Bimodal seismicity in the Himalaya controlled by fault friction and geometry

Abstract

There is increasing evidence that the Himalayan seismicity can be bimodal: blind earthquakes (up to Mw ~ 7.8) tend to cluster in the downdip part of the seismogenic zone, whereas infrequent great earthquakes (Mw 8+) propagate up to the Himalayan frontal thrust. To explore the causes of this bimodal seismicity, we developed a two-dimensional, seismic cycle model of the Nepal Himalaya. Our visco-elasto-plastic simulations reproduce important features of the earthquake cycle, including interseismic strain and a bimodal seismicity pattern. Bimodal seismicity emerges as a result of relatively higher friction and a non-planar geometry of the Main Himalayan Thrust fault. This introduces a region of large strength excess that can only be activated once enough stress is transferred upwards by blind earthquakes. This supports the view that most segments of the Himalaya might produce complete ruptures significantly larger than the 2015 Mw 7.8 Gorkha earthquake, which should be accounted for in future seismic hazard assessments.

Segmentation of the Main Himalayan Thrust Illuminated by Bayesian Inference of Interseismic Coupling

“…All other assumptions and hypothesis are kept identical to our reference model. This strong assumption is based on previous geodetic studies using GPS campaign and leveling measurements (Bollinger et al, ), a few continuous GPS stations (Bettinelli et al, ), and a recent physics‐based forward model of the Himalayan seismic cycle (Dal Zilio et al, ). Also, the location of the down‐dip end of the locked fault zone is consistent with the seismicity pattern observed in Nepal Himalaya (Cattin & Avouac, ), and the postseismic deformation following the 2015 7.8 Gorkha earthquake (Gualandi et al, ).…”

Complex Earthquake Sequences On Simple Faults

“…Other events, including the 2014 M w 8.2 Iquique, Chile earthquake (Schurr et al, ) ruptured a fraction of a locked patch, arresting along strike where earlier ruptures propagated. Partial ruptures have been attributed to geometrical or frictional fault heterogeneity (Dal Zilio et al, ; Li et al, ; Moreno et al, ; Qiu et al, ). Other studies invoke, as I do here, gradients in the stress field due to nonuniform loading (Herrendorfer et al, ; Michel et al, ), implying that partial ruptures may be rather common for sufficiently large VW regions.…”

Competing Effects of Mountain Uplift and Landslide Erosion Over Earthquake Cycles

“…For regions where interseismic loading operates at scales comparable to fault dimensions (e.g., creeping at the fault root below the locking depth) and causes significant surface deformation (e.g., Taiwan and the frontal Himalayas; Cattin & Avouac, ; Johnson et al, ; Stevens & Avouac, ), our model can only constrain the coseismic and postseismic components and the related volume balance, but lacks the interseismic component related to loading. Future work could add modeling of these interseismic processes (e.g., Cattin & Avouac, ; Dal Zilio et al, ; Simpson, ) into our framework, but is beyond the scope of this study.…”

“…However, topography responds not only to coseismic processes (uplift and landsliding), as considered in the work of Li et al () and Marc, Hovius, and Meunier (), but also to postseismic relaxation following coseismic deformation and isostatic compensation to erosional mass removal (Huang et al, ; King et al, ; Molnar, ; Watts, ). Interseismic processes may also play important roles (Cattin & Avouac, ; Dal Zilio et al, ; Godard et al, , ; Vergne et al, ).…”

“…Previous studies have quantified the effects of earthquake cycle processes using physical solutions for the mechanical behavior of dip‐slip fault systems with layered structures of different rheological properties (e.g., Cattin & Avouac, ; Dal Zilio et al, ; King et al, ; Simpson, ). These studies have been able to describe how first‐order topographic forms can emerge from repeated earthquake sequences, but they lacked quantitative constraints on earthquake‐triggered erosion.…”