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以主要的水平运动平移过另一部分。这种板块运动可以在地表用GPS数据和浅层断层的偏移量来测量，包括地壳地震产生的滑动。然而，这种运动的形式在岩石圈的地幔部分是不太确定的。走滑板块边界下的地幔变形模式包括从直接位于地表板块边界位置下方的狭窄剪切带到宽度超过100公里的广泛扩散变形带。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走滑断层系统（土耳其的北安纳托利亚断层和新西兰的南阿尔卑斯断层）下的大陆岩石圈的特性，这些结果将与之前在圣安德烈亚斯断层下的工作进行比较。特别是，在岩石圈底部从剪切运动转化为压缩运动（或相反）的波将被用来约束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，先前的研究表明，圣安地列斯中部岩石圈底部的剪切分布在一个宽度不到50公里的区域。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测量的岩石圈性质。这项研究的结果将为板块边界的物理性质以及板块运动如何在大陆岩石圈内调节提供新的见解。我们建议利用主要基于岩石圈-软流圈边界（LAB）转换的Sp相的地震成像来测量北安纳托利亚和南阿尔卑斯断层下的岩石圈厚度和岩石圈-软流圈速度梯度，并将采用走滑板块边界变形的数值模拟来为我们的结果提供物理框架。这种成像方法直接限制了地幔岩石圈底部的结构，包括由于与浅层断裂系统相关的贯穿剪切带而导致的LAB结构的可能变化。在先前对圣安德烈亚斯断层系统下的拉加带的研究中，我们发现拉加带的Sp相振幅和相应的横波速度下降在板块边界的西侧比东侧有系统地小。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公里的水平长度范围内。这些结果与不同性质的地幔岩石圈的并置一致，这些岩石圈横跨圣安德烈亚斯断层的中央，穿过一个狭窄的剪切带（宽50公里），延伸到岩石圈的底部。在北安纳托利亚和南阿尔卑斯断层系统下，Sp接收函数将被叠加成3D图像，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，以及对Sp和Ps相如何很好地解决LAB性质的横向变化的可靠评估。对圣安德烈亚斯、北安纳托利亚和阿尔卑斯断层系统下岩石圈和软流圈变形的数值模拟将使我们能够将Sp和Ps成像结果与剪切波分裂对地幔各向异性的限制相结合。它还将帮助我们根据岩石圈-软流圈粘度对比和变形引起的各向异性在产生明显LAB性质中的作用来解释地震结果。比较三个断层系统的结果将解决几个关键问题。大陆岩石圈深处的走滑剪切带有多宽？跨走滑断层系统的LAB构造对岩石圈和软流圈的流变学有什么意义？是什么过程导致了圣安地列斯断层系统以及阿尔卑斯和北安纳托利亚断层系统之间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?