OBSERVATIONAL RESEARCH ON CRUSTAL STRUCTURE AROUND M6.1 EARTHQUAKE FAULT

通过对1998年至2003年岩手岛火山西南6.1级地震前后人工地震试验激发的地震信号的分析和太平洋板块边界地震族地震波的分析，研究了1998年岩手岛火山西南6.1级地震前后地壳结构的时间变化。用安装在东北大学常设台(GNB、ANS、AKM)和临时台(MTI)的短周期宽带地震仪记录了活动炮击激发的地震信号。我们用交叉谱的方法估计了六次激发的相邻两条地震记录的相位差。然后，我们评估了两次激发过程中结构的地震速度变化。结果表明，6.1级地震发生时，位于断层北缘的MTI台和GNB台的震源1和炮点2的地震速度下降了约1.5%。从1998年(6.1级地震后)开始，观测到地震波速增加了约0-0.5%。到2002年。从2002年到2003年，又出现了约0.3%的速度增长。地震家族的互相关分析也表明6.1级地震和岩手岛火山前后的非均质性存在显著的时间变化。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和/或岩手山强烈的火山活动引起的构造变化有关。进一步确定了6.1级地震前后的三维P波速度结构和6.1级地震的滑动和高频波辐射区的空间分布。

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.