Seismically damaged regolith as self‐organized fragile geological feature
Seismically damaged regolith as self‐organized fragile geological feature
复制标题
地震受损的风化层是自组织的脆弱地质特征
DOI：
10.1029/2011gc003837
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发表时间：
2011
期刊：
影响因子：
3.7
通讯作者：
N. Sleep
中科院分区：
文献类型：
--
作者：
N. Sleep
地震活跃区域内浅层地下的横波速度自组织，使得典型的强动态剪应力略微超过库仑弹性极限。主要走滑断层的动态速度产生简单的尺寸关系。近场速度脉冲本质上是洛夫波。动态剪切应变是测量的粒子速度与深 S 波速度的比率。浅层动态剪切应力是该量乘以局部剪切模量。断层平行垂直面上的动态剪切牵引力在自由表面处是有限的。库仑破坏发生在有利定向的裂缝上以及完整岩石的内部。我通过用完整的坚硬岩石一直延伸到地表开始一系列地震来获得平衡剪切模量。坚硬岩石中施加的动态剪切应变会导致浅层库仑破坏，并在其尾流中留下裂缝。破裂的岩石比原始完整的岩石更柔顺。裂纹岩石的摩擦力也较弱，但剪切模量的变化影响较大。随后的每个事件都会导致额外的浅层裂纹，直到岩石变得足够柔顺，在数十到数百米的浅深度范围内达到库仑破坏。进一步的事件将材料维持在剪切模量作为其刚刚失效的函数。本文提供的形式合理地表示了加利福尼亚州卡洪通道和圣费尔南多谷附近挖出的沉积物中的 S 波速度。一个普遍的结论是，地震活跃地区的浅层岩石在典型的震动过程中会变得非线性。这个过程会引起横波速度的瞬态变化，但不会引起地震波的强烈非线性衰减。波幅明显大于典型波幅会强烈衰减并严重损坏岩石。
The S‐wave velocity in the shallow subsurface within seismically active regions self‐organizes so that typical strong dynamic shear stresses marginally exceed the Coulomb elastic limit. The dynamic velocity from major strike‐slip faults yields simple dimensional relations. The near‐field velocity pulse is essentially a Love wave. The dynamic shear strain is the ratio of the measured particle velocity over the deep S‐wave velocity. The shallow dynamic shear stress is this quantity times the local shear modulus. The dynamic shear traction on fault parallel vertical planes is finite at the free surface. Coulomb failure occurs on favorably oriented fractures and internally in intact rock. I obtain the equilibrium shear modulus by starting a sequence of earthquakes with intact stiff rock extending all the way to the surface. The imposed dynamic shear strain in stiff rock causes Coulomb failure at shallow depths and leaves cracks in it wake. Cracked rock is more compliant than the original intact rock. Cracked rock is also weaker in friction, but shear modulus changes have a larger effect. Each subsequent event causes additional shallow cracking until the rock becomes compliant enough that it just reaches Coulomb failure over a shallow depth range of tens to hundreds of meters. Further events maintain the material at the shear modulus as a function where it just fails. The formalism provided in the paper yields reasonable representation of the S‐wave velocity in exhumed sediments near Cajon Pass and the San Fernando Valley of California. A general conclusion is that shallow rocks in seismically active areas just become nonlinear during typical shaking. This process causes transient changes in S‐wave velocity, but not strong nonlinear attenuation of seismic waves. Wave amplitudes significantly larger than typical ones would strongly attenuate and strongly damage the rock.