Teleseismic depth estimation of the 2015 Gorkha−Nepal aftershocks

Abstract

The depth of 61 aftershocks of the 2015 April 25 Gorkha, Nepal earthquake, that occurred within the first 20 d following the main shock, is constrained using time delays between teleseismic P phases and depth phases (pP and sP). The detection and identification of these phases are automatically processed using the cepstral method developed by Letort et al., and are validated with computed radiation patterns from the most probable focal mechanisms. The events are found to be relatively shallow (13.1 ± 3.9 km). Because depth estimations could potentially be biased by the method, velocity model or selected data, we also evaluate the depth resolution of the events from local catalogues by extracting 138 events with assumed well-constrained depth estimations. Comparison between the teleseismic depths and the depths from local and regional catalogues helps decrease epistemic uncertainties, and shows that the seismicity is clustered in a narrow band between 10 and 15 km depth. Given the geometry and depth of the major tectonic structures, most aftershocks are probably located in the immediate vicinity of the Main Himalayan Thrust (MHT) shear zone. The mid-crustal ramp of the flat/ramp MHT system is not resolved indicating that its height is moderate (less than 5−10 km) in the trace of the sections that ruptured on April 25. However, the seismicity depth range widens and deepens through an adjacent section to the east, a region that failed on 2015 May 12 during an M w 7.3 earthquake. This deeper seismicity could reflect a step-down of the basal detachment of the MHT, a lateral structural variation which probably acted as a barrier to the dynamic rupture propagation.

Automatic analysis of the Gorkha earthquake aftershock sequence: evidences of structurally segmented seismicity

“…We observe that most of the depths of deeper events-presumably associated with the MHT-match the depth of the MHT shear zone as determined by Duputel et al (2016). We further show that, in general, the depths of the aftershocks are close to those obtained by Letort et al (2016) using time delays between teleseismic P and depth phases, confirming an average deepening of the events from west to east (from 15 to 18 km).…”

“…Figure S3. Depth differences between Letort et al (2016) catalogue and ours for 25 commons events. Note the linear bias with depth.…”

“…First, we compared our depth estimates to those obtained by Letort et al (2016) using time delays between teleseismic P and depth phases with the cepstral method developed by Letort et al (2015). Among the 60 events of the aftershock sequence studied by Figure 13.…”

“…The Lamosangu antiform (shades of grey) probably dips to the west which is highlighted by an alignment of earthquakes. Letort et al (2016) only 25 were sufficiently well located (errors <7 km in X-, Y-and Z-directions) to be properly compared. The difference between the depths is represented on Fig.…”

Probabilistic seismic hazard assessment of Nepal using multiple seismic source models

“…Moreover, some recent studies have found that the hypocenters of most of the earthquakes in Nepal are located at depths ranging between 0 and 30 km (Pandey et al, ; Priestley et al, ). In addition, hypocenter relocation results from a nearby broadband seismic array found that the focal depths for the Gorkha earthquake sequence ranged primarily between 10 km and 20 km (Adhikari et al, ; Bai et al, ; Letort et al, ). Thus, in this analysis, the average focal depth of 15 km is assigned as a constant depth for all the source models.…”