Temporal changes in the subsurface seismic velocity structure reflect the physical processes that modulate the properties of the media through which seismic waves propagate. These processes, such as healing of the surface damage zone and deep crustal deformation, are described by similar functional forms and operate on similar timescales, making it difficult to determine which process drives the observed changes. We examine earthquake-induced velocity changes using the measured lag-time time series τ(t) of the repeating earthquake sequences since the 2004 Mw 9.2 Sumatra and 2005 Mw 8.6 Nias earthquakes. The S-coda velocity changes (δVS, equivalent to -τS) recover steadily during the 2005−2015 period. The Rayleigh wave velocity changes (δVLR, or -τLR) undergo transient recovery, followed by a strong δVLR reduction in late 2007. δVS recovery is most likely driven by deep processes, whereas the temporal breaks in δVLR recovery in 2007 mostly reflect surface damage and healing induced by the strong ground motions of the 2004 Mw 9.2, 2005 Mw 8.6, 2007 Mw 8.4 and Mw 7.9 Bengkulu, and 2008 Mw 7.3 Simeulue earthquakes. The observed differences between the temporal variations in δVS and δVLR can distinguish deep processes from healing of the surface damage zone.

On October 2020, a Mw 7.6 earthquake struck to the south of the Shumagin Islands in Alaska, nearly 3 months after the Mw 7.8 Simeonof megathrust event. The initial models of the earthquake indicated a largely strike-slip rupture; however, the observed tsunami was much larger and widespread than expected for the focal mechanism. We investigate what sea surface deformation is necessary to recreate the tsunami waveforms using water-level inversion techniques. We find that the sea surface deformation does not resemble that expected from a purely strike-slip earthquake. We then carry out slip inversions with water level and static GNSS data as input. We explore the likelihood of megathrust co-seismic slip aiding tsunamigenesis. We propose that, concurrently with strike-slip faulting, it is likely that a considerable slip occurred on the megathrust westward and updip from the previous July 2020 event. We also propose that a smaller submarine landslide is likely to have occurred in an area prone to them. The Sand Point earthquake potentially released ~2 meters of accumulated slip in the western Shumagin Gap, but likely did not slip updip of ~15 km depth.

Strong ground motions from large earthquakes are capable of damaging near-surface sediments and promoting notable reductions in their seismic velocity structures. These velocity reductions can be monitored using either body waves or surface waves from repeatable seismic sources, such as repeating earthquakes or ambient seismic noise. Here we compile a decade-long catalog of repeating earthquakes since the 2004 Mw 9.2 Sumatra Earthquake, and monitor the temporal velocity changes from Rayleigh waves (δVLR) and Love waves (δVLQ). We observe a δVLR of –0.16% and δVLR/δVLQ ratio of ~6, inconsistent with velocity reductions in isotropic media. To reconcile the observations, we carry out analyses of sensitivity kernels of surface waves in isotropic and vertical transversely isotropic (VTI) media and forward waveform modeling. The modeling reveals that the observed large δVLR/δVLQ ratio can be explained by strong dbV (–4%) and weak dbH (–0.615%) reductions and an increase in radial anisotropy in the near surface. These changes are best explained by a 2% increase in crack density of aligned horizontal cracks in overpressured sediments near the compressive subduction zone environment. Temporal variations of δVLR/δVLQ ratios after consecutive great earthquakes are consistent with laboratory experiments under cyclic loading.