Consecutive Ruptures on a Complex Conjugate Fault System During the 2018 Gulf of Alaska Earthquake

Abstract

We developed a flexible finite-fault inversion method for teleseismic P waveforms to obtain a detailed rupture process of a complex multiple-fault earthquake. We estimate the distribution of potency-rate density tensors on an assumed model plane to clarify rupture evolution processes, including variations of fault geometry. We applied our method to the 23 January 2018 Gulf of Alaska earthquake by representing slip on a projected horizontal model plane at a depth of 33.6 km to fit the distribution of aftershocks occurring within one week of the mainshock. The obtained source model, which successfully explained the complex teleseismic P waveforms, shows that the 2018 earthquake ruptured a conjugate system of N-S and E-W faults. The spatiotemporal rupture evolution indicates irregular rupture behavior involving a multiple-shock sequence, which is likely associated with discontinuities in the fault geometry that originated from E-W sea-floor fracture zones and N-S platebending faults.The 23 January 2018 Gulf of Alaska earthquake (moment-magnitude M W 7.9 1 ) struck offshore Kodiak Island (55.9097°N, 149.0521°W, 10.4 km depth; Alaska Earthquake Center, AEC 1 ), in the seaward-region of the Alaska-Aleutian subduction zone. The Global Centroid Moment Tensor (GCMT) project 2, 3 reported that the 2018 Gulf of Alaska earthquake had strike-slip faulting with a large non-double-couple component (47%). Aftershock seismicity determined by the AEC 1 shows a lineation extending about 120 km N-S near the epicenter and two aftershock clusters centered about 60 km northeast and about 50 km west from the epicenter (Fig. 1). The GCMT solutions of aftershocks are dominated by strike-slip faulting, but include normal and reverse faulting (Fig. 1).Several pioneering studies that built finite-fault models based on the aftershock distribution demonstrated that the 2018 Gulf of Alaska earthquake ruptured a quasi-orthogonal multiple-fault system oriented approximately N-S and E-W 4-8 . However, it is difficult to adopt a reasonable fault model because the fault model parametrization, number of fault segments, and fault geometries differ by study, partly due to the spatial spread of the aftershock distribution (Fig. 1). Based on the static slip distribution estimated from Global Navigation Satellite System and tsunami data, major slips occurred on E-W-striking segments 5,7,8 . Finite-fault inversions estimated that the maximum slip occurred around the boundary between the crust and uppermost mantle in the N-Soriented segment 4,6 , which would have played a significant role in tsunami generation. However, it remains challenging to adequately explain the complex characteristics of the observed teleseismic body waveforms by conventional finite-fault inversion methods due to the uncertainty on the fault geometry, which lead to significant model errors.In the framework of finite-fault waveform inversion, uncertainties on the Green's function and fault geometry have been the major sources of model errors [9][10][11][12][13] . Those due to uncertai...

Complex rupture process of the 2014 Thailand Mw 6.2 earthquake on the conjugate fault system of the Phayao fault zone

“…To mitigate the bias due to the smoothing constraints, we applied a new framework of the relative weight smoothness constraint developed by Yamashita et al (2021): it adds an inverse relative weight parameter (1/ " ) to the standard deviation of each basis component that is proportional to the double-couple component of the GCMT moment tensor solution (Fig. S3).…”

“…S1 and S2). This new framework has been proven efficient for the analyses of the source process of the 2018 Gulf of Alaska earthquake (Yamashita et al, 2021) and the 2020 Caribbean earthquake (Tadapansawut et al, 2021). There it solves the problem of oversmoothing the slip distribution of the major components, and allows more clear capture of the rupture propagation.…”

“…A comparison of the results between the non-weighted smoothing constraint (Shimizu et al, 2020) and the relative weighted smoothing constraint (Yamashita et al, 2021) methods is presented in Fig. S4.…”

“…To resolve the possible complex fault geometry, we apply a new framework of the flexible finite-fault inversion algorithm for teleseismic body waveforms (Yamashita et al, 2021). We introduce a relative weight smoothness constraint that is proportional to the components of each basis moment tensor (Kikuchi and Kanamori, 1991) into the potency density tensor inversion of Shimizu et al (2020) which can mitigate the effect of the modelling errors originating from the uncertainty of Green's function (Yagi and Fukahata, 2011) as well as the uncertainty of fault geometry (e.g., Ragon et al, 2018) (see details in the Method section).…”

“…This method can simultaneously estimate the distribution of the focal mechanism and the slip along the assumed model plane; it enables the reconstruction of complex rupture processes, including those occurring along faults containing fault bends and those consisting of multiple subevents, without a priori assumption of the fault geometry (Tadapansawut et al, 2021;Yamashsita et al, 2021). The improved flexible finite-fault inversion framework has been applied to large earthquakes such as the 2020 MW 7.7 Caribbean earthquake (Tadapansawut et al, 2021) and the 2018 MW 7.9 Gulf of Alaska earthquake (Yamashita et al, 2021), but it has never been applied to smaller-scale M6-class earthquakes like the 2014 Thailand earthquake. In the following sections, we first perform a numerical test to evaluate the resolvability of the rupture process for a moderate-sized event.…”