Heather M. Savage Christine Regalla Srisharan Shreedharan Catherine Ross Hanaya Okuda Uisdean Nicholson Kohtaro Ujiie Ron Hackney Marianne Conin Pei Pei Sara Satolli Junli Zhang Patrick Fulton Matt Ikari Shuichi Kodaira Lena Maeda Natsumi Okutsu Sean Toczko Nobu Eguchi Jamie D. Kirkpatrick 2025 The 2011 Mw9.1 Tohoku-oki earthquake is exceptional among great earthquakes for having peak slip of ~50-70 m on the shallowest portion of the plate boundary megathrust. International Ocean Discovery Program Expedition 405 drilled multiple holes through the megathrust in the large slip region and at a Pacific Plate input site. The megathrust preferentially develops at the top or base of the pelagic clay in the input section where pronounced contrasts in physical properties are present. This results in a narrow, weak fault located at a major mechanical contact between frontal prism mud and subducted clay. Localization imposed by the input section enhances the tendency for shallow seismic slip, showing the Japan Trench may be more susceptible to ruptures with large shallow slip than margins without weak clays. application/pdf 10.1126/science.ady0234 en American Association for the Advancement of Science Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench article