Abstract
To clarify the effect of surface geology on seismic motion in Ofunato City, Iwate Prefecture, a survey on detailed seismic intensity distribution was done using questionnaires for the off-Miyagi earthquake ( Mj 7.1 ) occurred at May 26, 2003. The questionnaire revised by Ohta et al.(1998) was used for 5 calculating seismic intensities. 3,387 questionnaires were distributed for parents of students of 14 elementary schools and 8 junior high schools in Ofunato City. The seismic intensities estimated from questionnaires were averaged for 250m square meshes to clarify the spatial variation of seismic intensities. To avoid differences among individuals for questionnaire survey, the effective meshes where the number of the questionnaire was more than three were used for analysis. The number of the 10 effective mesh was 212. The seismic intensities were ranging from 6.4 to 4.1, and the average was 5.1 in Ofunato City. It was revealed that the seismic intensity at the central area of Ofunato City was large, and the one around the area was small.
In order to clarify the difference of seismic intensities for each area in Ofunato City, we carried out conventional microtremor array observations at 14 sites. The array consists of four vertical 15 seismometers with an interval of 6 m. Phase velocities of Rayleigh waves were estimated from array records of vertical microtremors by spatial auto-correlation (SPAC) method. Average S-wave velocities (AVS) between a surface and 30 m depth were estimated from phase velocities of Rayleigh waves with a wavelength of 40 m by using empirical relations proposed by Nagao and Konno (2000). We compared to the estimated AVS distribution with the seismic intensity distribution. The correlation coefficient 20 between the seismic intensity averaged with a 250m mesh and AVS30 was -0.68. Because the geological structure changes abruptly in a small area in Ofunato City, we calculated the seismic intensities averaged from the observation point to a radius of 100 m to 500 m, and examined the correlation between the seismic intensities and the average S wave velocities (AVS10 to AVS30). It was revealed that the AVS was small at the area where the seismic intensity was large. As a result, it 25 was found that the correlation coefficient between the seismic intensity averaged with a radius of 250m and AVS30 was -0.82. It was also clarified that the correlation coefficient of AVS20 was -0.81 as well as the coefficient of AVS30.
