Re-estimation of the location and size of the pre-instrumented 1 September 1803 Garhwal-Kumaon Himalaya earthquake: Evidence from site characterization and strong motion seismology

The historical 1 September 1803 Garhwal-Kumaon Himalaya earthquake (Mw 8.1) was a solitary Himalayan earthquake reported felt widely in the Indo-Gangetic Plain with severe shaking observed from Punjab to Bengal. There was no common consensus among 9 different researchers on the source location and size of the Preinstrumented 1 September 1803 earthquake. In this study, we attempted to resolve the issues based on source, path, and site characterization by estimating peak ground accelerations (PGAs) and correlation between estimated and reported intensities based on the concept of strong motion seismology. In order to arrive at a plausible epicenter location of the pre-instrumented 1803 earthquake, we conducted a detailed postmortem, for the first time of its all of 9-different source locations propounded by different researchers and examined those locations based on the nature and extent of damage pattern; extent of earthquake shaking; intensity at source rock; source characterization; path characterization; and estimates of peak ground acceleration (PGA) at different earthquake hypocenters. In order to achieve a better correlation, the hypocentre is considered at 11 locations at MHT from Higher Himalaya to Sub Himalaya. The surface PGA and MMI are estimated at all locations where the intensity was reported for the 1803 earthquake by considering about 100 scenario earthquakes. Out of nine locations proposed by previous researchers, the five locations fall in the highest and one in the higher seismic attenuation zone that negates the nature and extent of earthquake shaking up to the distance of more than 1200 km at Kolkata then Calcutta where the intensity was reported to be IV. The other locations propsed in least atteuation zone were of less magnitude. We, therefore infer that the 1803 earthquake occurred at the depth of 20 km located in the least seismic attenuation zone in the NW Himalaya with a location of 31.2oN, 78.7 degrees E (130 km north of the HFT, 50 km north of the 1991 Uttarkashi earthquake) with a rupture dimension of 190 x 81 Sq. km with a slip of 3.0 m that corresponds to a magnitude of Mw 8.1. We also inferred that the mainshock rupture was directed toward the south along the Main Himalayan Thrust and the rupture found stopped at 50 km north of the HFT below the Ton Thrust in the Lesser Himalaya. This study also confirms that the rupture of the 1803 Garhwal-Kumaon earthquake has not reached up to the HFT. The 1991 Uttarkashi and 1999 Chamoli earthquakes are inferred to be located at the southern end of the 1803 Garhwal Kumaon earthquake rupture area. A slip deficiency of 1.3 m has been computed with reference to the source zones of the 1803 Garhwal-Kumaon, 1991 Uttarkashi, and 1999 Chamoli earthquakes, suggesting a maximum magnitude potential of Mw 7.2 The Tehri Dam is located approximately 20 km south of the 1991 Uttarkashi earthquake source zone and is believed to lie outside the rupture zone of the 1803 Garhwal-Kumaon earthquake.

Automated summary

This study attempts to resolve the source, path, and site characterization issues of the 1803 Garhwal-Kumaon Himalaya earthquake by estimating peak ground accelerations and correlation between estimated and reported intensities. The study suggests that the earthquake occurred at a depth of 20 km in the least seismic attenuation zone with a magnitude of 8.1.