据我国历史资料记载,强烈地震发生前的瞬间,存在着初期振动,并伴有地声、地光等现象。如1556年1月23日陕西关中大地震和1679年9月2日三河平谷大地震都有类似现象的记载。1976年7月28日唐山7.8级地震时,又被许多目击者所证实。这些先于强烈振动十几秒出现的现象是大地震即将来临的最后一次预警信号。从震源机理上分析,大地震断层的破裂错动一般不是一瞬间即告完成,而是有一个由小到大,从起始到终止的过程。在这个过程之中,就会伴随出现一些由于地下断层预滑和微破裂造成的初期微动、地声和地光等有关现象。人们把大震前短暂时间内所发生的,能够预示强烈地震必将到来的临震宏观现象叫做大震预警现象,它是临震应急避震的科学理论依据和减少地震伤亡的最后机会。从预警现象开始到发生大震的时间段为预警时间。地震学者从实际出发,把人们从发现预警现象开始到房屋倒塌为止所经历的时间定义为广义预警时间。它的长短取决于所处的地震烈度、住房条件等客观因素,也取决于人们发现预警现象早晚,受多种条件制约,只具有统计意义。但这个时间的长度确是人们能否进行应急避震求生的先决条件,具有实用价值。 According to historical records in our country, there was an initial vibration at the moment before the strong earthquake, accompanied by the sound of the earth and the light of the earth. Such as the January 23, 1556 Shaanxi Guanzhong earthquake and September 2, 1679 Sanhe Pinggu earthquake have similar records. July 28, 1976 Tangshan 7.8 earthquake, it has been confirmed by many witnesses. These phenomena, which occurred more than a few tenths of a second before the strong vibration, were the last warning signal of the impending earthquake. According to the focal mechanism analysis, the rupture and dislocation of a large earthquake fault is usually not completed in an instant, but there is a process from small to large and from start to finish. During this process, some related phenomena such as initial fretting, ground noise and ground light due to pre-slip and micro-rupture of underground faults will be accompanied. People took a short time ago before the earthquake can predict a strong earthquake is bound to come near the phenomenon of the Pro-seismic macro-warning phenomenon known as the earthquake, it is the scientific basis for emergency shock and the last chance to reduce earthquake casualties. The period from the early warning to the occurrence of a major earthquake is the warning time. Seismologists proceed from reality and define the time that people experience from the time when the early warning phenomenon is found to the time when the house collapses as the generalized warning time. Its length depends on the objective factors such as earthquake intensity and housing conditions, and also depends on the fact that people find the early warning phenomenon early and late, subject to various conditions, and have only statistical significance. However, the length of this time is indeed a prerequisite for people’s ability to make emergency shelter and have practical value.