SEISMOLOGICAL STUDIES OF THE NW HIMALAYA BASED ON EARTHQUAKE DATA RECORDED IN THE GARHWAL HIMALAYA

Abstract

This thesis is based on analysis of seismological data recorded with portable seismograph arrays operated during 1984-86 in the Garhwal segment ofthe northwest Himalaya. Thearrays were operated to monitor the local small and micro-earthquakes of the Garhwal region. The first and main part of the thesis is a report on the analysis of these data. The arrays also picked up earthquakes occurring in the Hindu Kush region of Afghanistan about 900 km tothe north-west. The second andsmaller part of the thesis in an analysis of thetravel times of Pwaves from 22Hindu Kush earthquakes as recorded in the Garhwal Himalaya during 1985-86 season. SEISMICITY AND FAULT PLANE SOLUTION STUDIES In all datafrom 152 small and micro-earthquakes of theGarhwal Himalaya picked up by the seismograph arrays during 1984-86 have been analysed. The work involved examination of seismograms, picking ofphases and their collation to identify earthquakes whose hypocentres could be located from the available readings. Subsequently the locations were also carried out using a program prepared for the purpose in the Department during an early study. The results ofthese analyses were combinedwith the previously available results for 193 small and micro-earthquakes recorded during 1979- 80 in an adjoining segment of the Garhwal Himalaya. The 345 earthquakes define a relatively narrow belt of earthquake epicentres which straddles the Main Central Thrust in the Garhwal Himalaya. This belt coincides with the belt of moderate and small earthquakes located using teleseismic data. The focal depth ofthese earthquakes range between 0 and 30 km, though a vast majority ofthem (85%) occur within the upper16km. Three composite fault plane solutions were determined from reliable first P motion data. The first composite is a strike-slip solution with nodal planes oriented northsouth and east-west. The second fault plane solutions is of the reverse/thrust type ABSTRACT with nodal planes striking subparallel to the local strike of the Himalaya. The nodal plane dipping at 60° towards N42° is picked as the fault plane. The third fault plane solution has nodal planes striking north-south and showing normal fault type slip on them. This fault plane solution has affinity with thefault plane solution of the Kinnaur earthquake of January 15, 1975 with a Ms of 6.0. The reverse/thrust solution is interpreted as suggesting that the Higher Himalaya are currently rising relative to the Lesser Himalaya across a fault zone defined by the seismicitybelt. The strike slip solution is considered to be evidence for lateral horizontal adjustments within the Garhwal Himalaya. The normal fault solution appears to represent concurrent extensional tectonics in the region. But overall, the data support the view that the earthquakes are occurring in response to the under thrusting ofthe Indian Shield material beneath the Himalayan rocks due to the convergence of Indian and Eurasian plates. TRAVEL TIME STUDY The 22 Hindu Kush earthquakes had focal depths in the range of50to245 km. The 154 P travel time readings obtained from them were interpreted using a computer program based on least squared inversion andwritten specifically for the purpose. The uppermantle P wave speed along the north-west Himalaya obtained from this study has a value of8.1 km/s. It appear to persists upto a depthofabout 100km. Below that level, it appears to increase to 8.35 km/s. The upper mantle P wave speed is found to be compareable to that obtainedalong a DSS profile in Kashmir. However it is lower by0.35 km/s. from the value estimated by Ni and Barazangi in 1982.