京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22759号 / 工博第4758号 / 新制||工||1744(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 三村 衛, 教授 渦岡 良介, 准教授 肥後 陽介 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18241号 / 工博第3833号 / 新制||工||1587(附属図書館) / 31099 / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 木村 亮, 教授 三村 衛, 准教授 木元 小百合 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Planet earth is restless and one cannot control its inside activities and vibrations those leading to natural hazards. Earthquake is one of such natural hazards that have affected the mankind most. Most of the causalities due to earthquakes happened not because of earthquakes as such, but because of poorly designed structures which could not withstand the earthquake forces. The improper building construction techniques adopted and the high population density are the major causes of the heavy damage due to earthquakes. The damage due to earthquakes can be reduced by following proper construction techniques, taking into consideration of appropriate forces on the structure that can be caused due to future earthquakes. The steps towards seismic hazard evaluation are very essential to estimate an optimal and reliable value of possible earthquake ground motion during a specific time period. These predicted values can be an input to assess the seismic vulnerability of an area based on which new construction and the restoration works of existing structures can be carried out. A large number of devastating earthquakes have occurred in India in the past. The northern region of India, which is along the plate boundary of the Indian plate with the Eurasian plate, is seismically very active. The north eastern movement of Indian plate has caused deformation in the Himalayan region, Tibet and the North Eastern India. Along the Himalayan belt, the Indian and Eurasian plates converge at the rate of about 50 mm/year (Bilham 2004; Jade 2004). The North East Indian (NEI) region is known as one of the most seismically active regions in the world. However the peninsular India, which is far away from the plate boundary, is a stable continental region, which is considered to be of moderate seismic activity. Even though, the activity is considered to be moderate in the Peninsular India, world’s deadliest earthquake occurred in this region (Bhuj earthquake 2001). The rapid drifting of Indian plate towards Himalayas in the north east direction with a high velocity along with its low plate thickness might be the cause of high seismicity of the Indian region. Bureau of Indian Standard has published a seismic zonation map in 1962 and revised it in 1966, 1970, 1984 and 2002. The latest version of the seismic zoning map of India assigns four levels of seismicity for the entire Country in terms of different zone factors. The main drawback of the seismic zonation code of India (BIS-1893, 2002) is that, it is based on the past seismic activity and not based on a scientific seismic hazard analysis. Several seismic hazard studies, which were taken up in the recent years, have shown that the hazard values given by BIS-1893 (2002) need to be revised (Raghu Kanth and Iyengar 2006; Vipin et al. 2009; Mahajan et al. 2009 etc.). These facts necessitate a comprehensive study for evaluating the seismic hazard of India and development of a seismic zonation map of India based on the Peak Ground Acceleration (PGA) values. The objective of this thesis is to estimate the seismic hazard of entire India using updated seismicity data based on the latest and different methodologies. The major outcomes of the thesis can be summarized as follows. An updated earthquake catalog that is uniform in moment magnitude, has been prepared for India and adjoining areas for the period till 2010. Region specific magnitude scaling relations have been established for the study region, which facilitated the generation of a homogenous earthquake catalog. By carefully converting the original magnitudes to unified MW magnitudes, we have removed a major obstacle for consistent assessment of seismic hazards in India. The earthquake catalog was declustered to remove the aftershocks and foreshocks. Out of 203448 events in the raw catalog, 75.3% were found to be dependent events and remaining 50317 events were identified as main shocks of which 27146 events were of MW ≥ 4. The completeness analysis of the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website A quantitative study of the spatial distribution of the seismicity rate across India and its vicinity has been performed. The lower b values obtained in shield regions imply that the energy released in these regions is mostly from large magnitude events. The b value of northeast India and Andaman Nicobar region is around unity which implies that the energy released is compatible for both smaller and larger events. The effect of aftershocks in the seismicity parameters was also studied. Maximum likelihood estimations of the b value from the raw and declustered earthquake catalogs show significant changes leading to a larger proportion of low magnitude events as foreshocks and aftershocks. The inclusions of dependent events in the catalog affect the relative abundance of low and high magnitude earthquakes. Thus, greater inclusion of dependent events leads to higher b values and higher activity rate. Hence, the seismicity parameters obtained from the declustered catalog is valid as they tend to follow a Poisson distribution. Mmax does not significantly change, since it depends on the largest observed magnitude rather than the inclusion of dependent events (foreshocks and aftershocks). The spatial variation of the seismicity parameters can be used as a base to identify regions of similar characteristics and to delineate regional seismic source zones. Further, Regions of similar seismicity characteristics were identified based on fault alignment, earthquake event distribution and spatial variation of seismicity parameters. 