OBSERVATIONAL RESEARCH ON CRUSTAL STRUCTURE AROUND M6.1 EARTHQUAKE FAULT

通过分析1998 - 2003年人工地震实验激发的地震信号和太平洋板块边界地震科地震波，探讨1998年岩手火山西南方向发生6.1级地震前后地壳结构的时间变化。在东北大学固定台站（GNB， ANS， AKM）和临时台站（MTI）安装的短周期和宽带地震仪记录了主动炮激发的地震信号。我们用交叉谱法估计了6次发射的相邻两个地震图的相位差。然后，我们计算了两次射击时结构的地震速度变化。结果表明，在断层北缘的MTI和GNB站，发生6.1级地震的第1炮和第2炮的地震速度下降了约1.5%。然后，从1998年（在M6.1方程之后）到2002年，观测到地震速度增加了约0-0.5%。从2002年到2003年，再次观测到大约0.3%的速度增加。地震科的互相关分析也显示出6.1级地震和岩手火山前后的非均质性有显著的时间变化。我们推测，1998 - 2002年观测到的这些浅层变化与6.1级地震的应力释放和积累以及岩手山在这一时期强烈的火山活动引起的结构变化有关。我们进一步确定了6.1级地震周边的三维纵波速度结构，以及6.1级地震的滑动和高频波辐射带的空间分布。

We investigate temporal changes of crustal structure around the 1998 M6.1 earthquake occurred south west of Volcano Iwate by analyzing seismic signals excited by artificial seismic experiments from 1998 to 2003 and by analyzing seismic waves of earthquake families occurring on the Pacific plate boundary. The seismic signals excited by the active shots were recorded by short period and broad band seismometers installed at permanent stations (GNB, ANS, AKM) of Tohoku University and a temporal station (MTI). We estimate phase difference of nearby two seismograms for the six shots by using a cross spectrum method. Then, we evaluate the seismic velocity changes of the structure during the two shots. The results show that the seismic velocity decreases about 1.5% at MTI and GNB stations, which are located at the northern edges of the fault, for shot 1 and shot 2, during which the M6.1 earthquake occurred. Then, the seismic velocity increases of about 0-0.5% is observed for the data from 1998 (after the M6.1 eq.) to 2002. From 2002 to 2003, about 0.3% velocity increase is observed again. Cross-correlation analyses of the earthquake families also show a significant temporal change of heterogeneity around the M6.1 earthquake and Volcano Iwate. We infer that these changes in shallow that are observed from 1998 to 2002 are related to stress release and accumulation of the M6.1 earthquakes and/or structure changes due to the intense volcanic activity of Mt Iwate at this period. We further determined a three-dimensional P-wave velocity structure around the M6.1 earthquake and the spatial distribution of slip and high-frequency wave radiation zone for the M6.1 earthquakes.