Hao Jinlai, Zhang Jinhai, Yao Zhenxing
Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
Natl Sci Rev. 2019 Oct;6(5):1016-1023. doi: 10.1093/nsr/nwy117. Epub 2018 Oct 8.
Previously, inadequate earthquake catalogs and the lack of method made it challenging, if not impossible, to determine the dominant period of earthquake occurrence. With the advent of densely distributed seismic stations in Japan since 2002, 410 642 M1.0+ earthquakes have been cataloged under the mainland within 30-km depth, which provide a superb dataset to identify the periodicity of earthquakes. We processed this catalog using time-frequency analyses and daily stacking, which are powerful in extracting weak periodical signal from strong random noises. We concentrated on the time sector from 0:00 to 6:00 (i.e. from midnight to daybreak), which is a reliable time window for much higher detectability of weak earthquakes, since it has the lowest affects from cultural noises. We successfully observed two apparent periodicities of 12- and 24-hour, which are much smaller than the fortnightly periodicity presented previously in the literature. Synthetic earth tides, after intentionally ignoring the contribution from the Moon, present similar dominant periods as the earthquakes. This may indicate that the dominant period of earthquakes is statistically associated with the Sun rather than the Moon. The daily stacking number of earthquakes using a 15-minute or 1-hour interval shows a peak around 1:30, rather than usually expected 3:00 to 4:00. In addition, bigger earthquakes show more evident variations in the stacking results, and the trend is very consistent for various lower limits of earthquake magnitude from M1.0 to M4.0. These discoveries settled the disputes on the existence of the periodicity of earthquakes since 1886 and may open a window to unravel the mystery of earthquakes.
此前，地震目录不完善以及缺乏方法使得确定地震发生的主导周期即便不是不可能，也是具有挑战性的。自2002年以来，随着日本密集分布的地震台站的出现，在30千米深度范围内的日本本土已记录了410642次震级1.0级及以上的地震，这为识别地震的周期性提供了一个极好的数据集。我们使用时频分析和每日叠加对该目录进行了处理，这在从强随机噪声中提取微弱的周期性信号方面很有效。我们关注的时间区间是从0:00到6:00（即从午夜到黎明），由于它受人为噪声的影响最小，所以是一个能更高概率探测到弱震的可靠时间窗口。我们成功观测到了12小时和24小时这两个明显的周期，它们比之前文献中提到的半月周期要小得多。在有意忽略月球的贡献后，合成地球潮汐呈现出与地震相似的主导周期。这可能表明地震的主导周期在统计上与太阳而非月球相关。以15分钟或1小时为间隔的地震每日叠加次数在1:30左右出现峰值，而不是通常预期的3:00到4:00。此外，较大的地震在叠加结果中显示出更明显的变化，并且对于震级从1.0级到4.0级的各种下限，这种趋势非常一致。这些发现解决了自1886年以来关于地震周期性存在的争议，并可能为揭开地震之谜打开一扇窗。
