1978年5月18日海城6级地震前地震活动的某些特征

1978年5月18日海城地震(Ms=6.0)发生在1975年2月4日海城破坏性地震(Ms=7.3)余震区的空区内.1975年2月4日海城7.3级强震发生之后,其余震(Ms2.0)的月频度衰减异常缓慢(P=0.73),而6级地震发生之后 P 值明显增大,即6级地震发生在月频度衰减曲线的拐点上.这次6级地震之前不同于7.3级强震前,在震源区没有丰富的前震活动.6级地震是7.3级强震的最大余震,作者认为它是区域应力在未曾充分破裂的空区内再次集中积累而发生的余震. 

CERTAIN CHARACTERISTICS OF THE SEISMIC ACTIVITY PRECEDING THE HAICHENG EARTHQUAKE OF MAY 18, 1978 OF MAGNITUDE Ms=6.0

The Haicheng Earthquake of May 18, 1978 occurred in a Gap of the aftershock area of the Strong Haicheng Earthquake of magnitude 7.3 of February 4, 1975.The monthly rate of attenuation of the aftershocks (Ms2.0) was abnormally Iow(p = 0.73). However, after the earthquake of May 18, 1978, the value of p obviously increased. That is to say, it occurred at the inflection point of the attenuation curve of the monthly rate of aftershocks.Contrarily to the mainshock of magnitude 7.3, before this earthquake, not much fore-shock activity appeared. It constitutes the strongest aftershock of the mainshock. The author believes that it is an aftershock caused by the concentrated accumulation of tectonic regional stress within the unbroken gap area.