an NNW-SSE segment near the hypocenter.…”

“…et al (2018) also performed short-period back-projection inversion and modeled GPS displacements, setting models to match relocated aftershock trends. Unlike the four-fault model of Lay et al (2018), their final model contains five segments, with most slip on two WSW-trending fault segments. However, Ruppert et al (2018) provided a coseismic slip distribution with a lower resolution.…”