Presentation by Liam Moser.
What Limits Megathrust Rupture? New Perspectives on the Subduction Seismogenic Zone from Earthquake Tomography and Fault Mechanics
Subduction zones host Earth’s largest earthquakes, with the downdip limit of the seismogenic zone constraining the onshore intensity of ground shaking and the updip limit controlling tsunami risk. In this talk, I present two novel models for these limits. First, I focus on the downdip limit, where the maximum earthquake depth is typically explained by a temperature-controlled transition in rate-and-state friction properties or by lithological changes associated with the overriding plate Moho. Given the availability of improved thermal models and earthquake catalogs, I revisit this classic model. I find that seismogenesis is not strongly controlled by temperature: in fact, the downdip limit is better explained by a fixed depth. I then demonstrate that a frictional-viscous transition with a weak and temperature-insensitive viscous mechanism, such as talc low-temperature plasticity, is most consistent with observations of the downdip limit and constraints on megathrust strength. Second, I show a case study in the Alaska Peninsula subduction zone of how the seismogenic zone is modulated by the hydration of, and fluid release from, the downgoing plate. To do this, I present 3‐D compressional and shear‐wave velocity models, and their ratio Vp/Vs, using local earthquake tomography. Low velocity anomalies along the slab spatially correlate with the coseismic slip of the 2020 Mw7.8 Simeonof, 2021 Mw8.2 Chignik, and 2023 Mw7.2 earthquakes. From this correlation, I propose a fault mechanics model based on earthquake nucleation length that links high pore-pressure zones along the megathrust, resulting from slab dehydration, to rupture barriers that arrest seismic slip.
