1962 年および 1983 年三宅島噴火に伴った地震活動の比較

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With the aim of comparing seismicities and finding precursory phenomena for the volcanic eruption, a joint hypocenter determination method is applied to the earthquake swarms which were accompanied with the eruption of Miyakejima in 1962 and 1983. In contrast to rather diverse interpretations of the 1962 swarm given by existing studies, the result of our new analysis shows much tightly clustered distribution of hypocenter, about 10 km width and 15 km long, in the north-west quadrant of the volcano. A small seismic gap across the cluster was recognizable. Spatial distribution of the swarm was consistent with the result of macroseismic observation and extent of damages on the island caused by the strong shocks. Before and after the eruption, we couldn't find any appreciable change of hypocenter distribution which will be effective for a prediction of the eruption. Most of earthquakes which occurred during and after the 1983 eruption were located in the region off the south coast of Miyakejima. The seismic activity may be classified into foreshock-mainshock-aftershock type and is not so intense as that of 1962, with respect to duration of activity and number of strong shocks, although magnitude of the largest event (6.2 by JMA) was larger in 1983 than that of 1962 (5.9 by JMA). Slight migration of epicenters toward the south-east direction was recognized after the eruption terminated, that might be related to the drain back of magma head along the southern fissure line. Focal mechanisms of large shocks were of strike-slip types with maximum pressure axis in the NW-SE direction in the northern area of the volcano and NNW-SSE direction in the southern area. Focal mechanisms of minor events inferred from initial motion of P waves at Miyakejima and other stations were consistent with those of the big events. Hence, there is no need to apply special focal mechanisms, such as a negative single force model nor a tensile-shear crack model, to explain the observational data.
特定非営利活動法人日本火山学会の論文
1985-10-01

1962 年および 1983 年三宅島噴火に伴った地震活動の比較

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