Nevitt Johanna M, Brooks Benjamin A, Catchings Rufus D, Goldman Mark R, Ericksen Todd L, Glennie Craig L
U.S. Geological Survey, P.O. Box 158, Moffett Field, CA, 94035, USA.
Department of Civil and Environmental Engineering, University of Houston, Houston, TX, 77204, USA.
Sci Rep. 2020 Mar 19;10(1):5031. doi: 10.1038/s41598-020-61400-9.
Poor knowledge of how faults slip and distribute deformation in the shallow crust hinders efforts to mitigate hazards where faults increasingly intersect with the expanding global population at Earth's surface. Here we analyze two study sites along the 2014 M 6.0 South Napa, California, earthquake rupture, each dominated by either co- or post-seismic shallow fault slip. We combine mobile laser scanning (MLS), active-source seismic tomography, and finite element modeling to investigate how deformation rate and mechanical properties of the shallow crust affect fault behavior. Despite four orders-of-magnitude difference in the rupture velocities, MLS-derived shear strain fields are remarkably similar at the two sites and suggest deceleration of the co-seismic rupture near Earth's surface. Constrained by the MLS and seismic data, finite element models indicate shallow faulting is more sensitive to lithologic layering and plastic yielding than to the presence of fault compliant zones (i.e., regions surrounding faults with reduced stiffness). Although both elastic and elastoplastic models can reproduce the observed surface displacement fields within the uncertainty of MLS data, elastoplastic models likely provide the most reliable representations of subsurface fault behavior, as they produce geologically reasonable stress states and are consistent with field, geodetic, and seismological observations.
对浅地壳中断层如何滑动以及如何分布变形的了解不足，阻碍了在断层与地球表面不断增长的全球人口越来越多地相交的地区减轻灾害的努力。在这里，我们分析了沿着2014年加利福尼亚州南纳帕6.0级地震破裂带的两个研究地点，每个地点分别以同震或震后浅部断层滑动为主。我们结合移动激光扫描（MLS）、有源地震层析成像和有限元建模，来研究浅地壳的变形速率和力学性质如何影响断层行为。尽管破裂速度相差四个数量级，但MLS得出的剪切应变场在这两个地点非常相似，并表明同震破裂在地球表面附近减速。受MLS和地震数据的约束，有限元模型表明，浅部断层活动对岩性分层和塑性屈服比对断层柔顺带（即围绕断层的刚度降低的区域）的存在更敏感。尽管弹性模型和弹塑性模型都能在MLS数据的不确定性范围内再现观测到的地表位移场，但弹塑性模型可能提供了最可靠的地下断层行为表征，因为它们产生了符合地质情况的应力状态，并且与野外、大地测量和地震学观测结果一致。
