答“评‘紫坪铺水库造成孔隙弹性耦合变化及其对2008年汶川地震触发作用’二维模拟的局限性”

程惠红等(2015)在"评‘紫坪铺水库造成孔隙弹性耦合变化及其对2008年汶川地震触发作用’二维模拟的局限性"一文中提出,陶玮等(2014)采用二维模型模拟紫坪铺水库造成孔隙弹性耦合变化是一个不足,夸大了汶川地震震源处库仑应力增长值.我们认为采用二维模型模拟水库造成汶川地震震源处库仑应力变化确实可能存在一定偏差,但不会如程惠红等认为的"与三维模型相比夸大三倍".这是因为在程惠红等引用的论证中,二维模型计算中选取了接近水库最大剖面宽度而不是水库平均剖面宽度作为加载量参数,造成计算得到震源处库仑应力明显夸大.此外,陶玮等(2014)提出论断的主要依据不是"震源处"库仑应力值的大小,而是紫坪铺水库蓄水"由浅入深影响到整条断层,尤其对浅层范围的加载作用明显,达上百千帕,为整个断层面的失稳提供了基础".初始破裂的发生既可能是由水库蓄水引起,也可能是并非水库蓄水造成的一次普通构造小震,但其发生引发断层面上部已被水库蓄水弱化部分的连锁失稳而发生大震.即汶川地震既可能为直接触发也可能为间接触发,而我们的研究结果认为地震的发生完全可能由间接触发产生.若仅纠结于"震源处"的库仑应力变化,则忽视了水库蓄水影响的主体.水库蓄水对地震触发作用是一个复杂问题,相对这一问题所涉及的一系列不确定性因素来说,二维模拟的局限性所造成的影响并不很大也不是最大的,也不妨碍我们据此得出紫坪铺水库蓄水促进汶川地震发生的结论.

Reply to "The 2D limitation of ‘Triggering effect of the Zipingpu Reservoir on the 2008 Mw7.9 Wenchuan,China, Earthquake due to poroelastic coupling’"

In the comments Cheng et al. (2015) made on our paper (Tao et al., 2014) they claim that by using a 2D instead of a 3D finite element model to simulate the impoundment effect of the Zipingpu Reservoir, we exaggerated by about 3 times the pore pressure and the Coulomb stress changes at the hypocenter of the 2008 Wenchuan Earthquake. We agree that there would be some deviations between the results of a 2D model and a 3D model simulation, but the difference would not be as large as they claimed. This is because that in the 2D simulation that Cheng et al. cited, a section profile close to the maximum width (much greater than the average width) of the Zipingpu Reservoir was chosen as the representative section profile in the computation of reservoir loading, causing exaggerated estimations of the pore pressure and Coulomb stress changes at the hypocenter in the 2D modeling. Furthermore, Tao et al. (2014) drew their conclusions based mainly on that "the impoundment of the Zipingpu Reservoir increased the Coulomb stress loading of the Longmenshan Fault on the whole", especially "increased the Coulomb stress at the shallow part of the Longmenshan Fault up to hundreds of kPa, significantly in favor of the failure of the fault". This major conclusion is not based on the calculated Coulomb stress changes at the reported hypocenters. The initial rupture of Wenchuan Earthquake could be a small event which might or might not be induced by the impoundment of the reservoir, but its rupture could trigger cascade failures at the upper part of the fault system that had been significantly weakened by the reservoir impoundment already. Therefore, the Wenchuan Earthquake could be directly or indirectly triggered by the impoundment of the Zipingpu Reservoir. If only focusing on the triggering effect at the "hypocenter", it would be easy to miss the much greater effect that the reservoir imposed on the upper part of the fault system. Reservoir triggering of earthquake is a complex problem, with many uncertainties involved in the model. Among these uncertainties, the one resulted from using a 2D simulation is neither the most significant nor the largest, and would not affect the major conclusions that we drew about triggering of the Wenchuan Earthquake by the impoundment of the Zipingpu Reservoir.