ATTENUATION CHARACTERISTICS OF UTTARAKHAND HIMALAYA USING STRONG MOTION DATA

Abstract

Himalaya is the most seismically active region in the world. This is the results of collision between the Indian and Eurasian plate, which is still, operated. The collision of these continental plates results in crustal shortening along the northern edges of the Indian plate. This process has given rise to three major thrust planes: the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT) (Gansser, 1964; Molnar and Chen, 1983).The central seismic gap region of the Himalaya which lies in the northern part of the Indian subcontinent is exposed to great seismic hazard. The State of Uttarakhand falls in the central seismic gap and hence any large earthquake in this region will pose great destruction. This region has witnessed two such damaging earthquakes in recent past viz. the Uttarkashi earthquake (Ms 7.1) of 20 October 1991 and the Chamoli earthquake (Ms 6.6) 28 March 1999. Devastation caused by these earthquakes has drawn interest of seismologist to investigate the attenuation characteristics of the medium in this region. It has been seen that the attenuation characteristics of medium plays an important role in the earthquake ground motion at a particular site. Different types of rocks have different attenuating properties. Attenuation characteristics of the medium control the decay of the seismic energy in the lithosphere. Attenuation of the seismic wave is the reduction in the amplitude or energy caused by heterogeneity and anelasticity in the earth. This attenuation can be quantitatively defined by the inverse of the dimensionless quantity known as quality factor Q, this is the ratio of stored to dissipated energy during one cycle of the wave (Johnston and Toksoz 1981). The strong motion data is fundamental for earthquake engineering studies such as attenuation properties, advance structural analysis, seismic hazard analysis and calibration of ground motion predication relationships. Several strong motions have been observed in the Himalaya region in the past. The destruction of these earthquakes is very high. The State of Uttarakhand has been occupied by the Himalayan mountain chain. The faults in this area are capable of generating large magnitude ground motion that would subject adjacent areas to significant ground shaking. Few studies have been carried out to understand the attenuation characteristics of the subsurface medium in this part of the Himalaya, which is mainly due to the scarcity of strong motion data. The main goal of this study is to improve the scientific ii understanding of the physical processes that control strong shaking and to develop reliable estimate of seismic hazard for reduction of loss of life and property during future earthquakes through improved earthquake resistant structure. Strong motion data is important for seismic hazard assessment in the Uttarakhand Himalaya. In the present study attenuation relations have been developed for the Kumaon and Garhwal Himalaya using strong motion data recorded by two regional networks operated in these regions. Damped least square inversion technique given by Livenberg (1944) has been used for obtaining peak ground attenuation relations. Dependency of the developed attenuation relations on distance parameters has been also checked. Obtained attenuation relation for the Kumaon and Garhwal Himalaya is further used to test the normality and model adequacies, which satisfied this test. Although worldwide attenuation relationships given by various researchers have not been satisfied this test with the Himalaya earthquakes data set. This test confirms the suitability of the developed attenuation relations for regional studies. To validate the developed attenuation relations, strong ground motion records have been simulated for the 1991 Uttarkashi and the 1999 Chamoli earthquakes which occurred in recent past in the Uttarakhand Himalaya. Semi empirical simulation technique given by Midorikawa (1993) has been used in this study. In the recent years semi empirical simulation technique of strong ground motion has developed as an effective tool to simulate strong motion records. Semi empirical technique has been extensively tested for its applicability in simulation of strong ground motion by Midorikawa (1993), Joshi and Patel (1997), Joshi et al. (1999, 2001), Kumar et al. (1997), Joshi (2001, 2004), Joshi and Midorikawa (2004), Joshi and Mohan (2008). This simulation technique is in turn dependent on attenuation relation of peak ground acceleration. The source parameters of the Uttarkashi and Chamoli earthquakes have been used for earlier studies. Strong motion records of the Uttarkashi and Chamoli earthquakes have been simulated at nine stations, which recorded these earthquakes. Comparison of the observed and simulated peak ground acceleration is made in terms of root mean square error (RMSE). Simulation records of these earthquakes give the good results, which validated developed attenuation relation. Attenuation study plays an important role for safe deign of engineering structure in seismically active regions. In the present thesis, comparative study of attenuation trend in the iii Kumaon and Garhwal Himalaya has been investigated using regional strong motion data. Coda wave quality factor (Qc(f)) has been observed for these regions. Similar strong motion dataset of nine earthquakes recorded at six stations has been used for both regions. Single backscattering technique given by Aki and Chouet (1975) has been used in the present study. Observed coda wave quality factor (Qc(f)) value has been compared with other relations given for the Indian and other worldwide regions, which falls in the range and justified these results. Observed coda wave quality factor (Qc(f)) for both region gives the different trend of attenuation characteristics of seismic wave. The Kumaon Himalaya gives the less coda wave quality factor (Qc(f)) as compared to the Garhwal Himalaya region i.e. the Kumaon Himalaya has high attenuation medium beneath the surface. This attenuation trend has also observed in developed relations of peak ground attenuation in this work. A modified seismic hazard technique of seismic hazard zonation initially given by Joshi and Patel (1997) has been used for seismic hazard zonation of the Uttarakhand Himalaya region in the present study. This technique has been used in different parts of the Himalaya. Worldwide attenuation relations of peak ground acceleration given by Abrahamson and Litehiser (1989) have been used by earlier researchers for seismic zonation. Modified seismic hazard zonation technique and developed attenuation relations have been used in the present work. Seismic hazard zonation map for probability of exceendance of peak ground acceleration of 100 and 200 gals have been prepared in the present work. These map shows that the many place of the Uttarakhand regions falls in high hazardous zone.