它们是周期性出现的还是随机出现的？
某一沉积事件周期性出现的频繁程度可用时间序列中的频率来表示。
The periodic occurrence of any sedimentary event may be expressed by the frequency in the time series.
开始时可能是因为过度劳累或患感冒，之后没有任何诱因也会周期性出现。
The beginning may be due to over exertion or cold, then there is no incentive there will be cyclical.
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