On a Formation Mechanism of Topography and Its Relation to Earthquake Occurrence in Southwest Japan
On a Formation Mechanism of Topography and Its Relation to Earthquake Occurrence in Southwest Japan
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日本西南部地形形成机制及其与地震的关系
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发表时间：
1984
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通讯作者：
Kazuo Mino
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作者：
Kazuo Mino
对日本西南部的地形和地震发生进行了详细分析，以探讨其形成机制。应用两种方法来分析形貌。一种是二维傅里叶分析，另一种是带通滤波。这里使用的数据是日本国土测绘院公布的平均高程，用经度和纬度分别为1.5'和1'的单位矩形内的网格数据表示。二维傅里叶分析结果表明，40 km左右的波长似乎是一个边界值，对应的区域相对较宽，所有区域都观察到了大于40 km的波长，而小于40 km的波长则表现出较大的局域性。参考二维傅里叶分析的结果，应用了Seya的带通滤波。提取的形貌分为长波长、中波长和短波长。存在两种长波长（70-150 公里）的主要地形。一种是在中国和四国地区进行电子战罢工，另一种是在中部地区进行国民服役罢工。在短波长和中波长范围内，中部地区和日本西南部外围地区的地形表现出与长波长大致相同的较规则的图案，但在短波长和中波长范围内，中国和近畿地区的详细地形更为复杂。中国和近畿地区的上地壳似乎可能由于南北向和东西向挤压的双重褶皱而被分成四边形，并以走滑型断层为边界。这些地壳块体的边界被认为是相对较宽的破碎带，可能是由块体运动产生的。大地震多发生在褶皱隆起区与沉降区的交界处，这可能与褶皱发育引起的断层相对应。微地震分布似乎与长波长的地形有密切关系，另一方面也与非常短波长的地形有密切关系。这两种现象是由于微震发生极其频繁且震源规模非常小造成的。河流和尖锐凹口的出现常常与微震分布有关。这表明这些细微结构是地壳中存在的应力作用在表面的表现。日本西南部的所有这些地形特征从根本上可以用自第四纪晚期以来持续的太平洋板块和菲律宾海板块俯冲造成的东西向和南北向的两次挤压来解释。现今的构造应力被认为是地质时代构造应力的延伸，它仍然不仅导致地形的形成，而且导致包括地震在内的地壳活动。
Topography and earthquake occurrence in Southwest Japan were analysed in detail to investigate their formation mechanism. Two methods were applied to analyse topography. One is a two-demensional Fourier analysis and the other is a band-pass filtering. The data used here are the mean elevation published by the Geographical Survey Institute of Japan, which are represented by the mesh data in a unit rectangle of 1.5' and 1' along longitude and latitude, respectively. The results of the two-dimensional Fourier analysis indicate that a wavelength of about 40 km appears to be a boundary value corresponding to relatively wide areas, wavelengths longer than 40 km are observed over all regions, and those shorter than 40 km show large locality. Seya's band-pass filtering was applied, referring to the results of the two-dimensional Fourier analysis. Extracted topography is classified into those with long, intermediate and short wavelengths. There exist two predominant topographies with long wavelengths (70-150 km). One is that with an EW strike in the Chugoku and Shikoku districts, another with an NS strike in the Chubu district. In the range from short and intermediate wavelengths, the topography in the Chubu region and the Outer Zone in Southwest Japan shows rather regular pattern being approximately the same as that with the long wavelengths, but the detailed topographies with short and intermediate wavelengths are more complicated in the Chugoku and Kinki districts. It appears that the upper crust in the Chugoku and Kinki districts may be divided into quadrilaterals probably due to double foldings by NS and EW compressions and bordered by faults of strikeslip type. Boundaries of these crustal blocks are considered as relatively wide crushed zones which were probably produced by block motions. Large earthquakes are mostly located on the boundaries between uplift and subsidence areas of the fold, which may correspond to the faults caused by the development of the fold. Microearthquake distribution appears to have a close relation to the topography with long wavelengths and, on the other hand, to that with very short wavelengths as well. These two phenomena are owing to extremely frequent occurrence and very small source dimensions of microearthquakes. Rivers and sharp notches often appear related to microearthquake distribution. This shows that these fine structures are manifestations on the surface of the present stress acting in the crust. All of these features of topography in Southwest Japan are fundamentally explained by two compressions in EW and NS directions by subductions of the Pacific plate and Philippine Sea plate which have continued since the late Quaternary. The present tectonic stress, which is considered to be an extension of that from the geological times, still causes not only the formation of the topography but crustal activity including earthquake occurrence.