Attenuation Tomography Based on Strong Motion Data: Case Study of Central Honshu Region, Japan
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Pure and Applied Geophysics
Attenuation Tomography Based on Strong Motion Data: Case Study of Central Honshu Region, Japan PARVEEN KUMAR,1 A. JOSHI,1 and O. P. VERMA1 Abstract—Three-dimensional frequency dependent S-wave quality factor (Qb(f)) value for the central Honshu region of Japan has been determined in this paper using an algorithm based on inversion of strong motion data. The method of inversion for determination of three-dimensional attenuation coefficients is proposed by HASHIDA and SHIMAZAKI (J Phys Earth. 32, 299–316, 1984) and has been used and modified by JOSHI (Curr Sci. 90, 581–585, 2006; Nat Hazards. 43, 129–146, 2007) and JOSHI et al. (J. Seismol. 14, 247–272, 2010). Twenty-one earthquakes digitally recorded on strong motion stations of Kik-net network have been used in this work. The magnitude of these earthquake ranges from 3.1 to 4.2 and depth ranging from 5 to 20 km, respectively. The borehole data having high signal to noise ratio and minimum site effect is used in the present work. The attenuation structure is determined by dividing the entire area into twenty-five three-dimensional blocks of uniform thickness having different frequency-dependent shear wave quality factor. Shear wave quality factor values have been determined at frequencies of 2.5, 7.0 and 10 Hz from record in a rectangular grid defined by 35.4°N to 36.4°N and 137.2°E to 138.2°E. The obtained attenuation structure is compared with the available geological features in the region and comparison shows that the obtained structure is capable of resolving important tectonic features present in the area. The proposed attenuation structure is 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. Key words: Quality factor, attenuation, strong motion network, inversion.
1. Introduction The attenuation property of the medium is related to the seismicity and regional tectonic activity of a particular area. The attenuation can be quantitatively defined by the inverse of the dimensionless quantity
1 Indian Institute of Technology, Roorkee, Uttaranchal, India. E-mail: [email protected]; anushijos@yahoo. co.in;
known as quality factor (Q) which is the ratio of stored energy to the dissipated energy during one cycle of the wave (JOHNSTON and TOKSO¨Z 1981). The amplitude of earthquake ground motion at observation site is influenced by the source characteristics, travel path and local site conditions. The effects of travel path on earthquake ground motion are directly related to the attenuation property of medium. It has been shown by MIDORIKAWA (1980) that Qb and shear wave velocity (Vs) are empirically related with each other. Therefore, the estimate of Qb gives direct estimate of both the rock properties and the seismic hazard. Analysis of strong motion accelerogram usually shows that peak ground acceleration is associated with the arrival of S-waves (HADLEY and ORCUTT 1982). Therefore S-wave quality factor (Qb) is used t
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