Evidence of blind thrusting in the frontal part of the Central Himalaya and its significance for rupture evolution and size of a medieval earthquake

Abstract

The central Himalaya has remained the prime focus for paleoseismic investigations to decipher the timing and extent of past great earthquakes. Several damaging earthquakes in the medieval period have been reported from the central Himalaya from historical data and paleoseismic studies, but their source and rupture extent remain debated. Results of paleoseismic investigation near longitude 84°E along the Himalayan frontal thrust in the Central Himalaya, south of the Chitwan intermontane valley, in northern Bihar, India are presented in this study. Trench excavated at the base of a frontal scarp near the Sofa temple in the Manguraha-Mahajogin Village reveals evidence for folding of the young sediment sequences in the interval between 600 and 1600 CE. We associate this folding with blind thrusts in the subsurface that may have been incremented in one or more earthquakes during this interval. Nevertheless, it cannot be ruled out that other ruptures may be present in the area. It is not possible from our data to determine whether deformation at Sofa Temple site occurred in the 1100, 1255, 1344 or 1505 earthquakes. Therefore, we explore various scenarios that may have produced the deformation. It is possible that incremental fold growth occurred during all four earthquakes. But, in the most likely scenario, if the fold growth occurred in 1100 CE and was associated with rupture during that earthquake at other locations along the MFT, it would require an earthquake of magnitude ∼8.9 ± 0.2. While co-seismic offsets associated with the 1100 CE and 1255 CE event have been recorded at nearby trench locations, the lack of surface ruptures at the site of the present Sofa temple trench highlights on the need to investigate lateral heterogeneities in the Himalayan décollement that could lead to inconsistencies along the rupture plane.