104 regional seismic source zones were delineated which are inevitable input to seismic hazard analysis. Separate subsets of the catalog were created for each of these zones and seismicity analysis was done for each zone after estimating the cutoff magnitude. The frequency magnitude distribution plots of all the source zones can be found at http://civil.iisc.ernet.in/~sitharam . There is considerable variation in seismicity parameters and magnitude of completeness across the study area. The b values for various regions vary from a lower value of 0.5 to a higher value of 1.5. The a value for different zones vary from a lower value of 2 to a higher value of 10. The analysis of seismicity parameters shows that there is considerable difference in the earthquake recurrence rate and Mmax in India. The coordinates of these source zones and the seismicity parameters a, b & Mmax estimated can be directly input into the Probabilistic seismic hazard analysis. The seismic hazard evaluation of the Indian landmass based on a state-of-the art Probabilistic Seismic Hazard Analysis (PSHA) study has been performed using the classical Cornell–McGuire approach with different source models and attenuation relations. The most recent knowledge of seismic activity in the region has been used to evaluate the hazard incorporating uncertainty associated with different modeling parameters as well as spatial and temporal uncertainties. The PSHA has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models (source models, maximum magnitude in hazard computations, and ground-motion attenuation relationships). The hazard maps have been produced for horizontal ground motion at bedrock level (Shear wave velocity ≥ 3.6 km/s) and compared with the earlier studies like Bhatia et al., 1999 (India and adjoining areas); Seeber et al, 1999 (Maharashtra state); Jaiswal and Sinha, 2007 (Peninsular India); Sitharam and Vipin, 2011 (South India); Menon et al., 2010 (Tamilnadu). It was observed that the seismic hazard is moderate in Peninsular shield (except the Kutch region of Gujarat), but the hazard in the North and Northeast India and Andaman-Nicobar region is very high. The ground motion predicted from the present study will not only give hazard values for design of structures, but also will help in deciding the locations of important structures such as nuclear power plants. The evaluation of surface level PGA values is of very high importance in the engineering design. The surface level PGA values were evaluated for the entire study area for four NEHRP site classes using appropriate amplification factors. If the site class at any location in the study area is known, then the ground level PGA values can be obtained from the respective map. In the absence of VS30 values, the site classes can be identified based on local geological conditions. Thus this method provides a simplified methodology for evaluating the surface level PGA values. The evaluation of PGA values for different site classes were evaluated based on the PGA values obtained from the DSHA and PSHA. This thesis also presents VS30 characterization of entire country based on the topographic gradient using existing correlations. Further, surface level PGA contour map was developed based on the same. Liquefaction is the conversion of formally stable cohesionless soils to a fluid mass, due to increase in pore pressure and is prominent in areas that have groundwater near the surface and sandy soil. Soil liquefaction has been observed during the earthquakes because of the sudden dynamic earthquake load, which in turn increases the pore pressure. The evaluation of liquefaction potential involves evaluation of earthquake loading and evaluation of soil resistance to liquefaction. In the present work, the spatial variation of the SPT value required to prevent liquefaction has been estimated using a probabilistic methodology, for entire India. To summarize, the major contribution of this thesis are the development of region specific magnitude correlations suitable for Indian subcontinent and an updated homogeneous earthquake catalog for India that is uniform in moment magnitude scale. The delineation and characterization of regional seismic source zones for a vast country like India is a unique contribution, which requires reasonable observation and engineering judgement. Considering complex seismotectonic set up of the country, the present work employed numerous methodologies (DSHA and PSHA) in analyzing the seismic hazard using appropriate instrument such as the logic tree to explicitly account for epistemic uncertainties considering alternative models (For Source model, Mmax estimation and Ground motion prediction equations) to estimate the PGA value at bedrock level. Further, VS30 characterization of India was done based on the topographic gradient, as a first level approach, which facilitated the development of surface level PGA map for entire country using appropriate amplification factors. Above factors make the present work very unique and comprehensive touching various aspects of seismic hazard. It is hoped that the methodology and outcomes presented in this thesis will be beneficial to practicing engineers and researchers working in the area of seismology and geotechnical engineering in particular and to the society as a whole.

