Seismic risk microzonation mapping using microtremor recordings for an urban area in the vicinity of Lamni Fault, southern Kashmir Basin 

Abstract

A high-angle reverse fault was identified near the Aharbal Falls in the Shopian District of the southern Kashmir Basin and was subsequently named as Lamni Fault by Zahoor et al. (2023). The rise of the Pir Panjal range due to the higher tectonic activity in the southern part of Kashmir (Dar et al., 2014) has led to the overthrusting of the Early Permian Panjal Traps over the Pleistocene Fluvio-Glacial Deposits. The presence of the fault was experimentally validated through the conduction of single-station microtremor horizontal-to-vertical spectral ratio (MHVSR) method, supported by the results from multichannel simulation with one-receiver (MSOR) surface wave tests across the suspected fault zone in Shopian. The fault zone was demarcated at the location by identifying anomalously high H/V amplitudes (>8) at high frequencies (>4Hz) as opposed to the low values (2-3) in the surrounding host rock. As an extension of the work, we seek to conduct the microzonation of the region laced by the Lamni Fault in the Sedow area of Shopian, Kashmir, using the atypical HVSR amplitudes as the markers of high risk. The MHVSR method has been successfully employed for investigating buried and exposed faults like Erft-Sprung normal fault, Tremestieri normal fault, southern Crete of Greece, Longmen Shan fault zone, etc. (Hinzen, 2004; Lombardo and Rigano 2006; Moisidi et al., 2012; Zhang et al., 2019). The low-velocity fractured fault zone is known trap seismic waves and hence lead to large amplifications of the waves which is thus reflected in the HVSR curves as well as in earthquake recordings e.g., in San Andreas fault zone (Li et al., 2000) and San Jacinto fault zone (Roux et al., 2016). Utilising this behaviour of the fractured fault zones, microzonation of the Sedow locality which lies in the vicinity of the detected Lamni Fault was conducted by performing about 50-60 single-station MHVSR tests over an area of about 2 km x 2 km. The area has several residential buildings along with local government school buildings as well as a mosque, thus demanding serious consideration of the increased seismic hazard due to the presence of the fault. Zones of anomalous H/V amplitudes were found in the region surrounded by stable low estimates. This may have serious implications for the seismic hazard estimates in the region surrounding the fault zone also supported in earlier studies in other regions of the world (e.g., Spudich and Olsen, 2001; Donati et al., 2001; Rovelli et al., 2002). It is recommended that a zone be defined at a significant width around the fault zone in which constructions may be avoided or else special considerations for the increased seismic hazard be considered for the existing buildings. Such a buffer zone has been named the Alquist-Priolo zone, APZ in earlier studies (Spudich and Olsen, 2001; Bryant, 2010). The APZ is considered an important inclusion in seismic hazard and microzonation process to aid in urban planning and development in a region.