Two local earthquakes on April 11 and 21, 1979, which occurred 10km south off Hakodate, recorded seismic intensities III in JMA scale in and around Hakodate city.
This study was designed to confirm that a difference on the patterns of seismic zoning maps exists between at a remarkable event and such a local one. So far as is known, comparatively little work has been done about questionnaires for local events with magnitudes 4.0 or less. Immediately after the earthquakes, the questionnaires were distributed to the persons living in and around Hakodate city. Answers to the questionnaires were retured from about 3000 persons a few days after, and seismic microzoning maps were constructed. The maps show that seismic intensities depend strongly on the subsurface structures at sites; for example, the intensities in unstable places of alluvial formations are, on an average, about 0.3 less than those in relatively stable places of diluvial formations. On the contrary, the former places show about 0.3 greater than the later for the cose of the remarkable event of the 1968 Tokachi oki earthquake (M=7.9).
In addition to the difference of the seismic intensities related to the geological age, the local intensities vary with S-wave velocities in the subsurface structures. The seismic exploration data previously obtained provide with no interface of S-wave velocity discontinuity down to a depth of 40m at least in the region, Nanaehama, while those provide with an clear S-wave velocity interface at a depth of about 15m in the region, Kamiyunokawa. The local intensities in Nanaehama are not so large for the present events though it places in unstable alluvial formations, while, the local intensities around Kamiyunokawa show nearly the maximum in the whole Hakodate. These facts strongly suggest that the subsurface structures act as band-pass or high-cut filters owing to the existances of the clear boundaries of S-wave velocity discontinuities.
Moreover, the predominat periods of incoming seismic waves should also be connected with the response of the subsurface structures. The predominant periods of particle velocities of these local earthquakes were compared with those of the 1968 Tokachi oki eartquake using the empirical relation after SATO (1979). The comparison shows that there exists a considerable difference among these periods; the longer the period, the deeper structure is reflected on it. This shows that the contribution of the inherent characteristics of the incident wave to the seismic intensity is not negligible.
Nevertheless, the local intensities is, on the average, related simply to epicentral distances as
I=-0.1r+3.9
where r and I represent the epicentral distance in km and the average intensity at the distance r, respectively.