Study on lower crustal structure of Northeastren Japan Arc based on waveforms by quarry explosions.

我们收集了东北大学地震观测站记录的采石场爆炸产生的波形，周期为4年。经过动校正后，得到了具有共同中点(站和采石场之间的中点)的波。通过显示这些‘CMP道集’，我们检查了中点以下是否有反射器。在太平洋沿岸莫霍面附近、火山前锋以东20-30公里处和火山前锋以西的中地壳附近发现了反射体。1997年10月，在东北地区连接龟石和磐城的东西向线上进行了地震爆炸实验，在山形县北部设置了一条长约20公里的N-S观测线，记录了在东西向线上爆炸的情况。这条N-S线上设置了33个车站。在花崎(震中距S北线93-103公里)、佐内(80-93公里)和磐城(62-80公里)的炮击中发现了被认为反映在地壳中部(约15公里)的明显的后期震相。来自Iwaki的后续阶段的幅度非常大，是第一个动作的5倍多。然而，来自奥马加里(54-71公里)的波没有显示出明显的后期阶段。东北大学站在N-S线附近获得的记录也显示出类似的特征。通过对初动振幅的分析发现，当波浪穿过秋田县南部日本海沿岸的地壳浅部时，振幅衰减。第一个运动的衰变可能会使磐城的后续阶段变得非常突出。

We collected waveforms which were generated by explosions at quarries and recorded by earthquake observation stations of Tohoku University for the period amount of 4 years. After NMO correction, waves with the common mid point (middle point between a station and a quarry) were gathered. By displaying these 'CMP gather', we checked whether there were reflectors beneath the mid point. Reflectors were found around the Moho depth near the coast of the Pacific Ocean, at depths of 20-30km in the region east of the volcanic front and in the mid crust west of the volcanic front.In Oct., 1997, seismic explosion experiments was carried out along the E-W line connecting Kamaishi and Iwaki in the Tohoku District, We set a N-S observation line of about 20km long in the northern part of Yamagata Prefecture and recorded explosions detonated on the E-W line. 33 stations were put on this N-S line. Clear later phases which were thought to be reflected in the middle part of crust (at depth of about 15km ) were found from shots at Hanamaki (93-103km epicentral distance to the N-S line ) , Sawauchi (80-93km) and Iwaki (62-80km). Amplitudes of later phases from Iwaki were very large and more than 5 times of the first motion. Waves from Omagari (54-71km), however, revealed no distinct later phases. Records obtained by Tohoku University stations near the N-S line showed the similar characteristics. By analyzing amplitudes of first motion, it was found that amplitude decayed when waves passed through the shallow part of the crust along the Japan Sea coast in the south Akita Prefecture. Decay of the first motion may make later phases from Iwaki very prominent.