Mechanism of underthrusting in southwest Japan: A model of convergent plate interactions
Mechanism of underthrusting in southwest Japan: A model of convergent plate interactions
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日本西南部的逆冲机制：汇聚板块相互作用模型
DOI：
10.1029/jb076i029p07260
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发表时间：
1971
影响因子：
--
通讯作者：
C. Scholz
中科院分区：
文献类型：
--
作者：
T. Fitch;C. Scholz
在1946年12月21日发生的8.2级南海道地震中，一种与逆冲作用相一致的弹性回弹机制解释了日本西南部大部分地区地震和震前海拔变化之间的逆转。这一事件和1944年的东南海8.0级地震几乎破坏了亚洲和菲律宾海板块在琉球和伊豆-小笠原弧之间的整个边界。在板块构造方面，这次地震的震源区域内的上升的海洋阶地的形成可以解释为菲律宾海板块被亚洲板块覆盖，因为前者在日本西南部被消耗。没有倾斜的地震带、活火山和发育良好的深海海沟，这表明日本西南部的俯冲作用在地质学上是近代开始的，可能不到一百万年前。然而，估计的滑动速率为8±4厘米/年，这表明该地区典型的岛弧结构正在迅速演变。1960年智利和1964年阿拉斯加巨型逆冲断层震源区内类似的地震和震前运动，尽管依据的证据相对较少，但表明推断出的日本西南部的俯冲机制是沿沿着其他板块会聚带相互作用的合理模型。过去在这些区域内类似地震序列的准周期性复发表明，对应变积累的频繁监测可以为这些和其他俯冲区域的地震风险提供更好的衡量标准。
根据大地测量资料推断，1946年南海岛地震期间的断层运动即使不完全是倾滑，也是主要的。破裂从震中向西南方向沿着推断的N70°E走向传播了近300公里，并远远超出了余震区的范围。推测断层的前缘被假定为在南海海槽内壁上破坏表面。为了拟合数据，必须使用模拟为复合位错的断层。首选的断层模式是向西北倾斜30°至40°的俯冲。从位错模型计算的滑动增加5至18米的破裂传播的方向。在地震之前，从19世纪90年代到20世纪30年代，应变以恒定的速度增加。地震后，一种复杂的运动模式以迅速衰减的速度持续了大约三年。这些运动可以解释为在地震期间破裂的几乎整个断层面上的反向滑动和断层较深部分的延迟前滑。从力学上讲，这些运动被解释为主震发生时断层表面浅部欠阻尼滑动和深部过阻尼滑动的响应。
An elastic rebound mechanism consistent with underthrusting at the time of the magnitude 8.2 Nankaido earthquake of December 21, 1946, accounts for a reversal in sense between seismic and pre-seismic changes in elevation throughout a large part of southwest Japan. This event and the magnitude 8.0 Tonankai earthquake of 1944 ruptured almost the entire boundary of the Asian and Philippine Sea plates between the Ryukyu and Izu-Bonin arcs. In terms of plate tectonics, the formation of uplifted marine terraces within the focal region of this earthquake can be explained by overriding of the Philippine Sea plate by the Asian plate as the former is consumed beneath southwest Japan. The absence of an inclined seismic zone, active volcanoes, and a well-developed deep-sea trench suggests that underthrusting in southwest Japan started in geologically recent time, possibly less than one million years ago. However, an estimated slip rate of 8±4 cm/yr suggests that typical island arc structures are rapidly evolving in this region. Similar seismic and pre-seismic movements within the focal regions of the 1960 Chilean and the 1964 Alaskan megathrusts, although based on comparatively little evidence, suggest that the inferred mechanism of underthrusting in southwest Japan is a reasonable model for interactions along other zones of plate convergence. Quasi-periodic recurrence of similar earthquake sequences in the past within each of these regions suggests that frequent monitoring of strain buildup can provide an improved measure of earthquake risk in these and other regions of underthrusting.
The fault motion during the 1946 Nankaido earthquake as inferred from geodetic data was predominantly, if not entirely, dip-slip. The rupture propagated almost 300 km southwest from the epicenter along an inferred strike of N70°E and extended well beyond the limits of the after-shock zone. The leading edge of the inferred fault is assumed to break the surface on the inner wall of the Nankai trough. In order to fit the data, faults modeled as compound dislocations must be used. The preferred fault model is an underthrust dipping 30° to 40° toward the northwest. Slip calculated from a dislocation model increases from 5 to 18 meters in the direction of rupture propagation. Before the earthquake, from the 1890's to the 1930's, strain increased at a constant rate. After the earthquake, a complex pattern of movements continued for about three years at rapidly decaying rates. These movements can be explained by reversed slip on nearly the entire fault surface that ruptured during the earthquake and by delayed forward slip on the deeper parts of the fault. Mechanically these movements are interpreted as a response to underdamped slip on shallow parts and overdamped slip on deeper parts of the fault surface at the time of the main shock.