To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure.

中文翻译：

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密集地震观测揭示2016年鸟取县中部地震余震活动的平稳性

为了阐明地震发生与流体之间的关系，我们分析了来自密集余震观测网络的数据，该网络在 2016 年鸟取县中部地震的余震区在主震发生后（10 月 21 日）立即安装了 69 个高增益短周期地震仪。 . 我们非常精确地确定了10月22日至12月15日期间余震的震源和震源机制，然后研究了震源空间分布和震源机制T轴方位角的时间变化。余震震源和T轴方位角分布在时间上基本稳定，但余震区西缘浅层有限部分余震随WSW T轴方位角向西快速减少。如果流体由于断层破裂而从下地壳上升，余震的位置和震源机制可能会随着时间的推移而发生变化，特别是在余震区的最深处。然而，这些参数的时间变化在深度上并不明显。这些观察结果表明，鸟取县中部地震的余震活动主要受应力集中控制，而不是由高流体压力引起的强度降低。