Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake

Abstract

Optical Offsets 7To determine the co-seismic horizontal displacement field due to the Gorkha earthquake, we use optical 8 image correlation to measure the displacement of pixels between pre-and post-earthquake satellite im-9 ages. We are able to resolve sub-pixel displacements of less than 1/15th of the Landsat8 pixel resolu-10 tion (i.e. < 1 m) using the COSI-Corr software package images, which helps to increase the signal-to-noise ratio, (4) the deformation field is resolved perpendicular to 16 the look angle (i.e. the horizontal plane for nadir images), thereby providing measurements complementary 17to InSAR (which is sensitive to vertical displacements), (5) the nadir look angle is insensitive to topographic 18 residuals produced during orthorectification of the satellite images (such residuals are produced when a lower 19 resolution digital elevation model, DEM, is used during the orthorectification process), and (5) Landsat8 20 images are freely available from the USGS as an orthorectified product -see 5 for additional details. 21Landsat8 images are typically acquired at 10am each morning. Consequently, the illumination charac-22 teristics (i.e. shadows) vary in every image acquired throughout the year according to the position of the 23 sun. Because shadows produce sharp edges in satellite images, they strongly influence the correlation. There-24 fore, to reduce the effect of differing shadows biasing the displacement field, we correlate Landsat8 images 25 acquired at a similar time of year, thereby yielding images with similar illumination characteristics (i.e. sun 26 azimuth and elevation). In addition to having similar illumination characteristics, we also require images with 27 minimal cloud cover. From the Landsat8 archive, we found two suitable images from the (pre-earthquake) 2813th May 2014 (sun azimuth: 109• , sun elevation: 68

Seismotectonics and rupture process of theMW 7.1 2011 Van reverse-faulting earthquake, eastern Turkey, and implications for hazard in regions of distributed shortening

“…This is also consistent with Evangelidis & Kao (2013) who imaged a high-frequency (HF) source at this depth and location which they relate to rupture termination or geometric complexity. A similar trend was observed in the location of the HF seismic radiation sources in the 2015 Gorkha Earthquake, Nepal (Avouac et al 2015); Elliott et al (2016) show that the HF sources are co-located with a change in dip of the fault plane, though in that case, they were in the middle of the rupture zone.…”

“…Lithological layers such as salt and shale detachment horizons act to compartmentalize slip in narrow depth extents (Nissen et al 2010;Elliott et al 2015a), and the intersection of faults at depth has been suggested as a possible (albeit potentially temporary) barrier to rupture propagation (Elliott et al 2011(Elliott et al , 2016. There has been much discussion on the effect of alongstrike segmentation on the length of ruptures in strike-slip earthquakes (e.g.…”

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

“…9). This has been conjectured to be either structurally controlled by the steeply dipping shallower ramp on the MHT (Elliott et al 2016) or by the lower level of stresses updip of the ruptured zone. Finally, the last stage splitting of the rupture front possibly indicates a stronger patch on the MHT leading to transfer of motion onto a different fault, which has been illuminated through the multiple subevent source modelling.…”

“…Rest of the figure caption is same as Fig. 3. in the High Himalaya (Elliott et al 2016) The third major aftershock (M w 5.1) occurred ∼5.5 hr after the previous one and originated close to the southern edge of the mainshock rupture area (Fig. 8).…”

“…The downdip limit of the rupture has been conjectured to coincide with the junction between the frontal flat and deeper steeply dipping ramp on the MHT. Geodetic data combined with geologic, geomorphological and geophysical analysis (Elliott et al 2016) revealed that the main-shock slip occurred on the shallow flat portion of the MHT, between 5 and 15 km, and did not require any outof-sequence thrusting. Study of aftershock distribution (Adhikari et al 2015) highlighted three distinct southward extending belts of seismicity, reflecting possible regions of stress concentrations following the main-shock rupture.…”