Tee present research focuses on seismic hazard studies for the states of Tripura and Mizoram in the North-East India with taking into account the complex sesismotectonic characteristics of the region. This area is more prone to earthquake hazard due to complex subsurface geology, peculiar topographical distribution, continuous crustal deformation due to the under thrusting of Indian and the Eurasian plates, a possible seismic gap, and many active intraplate sources identified within this region. The study area encompasses major seismic source zones such as Indo Burmese Range (IBR), Shillong Plateau (SP), Eastern Himalayan arc (EH), Bengal Basin (BB), Mishmi Thrust (MT) and Naga Thrust (NT). Five historical earthquakes of magnitude Mw>8 have been listed in the study area and 15 events of magnitude Mw>7 have occurred in last 100 years. Indian seismic code BIS-1893-2002 places the study area with a high level of seismic hazard in the country (i.e. seismic zone V). More than 60% of the area is hilly steep-terrain in nature and the altitude varies from 0 to 3000 meters. Recent works have located a seismic gap, known as the Assam gap since 1950 between the EH, SP, and IBR with the Eurasian plate. Various researchers have estimated the return period, and a large size earthquake is expected in this region any time in future. The area is also highly prone to liquefaction, since rivers in Tripura (Gomati, Howrah, Dhalai, Manu, Bijay, Jeri, Feni) and the rivers in Mizoram (Chhimtuipui, Tlawng, Tut, Tuirial and Tuivawl etc.) are scattered throughout the study area where soil deposits are of sedimentary type. In 2011, both the states together have experienced 37 earthquakes (including foreshocks and aftershocks) with magnitudes ranging from 2.9 to 6.9. Of these events, there were 23 earthquakes (M>4) of magnitudes M6.4 (Feb 4th 2011), M6.7 (March 24th 2011), M6.9 (Sept.18th 2011), M6.4 (October 30th 2011), M6.9 (Dec 13th 2011), M5.8 (Nov 21st 2011), M5 (Aug 18th 2011), M4.9 (July 28th 2011), M4.6 (Dec 15th 2011), M4.6 (Jan 21st 2011), M4.5 (Dec 9th 2011), M4.5 (Oct 21th 2011), M4.5 (Oct 17th 2011), M4.5 (Sept 18th 2011), M4.3 (Oct 10th 2011), M4.3 (Sept 22nd 2011), M4.3 (April 4th 2011), M4.2 (Sept 9th 2011), M4.2 (Sept 18th 2011), M4.1 (April 29th 2011), M4.1 (Feb 22nd 2011), M4 (June 9th 2011), and M4 (Dec 2nd 2011) which occurred within this region [source: IMD (Indian Metrological Department), India]. The earthquake (M6.9) that occurred on Sept. 18th 2011 is known as the Sikkim earthquake, and it caused immense destruction including building collapse, landslides, causalities, disrupted connectivity by road damages and other infrastructural damages in Sikkim state as well as the entire North-East India. In the cities of Agartala and Aizawl of Tripura and Mizoram, construction of high rise building is highly restricted by the Government. Being the capital city, many modern infrastructures are still pending for growth of the city planning. Although many researchers have studied and reported about the status of seismicity in North-East Region of India, very few detailed studies have been carried out in this region except Guwahati, Sikkim and Manipur where almost the whole of the study area is highly vulnerable to severe shaking, amplification, liquefaction, and landslide. From the available literature, no specific study exists for Tripura and Mizoram till date. In the present research, seismic hazard assessment has been performed based on spatial-temporal distribution of seismicity and fault rupture characteristics of the region. The seismic events were collected from regions covering about 500 km from the political boundary of the study area. The earthquake data were collected from various national and international seismological agencies such as the IMD, Geological Survey of India (GSI), United State Geological Survey (USGS), and International Seismological Centre (ISC) etc. As the collected events were in different magnitude scales, all the events were homogenized to a unified moment magnitude scale using recent magnitude conversion relations (region specific) developed by the authors for North-East Region of India. The dependent events (foreshocks and aftershocks) were removed using declustering algorithm and in total 3251 declustered events (main shocks) were identified in the study area since 1731 to 2011. The data set contains 825 events of MW < 4, 1279 events of MW from 4 to 4.9, 996 events MW from 5 to 5.9, 131 events MW from 6 to 6.9, 15 events MW from 7 to 7.9 and 5 events MW ≥8. The statistical analysis was carried out for data completeness (Stepp, 1972). The whole region was divided into six seismic source zones based on the updated seismicity characteristics, fault rupture mechanism, size of earthquake magnitude and the epicentral depth. Separate catalogs were used for each zone, and seismicity parameters a and b were estimated for each source zone and other necessary parameters such as mean magnitude (Mmean), Mmax, Mmin, Mc and recurrence periods were also estimated. Toposheets/vector maps of the study area were collected and seismic sources were identified and characterized as line, point, and areal sources. Linear seismic sources were identified from the Seismotectonic atlas (SEISAT, 2000) published by the GSI, in addition to the source details collected from available literature and remote sensing images. The SEISAT map contains 43 maps presented in 42 sheets covering entire India and adjacent countries with 1:1million scale. Sheets representing the features of the study area were scanned, digitized and georeferenced using MapInfo 10.0 