Significant lateral dip changes may have limited the scale of the 2015 M_w 7.8 Gorkha earthquake

Abstract

The 2015 Mw 7.8 Gorkha earthquake has drawn interest due to its complex fault geometry. Both geodetic and geologic studies have focused on the dip variations. In this study we invert the coseismic geodetic data for the 2‐D dip variations of the earthquake. The best fit model confirms that the dip varies with depth, and suggests that there is a significant lateral dip anomaly along strike. The depth‐dependent dip variation suggests that the earthquake ruptured a ramp‐flat fault. The shallow ramp may have prevented the rupture breaking through the surface. In addition, a lateral large‐dip anomaly is found in the northeastern corner of the slip area, which supports the previous findings of inferred interseismic fault coupling, coseismic high‐frequency radiations, and the aftershock mechanisms. This lateral dip anomaly is likely associated with local tearing within the Indian slab. It may have blocked the east‐southeastward rupture propagations of the Gorkha earthquake, implying important controls on the earthquake scale and the spatial limits of ruptures.

Constraints on the Geometry and Frictional Properties of the Main Himalayan Thrust Using Coseismic, Postseismic, and Interseismic Deformation in Nepal

“…The most significant deviation from this geometry (best fit dip of 35 • ) is suggested by coseismic data from profile E that passes through the uplift anomaly identified by Whipple et al (2016). This change in dip may be real and highlight a variable geometry along strike, as has been suggested by previous authors (e.g., Hubbard et al, 2016;Zhang et al, 2017). Alternatively, rather than the dip of a single, planar MHT changing along strike, a more complicated downdip geometry may be able to explain the data from profile E. We therefore try a series of more complex geometries to model data from profile E in subsequent sections.…”

“…The geological modeling approach used by Hubbard et al (2016) may be one way of conducting such interdisciplinary studies. Their model satisfies the constraints provided by geology and is corroborated by the distribution of slip in the Gorkha earthquake (Grandin et al, 2015;Qiu et al, 2016), potential dip variations in the earthquake itself (Zhang et al, 2017) and the locations of aftershocks (Wang et al, 2017). However, corroboration is not necessarily equivalent to confirmation of such a model.…”

“…as the location of microseismicity (Pandey et al, 1995), the location of a rapid change in uplift rates (Lavé & Avouac, 2001), and the geology (e.g., Hubbard et al, 2016). The complexity of these geometries varies from a single geometry along strike (Elliott et al, 2016;Sreejith et al, 2016) to full 3-D models with multiple ramps and along-strike variation (Hubbard et al, 2016;Qiu et al, 2016;Zhang et al, 2017). Laterally continuous FRF models have also been used to model postseismic deformation by Mencin et al (2016) and Sreejith et al (2016).…”

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

“…This effect is not considered in previous (analytical) geometry studies (e.g., Bletery et al, ). Thus, 3‐D models with laterally varying dip angle of the ramp would help us to evaluate the variation of seismic potential in lateral direction (e.g., Whipple et al, ; Zhang et al, ) and may provide insights on the mechanisms of lateral seismic segmentation of large megathrust system.…”

“…The Gorkha earthquake ruptured the downdip portion of the MHT in a region encompassing along‐dip geometric changes, or “ramp‐flat” structures (Zhao et al, ; Schulte‐Pelkum et al, ; Hubbard et al, ), which are frequently observed in continental collision zones or subduction zones, for another instance, in the Zagros Mountains (e.g., Barnhart et al, ). Geodetic and seismic investigations of the Gorkha earthquake revealed that coseismic slip concentrated near the geometric transition of the flat to the ramp segment at ~15 km depth, and negligible, seismic or aseismic, slip propagated to the shallow depth (<10 km; Elliott et al, ; Mencin et al, ; Qiu et al, ; Zhang et al, ). The lack of fault slip in the updip flat portion of the MHT indicates that this portion of the megathrust absorbed considerable elastic strain that has not yet been released.…”