他们的房子修建得很坚固，经受住了主震。
Their house, which was solidly built, resisted the main shock.
信号震和主震沿同一构造体系活动；
Signal earthquake and the major earthquake seem to be controlled by the same structural system.
余震可以在主震发生后的一个月或者几天之内发生。
It can happen with a month, or within days, from the main shock.
北北东—南南西向分布的余震发生在主震的断层面上（或其附近）；
The NNE-SSW distributed aftershocks occurred on, or near, the fault plane of the main shock;
北北东—南南西向分布的余震发生在主震的断层面上（或其附近）；
The NNE-SSW distributed aftershocks occurred on, or near, the fault plane of the main shock;
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