version. After this, tectonic features and seismicity events were superimposed on the map of the study area to prepare a Seismotectonic Map with a scale of 1:1million. In seismic hazard assessment, a state of art well known methodologies (deterministic and probabilistic) was used. In deterministic seismic hazard analysis (DSHA) procedure, hazard assessment is based on the minimum distance between sources to site considering the maximum magnitude occurred at each source. In hazard estimation procedure a lot of uncertainties are involved, which can be explained by probabilistic seismic hazard analysis (PSHA) procedure related to the source, magnitude, distance, and local site conditions. The attenuation relations proposed by Atkinson and Boore (2003), and Gupta (2010) are used in this analysis. Because in this region two type activities are mostly observed, regions such as SP, and EH are under plate boundary zone whereas IBR is under subduction process. These equations (GMPEs) were validated with the observed PGA (Peak ground acceleration) values before use in the hazard evaluation. The hazard curves for all six major sources were prepared and compiled to get the total hazard curve which represents the cumulative hazard of all sources. Evaluation of PGA, Sa (0.2s and 1.0s) parameters at bedrock level were estimated considering a grid size of 5 km x 5 km, and spectral acceleration values corresponding to a certain level of probability (2% and 10%) were done to develop uniform hazard spectrum (UHS) for both the cities (Agartala and Aizawl). To carry out the seismic microzonation of Agartala and Aizawl cities, a detailed study using geotechnical and geophysical data has been carried out for site characterization and evaluation of site response according to NEHRP (National Earthquake Hazard Response Program) soil classifications (A, B, C, D, and E-type). Seismic site characterization, which is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to have the idea about the average dynamic behavior of soil deposits, and thus helps to evaluate the surface level response. A series of geophysical tests at selected locations have been conducted using multichannel analysis of surface waves (MASW) technique, which is an advanced method to obtain direct shear wave velocity profiles from in situ measurements for both the cities. Based on the present study a major part of Agartala city falls under site class D, very few portions come under site class E. On the other hand, Aizawl city comes under site class C. Next, a detailed site response analysis has been carried out for both the cities. This study addresses the influence of local geology and soil conditions on incoming ground motion. Subsurface geotechnical (SPT) and geophysical (MASW) data have been obtained and used to estimate surface level response. The vulnerable seismic source has been identified based on DSHA. Due to the lack of strong motion time history of the study area, synthetic ground motion time histories have been generated using point source seismological model (Boore 2003) at bedrock level based on fault rupture parameters such as stress drop, quality factor, frequency range, magnitude, hypocentral distance etc. Dynamic properties such as the shear modulus (G) and damping ratios (ζ) have been evaluated from the soil properties obtained from SPT bore log data collected from different agencies such as PWD (Public works Department), and Urban Development Dept. of the State Government, in situ shear wave velocity has been obtained from MASW survey in different locations, and following this, a site response analysis has been carried out using SHAKE-2000 to calculate the responses at the ground surface in combination of different magnitudes, distances and epicentral depth for a particular site class. An amplification factor was estimated as the ratio of the PGA at the ground surface to the PGA at bedrock level, a regression analysis was carried out to evaluate period dependant site coefficients, and hence, the period dependant hazard impact on the ground surface could be calculated to obtain the spatial variation of PGA over the study area. Further, liquefaction potential of the site (Agartala) was also evaluated using available SPT bore log data collected and using presently estimated surface level PGA. The results are presented in the form of liquefaction hazard map representing as a Factor of safety (FS) against liquefaction with various depths such as 1.5m, 10m, and 15m respectively. It has been seen that Agartala city shows highly prone to liquefaction even up to 15 m depth. Hence, site specific study is highly recommended for implementing any important project. The liquefaction hazard assessment could not be conducted for the Aizawl city because of non availability of the SPT-N data, however, the city stands on hills/mountains, and therefore, such a study is not applicable in this area. Further, seismic microzonation maps for both the cities have been prepared considering Analytical Hierarchy Process (AHP) which support to the Eigen value properties of the system. Two types of hazard maps have been developed, one using deterministic and another using the probabilistic seismic microzonation maps. These maps can be directly used as inputs for earthquake resistant design, and disaster mitigation planning of the study area. However, an investigation has also been made in forecasting a major earthquake (Mw>6) in North-East India using several probabilistic models such as Gamma, Weibull and lognormal models. IBR and EH show a high probability of occurrences in the next 5 years (i.e. 2013-2018) with >90% probability.