EXECUTIVE SUMMARY On request and initiative of the Hon’ble Chief Minister of Assam, Sri Tarun Gogoi, the Department of Science & Technology, Government of India constituted a national level Expert Group vide DST Office Order No. DST/ Exp Group/Guwahati-microzonation/2002 dated August 19, 2002 (Annexure-I) to carry out the Seismic Microzonation of Guwahati Region. The Expert Group met for the first time at AMTRON in Guwahati on 12th and 13th of September, 2002 and thereby set the plan of action and the broad framework for the participating institutions. It was acknowledged that the Seismic Microzonation of Guwahati Region posed a challenge hitherto unattempted, when compared with the similar work being carried out by DST for the city of Jabalpur and New Delhi. In case of Guwahati, most of the data would have to be collected afresh, meaning it would require extensive ground survey, including geophysical, geomorphological, land use, land cover, ground noise response spectra, basement configurations and landslide hazard mapping, deployment of strong motion accelerographs, studies on Peak Ground Acceleration etc. Accordingly, the Expert Group set out the tasks and priorities for the participating members and institutions. The Groups so far held eight sittings (two at AMTRON, two at IIT Guwahati & four at IIT Kharagpur Salt Lake City Campus, Kolkata), and has monitored the progress of work, held critical review of the outcome, and thus, ensured quality output at every stage. IDENTIFICATION OF WORK COMPONENTS FOR THE FIRST PHASE Having the foregoing information in the background the following items of work were identified and carried out as the first step towards seismic microzonation of Guwahati region mostly based on available data. 1. Preparation of Base map on 1:25,000 scale of Guwahati Region on a digital platform by AMTRON. 2. Compilation and synthesis of geological and geomorphological map of Guwahati region on the base of SOI toposheets on 1:25,000 scale by GSI. 3. Preparation of bed rock contour map of the area on 1:25,000 scale from the results of vertical electrical resistivity sounding vii surveys carried out by GSI during 1986-87 and 1987-88, supplemented by similar data generated by GSI during 2002-03 field seasons. These data were compared and contrasted with similar set of data obtained by DGM from about 30 bore holes in the basin area. Finally a synthesized basement contour map was prepared by examining both the sets of data and superimposed the same on the geological map of the area. 4. Preparation of landslide hazard zonation map of the hilly tract of the area on 1:25,000 scale by GSI. 5. A seismotectonic map, prepared by GSI of the area of 200km radius around Guwahati based mostly on the data published by GSI in the Seismotectonic Atlas of India (2000). 6. Recent satellite imageries of the area studied by AMTRON to verify and update the change in landforms and land use pattern from that given in the SOI toposheets surveyed during 1986-87 to achieve an updated landuse map of the study area. 7. Instrumental site response studies by using ambient noise (Nakamura ratio technique) were carried out by GSI, IMD and RRL-J to determine the maximum site amplification factors and corresponding peak frequency covering almost the entire area at 141 sites. 8. 200 Borehole geotechnical studies by Assam Engineering College. 9. Compilation of results of macroseismic surveys of the past major earthquakes affecting Guwahati town by D.R. Nandy, Retd. Director, GSI. 10. All the data digitized and stored at AMTRON, Guwahati. 11. Map analysis (raster & vector) for Microzonation by IIT Kharagpur. 12. Empirical site response synthesis by IIT Kharagpur. 13. Estimation of scenario earthquake magnitude from the earthquake catalogue of 140 years by IIT Kharagpur. 14. Site amplification, classification, strong motion spectral acceleration analysis and synthesis by IIT Kharagpur, IIT Guwahati, Assam Engineering College, Geological Survey of India, IMD and RRL Jorhat. 30 15. Site classification map on GIS platform using shear wave velocity (VS ), viii site response, predominant frequency and factor of safety by IIT Kharagpur. 16. Demography and Preliminary Seismic Population Risk Assessment by Jorhat Engineering College and IIT Kharagpur. 17. Integration of all the thematic layers and preparation of Seismic Microzonation map on GIS platform in the scale of 1:25,000 by IIT Kharagpur. 18. Integration of Seismic Microzonation and demographic distribution maps on GIS platform for the preparation of Preliminary Seismic Population Risk map in the scale of 1:25,000 by IIT Kharagpur. 