歴史時代の白頭山の火山活動

Abstract

Recently, it was reported that Mt. Beakdu experienced at least four or five large volcanic eruptions in geological and historical times, and that the lake Cheonji had been formed by the collapse of a part of the mountain's summit (Wei et al., 1998). The last of four eruptions occurred in historical times. Geologists attempted to estimate the period of the eruptions using radio carbon isotope dating, but the results showed a variety of periods ranging from approximately AD 8th to 14th centuries, which are the dates of the Balae and Goryo dynasties. Unfortunately, there are no distinct records of eruptions during this period. In the present study, we suggest that the last great volcanic eruption occurred in winter when there was a strong northwestern seasonal wind, based on the distribution of pumice on satellite images and the thickness of pumice layers measured at sites in relationship to the climatic environment. On the other hand, some researchers interpreted that five events described in the Joseon Dynasty relate to volcanic eruptions of Mt. Baekdu. Those events occurred in 1413, 1593, 1668, 1702, and 1903. Their interpretations are widely cited in journals and books; however, based on critical reviews of historical literature including Joseon-wangjo-sillog ("Chronology of the Joseon Dynasty"), we found that three of the events were not related to volcanic eruptions of Mt. Beakdu. Events in 1413 and 1668 were Asian yellow sand storm. The event in 1903 recorded in Chinese literature (Liu, 1909) was found to be a shower of rain and hail accompanied by thunder and lightning. Only the two events in 1597 and 1702 were confirmed to be related to volcanic activities of Mt. Beakdu. According to Joseon-wangjo-sillog, a large earthquake with the maximum intensity of 9.0 on the Modified Mercalli Intensity scale (MMI) and its aftershocks occurred at the boulder region of Samsu county, Hamgyeongdo Province, in 1597. The document reveals that they were detected in Hamgyeondo (MMI6) and in Chungcheong-do (MMI5) over three days. The mainshock was accompanied by a volcanic explosion at Wangtian, which is located 35km southwest from Mt. Baekdu. The site is one of the three eruption centers of the Mt.Beakdu volcanic body. A document from China reveals that a large earthquake with an estimated magnitude of 7.0 and aftershocks occurred in the Gulf of Bohai on the same day as the Samsu earthquakes in Hamgyeongdo province. The shakes and disturbances observed eight times in Hamgyeong-do province might not be directly related to the large earthquake in the Gulf of Bohai. However, two series of earthquakes reported at two locations on the same day imply that there may be close relationships between the genesis of the two events. Based on phenomena observed recently, such as increased frequency of earthquakes, upheaval of ground level, releases of volcanic gases, and increased temperature of hot springs near the summit of Mt. Beakdu, the possibility of eruptions or explosions at the mountain in the near future has been suggested. Recently, scientists from the United States of America, Japan, Canada, and Germany were invited to investigate activity of the mountain. They agreed that the mountain is a dormant volcano and presents a temporal hazard.