1996年8月秋田・宮城県境付近の地震群のK-NET強震記録から推定した震源・伝播・サイト特性

Abstract

We estimated source spectra, an attenuation function, and site amplification factors using the K-NET records for six earthquakes (MJ=4.4-5.9) that occurred in the border of Akita and Miyagi Prefectures in August, 1996. These earthquakes occurred inland at the focal depths from 6 to 11km. The hypocentral distances of the stations used in this study range from 5 to 200km. We analyzed 212 source-station pairs×3 components, that is, radial, transverse, and vertical components. The Q-1 estimated by assuming the geometrical spreading to be the reciprocal of the hypocentral distance showed strong decay with frequency. It was modeled by Q-1=0.005f-1.5 in the frequency range from 0.3 to 1Hz in which Q-1 had almost the same value among the three components. We also found that the Q-1 has no clear difference between the eastern and western regions to the volcanic front. The site amplification factor at IWT09 (Daito station) in the Kitakami region whose geology is categorized into pre-Tertiary rock is smallest in the frequency range from 0.1 to 4Hz. Therefore we examined site amplification factors and source spectra in the frequency range from 0.1 to 4Hz using IWT09 as a reference site. The site amplification factors relative to IWT09 vary from site to site very strongly and their standard deviation for all the used 59 sites is the largest, about a factor of 2.5 of the average, for the horizontal components in the frequency range from 0.5 to 2Hz. For the vertical components that is a factor of 2.2 of the average in the similar frequency range. The source spectra for all the six earthquakes have predominant peaks at the frequency about 0.3Hz for all the three components. We found that the remarkable dominance of the horizontal components in the frequency of 0.3Hz at MYG05 (Naruko, the nearest station with an epicentral distance of about 10km) are caused by the source spectra as well as its site amplification. The Brune's stress drops were estimated to be 5 bar, much smaller than the average stress drop of 200 bar estimated for subduction-zone earthquakes in the Tohoku district. Acceleration magnitude Mpga, which is determined to match the observed peak ground acceleration with an empirical attenuation relation, is 0.3 to 0.7 smaller than the JMA magnitude MJ. The relation between the Mpga and the seismic moment M0 of the Harvard CMT solution, also suggested low static stress drops from 5 to 15 bar. However, the relation between the MJ and M0 is consistent with the SATO's empirical relation which is equivalent to the static stress drops of 50 bar. We interpreted this discrepancy to be caused by the prominent generation of waves in the frequency range around 0.3Hz at which MJ is determined.