19. Data repository, web based GIS and hosting on a special website by AMTRON. Due to untiring effort of the Group and initiatives of DST in sorting out issues arising out of inter institutional coordination; the basic groundwork has been completed, collated and compiled for Guwahati Region. Finally following maps have been prepared and included in the Atlas Map I Satellite image of Guwahati Region Map II Landuse map of Guwahati Region Map III Geological & Geomorphological map of Guwahati Region Map IV Basement contour map of Guwahati Region Map V Basement zonation map of Guwahati Region Map VI Seismotectonic map of Guwahati Region covering 200km radius of Guwahati Area Map VII Landslide hazard zonation map of Guwahati Region Map VIII Bedrock Section Profile (1) Map IX Bedrock Section Profile (2) Map X Bedrock Section Profile (3) Map XI Road network map of Guwahati Region Map XII Borehole location map of Guwahati Region ix Map XIII Ambient noise survey location map (Predominant frequency observation sites) Map XIV Boreholes, Ambient noise survey and Strong motion station map 30 Map XV Shear wave velocity(Vs ) contour map of Guwahati Region 30 Map XVI Shear wave velocity(Vs ) zonation map of Guwahati Region Map XVII Factor of Safety contour map of Guwahati Region Map XVIII Factor of Safety zonation map of Guwahati Region Map XIX Bulk density contour map of Guwahati Region Map XX Predominant frequency contour map of Guwahati Region Map XXI Predominant frequency distribution map of Guwahati Region Map XXII Site response contour map of Guwahati Region Map XXIII Site response distribution map of Guwahati Region Map XXIV Site classification of Guwahati Region based on 30 integration of Vs , predominant frequency, site response and factor of safety Map XXV Peak Ground Acceleration (PGA) map of Guwahati Region. PGA is estimated through Spectral Strong Motion Synthesis by Brune ω2-circular Crack source model for an SEM of Mw 8.7 nucleating from the 1897 Great Shillong Earthquake of Mw 8.7 Map XXVI Peak Ground Acceleration (PGA) map of Guwahati Region. PGA is estimated through Spectral Strong Motion Synthesis by Synthetic Green’s Function Simulation for an SEM of Mw 8.7 nucleating from the 1897 Great Shillong Earthquake of Mw 8.7 Map XXVII Seismic Microzonation map of Guwahati Region (PGA from Green’s Function Simulation) x Map XXVIII Seismic Microzonation map of Guwahati Region with PGA contours overlaid Map XXIX Demographic distribution map of Guwahati Region Map XXX Preliminary Seismic Population Risk map of Guwahati Region This report gives indepth analysis of the data collated and compiled for the above mentioned themes. The data has been given in the Data volume, 27 Map Plates and 3 Section profiles. The Site response was computed empirically at all 141 predominant frequency observation sites and as well as at five strong motion stations using waveform data. Site response contour map was drawn taking all the sites into consideration. This map matches very well with the surface geology and 30 shear wave velocity map (Vs ). Earthquake catalogue derived from ISC and USGS from 1866 - 2006 were used for computing the Scenario Earthquake Magnitude (SEM) for Guwahati region. It is to be noted that the historical earthquakes 1897 Shillong, 1918 Srimangal, and 1950 Assam constrained this catalogued estimation. Peak Ground Acceleration (PGA) is estimated 2 through Spectral Strong Motion Synthesis by both Brune ω -circular Crack source model and impulsive source function using Empirical Green’s Function Simulation for an SEM of Mw 8.7 nucleating from hypocenter of the 1897 Great Shillong Earthquake. PGA is contoured for the region with the SEM at the focus of the 1897 Shillong Earthquake to create a seismic hazard scenario in the region. It is found that PGA varies monotonically between 0.15g - 0.93g (1g = 1000gal) in the region. Through the overlaying and subsequent integration of Geology and Geomorphology (GG), Basement (B), Landslide 30 Zones (L) and Seismological themes such as Shear Wave Velocity (Vs ), Site Response (SR), Peak Ground Acceleration (PGA) and Predominant Frequency (PF) on GIS platform, the final seismic hazard Microzonation map is prepared. Five zones are mapped where the average PSHI index is 0.55, 0.44, 0.35, 0.26 and 0.16. We termed these zones as very high, high, moderate, low and very low hazard regions. An integration of this microzonation vector layer with the demographic distribution of the urban center yielded a preliminary seismic population risk map of the region. Prof. Sankar Kumar Nath IIT Kharagpur xi CHAPTER 1 Introduction and Background Information 1.1 INTRODUCTION The northeast India and its adjoining regions are characterized by high seismic activity. This region encompasses the northern part of the Assam-Arakan geological province and includes the eastern Himalayas, the Indo-Myanmar arc, the Mishmi Massif, the Shillong Plateau and adjoining parts, the Tripura folded belt, the Assam intermountain depression, and the northern part of the Bengal basin. The earlier seismicity studies of the region were mostly those of investigations of the major earthquakes originating from the region. One of the first scientific study could be associated with the classic work of Oldham (Oldham,1899) on the 1897 Great Assam Earthquake.