Investigating the mantle expression of continental strike-slip fault systems with scattered wave imaging of the lithosphere-asthenosphere boundary

在大陆走滑板块边界，如加州的圣安德烈亚斯断层系统，大陆岩石圈的一部分主要以水平运动平移到另一部分。 这种板块运动可以在地表用GPS数据测量，也可以通过浅层断层的偏移量来测量，包括地壳地震产生的滑动。 然而，这种运动在岩石圈地幔部分的形式是不太确定的。 走滑板块边界下的地幔变形模型范围从位于地表板块边界位置正下方的狭窄剪切带到超过100公里宽的扩散变形带。 在这个研究项目中，来自遥远地震的地震波将被用来测量两个主要的走滑断层系统，土耳其的北方安纳托利亚断层和新西兰的阿尔卑斯山南部断层下的大陆岩石圈的属性，这些结果将与先前在圣安德烈亚斯下的工作进行比较。 特别是，在岩石圈底部从剪切运动转换为压缩运动（反之亦然）的波将被用来限制岩石圈的厚度和较冷、相对刚性的岩石圈与较暖的软流圈之间的地震波速度梯度。 如果岩石圈厚度和/或岩石圈-软流圈速度梯度在走滑板块边界以下的小水平长度尺度上发生变化，这些发现将支持窄剪切带模型。 例如，先前的工作表明，在圣安德烈亚斯中心下面的岩石圈底部的剪切分布在一个不到50公里宽的区域。地幔岩石圈性质的更渐进的水平变化将支持更广泛的板块边界地幔剪切带的存在。 走滑断层系统下地幔变形的数值模拟将有助于解释地震学测量的岩石圈性质。 这项研究的结果将提供新的见解板块边界的物理性质和板块运动是如何容纳在大陆岩石圈。我们建议测量岩石圈厚度和岩石圈-软流圈速度梯度下的北方安纳托利亚和南阿尔卑斯山断层地震成像主要基于Sp阶段，转换在岩石圈-软流圈边界（LAB），我们将采用走滑板块边界变形的数值模拟，为我们的结果提供一个物理框架。 这种成像方法直接约束结构在地幔岩石圈的基础上，包括可能的变化，由于贯穿剪切带与浅断层系统的LAB结构。 在圣安德烈亚斯断层系统下的LAB上的先前工作中，我们发现在LAB处的Sp相位振幅和相应的剪切波速度下降在板块边界的西侧比其东侧系统地小。 在加州中部，LAB速度梯度的变化发生在圣安德烈亚斯断层正下方不到50 km的水平长度范围内。 这些结果是一致的地幔岩石圈具有不同的属性，在中央圣安德烈亚斯断层横跨一个狭窄的剪切带（50公里宽），延伸到岩石圈的基础上的并列。 在北方安纳托利亚和南阿尔卑斯山断层系统之下，Sp接收器功能将被叠加到3D图像中，以确定LAB不连续深度和振幅，以及LAB属性是否在每个断层上系统地变化。 还将评估Ps阶段的限制。 基于合成地震图的建模将提供与LAB相关的垂直剪切速度梯度的定量估计，以及Sp和Ps相位如何解决LAB属性的横向变化的可靠评估。 圣安德烈亚斯，北方安纳托利亚，阿尔卑斯山断层系统下的岩石圈和软流圈变形的数值模拟将使我们能够整合的Sp和Ps成像的结果与约束地幔各向异性剪切波分裂。 这也将有助于我们解释地震学的结果，岩石圈软流圈粘度对比和变形引起的各向异性在创建明显的LAB属性的作用。 比较三个断层系统的结果将解决几个关键问题。大陆岩石圈深部走滑剪切带有多宽？走滑断层系统中的LAB结构对岩石圈和软流圈流变学有什么影响？什么过程是负责在整个圣安德烈亚斯断层系统和潜在的阿尔卑斯山和北方安纳托利亚断层系统的LAB属性的横向对比？

At continental strike-slip plate boundaries, such as the San Andreas fault system in California, one portion of continental lithosphere translates past another with primarily horizontal motion. This plate motion can be measured at the surface with GPS data and by offsets on shallow faults, including the slip produced by crustal earthquakes. However, the form of this motion in the mantle portion of the lithosphere is much less certain. Models of mantle deformation beneath strike-slip plate boundaries range from narrow shear zones that lie directly beneath the position of the plate boundary at the surface to broad zones of diffuse deformation that are more than ~100 kilometers wide. In this research project, seismic waves from distant earthquakes will be used to measure the properties of the continental lithosphere beneath two major strike-slip fault systems, the Northern Anatolian fault in Turkey and the Southern Alpine fault in New Zealand, and these results will be compared to prior work beneath the San Andreas. In particular, waves that convert from shear motion to compressional motion (or viceversa) at the base of the lithosphere will be used to constrain the thickness of the lithosphere and the gradient in seismic wave velocity between the colder, relatively rigid lithosphere and the warmer asthenosphere. If lithospheric thickness and/or the lithosphere-asthenosphere velocity gradient change over small horizontal length-scales beneath strike-slip plate boundaries, these findings would support the narrow shear zone model. For example, prior work indicates that shear at the base of the lithosphere beneath the central San Andreas is distributed across a zone that is less than 50 kilometers