Updating the Crustal Fault Model for the 2023 National Seismic Hazard Model for Alaska

Abstract

We present the crustal fault model for Alaska, based on geologic observations, as a primary input for the 2023 revision of the U.S. Geological Survey National Seismic Hazard Model (NSHM). We update the 2013 Alaska Quaternary fault and fold database (Koehler, 2013) with subsequent findings to produce a simplified model of 105 fault sections and four fault zone polygons with basic geologic parameters including slip sense and rate. Significant updates from prior maps include: 1) A slip rate of ~53 mm/yr on the Queen Charlotte Fault system indicating it accommodates all of the plate boundary motion. 2) Quantified long-term slip rates on megathrust splay faults in the southern Prince William Sound region and near Kodiak Island. 3) Improved details of structures in the Chugach-St. Elias orogen. 4) Revised characterization of Castle Mountain Fault from right-lateral slip to a predominantly reverse fault. 5) Improved Interior Alaska tectonic models that clarify relationships between the Denali Fault, Totschunda Fault, and thrust faults on both sides of the Alaska Range. 6) Identified large earthquake sources in the eastern Brooks Range. 7) Omission of the Chatham Strait section of the Denali Fault. We also describe the growing body of Alaskan lacustrine paleoseismic records of strong shaking, which may offer a test of ground motion recurrence predicted by the 2023 NSHM for crustal, megathrust, and intraslab events. The fault model underscores that the collision of the Yakutat microplate is the dominant driver of active crustal faulting in most of Alaska.