A Study on the Effect of Deep Sedimentary Layrs below the Plain on Earthquake Motions

根据1995年大阪平原、奈良盆地和近平原地区1995年Hyogoken-Nanbu 7.2级地震和1997年3月16日Aichiken Toubu 5.8级地震的观测记录，主要研究了深部沉积层对地震运动特征的影响。地震观测结果表明，被山脉包围的沉积平原的地震运动特征受平原规模和地震动强度的影响很大。对这些观测结果的分析表明：1)1.0s以上的较长周期范围取决于地震动强度和沉积层厚度。优势周期随层深和震级的增加而增大。2)1.0秒以下较短的优势周期范围取决于约100米深度内沉积层的厚度。3)大阪平原的优势周期比奈良盆地的长，后者的沉积层较薄。对于7.2级的Hyogoken-Nanbu地震，大阪平面的优势周期为4.0秒，奈良盆地的优势周期为2.5秒。4)沉积平原的地震活动持续时间取决于平原的大小，特别是区域。大阪平原的持续时间是面积较小的奈良盆地的两倍多。

This study mainly covered the influence of deep sedimentary layrs on earthquake motion characteristics, based on observation records taken from the 1995 Hyogoken-Nanbu Earthquake (Magnitude 7.2) and the 1997 March 16th Aichiken Toubu Earthquake with a magnitude of 5.8 in the Osaka Plain, Nara basin and nearly plains. Earthquake observation results show that earthquake motion characteristics in a sedimentary plain surrounded by mountains are greatly influenced by the scale of plain and magnitude of the earthquake motions.The analysis of these observations are summarized as follows :1)The range of longer period above 1.0 sec.is dependent upon the magnitude of earthquake motions and the thickness of the sedimentary layr. The predominant period increases with the increase in magnitude and depth of layrs.2)The range of shorter prevailing periods under 1.0 sec.is dependent upon the thickness of sedimentary layr within a depth of approx.100 meters.3)The predominant period in the Osaka Plain is longer than that in the Nara Basin, which has a thinner sedimentary layr. In the case of the Hyogoken-Nanbu Earthquake which has a magnitude of 7.2, the predominant period was 4.0 sec.in the Osaka Plan, and 2.5 sec.in the Nara Basin.4)The duration time of earthquake motion in a sedimentary plain depends on size of plain particularly the area. The duration time in the Osaka Plain is more than twice that of the Nara Basin, which has a smaller area.