The 2025 Mw 8.8 Kamchatka Megathrust Earthquake: Evidence for an Extended Rupture from Teleseismic Backprojection and Hydroacoustic Observations

On 29 July 2025, an Mw 8.8 megathrust earthquake ruptured the Kamchatka subduction zone, generating strong ground shaking across the Kamchatka Peninsula and a Pacific-wide tsunami. We investigate the rupture evolution of this event using high-frequency teleseismic backprojection and hydroacoustic T-wave observations. Teleseismic backprojection reveals a predominantly southwestward rupture extending ∼500 km from the epicenter, with a rupture duration of ∼190 s and subshear propagation velocity (mean ∼2.1 km/s). Similarly, results of the T-wave records from the H11N hydrophone array indicate a consistent rupture length and orientation (∼500 km to the south), providing robust seismoacoustic confirmation of the rupture extent. The inferred rupture length from both methods exceeds previous empirical scaling relationships for Mw 8.8 subduction earthquakes. Interestingly, the rupture extent seems to be indistinguishable from that of the 1952 Kamchatka rupture, within uncertainties, implying that segment limits are controlled by structural heterogeneity of the margin, rather than details of the prestress. Our results demonstrate the need to combine teleseismic and hydroacoustic observations to robustly characterize rupture dimensions of future great megathrust earthquakes.