Structural Control on Downdip Locking Extent of the Himalayan Megathrust

Abstract

Geologic reconstructions of the Main Himalayan Thrust in Nepal show a laterally extensive midcrustal ramp, hypothesized to form the downdip boundary of interseismic locking. Using a recent compilation of interseismic GPS velocities and a simplified model of fault coupling, we estimate the width of coupling across Nepal using a series of two‐dimensional transects. We find that the downdip width of fault coupling increases smoothly from 70 to 90 km in eastern Nepal to 100–110 km in central Nepal, then narrows again in western Nepal. The inferred coupling transition is closely aligned with geologic reconstructions of the base of the midcrustal ramp in central and eastern Nepal, but in western Nepal, the data suggest that the location is intermediate between two proposed ramp locations. The result for western Nepal implies either an anomalous coupling transition that occurs along a shallowly dipping portion of the fault or that both ramps may be partially coupled and that a proposed crustal‐scale duplexing process may be active during the interseismic period. We also find that the models require a convergence rate of 15.5 ± 2 mm/year throughout Nepal, reducing the geodetic moment accumulation rate by up to 30% compared with earlier models, partially resolving an inferred discrepancy between geodetic and paleoseismic estimates of moment release across the Himalaya.

Geometrical and Frictional Effects on Incomplete Rupture and Shallow Slip Deficit in Ramp‐Flat Structures

“…Comprehensive analysis of interseismic deformation indicates that the crustal ramp is an active structure beneath the topographic front of the Higher Himalaya (Berger et al, ; Grandin et al, ; Lindsey et al, ). The ramp seems spatially coincident with the clustered background microseismicity (Pandey et al, ), indicating that high shear stress is building up near such geometrical variation under steady tectonic loading.…”

“…Previous earthquakes in this region exhibited similar behavior, leading to the hypothesis that shallow portion of the megathrust may act as strain reservoir, with elastic energy tranferred and stored from deep, more frequent but smaller ruptures until a great earthquake eventually sweeps through these shallow regions (Bilham et al, 2017). These observations suggest that downdip seismogenic segmentation of the MHT is geometrically controlled (e.g., Qiu et al, 2016;Lindsey et al, 2018), as has been suggested in other regions (e.g., Bletery et al, 2016;Duan, 2012;Yang et al, 2013). However, slip distributions of the Gorkha earthquake mimic incomplete ruptures in other regions where incomplete rupture seem to be depth-controlled, which is consitent with the notion that a plate boundary thrust is zoned according to frictional strength variations (e.g., Konca et al, 2008;Perfettini et al, 2010;Lay et al, 2012;Moreno et al, 2018).…”

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

“…To assess the extent of interseismic coupling, we use an updated set of GPS-derived velocities spanning the 1990s to 2015 (Kreemer et al, 2014). This data set was recently used to estimate the width of coupling across the MHT through a series of two-dimensional transects (Lindsey et al, 2018) (supporting information, Figure S1). To better resolve the pattern of coupling we also include leveling measurements from the Survey of Nepal collected between 1977 and 1990 (Jackson & Bilham, 1994).…”

The 2015 Gorkha Earthquake: Earthquake Reflection Imaging of the Source Fault and Connecting Seismic Structure With Fault Slip Behavior

“…Previous studies have mentioned that resolving the detailed geometry of the plate boundary is important for assessing the size and location of future megathrust earthquakes (e.g., Hubbard et al, 2016;Qiu et al, 2016). Lindsey et al (2018) connect control of the transition from locked to creeping to the midcrustal ramp in central Nepal. Our results suggest that a heterogeneous crustal structure around the plate boundary may be associated with changes in fault slip behavior, including control on the extent of updip rupture and variations in coseismic slip and seismicity.…”