石灯籠の破損から歴史地震の強震動の特徴を推定することは可能か？

——北野天満宮及び石清水八幡宮における検討——

抄録

Stone lanterns in temples and shrines often collapse and suffer damages when these sites are hit by a strong seismic ground motion. There have been several reports in Japan that large numbers of stone lanterns collapse in the direction of strong shaking at large earthquakes. Descriptions that stone lanterns were damaged by strong earthquakes frequently appear in historic Japanese documents. In this report, we examine whether it is possible to retrieve characteristics of historic ground motions from the ages and damages of stone lanterns at Kitano-Tenmangu Shrine and Iwashimizu-Hachimangu Shrine, Kyoto. We assume that if ground motions cause severe damages to stone lanterns, they would be removed from the site and be newly rebuilt thereafter, and occurrence of such events would be imprinted on the age distribution of stone lanterns. If the damages are minor, the damaged parts would be reused when stone lanterns are rebuilt at the site, and the damages of stone lanterns we find today could be used as records of occurrence of historical strong seismic ground motions and the direction of collapses. Scarcity of stone lanterns which were built before 17th century at Iwashimizu-Hachimangu Shrine as well as their increase at Kitano-Tenmangu Shrine in mid 19th century are likely results of strong earthquake and resultant damages at these sites. Ratios between numbers of damaged and undamaged stone lanterns at two sites vary with respect to their ages but they do not correlate with records of historic earthquakes in Kyoto. Damages of stone lanterns we observe today appear to distribute evenly with respect to the cardinal directions, whereas these damages are more often found in the rear of the stone lanterns. While the damages we find today could include those due to the historic earthquakes, stone lanterns at these sites are likely to be so maintained that damaged parts are rotated into the direction which makes the major damages less visible. We conclude that it is very difficult to estimate direction of historic strong motion at these sites from the statistics of damages of stone lanterns which we observe today.