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

基于采石场爆炸波形的日本东北弧下地壳结构研究

• 批准号: 09640503
• 负责人: HASEMI Akiko
• 金额: $ 2.11万
• 依托单位: Yamagata University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 1997
• 资助国家: 日本
• 起止时间: 1997 至 1998
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-09640503/
• 关键词: lower crust; artifitial source; Tohoku District; reflectors; 東北日本; 地震波形

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.

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，为期4年。动校正后，采集了具有公共中点（站场和采石场之间的中点）的波。通过显示这些“CMP”，我们检查了中点下方是否有反射器。在太平洋沿岸莫霍面附近、火山前缘以东20- 30 km深度和火山前缘以西中地壳中发现了反射体。1997年，在东北地区连接釜石和磐城的东西线上进行了沿着的地震爆炸实验，我们在山形县北方设置了一条长约20 km的南北观测线，并记录了东西线上的爆炸。这条南北线共设33个车站。在花卷（震中距南北线93- 103 km）、佐坂内（80- 93 km）和磐城（62- 80 km）的地震中发现了明显的后期震相，这些震相被认为反映在地壳中部（深度约15 km）。岩城地震的后期震相振幅很大，是第一次震相振幅的5倍以上。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。东北大学站在南北线附近的记录也显示出类似的特征。通过对初动振幅的分析，发现当波浪通过秋田县南部日本海沿岸的沿着地壳浅部时，振幅衰减。第一个运动的衰减可能会使来自岩城的后期阶段非常突出。