四川六级以上地震前地震活动条带的特征
THE CHARACTERISTICS OF SEISMICITY BELTS BEFORE STRONG EARTHQUAKES WITH MAGNITUDE M≥6 IN SICHUAN
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摘要: 本文比较了地震带、地震活动成带转移和强震前地震活动条带的概念。对于1970年下半年以来,四川及邻近地区发生的6级以上强震前的背景性地震活动条带和弱震密集条带,进行了清理和研究。背景性地震活动条带指强震前十几个月到几年出现的由4级以上地震组成的地震活动条带。弱震密集条带是强震前几个月出现的,由微弱地震组成的地震活动条带。 3次7级以上地震和盐源、宁蒗6.9级地震前出现了背景性地震活动条带。其中炉霍7.9级和松潘—平武7.2级地震前出现两条互相近乎垂直的背景性条带。永善7.1级和盐源—宁蒗6.9级地震前只有一条背景性条带。8次6级以上地震中有7次震前出现了弱震密集条带。强震往往发生在两条地震活动条带的交汇部位附近或一条地震活动条带的端部或空段。 本文对于7个弱震密集条带的震例作了各时空参数与未来强震震级的统计分析。弱震密集条带的面积S、持续时间ΔT与震级M分别在信度α=0.05和0.10的情况下,通过线性相关检验,用最小二乘法求得的直线关系为: M=3.40lgS—3.91±1.3 M=1.84lgΔT+6.01±1.3 式中ΔT的单位是月,S的单位是103公里2。 最后,本文初步探讨了震前地震活动条带与构造、强震震源机制的关系及其在地震预报中的作用。Abstract: In the present paper we compare the concept of seismicity belt before strong earthquakes with that of seismic zone, and of the earthquake-zoned transfer. We investigate the background seismicity belt and the microseismic active belt before M≥6 earthquakes. By background seismicity belt we mean a kind of seismicity belt in which M≥4 earthquakes occur during a period of over 10 months to several years before a strong earthquake. A microseismic seismicity belt is another kind of seismicity belt in which weak earthquakes occur during a period of several months before a strong earthquake.Background seismicity belts appeared before three earthquakes with magnitude M≥7 and the Yanyuan—Ninglang earthquake with magnitude 6.9. Of these, two perpendicular background seismicity belts appeared before the Luhuo earthquake with magnitude 7.9 and the Songpan—Pingwu earthquake with magnitude 7.2. There was only one background seismicity belt before the Yongshan earthquake of magnitude 7.1 and the Yanyuan—Ning Lang earthquake with magnitude 6.9. There occurred micro-seismic active belts before seven out of the eight M≥6 earthquakes. Strong earthquakes often occur near the intersection of two belts or one of the two ends of a belt or in the gap within a belt.In this paper, we analyse the statistic relation between the microseismic active belt paremeters (length, or width, or appearance duration etc.) and the magnitude of the strong earthquake that followed, using data of the seven earthquakes before which microseismic active belts appeared. The study shows that the magnitude M and duration ΔT and area S of a microseismic active belt with confidence Level =0.05 and 0.10 respectively are linearly related. The empirical formulas obtained by least squares method are M=3.401gS—3.91±1.3 M=1.841g△T+6.01±1.3 where, the unit of S is 103 km2 and the unit of △T is one month.Finally, we also study the relations between the seismicity belt and the tectonics or the focal mechanism of the strong earthquake that followed, and discuss the role of seismicity belt in earthquake prediction studies.
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