Locked Frontal and Lateral Ramps on the Main Himalayan Thrust Beneath NW Himalaya Illuminated by Precisely Located Seismicity

• Published in: Geophysical research letters 2024-11, Vol.51 (21), p.n/a
• Main Authors: Shamim, S. K., Ghosh, Ayon, Mitra, Supriyo, Priestley, Keith, Sharma, Swati, Wanchoo, Sunil Kumar
• Format: Article
• Language: English
• Subjects: Convergence; Depth; Dipping; Earthquakes; Exhumation; Fault zones; Geological faults; mid‐crustal ramps; NW Himalaya; Ramps; Relocation; Rupture; Rupturing; S waves; Seismic activity; Seismicity; Thrust; Waveforms
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL110160

The Kashmir “seismic gap” in NW Himalaya is marked by hinterland‐to‐foreland reduction in GPS‐geodetic arc‐normal convergence‐velocity and increase in horizontal strain‐rate, associated with occurrence of moderate‐to‐small earthquakes. We analyze continuous waveforms from Jammu and Kashmir seismological network (2015–2017) to detect and preliminarily locate 1064 events, followed by probabilistic non‐linear relocation of 360 well‐located local earthquakes, with magnitudes 0.6–4.7 and hypocentral depths 0–60 km. Hypocenters shallower than 20 km lie on or above the Main Himalayan Thrust (MHT), clustered beneath the Kishtwar Higher‐Himalaya. Hinterlandward dipping seismicity‐clusters coincide with a MHT mid‐crustal frontal ramp, marking a ∼ ${\sim} $50 km wide locked‐to‐creep transition. The frictionally‐locked up‐dip MHT segment is ∼ ${\sim} $100 km wide, capable of hosting a Mw∼ ${M}_{w}\sim $ 8.4 earthquake, if ruptured completely. Clustered seismicity SW of the Kishtwar Window and to its east illuminate MHT lateral‐ramps, which may limit the rupture width and/or modulate the rupture propagation. Entirely seismogenic underthrust Indian‐crust poses additional hazard from large earthquakes within it. Plain Language Summary In the NW Himalaya, the region between the 1905 Kangra and 2005 Kashmir earthquake rupture zones is known as the Kashmir ’seismic gap’. The last major earthquake in this “gap” was the 1555 magnitude ∼ ${\sim} $8 event. From the measured convergence rate across the NW Himalaya it is estimated that this region has accumulated sufficient strain‐energy to cause a major‐to‐great earthquake. We established the Jammu and Kashmir seismological network in 2013 to study the seismicity in this “gap.” Continuous ground‐motion data from 2015 to 2017 has been used to detect and locate 1064 events. After visual assessment and accurate P‐ and S‐wave arrival‐time picks, we relocated earthquakes using a probabilistic non‐linear location method. These 360 well‐located local earthquakes have magnitudes of 0.6–4.7 and source depths of 0–60 km. Depth distribution of these earthquakes indicate that the entire crust deforms brittlely. The majority of these earthquakes are shallower than 20 km and are clustered beneath the Jammu‐Kishtwar Higher‐Himalaya. They originated on or above the Main Himalayan Thrust (MHT), a fault zone which separates the underthrusting Indian crust from the overriding Himalaya. The distribution of the clustered seismicity illuminates struc