异常地下应变信号观测及其物理机制

This proposal proposes to systematically search for two types of long-period anomalous strain signals observed in the PBO borehole strainmeters installed in the Western North America, with one kind of signal related to near-surface pore pressure and the other occurring after some moderate earthquakes. The proposal will analyze the temporal and spatial characteristics of these two types of long-period strain signals, establish a physical relationship of the first type of signal with rainfall and pore pressure changes based on poroelastic theory, determine the location and characteristics of the source for the second type of signal, explore possible mechanisms of fault afterslip and earthquake-triggered creep, and evaluate the contributions of Earth’s poroelastic and viscoelastic effects on near-surface deformation. The investigation of these newly discovered strain signals and their origins will identify new physical mechanisms of near-surface deformation and improve our understanding of near-surface geophysical processes and their controlling factors.

本申请拟从北美西部PBO钻孔应变数据中系统地寻找两类长周期异常应变信号，其中第一类明显与近地表孔隙压力相关，而第二类则出现在中级地震之后。项目拟分析这两类长周期异常应变信号的时间变化和空间分布特性，通过孔弹介质力学数值模拟，建立第一类异常应变信号与降水、孔隙压力变化之间的物理联系，结合GPS观测确定第二类异常应变信号源的位置和属性，分析信号源与地震之间的关系，探讨断层面震后余滑、地震触发其它区域蠕动等可能性，分析地球介质粘弹、孔弹特性等因素对地下应变的贡献。本项目拟研究的这两类长周期应变信号均属学术界首次发现。项目对其起因和物理机制的研究将提出新的物理机制，同时也将提高我们对地球浅表应变变化过程和产生机制的理解，更好地了解近地表各种因素对地表形变的影响。

本项目系统分析了北美PBO钻孔应变资料，发现了强烈的长周期震后应变和孔隙压力异常信号，提出了相应的物理机制。这些信号出现在2010年至2017年间发生的3次Mw 4.5以上的局部地震和1次Mw 7.2的区域地震之后，并持续了40天到100天之久。项目提出一种物理机制，将震后应变和由地震触发孔隙压力变化引起的孔弹性形变和地震在邻近断层触发的慢滑移联系起来。研究表明，除了地震可能引起的断层慢滑移外，地震之后的孔隙压力变化可以在震后形变中扮演着更为重要的角色。