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

我们收集了采石场爆炸产生的波形，并由东北大学地震观测站记录了4年的波形。经过NMO校正后，收集了具有共同中点（站与采石场之间的中点）的波。通过显示这些“CMP集合”，我们检查了中点下方是否有反射器。在太平洋海岸附近的莫霍深度附近，火山锋以东20-30km的深度以及火山锋以西的地壳中部，都发现了反射体。1997年10月，在东北地区釜石和岩城之间的东西向线进行了地震爆炸实验，在山形县北部设置了长约20公里的南北观测线，记录了东西向线上发生的爆炸。这条南北向线路上有33个车站。在花木（震中距南北线93-103公里）、Sawauchi（震中距80-93公里）和Iwaki（震中距62-80公里）的拍摄中发现了被认为反映在地壳中部（深度约15公里）的清晰的后期阶段。岩城地震后期的振幅非常大，是第一次地震的5倍多。然而，来自Omagari（54-71公里）的波没有显示出明显的后期阶段。东北大学北南线附近的台站记录也显示了类似的特征。通过对首次运动振幅的分析发现，当地震波穿过日本秋田县南部沿海的地壳浅层时，振幅衰减。第一个运动的衰减可能使Iwaki的后期阶段非常突出。

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.