Figure 1. Study area along the Alaska-Aleutian subduction zone (AASZ) showing the locations of fault sections and approximate historical rupture areas. For older events, rupture areas are inferred from aftershock zones (Tape and Lomax, 2022), and recent events are taken from fault rupture models (Freymueller et al., 2021; Tape & Lomax, 2022; Ye et al., 2022). Slab interface contours are from Hayes et al., 2018. Bathymetry and shaded relief from GEBCO Compilation Group (2023).
The last update to the Alaska portion of the U. S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) in 2007 used the best available information to define recurrence rates for seven fault sections along the subduction interface, primarily based on rupture areas of historical subduction earthquakes (Wesson et al., 2007). Maximum magnitudes were assigned to each fault section, and seismicity and limited paleoseismic data were used to estimate rupture recurrence (Wesson et al., 2007). At the time of the Wesson et al. (2007) update, paleoseismic and paleotsunami records extended westward only as far as Kodiak Island, and so seismicity rates exclusively were used west of Kodiak to approximate the recurrence of large ruptures. Geodetic data (Freymueller et al., 2008) were not incorporated in the 2007 update.
There have been several advances in the treatment of subduction zone hazard since the last update of the Alaska portion of the NSHM in 2007. In New Zealand, geodetic data were incorporated into the Hikurangi subduction interface model of Stirling et al. (2012), where rupture segments were defined based on the pattern of interseismic coupling, slow slip events, and historical seismicity and earthquake recurrence rates were inferred from plate convergence rates and coupling coefficients. The most recent New Zealand seismic hazard model also uses geodetic data, and leverages geologic data as a comparison, but not as a constraint, in the inversion for rupture rates (Coffey et al., 2022). By contrast, in Cascadia geodetic data are not used explicitly in recurrence models, but instead a rich onshore and offshore paleoseismic record is available to assign entire-zone and partial-rupture recurrence rates (Frankel et al., 2015). Recent global subduction zone recurrence models (Pagani et al., 2021) rely primarily on seismicity, especially where paleoseismic data are lacking.
Here, we construct a recurrence model for the Aleutian-Alaskan subduction zone using both geologic and geodetic data because these datasets provide different, but complementary, views of rupture behavior. Along the energetic coasts of Alaska, geologic data capture only the largest ruptures, generally with preserved evidence of vertical deformation above detection limits of > 0.2 m (Hawkes et al., 2010; Shennan et al., 2016) or tsunami runup > 5 m above the modern tidal range (Nelson et al., 2015; Witter et al., 2016, 2019) The geologic data also represent events that typically rupture multiple fault sections, which is demonstrated by historical events and inferred for prehistoric earthquakes. The geodetic data is used to approximate strain accumulation and release by a single fault section. Thus, recurrence rates of ruptures inferred from geodetic data are necessarily shorter and the inferred earthquake magnitudes are smaller than events recorded by geology. Our goal is to provide parameters useful for seismic hazard analyses, such as for the next update of the USGS NSHM. The model focuses on subduction interface ruptures rather than outer rise, crustal, or intraslab events.