地震剥落——花岗岩地区的一种典型震害

1976年5月29日20点23分和22点00分在我国云南省龙陵地区连续发生了7.5、7.6级强烈地震。宏观震中分别为镇安、金竹坪、邦公以及平达一带,震中烈度为九度。震区花岗岩分布广泛,花岗岩风化壳表层的地震剥落破坏了稻田和地面各种设施,堵塞了公路和沟渠,成为这次地震的典型震害。分析这次地震剥落特点及其产生的地质条件,对类似地区未来的抗震防震及震害预测都是有实际意义的。

A TYPICAL EARTHQUAKE HAZARD IN GRANITE AREA ——EARTHQUAKE ROCK-FALL

On May 29, 1976 (at Beijing time 20h23m and 22h00m) two strong earthquakes of magnitude 7.5 and 7.6 respectively occurred in Long Ling of Yunnan province. The epicentral area (Intensity Ⅷ-Ⅸ) lies in a middle mountain region with granite outcrops. The widespread rock-fall from the weathered granite crust forms the chief hazard caused by the strong shocks.This paper emphatically Analyses the geological conditions causing earthquake rock-fall which occurred mostly on the two steeper flanks of a narrow ridge separating erosional valleys and in granite bodies of the Yen’shanian orogeny-, especially in those of the first and second stages of intrusion (γ53(1),γ53(2)). Comparing with the two subsequent intrusive stages (γ53(3), γ53(4)) the first and the second stages are characterizied by less homogeneous texture and relatively higher content of easily weathered minerals in mineral constituents. In addition, under similar geologic and geomorphologic conditions the earthquake rock-fall largely happened on such slopes where vegetation (particularly the deeply rooted one) is lacking.Finally, it is suggested that the Long Ling earthquake is similar in ground failure to those shocks occurring in mountain area in recent years, predominating in seismogenic gravitative failure. The earthquake rock-fall of such a large scale, however, has never been known before.