Aftershock Analysis of the 2018 Mw 7.1 Anchorage, Alaska, Earthquake: Relocations and Regional Moment Tensors

Abstract

The 30 November 2018 magnitude 7.1 Anchorage earthquake occurred as a result of normal faulting within the lithosphere of subducted Yakutat slab. It was followed by a vigorous aftershock sequence with over 10,000 aftershocks reported through the end of July 2019. The Alaska Earthquake Center produced a reviewed aftershock catalog with a magnitude of completeness of 1.3. This well‐recorded dataset provides a rare opportunity to study the relationship between the aftershocks and fault rupture of a major intraslab event. We use tomoDD algorithm to relocate 2038 M≥2 aftershocks with a regional 3D velocity model. The relocated aftershocks extend over a 20 km long zone between 47 and 57 km depth and are primarily confined to a high VP/VS region. Aftershocks form two clusters, a diffuse southern cluster and a steeply west‐dipping northern cluster with a gap in between where maximum slip has been inferred. We compute moment tensors for the Mw>4 aftershocks using a cut‐and‐paste method and careful selection of regional broadband stations. The moment tensor solutions do not exhibit significant variability or systematic differences between the northern and southern clusters and, on average, agree well with the mainshock fault‐plane parameters. We propose that the mainshock rupture initiated in the Yakutat lower crust or uppermost mantle and propagated both upward into the crust to near its top and downward into the mantle. The majority of the aftershocks are confined to the seismically active Yakutat crust and located both on and in the hanging wall of the mainshock fault rupture.

Simultaneous Rupture on Conjugate Faults During the 2018 Anchorage, Alaska, Intraslab Earthquake (MW 7.1) Inverted from Strong-Motion Waveforms

“…3b), we found that the southern and northern clusters dip with angles of ~ 30° and ~ 60°, respectively. These dip angles are consistent with those of the two nodal planes in the focal mechanisms for the mainshock and aftershocks (Ruppert et al 2020). The mainshock hypocenter is likely on the plane of the southern cluster rather than on that of the northern cluster.…”

“…These facts suggest that the Anchorage mainshock was an event with two conjugate faults corresponding to the two aftershock clusters. The suggestion of Ruppert et al (2020) that there are no systematic differences in the focal mechanisms between the southern and northern aftershock clusters also supports this fault con guration.…”

“…Even though these studies focused on the rupture during the Anchorage mainshock, the fault geometries in their rupture models insu ciently reproduce the spatially complex aftershock pattern (Figure 3), as detailed later. Based on this pattern, Ruppert et al (2020) and West et al (2020) suggested the possibility of a simultaneous rupture on two conjugate faults during the mainshock. The fault geometry in our inversion considers this possibility and decreases the level of disagreement between the mainshock fault geometry and the aftershock distribution.…”

“…We determined the fault planes in our inversion based on the mainshock and the aftershock distributions as relocated by Ruppert et al (2020). As shown in Fig.…”

Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

“…The mainshock and aftershocks generally become deeper after the relocation ( Figure S2). Our relocated mainshock hypocenter is shallower and more eastward than those determined with the double-difference location method (Ruppert et al, 2020) and the finite-fault inversion (C. Liu et al, 2019), partly due to the different velocity models used by these studies. The different colors of the dots in (a) denote focal depths whose scale is shown atop (a), and the sizes of the dots denote magnitudes whose scale is shown beside (a).…”