1872 年浜田地震による石見畳ヶ浦の波食棚隆起可能性の検討：タフォニの分布と形成期間に基づく一考察

The Hamada earthquake of 1872 was a big natural disaster in the San’in region that caused 536 casualties. Some of earlier researches suggested that the wave-cut bench at Iwami-tatamigaura, was uplifted during that event. The surface of the bench is characterized by linear arrangements of calcareous concretions which are described as oblate ellipsoids with height, major axis and minor axis of around 30, 50 and 40 cm, respectively. Tafoni (singular tafone), which are produced by salt weathering processes, develop on the surface of the calcareous concretions. Tafone depth can be described as a function of time, providing a measure of the duration during which individual concretions are exposed to salt weathering. The time of tafoni formation will therefore be equal to the exposure of the bench to subaerial conditions, because salt weathering never happens in a submarine environment.
Tafoni depths were measured to calculate the period of tafoni formation, to discuss possible uplift of the bench during the 1872 Hamada Earthquake. An equation proposed by Sunamura and Aoki (2011) was used to calculate the formation period of tafoni. The tafoni depths were substituted into the equation together with physical and mechanical properties of the concretions. The relationship between altitudes of concretions and tafoni depths on their surface was also investigated to discuss local uplift or subsidence of the bench. The measurements from Iwami-tatamigaura were compared with those from a proxy site for undisturbed tafoni growth, providing a calibrated vertical profile of tafone depth from a comparable environment. Results show that the duration for tafoni formation is calculated to be ca. 5,000 ～ 7,000 years, indicating that the tafoni formation started during the Jomon transgression.
The maximum rate for tafone formation is estimated to be 5.9×10 － 3 mm yr － 1 at Iwamitatamigaura, resulting in the maximum tafone depth to be 0.88 mm that can be formed for 150 years since the occurrence of the 1872 Hamada Earthquake to the present moment. This depth is much smaller than the actual measured depths. The comparison of the vertical distribution of tafone depth, which is defined as the relationship between normalized tafone depth (the ratio of tafone depth to the maximum depth measured in this study) and its normalized altitude (altitude of a tafone divided by mean high water level), showed similar vertical profiles between Iwami-tatamigaura and the proxy site. These results indicate that the 1872 Hamada Earthquake caused no particular uplift or subsidence of the wave-cut bench at Iwami-tatamigaura.