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在日本南部九州鹿儿岛县西北部发生了两个浅源中等(M～6)地震。这里讨论了第一个地震(1997年3月26日)产生的应力变化对第二个地震(1997年5月13日)发生的影响。为了形成本文讨论的基础,以两步推测这两次地震的破裂特征。首先根据强地面运动数据来构建动态震源模型,然后根据得到的位错分布估计静态应力降的分布。3月地震的破裂过程简单,由单一凹凸体(高应力降区)的破裂很好地描述了。而5月地震在共轭断层上破裂了多个凹凸体。两个地震的最大静态应力降都是4MPa 左右,似乎低于日本其他板内地震的值。5月地震的震源和最大凹凸体,位于3月地震产生的应力影区之中。这样5月地震的破裂历史很难用静态应力变化模型来解释。还讨论了如流体迁移和动态应力的其他机制,但不能解释这种触发效应。本文提出,依赖于速率和状态的摩擦定律的剪切应力和正应力静态变化的耦合效应为一种可能的机制。
Two shallow-medium (M ~ 6) earthquakes occurred in the northwestern part of Kagoshima Prefecture in southern Japan. Here we discuss the impact of the first earthquakes (March 26, 1997) on the second earthquake (May 13, 1997). In order to form the basis of the discussion in this article, the rupture characteristics of these two earthquakes are inferred in two steps. First, the dynamic hypocenter model is constructed based on strong ground motion data, and then the static stress drop distribution is estimated based on the dislocation distribution obtained. The March earthquake’s rupture process is simple and well described by the rupture of a single asperity (high stress drop zone). The May earthquake ruptured several asperities on the conjugate fault. The maximum static stress drop of both earthquakes is about 4 MPa, which seems to be lower than the values of other intra-plate earthquakes in Japan. The source of the May earthquake and the largest asperity were in the stress shadow of the March earthquake. Such a history of the May earthquake rupture is hard to explain using static stress changes. Other mechanisms such as fluid migration and dynamic stress are also discussed, but this triggering effect can not be explained. This paper suggests that the coupling effect of shear stress and normal static stress, which depend on the friction law of velocity and state, is a possible mechanism.