A method for mapping crustal deformation and anisotropy with receiver functions and first results from USArray
A method for mapping crustal deformation and anisotropy with receiver functions and first results from USArray
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DOI：
10.1016/j.epsl.2014.01.050
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发表时间：
2014-09
影响因子：
5.3
通讯作者：
V. Schulte‐Pelkum;K. Mahan
中科院分区：
文献类型：
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作者：
V. Schulte‐Pelkum;K. Mahan
经历变形的岩石圈可能以织物的形式记录这一过程，例如延性变形产生的叶理和线理，以及地壳内或地壳底部（莫霍面）不同性质的材料之间的非水平界面。接收器函数分析技术隔离了源自地震台下方速度对比的远震模式转换。我们表明，各向同性对比度的下降以及较小（<10%）的各向异性会在接收器函数中产生一阶（即莫霍面大小）到达。我们证明，通过减去后方位角的平均值（0 度），可以将方位角变化的信号与径向分量接收器函数隔离。在各向同性倾斜界面或切入轴各向异性的情况下，径向信号与后方位角偏移 90° 的切向分量信号相匹配，两者均以后方位角 1 度变化为主。两个分量中极值和节点的位置限制了倾斜界面、倾斜叶理或垂直于快速线理的平面的走向。对于方位角各向异性，分量之间的偏移为 45°，并且均显示 2 度方位角变化。各向同性倾斜界面和切入轴各向异性之间的区别可以通过在各向异性情况下缺乏直接到达的极性反转方位角变化来进行。所描述的行为与快轴对称性和慢轴对称性以及绝对速度无关，并且在很大程度上与各向异性的强度、精确的各向异性对称性（例如形状因子）以及界面或叶状倾角无关。我们的方法简单地包括从径向分量接收器函数中减去台站平均值，将切向接收器函数移动后方位角上的固定量，并求解联合接收器函数集中的1度和2度到达。它允许确定延迟时间（按深度缩放）以及倾斜和各向异性特征的方向，而无需波形建模。将径向和切向分量与后方位角偏移相结合，可以填补方位角事件覆盖中可能存在的漏洞。我们根据怀俄明州风河逆冲断层的活动源结果以及针对南加州深层地壳各向异性的波形建模研究验证了该方法。然后，我们使用可移动阵列数据在美国大陆展示初步结果。第一阶特征是沿西海岸板块边界的高振幅和西部山间部分地区的高振幅、美国中部落基山/拉拉米德前缘向低振幅的过渡以及沿阿巴拉契亚-沃希托带的高振幅。叶状走向与山脉走向大致一致，但高信号幅度和匹配走向勾勒出目前在地表地形中很少表达的地质特征，例如俄克拉荷马州南部的奥拉科根、阿巴拉契亚山麓的高品位岩石和中部大陆裂谷。我们的结论是，岩石圈变形可以通过接收器函数来定位，而无需进行广泛的波形建模，从而可以首先切割大型数据集（例如来自 USArray 的数据集）。
Lithosphere that has undergone deformation may record this process in the form of fabric such as foliation and lineation from ductile deformation, as well as non-horizontal interfaces between materials with different properties within the crust or at the bottom of the crust (Moho). The receiver function analysis technique isolates teleseismic mode conversions originating at velocity contrasts beneath a seismic station. We show that dipping isotropic contrasts as well as small (<10%) anisotropy generate first-order (i.e. Moho-sized) arrivals in receiver functions. We demonstrate that the azimuthally varying signal can be isolated from radial component receiver functions by subtracting the average (degree 0) over backazimuth. The radial signal matches the tangential component signal with an offset of 90° in backazimuth in the case of isotropic dipping interfaces or plunging axis anisotropy, with both dominated by a degree-1 variation in backazimuth. The positions of the extrema and nodes in both components constrain the strike of the dipping interface, dipping foliation, or plane perpendicular to a fast lineation. For azimuthal anisotropy, the offset between components is 45° and both show a degree-2 azimuthal variation. A distinction between isotropic dipping interfaces and plunging axis anisotropy can be made via the lack of a polarity-reversed azimuthal variation of the direct arrival in the anisotropic case. The described behavior is independent of fast versus slow axis symmetry and absolute velocities and largely independent of strength of anisotropy, exact anisotropic symmetry such as shape factor, and interface or foliation dip. Our method simply consists of subtracting the station average from the radial component receiver functions, shifting the tangential receiver functions by a fixed amount in backazimuth, and solving for degree 1 and 2 arrivals in the joint set of receiver functions. It allows determination of delay time (which scales to depth) and orientation of dipping and anisotropic features without waveform modeling. Combining radial and tangential components with a shift in backazimuth fills holes that may otherwise exist in azimuthal event coverage. We validate the method against active source results from the Wind River Thrust fault in Wyoming and against a waveform modeling study targeting deep crustal anisotropy in southern California. We then present initial results across the continental U.S. using Transportable Array data. First-order features are high amplitudes along the West Coast plate boundary and elevated amplitudes in parts of the intermountain West, a transition to low amplitudes in the central U.S. at the Rocky Mountain/Laramide front, and high amplitudes along the Appalachian–Ouachita belt. Foliation strike roughly aligns with the strike of mountains, but high signal amplitudes and matching strikes outline geological features with little current expression in surface topography, such as the Southern Oklahoma aulacogen, high-grade rocks in the Appalachian Piedmont, and the Midcontinent Rift. We conclude that lithospheric deformation can be targeted with receiver functions without extensive waveform modeling, allowing first cuts through large data sets such as that from USArray.