汶川地震断裂带断层滑移作用--来自高磁化率断层岩的信息

FAULT SLIP IN THE WENCHUAN EARTHQUAKE FAULT ZONE——INFORMATION FROM FAULT ROCKS WITH HIGHER MAGNETIC SUSCEPTIBILITY

断层岩与地震有着密切相关,通常沿断裂带的断层面发育。本研究以2008年汶川地震(M_s8.0)的发震断裂(映秀-北川断裂带)发育的古地震造成的断层岩为研究对象,利用Bartington MS2仪器表面测量探头对发育于四川省都江堰市虹口乡(31°8.789′N,103°41.472′E)映秀-北川断裂带花岗岩中的两条断层岩开展了高密度测量。测试结果表明磁化率在2×10^(-6)SI到74×10^(-6)SI之间变化,所测剖面均显示出断层岩区域与高磁化率相互对应现象。这种现象可能是由于断层滑移作用产生的高磨擦温度导致断层岩中高磁化率新矿物的形成所致。断层滑移作用致使断层泥中矿物颗粒粒径的减小也可能使磁化率增强。高磁化率断层岩(断层泥)的存在可以作为地震断层的判别依据之一,本研究对于定量分析研究断层岩(断层泥)提供了一种新尝试。

Fault rocks are intimately associated with earthquakes and typically occur along fault surfaces. Fault rocks produced by paleoearthquakes along the 2008 Wenchuan earthquake seismogeneticic fault （Yingxiu-Beichuan Fault） provide real objects for this study. In the sudy,we measured the magnetic susceptibilities of fault rocks A and B produced by paleoearthquakes in the Yingxiu-Beichuan Fault（31°8. 789＇N, 103°41. 472＇E）. Fault rock A is 40cm thick and fault rock B is 100cm thick. Magnetic susceptibility was measured at lcm interval using the Bartington MS2 surface sensor. For fault rock A,four sections were measured,whereas for fault rock B,one section was measured. The magnetic susceptibilities measured in all five sections fluctuate mainly from 2 × 10^-6 SI to 74×10^-6 SI. It was found that the fault rocks have higher magnetic susceptibility peak values than granites by them. Moreover, The finer is the grain size of fault rocks, the more is their magnetic susceptibility. It was inferred that the higher magnetic susceptibility values of fault rocks might resulted from new magnetic minerals with high magnetic susceptibility produced by frictional heating during an earthquake. Gouges with high magnetic susceptibility provide scientific basis for judging if the fault formed by earthquake. This study is an attempt of quantitative analysis of fault rocks（ or fault gouges）.