Crowell, B. W. & Melgar, D. Slipping the Shumagin gap: A kinematic coseismic and early afterslip model of the M W 7.8 Simeonof Island, Alaska, earthquake. Geophys. Res. Lett. 47, e2020GL090308 (2020).

Liu, C., Lay, T., Xiong, X. & Wen, Y. Rupture of the 2020 M W 7.8 earthquake in the Shumagin gap inferred from seismic and geodetic observations. Geophys. Res. Lett. 47, e2020GL090806 (2020). Okal, E. A. & Hébert, H. Far-field simulation of the 1946 Aleutian tsunami. Geophys. J. Inter. 169, 1229–1238 (2007).Niazi, M. & Chun, K. Y. Crustal structure in the southern Bering Shelf and the Alaska Peninsula from inversion of surface-wave dispersion data. Bull. Seism. Soc. Amer. 79, 1883–1893 (1989). The table below contains all postcodes on a two day service. Please note all deliveries to Northern Ireland are also on a 3-5 days service. Given the guidance provided by the simple dipole modeling, we considered physical fault dislocation models for plausible geometries that can match the salient features of seafloor deformation from the dipole model that leads to successful match of the tsunami waveforms. This includes simultaneous assessment of the seismic and geodetic motions produced by such models for the sensitive high-rate GNSS recordings at nearby stations AC12 and AC28. The latter constraint is very important; there is essentially no geodetic or seismic signature of the second (dominant) tsunami source, and models that violate this can be rejected with confidence. We considered appropriately placed models with delayed slow thrust slip on the shallow megathrust (Methods, Supplementary Figs. 8, 9) or slow thrust slip on an upper plate splay fault with a strike parallel to the trench (Methods, Supplementary Figs. 10, 11) and allowed sufficiently long source process times to obscure the seismic and geodetic expressions while giving strong tsunami excitation, finding models that match the tsunami signals by extensive searches over model parameters (fault dimensions, slip, absolute location, etc.). However, those models that do match the tsunami observations acceptably all badly violate the geodetic observations at AC12 and AC28 (Supplementary Figs. 8, 10). This eliminates the more obvious candidate model geometries. Successful slow-slip faulting geometry

Yamazaki, Y., Kowalik, Z. & Cheung, K. F. Depth-integrated, non-hydrostatic model for wave breaking and run-up. Int. J. Num. Meth. Fluids 61, 473–497 (2009). Herman, M. W. & Furlong, K. P. Triggering an unexpected earthquake in an uncoupled subduction zone. Sci. Adv. 7, eabf7590 (2021). Ye, L., Lay, T., Kanamori, H., Yamazaki, Y. & Cheung, K. F. The 22 July 2020 M W 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust. Earth Planet. Sci. Lett. 562, 116879 (2021). Xu, W. et al. Transpressional rupture cascade of the 2016 M W 7.8 Kaikoura earthquake, New Zealand. J. Geophys. Res.: Solid Earth 123, 2396–2409 (2018). Yamazaki, Y., Cheung, K. F. & Kowalik, Z. Depth-integrated, non-hydrostatic model with grid nesting for tsunami generation, propagation, and run-up. Int. J. Num. Meth. Fluids 67, 2081–2107 (2011).

Drooff, C. & Freymueller, J. T. New constraints on slip deficit on the Aleutian megathrust and inflation at Mt. Veniaminof, Alaska from repeat GPS measurements. Geophys. Res. Lett. 48, e2020GL091787 (2021). Ji, C., Wald, D. J. & Helmberger, D. V. Source description of the 1999 Hector Mine, California, earthquake, Part I: Wavelet domain inversion theory and resolution analysis. Bull. Seism. Soc. Am. 92, 1192–1207 (2002). Figure 10 shows the regions that have been inferred to have strong geodetic coupling and weak geodetic coupling, which may play an important role in the lateral shearing within the Pacific plate 15, but there is very little resolution of the shallow megathrust coupling along the 1938 and 2021 Semidi ruptures or along the Shumagin segment. Seafloor geodesy may help to resolve whether there is strain release or a lateral gradient in strain accumulation on the megathrust near the 19 October 2020 event. This information is needed to understand the cause of lateral compression in the upper wedge implied by our slow slip source. If the process instead involved slumping across the shelf break rather than slow thrusting within the wedge, high-resolution bathymetric scans may help to resolve the occurrence of such mass wasting, but as we discuss, it is challenging to have substantial slumping go undetected by the nearby geodetic stations. Dense reflection profiling might resolve the faults involved in this complex event, and complex structures have been indicated in existing sparse profiles 17, but 3D imaging is likely needed to resolve structures with a strike close to perpendicular to the ridge.

