ATTENUATION STUDIES FROM INVERSION OF STRONG MOTION DATA

Abstract

The amplitude of seismic energy recorded at recording sites is influenced by the source characteristics, medium characteristic and local site conditions. The effects of travel path on earthquake ground motion are directly related to the attenuation property of medium. The knowledge of attenuation characteristic is an essential requirement for determination of the earthquake source parameters and simulation of strong ground motions. It plays an important role in estimation of the seismic hazard of a region. Attenuation property of earth medium is quantitatively defined by a dimensionless quantity know as quality factor ‘Q’ which defined as the fractional loss of energy per cycle (Knopoff, 1964). It is observed from analysis of strong motion accelerogram that peak ground acceleration is associated with the arrival of S-waves (Hadley et al., 1982). In this thesis shear wave quality factor (Qβ) is used to characterize the attenuation studies of Kumaon Himalaya, India and Central Honshu, Japan region. The inversion of strong motion data has been used for determination of the attenuation studies in the different parts of the Himalayan region by Joshi (2006a, 2006b and 2007) and Joshi et al. (2010). Joshi (2006a and 2007) and Joshi et al. (2010) have used inversion technique to determine three dimensional attenuation structure based on the shear wave quality factor. The inversion algorithm given by Joshi et al. (2010) for estimation of three dimensional distribution of attenuation property uses strong motion data from limited number of events. Modification in this technique has been made in the present thesis to consider large number of events for better estimation of attenuation characteristic of the region. Various numerical experiments have been made to check the stability of this inversion algorithm. In this work three dimensional attenuation structures have been determined for the Central Honshu, Japan and Kumaon Himalaya, India region to validate the present modified technique. Strong motion data has been used by Joshi (2006b) to obtain frequency dependent shear wave quality factor from inversion of spectral acceleration data. This algorithm estimates simultaneously both the frequency dependent attenuation relation and site effect. Seismic moment is used as one of the main input in the algorithm developed by Joshi (2006b) for the estimation of frequency dependent shear wave quality factor from inversion of strong motion data. Seismic moment used in this algorithm is computed or assumed from independent sources of well studied earthquakes. Modification in this algorithm is made to compute seismic moment directly from the records and refine its value in the inversion scheme. ii Three dimensional frequency dependent S-wave quality factor (Qβ(f)) values for the central Honshu region of Japan have been determined in this thesis using modified algorithm based on inversion of strong motion data. Twenty one earthquakes digitally recorded on strong motion stations of KiK-net network have been used in this work. The borehole data at rock sites having high signal to noise ratio and minimum site effect has been used in this work. The attenuation structure is determined by dividing the entire area into 25 three dimensional blocks of uniform thickness having different frequency dependent shear wave quality factor. Values of shear wave quality factor have been determined at different frequencies. The obtained attenuation structure is compared with the major tectonic features in the region. The comparison shows that the obtained attenuation structure is capable of resolving major tectonic features present in the area. The proposed attenuation structure is further compared with the probabilistic seismic hazard map of the region and shows that it bears some remarkable similarity in the patterns seen in seismic hazard map though two approaches of obtaining hazards are totally different. In this thesis, the frequency dependent shear wave quality factor (Qβ(f)) has been calculated using strong motion data of the Kumaon Himalaya. A dense network of strong ground motion recorders in the Kumaon region of the Uttarakhand Himalaya is operating since 2006. This network has recorded 294 earthquakes upto July, 2013. A total of forty events recorded on this network have been used for this work. The developed algorithm for inversion of strong motion data gives simultaneously both the shear wave quality factor and the site effect. The site effects obtained from the inversion are compared with the technique given by Lermo and Chavez-Garcia (1993). The comparison of site effect from inversion and H/V technique proposed by Lermo and Chavez-Garcia (1993) indicate that the obtained site effects are well within the standard limit of error. The Qβ(f) values at different stations are calculated by using both the North South (NS) and East West (EW) component of acceleration records. The Qβ(f) values obtained from both NS and EW components at 16 stations have been used to compute a regional relationship for the Kumaon Himalaya of form Qβ(f) = (28±2.1)f (1.2±0.09). Kumar et al. (2005b) suggests that low Qo (<200) and high n (>0.8) value indicate high tectonic and seismic active region. Therefore obtained Qβ(f) relation revealed that Kumaon region lies in tectonic and seismic active region. In this thesis, three dimensional attenuation structures based on the frequency dependent shear wave quality factor values have been determined for the Kumaon Himalaya region. Eighteen events recorded on Kumaon network have been used for the present work. Shear wave quality factor values have been estimated at different frequencies for two different rectangular blocks of iii surface dimension 85×55 km and 90×30 km which lie in the Kumaon Himalaya region. Both blocks are divided into 25 three dimensional blocks of uniform thickness having different Qβ(f) values. The three dimensional distributions of frequency dependent shear wave quality factor values in two different blocks provide attenuation property of the region. The observed contours of shear wave quality factor show comparable trends with the major tectonic units present in the region. The site amplification and frequency dependent shear wave quality factor determined at 16 stations from inversion of strong motion data have been used to compute the source parameters of the Sikkim earthquake (Mw = 6.9) of 18 September, 2011. This earthquake is recorded on six stations of strong motion network in the Uttarakhand Himalaya located about 900 km away from the epicenter of this earthquake. In this work the spectrum of S-phase recorded at these far field stations has been corrected for site amplification term and anelastic attenuation at source and recording site, respectively. The obtained source spectrum from acceleration records is compared with the theoretical source spectrum defined by Brune (1970) at each station for both horizontal components of the records. Iterative forward modeling of theoretical source spectrum give the average estimate of seismic moment (Mo), source radius (ro) and stress drop ( σ) as (3.2 ±0.8) ×1026 dyne-cm, 13.3 ±0.8 km and 59.2 ±8.8 bars, respectively for the Sikkim earthquake.