The 25 October 2010 Mentawai tsunami earthquake (M w 7.8) ruptured the shallow portion of the Sunda megathrust seaward of the Mentawai Islands, offshore of Sumatra, Indonesia, generating a strong tsunami that took 509 lives. The rupture zone was updip of those of the 12 September 2007 M w 8.5 and 7.9 underthrusting earthquakes. High-rate (1 s sampling) GPS instruments of the Sumatra GPS Array network deployed on the Mentawai Islands and Sumatra mainland recorded time-varying and static ground displacements at epicentral distances from 49 to 322 km. Azimuthally distributed tsunami recordings from two deepwater sensors and two tide gauges that have local high-resolution bathymetric information provide additional constraints on the source process. Finite-fault rupture models, obtained by joint inversion of the high-rate (hr)-GPS time series and numerous teleseismic broadband P and S wave seismograms together with iterative forward modeling of the tsunami recordings, indicate rupture propagation~50 km up dip and~100 km northwest along strike from the hypocenter, with a rupture velocity of~1.8 km/s. Subregions with large slip extend from 7 to 10 km depth~80 km northwest from the hypocenter with a maximum slip of 8 m and from~5 km depth to beneath thin horizontal sedimentary layers beyond the prism deformation front for~100 km along strike, with a localized region having >15 m of slip. The seismic moment is 7.2 × 10 20 N m. The rupture model indicates that local heterogeneities in the shallow megathrust can accumulate strain that allows some regions near the toe of accretionary prisms to fail in tsunami earthquakes.
The 13 November 2016 Kaikoura, New Zealand, Mw 7.8 earthquake ruptured multiple crustal faults in the transpressional Marlborough and North Canterbury tectonic domains of northeastern South Island. The Hikurangi trench and underthrust Pacific slab terminate in the region south of Kaikoura, as the subdution zone transitions to the Alpine fault strike‐slip regime. It is difficult to establish whether any coseismic slip occurred on the megathrust from on‐land observations. The rupture generated a tsunami well recorded at tide gauges along the eastern coasts and in Chatham Islands, including a ~4 m crest‐to‐trough signal at Kaikoura where coastal uplift was about 1 m, and at multiple gauges in Wellington Harbor. Iterative modeling of teleseismic body waves and the regional water‐level recordings establishes that two regions of seafloor motion produced the tsunami, including an Mw ~7.6 rupture on the megathrust below Kaikoura and comparable size transpressional crustal faulting extending offshore near Cook Strait.
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.
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