1920年海原8(1/2)级大地震的多重破裂特征
MULTIPLE RUPTURE CHARACTERISTICS OF THE 1920 HAIYUAN M8(1/2) EARTHQUAKES
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摘要: 根据作者等人野外实地考察,1920年海原8(1/2)级大地震的地层断层全长225km,共由6条几何参数不同的次级断层呈有规律的斜列状组合而成.每一条次级断层都可分为形变性质不同的三段,其中中段以水平走滑性质为主;两端以垂直形变为主,表现为逆断层和正断层性质.垂直和水平位移的测量数据也表明,每一条次级断层的中段水平位移最大,向两端逐渐减小,以至消失,而垂直位移正好相反,中段小,两端大.整个断层的地震位移显示出多个峰值的形变特征.这种形变特征表明,海原大地震的6条次级断层都是各个相对独立的水平位移间断面(即位错面),每个位错面,都可以代表一次独立的次级地震破裂事件.作者等据此认为,巨大的8(1/2)级海原大地震可能是由6次相对独立的次级地震事件接续发生的结果.Abstract: Field survey by the authors reveals that, the overall length of the earthquake fault for the great 1920 Haiyuan, China, M8(1/2) Earthquake is 225 km, and that the fault consists of 6 sub-faults which show different geometries and stretch regularly in en-echelon configuration. Each subfault can be divided into three segments with different deformation features: the middle segment is essentially strike-slip type, while the two end-segments show predominant vertical displacement, being essentially normal or thrust type respectively. Measurements give the result that maximum horizontal displacement appears at the middle part of each subfault, and dies away gradually near the two ends, while the vertical displacement distributes in just thte opposite way, being small at the middle and large at the ends. The whole fault exhibits several peak displacements.This implies that the 6 subfaults of the great Haiyuan Earthquake are all independent displacement dis-continuities (dislocations), and each of them represents a rupture event. In this regard the authors suggest that the great M8(1/2) Haiyuan Earthquake might be the result of 6 successive subevents which occurred independently of each other.