The Cenozoic evolution of the Himalaya-Tibet Plateau, dictated by the India-Asia convergence, remains a subject of substantial ambiguity. Here, a thermo-mechanical model is used to show the critical controls of decelerating convergence on the formation and stabilization of distinctive tectonic structures during prolonged collision. At high constant convergence rates, similar to the late Paleogene India-Asia motions, the lower plate crust is injected beneath the overriding crust, uplifting a plateau, first, then is exhumed towards the orogeny front. Conversely, low constant convergence rates, similar to the Neogene India-Asia motions, induce crustal thickening and plateau formation without underplating or exhumation of incoming crust. Strikingly, models simulating the decelerating India-Asia convergence history portray a dynamic evolution, highlighting the transitory nature of features under decreasing convergence, as the orogen shifts to a new equilibrium. In the transitional phase, the slowing of convergence decreases basal shearing and compression, leading to extension and heating in the orogen interiors. This allows diapiric ascent of buried crust and plateau collapse, as accretion migrates to a frontal fold-and-thrust belt. The models provide insights into the multi-stage evolution of the long-lived Himalayan-Tibetan orogeny, from fast early growth of the Tibetan Plateau, through its transient destabilisation and late-stage internal extension, behind the expanding Himalayan belt.

Accelerating aseismic slip events have been commonly observed during the rupture nucleation processes of the earthquake. While that accelerating aseismic slip is usually considered strong evidence for precursory activity, it remains unclear whether all accelerating aseismic slip events are precursory to an incoming earthquake. Two contrasting nucleation models have been introduced to explain the observations associated with the nucleation of unstable slip: the pre-slip and cascade nucleation models. Each of these two-end members, however, has its own limitations. In this study, we employ Discrete Element Method (DEM) simulations of a 2-D strike-slip fault to simulate various rupture nucleation and triggering processes. Our simulation results manifest that the final seismic event is a product contributed by multiple pre-slip nucleation sites, which may interact, causing clock advance or cascade nucleation rupture processes. We also introduce a strengthening perturbation zone to investigate the role of a single nucleation site in an imminent seismic event. The simulation results reveal a new type of non-precursory aseismic slip, representing the region favoring the generation of the precursory slip process but not correlating to the incoming main event, which differs from the previous interpretation of precursory slip. Furthermore, we include weakening perturbation zones in some simulations to demonstrate how small earthquakes may or may not trigger a nucleation site depending on spatial and temporal conditions. Our simulation results imply that such non-precursory but accelerating aseismic slip events may suggest a fault segment that appears weakly coupled but possesses the potential to be triggered seismically.

Dynamic modeling of sequences of earthquakes and aseismic slip (SEAS) provides a self-consistent, physics-based framework to connect, interpret, and predict diverse geophysical observations across spatial and temporal scales. Amid growing applications of SEAS models, numerical code verification is essential to ensure reliable simulation results but is often infeasible due to the lack of analytical solutions. Here, we develop two benchmarks for three-dimensional (3D) SEAS problems to compare and verify numerical codes based on boundary-element, finite-element, and finite-difference methods, in a community initiative. Our benchmarks consider a planar vertical strike-slip fault obeying a rate- and state-dependent friction law, in a 3D homogeneous, linear elastic whole-space or half-space, where spontaneous earthquakes and slow slip arise due to tectonic-like loading. We use a suite of quasi-dynamic simulations from 10 modeling groups to assess the agreement during all phases of multiple seismic cycles. We find excellent quantitative agreement among simulated outputs for sufficiently large model domains and small grid spacings. However, discrepancies in rupture fronts of the initial event are influenced by the free surface and various computational factors. The recurrence intervals and nucleation phase of later earthquakes are particularly sensitive to numerical resolution and domain-size-dependent loading. Despite such variability, key properties of individual earthquakes, including rupture style, duration, total slip, peak slip rate, and stress drop, are comparable among even marginally resolved simulations. Our benchmark efforts offer a community-based example to improve numerical simulations and reveal sensitivities of model observables, which are important for advancing SEAS models to better understand earthquake system dynamics.