Seismic observations and modelling of the August 2025 Tracy Arm, Alaska landslide, megatsunami, precursory seismicity, and seiche

On 10 August 2025, a large landslide (>64×10⁶ m³) collapsed more than 1,000 m onto South Sawyer Glacier and into Tracy Arm fjord in Southeast Alaska. The resulting tsunami ran up the opposing fjord wall to a height of 480 m, the second-highest tsunami ever recorded.The landslide was preceded by more than 24 hours of repeated microseismicity (up to M~2), with event rates increasing until ~1 hour before failure, signalling a transition to continuous slip of the overall rock mass.The landslide generated globally observed body and long-period seismic waves equivalent to an Mw 5.4 earthquake, making it one of the largest-magnitude landslides in decades. From regional and global seismic data, we infer a total mass of ~370 million metric tons, exceeding estimates from remote sensing and DEM analysis. This discrepancy suggests that water displacement during the initial tsunami contributed to the long-period global seismic signal.Following the landslide signal, we observe monochromatic seismic waves worldwide with dominant periods of 50, 52, 66, and 86 s. The 66 s mode is strongest and persists for >36 hours at regional stations. Surface-wave radiation patterns, numerical tsunami modelling, and SWOT satellite water-height observations support the genesis of a fjord-transverse landslide-induced seiche (LIS) in the central fjord. However, seismic radiation is more complex than that of the 2023 Dickson Fjord, Greenland LIS event, likely reflecting differences in the landslide location and its direction, fjord geometry, and interaction of multiple seiche modes.Despite heavy summer vessel traffic in Tracy Arm, there were no fatalities, making this a near miss. Seismic observations, combined with remote sensing, provide a critical pathway for forecasting and early warning of cascading landslide–tsunami events and for understanding ice–land–water interactions in polar environments.