wide. More gradual horizontal changes in the properties of the mantle lithosphere would support the presence of broader plate boundary mantle shear zones. Numerical modeling of mantle deformation beneath strike-slip fault systems will aid in interpreting the seismologically-measured lithospheric properties. The results of this research will provide new insight on the physical properties of plate boundaries and how plate motion is accommodated within the continental lithosphere. We propose to measure lithospheric thickness and the lithosphere-asthenosphere velocity gradient beneath the Northern Anatolian and Southern Alpine faults with seismic imaging primarily based on Sp phases that convert at the lithosphere-asthenosphere boundary (LAB), and we will employ numerical modeling of strike-slip plate boundary deformation to provide a physical framework for our results. This imaging approach directly constrains structure at the base of the mantle lithosphere, including possible variations in LAB structure due to through-going shear zones associated with shallow fault systems. In prior work on the LAB beneath the San Andreas fault system, we found that Sp phase amplitudes and the corresponding drop in shear-wave velocity at the LAB are systematically smaller on the western side of the plate boundary than to its east. In central California, the change in LAB velocity gradient occurs over a horizontal length scale of less than 50 km directly beneath the San Andreas fault. These results are consistent with the juxtaposition of mantle lithospheres with different properties across the central San Andreas fault across a narrow shear zone ( 50 km in width) that extends to the base of the lithosphere. Beneath the Northern Anatolian and Southern Alpine fault systems, Sp receiver functions will be stacked into 3D images to determine LAB discontinuity depths and amplitudes and whether LAB properties vary systematically across each of the faults. Constraints from Ps phases will also be assessed. Modeling based on synthetic seismograms will provide quantitative estimates of the vertical shear velocity gradients associated with the LAB and a robust assessment of how well Sp and Ps phases resolve lateral changes in LAB properties. Numerical modeling of lithospheric and asthenospheric deformation beneath the San Andreas, Northern Anatolian, and Alpine fault systems will allow us to integrate the results of the Sp and Ps imaging with constraints on mantle anisotropy from shear-wave splitting. It will also help us to interpret the seismological results in terms of lithosphere-asthenosphere viscosity contrasts and the role of deformation-induced anisotropy in creating apparent LAB properties. Comparison of results from the three fault systems will address several key questions. How wide are strike-slip shear zones in the deep continental lithosphere? What are the implications of LAB structure across strike-slip fault systems for the rheologies of the lithosphere and asthenosphere? What processes are responsible for the lateral contrast in LAB properties across the San Andreas fault system and potentially the Alpine and Northern Anatolian fault systems?