Signatures of the Existence of Frontal and Lateral Ramp Structures Near the Kishtwar Window of the Jammu and Kashmir Himalaya: Evidence From Microseismicity and Source Mechanisms

Abstract

We study the Kashmir seismic gap using data from a network of local broadband stations spanning southeastern Kashmir Valley and Jammu. We detected and located several hundred earthquakes using continuous data recorded in our network. The earthquakes (ML 1.0–5.0) were relocated using probabilistic and relative location methods to obtain a subset of events with depth and spatial uncertainty ≤1.5 km. The earthquakes were found to cluster along two adjacent lines parallel to the strike of the Himalaya but at different depths. The SW cluster was found to be shallower (4–15 km) than the NE cluster (13–18 km). The events in the SW cluster shallowed toward NW from SE. We calculated the source mechanism of larger earthquakes (ML≥3.0) using global and local data sets. The source mechanism results show dominant thrust motion of the earthquakes in the NE cluster. Considering nodal planes dipping to the northeast as fault planes, we obtained the model of a steep frontal ramp close to the locked portion of the Main Himalayan Thrust. The model obtained for SW cluster of seismicity showed the presence of a lateral ramp dipping to the SE. Normal faulting was observed in the SW cluster. Our analysis shows two possible causes for the existence of these normal faults—the existing strike-slip motion loading/unloading stresses on the lateral heterogeneities within the decollement or the transfer of potential energy by sediment yield of the river Chenab. The elevated flat situated northwest of the lateral ramp gives rise to very shallow microseismicity (4–7 km).