Unraveling the Mechanisms of Giant Intraplate Strike-Slip Earthquakes in Mongolia: The Roles of Slow Plate Rates

The largest strike-slip earthquakes ever recorded (M > 8) occurred in Mongolia in the 20th century, far from any plate boundaries. Rupture length-magnitude data indicate that the magnitude of these intraplate Mongolian events is, on average, ~0.5 magnitude larger than that of typical interplate earthquakes. The physical mechanisms that allow for such extra-large events remain mostly unresolved, largely due to the long return time of such events, hence the limited observational data. To address this, we employed a dual approach—numerical simulations with the PyQuake3D boundary element code (Tang et al., 2025) and theoretical analyses using the Rate and State Friction (Aging) Law on the spring slider—focusing on the role of slow plate rates. Our findings show that lower plate rates result in higher slip and greater stress drop, driven by enhanced fault restrengthening (healing). This healing, quantified by the state parameter, increases linearly with the inverse plate rate, in agreement with both analytical spring-slider models and 3D simulations. Critically, however, the observed GNSS plate rates of 1–3 mm/yr are insufficient to account for the ~0.5-unit magnitude excess relative to typical interplate earthquakes. We rigorously examine two scientific hypotheses: First, plate rates may be at residual levels (<1 mm/yr), perhaps reflecting far-field tectonic stresses or gravitational potential energy contrasts in Central/East Asia. Such extremely low driving rates could enable extended interseismic healing and thus unusually large stress drops and magnitudes. Second, the rupture width and depth of these intraplate earthquakes exceed those of typical interplate events. Our argument for this second scenario is strengthened by simulations of thermal pressurization: at high slip rates, rapid heating of pore fluids increases pore pressure and reduces the effective normal stress, thereby facilitating enhanced fault weakening and deeper rupture penetration. Our integrated numerical and theoretical approaches provide a robust basis for these hypotheses, advancing our understanding of the generation of remarkably large intraplate earthquakes and highlighting the importance of tectonic plate rate controlling earthquake magnitude.

Tang, R., Gan, L., Li, F., & Dal Zilio, L. (2025). PyQuake3D: A Python tool for 3-D earthquake sequence simulations of seismic and aseismic slip. Journal of Geophysical Research: Machine Learning and Computation, 2(4), e2025JH000871.