On-fault and remote observations of exceptionally long, supershear rupture in the 2025 M 7.7 Mandalay earthquake

Abstract

Straight faults are considered prone to large supershear ruptures and seismic gaps to likely host future earthquakes. Both predictions may have been fulfilled in the magnitude (M) 7.7 Mandalay earthquake of 28 March 2025 on the Sagaing Fault. Exceptional video footage of the fault rupture, high-rate Global Navigation Satellite System, and seismological and remote sensing data reveal a supershear slip pulse that formed early and ruptured a well-identified seismic gap but propagated well beyond it, resulting in an exceptional rupture length of 510 kilometers. By contrast, the slip pulse observed in the video footage has subshear characteristics. These observations can be reconciled by a preferred conceptual interpretation invoking depth-dependent weakening properties, stress heterogeneity, and possible dynamic triggering. These properties allow for energetic ruptures that stay confined to the depth range locked in the interseismic period but are not confined to seismic gaps along strike.