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

为了研究详细的破裂过程和强地面运动的产生机制，我们在伊豆半岛地区的伊藤地区部署了密集的强地面运动观测台阵，该地区的震群活动反复发生，并且在本项目期间可以预期另一次震群活动。然而，在1998年3月至6月的前一次震群之后的这四年中没有发生任何活动，因此我们还在北方伊豆群岛地区进行了移动的观测，2000年三宅岛的爆发引发了一系列地震。以伊豆半岛附近的关东地区为例，分析了1998年伊豆东邦冲地震（M5. 7）前关东盆地384个台站的强震记录。像这次地震这样发生在伊豆或相模湾的走滑事件，往往会在关东盆地内产生一种称为Love波的长周期强震主导面波，其产生和传播受盆地三维结构的控制。然后，我们进行了矩张量反演和层析分析的观测地震记录从三宅岛火山喷发引起的震群。在地震活动带中发现了震源机制的时空变化，并从观测记录的谱振幅导出了地震活动带中的三维衰减结构。对与火山口坍塌相关的地震运动的分析表明，火山口下方存在缓慢的膨胀和某种单一力量所代表的现象。

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.