Rupture Depth-Varying Seismicity Patterns for Major and Great (M_W ≥ 7.0) Megathrust Earthquakes

Large earthquakes on subduction zone plate boundary megathrusts result from intervals of strain accumulation and release. The mechanism diversity and spatial distribution of moderate-size aftershocks is influenced by the mainshock rupture depth extent. Mainshocks that rupture across the shallow megathrust to near the trench have greater intraplate aftershock faulting diversity than events with rupture confined to deeper portions of the megathrust. Diversity of intraplate aftershock faulting also increases as the size of the mainshock approaches the largest size event to have ruptured that region of the megathrust. Based on these tendencies, we identify "breakthrough" ruptures as those involving shallow rupture of the megathrust with volumetrically extensive elastic strain drop around the plate boundary that allows activation of diverse intraplate faulting influenced by long-term ambient deformation stresses. In contrast, homogeneity of the aftershock faulting mechanisms indicates only partial release of elastic strain energy and remaining potential for another large rupture.