Study on detailed rupture processes and generation mechanism of strong ground motions by dense array observations in an earthquake swarm area

为了详细研究强地面运动的破裂过程和产生机制，我们在伊豆半岛地区的伊藤附近地区部署了密集的强震观测阵列，该地区地震群活动多次发生，预计在本项目期间还会发生另一次地震。然而，在1998年3月至6月的上一次地震群之后的这四年里，没有任何活动发生，因此我们还在2000年宫崎岛火山爆发引发地震群的伊豆北部地区进行了流动观测。他们给我们带来了一个巨大的强震记录数据集，包括31次M5级地震和5次M6级地震。对于伊豆半岛地区的伊藤外地区，我们分析了1998年伊豆东湖大地震（5.7级）在项目之前在关东盆地384个站点观测到的强震记录。在伊豆或相模湾发生的这种走滑事件，如这次地震，往往会在关东盆地产生较长周期强运动的主导面波，称为Love波，它们的产生和传播受盆地三维结构的控制。然后，对三宅岛火山喷发引起的地震群观测到的地震图进行矩张量反演和层析分析。地震活跃区震源机制存在时空变化，地震活跃区的三维衰减结构也可由观测记录的频谱幅值推导。对与火山口坍塌有关的地震运动的分析表明，缓慢的膨胀和火山口下单一力量所代表的一些现象。

For investigating detailed rupture processes and generation mechanism of strong ground motions, we deployed a dense strong-motion observation array in the off-Ito area of the Izu peninsula region, where earthquake swarm activities were repeated and another one can be expected during the term of this project. However, any activity has not yer occurred in these four years after the previous swarm from March to June 1998, and so we also carried out mobile observations in the northern Izu islands region, where the eruption of Miyakejima induced a swarm of earthquakes in 2000. They brought us a huge dataset of strong motion records for many middle-size earthquakes including thirty-one M5 events and five M6 events.For the off-Ito area of the Izu peninsula region, we analyze the strong motion records observed at 384 stations in the Kanto basin for the 1998 Izu Toho Oki earthquake (M5.7) immediately before the project term. Such a strike-slip event in Izu or the Sagami bay as this earthquake often generates dominant surface waves called Love waves for rather long-period strong motions in the Kanto basion, and their generation and propagation are controlled by the three-dimensional structure of the basin. We then carry out moment tensor inversions and tomographic analyzes for the observed seismograms from the earthquake swarm induced by the Miyakejima eruption. Spatio-temporal variations in the earthquake source mechanism are found in the seismically active zone, and the three-dimensional attenuation structure in the zone is also derived from the spectral amplitudes of the observed records. The analysis of the seismic motions associated with the caldera collapse indicates slow inflation and some phenomenon represented by a single force beneath the crater.