Seismicity patterns and geodynamics of the Hindukush seismic zone: A comprehensive study through relocation of moderate to large earthquakes

The widespread variation of focal depths and fault plane solutions observed in the Hindukush region depicts continuous deformation along the Indian–Eurasian collision zone. For period of twelve years i.e. from 2010 to 2022, a total of 89 intermediate-depth earthquakes of magnitude (M_w)≥ 5.5 of the Hindukush Region were considered, relocated using both regional and tele seismic data with 90 per cent confidence limits of less than 20 km. Two distinct seismic activity clusters: First one at a deeper depth and second at a shallower depth having different P-axes were observed that verifies the internal structure and geometry of Hindukush zone as suggested in previous studies. Beneath the Hindukush collision zone, there exists a complex pattern of deformation, arising from a combination of compression, tension, shearing and necking states due to an unusual and a rare case of subduction that is not from oceanic plate. The Hindukush seismic zone extends from 70 to 300 km depth and mostly strikes east-west and then turns northeast. The relocated seismicity by merging data of seismic network close to Hindukush along with international data shows that the Hindukush zone may be divided vertically into upper and lower slabs separated by a gap at about 150 km depth at which strike and dip directions change sharply with significant structural changes. Seismicity rate is higher in the lower part of Hindukush, having large magnitude events in a small volume below 180 km forming complex pattern of source mechanisms. Contrary in upper part seismicity rate is lower and scattered. The Global CMT (Global Centroid-Moment-Tensor Project) source mechanisms of intermediate depth earthquakes have a systematic pattern of reverse faulting with the vertical T-axes, while shallow events do not have such pattern. The vertical T-axes of the intermediate-depth events may be attributed to negative buoyancy caused by subduction of the cold and denser slab.