Abstract
An inversion method was applied to crustal earthquakes dataset to find S-wave attenuation characteristics beneath the Eastern Tohoku region of Japan. Accelerograms from 85 shallow crustal earthquakes up to 25 km depth and magnitude range between 3.5 and 5.5 were analyzed to estimate the seismic quality factor Q s. A homogeneous attenuation model Q s for the wave propagation path was evaluated from spectral amplitudes, at 24 different frequencies between 0.5 and 20 Hz by using generalized inversion technique. To do this, non-parametric attenuation functions were calculated to observe spectral amplitude decay with hypocentral distance. Then, these functions were parameterized to estimate Q s. It was found that in Eastern Tohoku region, the Q s frequency dependence can be approximated with the function 33 f 1.22 within a frequency range between 0.5 and 20 Hz. However, the frequency dependence of Q s in the frequency range between 0.5 and 6 Hz is best approximated by Q s (f) = 36 f 0.94 showing relatively weaker frequency dependence as compared to the relation Q s (f) = 6 f 2.09 for the frequency range between 6 and 15 Hz. These results could be used to estimate source and site parameters for seismic hazard assessment in the region.
Highlights
IntroductionEastern Tohoku mainly comprises Iwate and Miyagi prefectures
As a result of inversion, the frequency dependence of the attenuation of seismic waves as well as source characteristics and site response can be found (Parolai et al 2000; Bindi et al 2006). These dataset from Eastern Tohoku region allows us to make stable and reliable estimates of attenuation properties as it covers a reasonable range of magnitudes, distances and focal depths
A strong frequency dependence of Qs is found between the frequency range of (0.5 and 20 Hz) and is given by the relation Qs (f) = 33 f 1.22
Summary
Eastern Tohoku mainly comprises Iwate and Miyagi prefectures. The region is situated over one of the most active tectonic regimes of the world where Pacific plate is subducting under the Eurasian plate, generating numerous earthquakes in this area. There is a close connection between the topography of the Tohoku region and the distribution of active faults. The topography of the Tohoku region is characterized by alternate mountain ranges and lowlands running in a north– south direction. The primary active faults of the Tohoku region lie at the boundaries of the mountains and the lowlands
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