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地壳流变结构控制作用下的龙门山断裂带地震发生机理

DOI: 10.6038/cjg20140207, PP. 404-418

Keywords: 龙门山断裂带,汶川地震,芦山地震,应力集中,黏性差异,Moho面突变

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Abstract:

青藏高原东缘低地形变速率的龙门山断裂带上相继发生了2008汶川Mw7.9级地震和2013芦山Mw6.6级地震.地震勘探与震源定位结果揭示了龙门山区域地震空间分布特征:纵向上,龙门山断裂带这两次地震主震均发生在龙门山断裂带上地壳的底部(14~19km),绝大部分余震均发生在上地壳范围(5~25km),而在其中、下地壳深度范围内鲜见余震发生;横向上,地震(Mw>3)在龙门山断裂带青藏高原一侧密集分布且曾有大震发生,而四川盆地地震稀少(Mw>3).为探讨龙门山断裂带地震发生机理,并解释以上龙门山区域地震空间分布特征,本文建立了龙门山断裂带西南段跨芦山地震震中区域的四种不同流变结构的龙门山断裂带三维岩石圈模型,以地表GPS观测资料为约束边界条件,数值模拟龙门山断裂带岩石圈在数千年以上长期匀速构造挤压作用下的应力积累特征,探讨了地壳分层流变性质对地壳应力积累的影响,分析了该区域地震空间分布与构造应力积累速率的关系.计算结果表明:该区域在数千年的应力积累过程中,脆性上地壳中应力表现近于恒定值的线性增长趋势,龙门山断裂带上地壳底部出现应力集中积累现象,这一应力集中现象可以解释龙门山断裂带汶川地震与芦山地震主震的发生,及其大部分余震在脆性上地壳中的触发;青藏高原一侧上地壳应力积累速率远远高于四川盆地的应力积累速率,这一应力积累分布现象可以解释龙门山区域青藏高原一侧地震密集而四川盆地地震稀少的地震空间分布特征;通过比较不同流变结构模型中的应力积累状态,认为导致这一应力积累空间分布状态的重要控制因素在于青藏高原中、下地壳较低的黏滞系数与四川盆地中、下地壳较高的黏滞系数的差异.在柔性的中、下地壳内,应力增长近于指数形式,稳定状态之后其应力增长速率近于零,构造应力积累难以达到岩石破裂强度,因而鲜见地震发生.地壳各层位的应力增长率差异与地震成层分布的现象共同揭示了龙门山区域岩石圈分层流变结构:脆性上地壳、韧性中、下地壳(青藏高原一侧较弱,四川盆地一侧较强)、韧性岩石圈上地幔.

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