Li, S. & Freymueller, J. T. Spatial variation of slip behavior beneath the Alaska Peninsula along Alaska-Aleutian subduction zone. Geophys. Res. Lett. 45, 3453–3460 (2018). Fukao, Y. et al. Detection of “Rapid” aseismic slip at the Izu-Bonin trench. J. Geophys. Res.: Solid Earth 126, e2021JB022132 (2021). Offshore Island deliveries will take longer than two days including Channel Islands, Isle of Man, Scottish Highlands and Islands and Scilly Isles. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981).

And ..... done! Awesome.

The specific geometry of the inferred slow thrust faulting, with along-trench compression in the upper plate, is surprising, and if this model is correct, it comprises an unexpected tsunami hazard in the region. The presence of weak sediments near the shelf break may have influenced slow-slip rupture with 15 m of slip over ~300 s, as found for this successful model, which has fault dimensions of 20 km × 20 km. Such large slip over localized area has been observed in shallow megathrusts environments, typically involving a tsunami earthquake 23 or aseismic transient slip 24. Transpressional environments have been observed to have large slow thrust faulting along with dominant strike-slip faulting as well 25. Models with a larger fault area (30 km × 30 km; 40 km × 40 km) and lower slip (7 m, 4 m) that have similar total moment may be viable, but it is challenging to fit all of the tsunami data as well as in Fig. 8 (e.g., Supplementary Figs. 16, 17). While lower slip is appealing, larger fault dimensions imply more observable faulting in the wedge, for which available bathymetry and reflection profiling now provide independent evidence. The non-unique modeling suggests slow slip of from 4 to 15 m on the westward-dipping upper plate thrust fault. Four levels of telescopic grids are needed to model the tsunami from the sources with increasing resolution to the Kahului tide gauge. An additional level is needed to resolve the more complex waterways leading to Hilo, King Cove, and Sand Point. Supplementary Fig. 7 shows the layout of the computational grid systems. The level-1 grid extends across the North Pacific at 2-arcmin (~3700 m) resolution, which gives an adequate description of large-scale bathymetric features and optimal dispersion properties for modeling of trans-oceanic tsunami propagation with NEOWAVE 35. The level-2 grids resolve the insular shelves along the Hawaiian Islands at 24-arcsec (~740 m) and the continental shelf of the Alaska Peninsula at 30-arcsec (~925 m), while providing a transition to the level-3 grids for the respective islands or coastal regions at 6-arcsec (~185 m) resolution. The finest grids at levels 4 or 5 resolve the harbors where the tide gauges are located at 0.3-arcsec (9.25 m) or 0.4 arcsec (12.3 m). A Manning number of 0.025 accounts for the sub-grid roughness at the harbors. The digital elevation model includes GEBCO at 30-arcsec (~3700 m) resolution for the North Pacific, multibeam and LiDAR data at 50 m and ~3 m in the Hawaii region, and NCEI King Cove 8/15-arcsec dataset and Sand Point V2 1/3-arcsec dataset, which also covers the Shumagin Islands. Long-period spectral analysis As seismic and geodetic data can provide complementary constraints on the rupture process, we used both data types to invert the rupture process of the 19 October 2020 event assuming first one and then two fault segments. We performed non-linear finite fault inversions 29, 30, involving the joint analysis of coseismic static offsets, hr-GNSS time series, and seismic waveforms. A simulated annealing algorithm was used to solve for the slip magnitude and direction, rise time, and average rupture velocity for subfaults on the two segments. For each parameter, we set specific search bounds and intervals. The subfault size is chosen as 5 km × 5 km, and the rake angles on the two fault segments are constrained to be right-lateral purely strike-slip and purely dip-slip, respectively. We allowed both the rise and fall intervals of the asymmetric slip rate function for each subfault to vary from 0.6 to 6.0 s; thus, the corresponding slip duration for each subfault is limited between 1.2 and 12 s. We let the slip vary from 0.0 to 8.0 m, and the average rupture velocity is allowed to vary from 0.5 to 3.0 km/s. Green’s functions for static displacements and seismic waveforms are computed using a 1-D layered velocity model 31. Equal weighting among the data functionals for GNSS statics and seismic waveforms was used in this study. Tsunami modeling Scholz, C. H. The Mechanics of Earthquakes and Faulting. 439 (Cambridge Univ. Press, New York, 1990). Mulia, I. E., Heidarzadeh, M. & Satake, K. Effects of depth of fault slip and continental shelf geometry on the generation of anomalously long-period tsunami by the July 2020 M W 7.8 Shumagin (Alaska) earthquake. Geophys. Res. Lett. 49, e2021GL094937 (2022).