Correlating the 410 km discontinuity low-velocity layer occurrence with velocity tomograms using Pds imaging with high-fold arrays

美国西部上地幔的两个新的地震观测是：1)从高折叠PASSCAL阵列的接收函数叠加数据中经常发现位于410公里不连续面上的低速层；2)观察到大约8个低速异常，从最上面的地幔延伸到410公里不连续面附近(Schmandt和Humphreys，2010)。对410公里不连续面上的低速层的最好解释是，它表明存在由水过滤模型产生的部分熔融层(Bercovici和Karato，2003)。我们认为，410公里不连续面上方的众多低速异常可能是浮力熔岩底辟的表现，这些底辟是从410公里不连续面浮力熔融层的顶部成核的。这种上升410个熔层底辟的情景已经被研究并被证明是可信的(Leahy和Bercovici，2008)。我们之前对410公里不连续面的成像工作表明，要可靠地成像410低速层，需要记录一年的高折叠PASSCAL阵列，站间距为30公里。我们建议使用以下PASSCAL/柔性阵列来成像410千米的不连续面：内华达山脉地球望远镜阵列、柔性阵列门多西诺实验、科罗拉多落基山脉实验和地震断面阵列以及高熔岩平原阵列。410公里的不连续面将使用网格搜索算法和简单的速度模型参数化来模拟；模型的不确定性将通过计算一维和二维模型参数的边缘概率密度函数来评估。我们的假设--410公里不连续面以上的低速异常表现为来自410熔层的上涌底辟--将通过将410熔层产状/厚度与P/S体波层析图(S)相关联来检验。我们将使用Schmandt和Humphreys的断层图像(在回顾中)，因为这个模型拥有我们打算分析的所有PASSCAL阵列的旅行时间数据，以及可运输阵列数据。美国西部是一个正在经历重大火山活动的地区，山脉隆起，并且正在积极扩张(例如，形成盆地和山脉)。这种构造活动的一部分很可能是由深部地幔过程驱动的，例如上涌的地幔热柱和俯冲的大洋岩石圈。这一提议旨在检验一个新的假设，即深部地幔过程如何影响美国西部的构造活动：具体地说，一些构造活动可能是由位于410公里深地幔速度不连续面上的部分熔融岩层脱落的上升底辟驱动的。这种速度不连续反映了地幔橄榄石组分的固态相变，由于这种相变前后橄榄石组分的水溶度不同，可能是从410公里不连续面下方向上流动的充分水化的地幔形成了部分熔融层(少数百分比的熔体孔隙度)。为了验证这一假设，将使用美国西部高密度PASSCAL和灵活阵列地震部署的地震数据绘制410公里熔层的空间分布。将这张新的410熔体层图与该区域下方速度结构的新层析图像进行比较，将量化众多上地幔低速管、低速管和低速管之间的相关性。出现了410公里的熔融层。发现上地幔低速异常是否表现为410个熔融底辟的社会益处将是更好地理解美国西部火山活动的起源。

Two new seismic observations of the upper mantle beneath the western US are: 1) a low velocity layer atop the 410 km discontinuity is often found from stacking receiver function data from high fold PASSCAL arrays; 2) about eight low velocity anomalies are observed that extend from the uppermost mantle to near the 410 km discontinuity (Schmandt and Humphreys, 2010). The best interpretation of the low velocity layer atop the 410 km discontinuity is that it manifests the presence of a partially molten layer created by the water-filter model (Bercovici and Karato, 2003). We suggest that the numerous low velocity anomalies above the 410 km discontinuity may be the manifestation of buoyant melt diapirs that nucleate from the top of a buoyant 410 km discontinuity melt layer. This scenario of upwelling 410 melt layer diapirs has been studied and shown to be plausible (Leahy and Bercovici, 2008). Our prior work imaging the 410 km discontinuity demonstrates that high fold PASSCAL arrays with 1 year of recording and a station spacing of 30 km are required to reliably image a 410 low velocity layer. We propose to image the 410 km discontinuity using the following PASSCAL/Flexible arrays: the Sierra Nevada Earthscope Array, the Flex-Array Mendocino Experiment, the Colorado Rockies Experiment and Seismic Transects array, and the High Lava Plains Array. The 410 km discontinuity will be modeled using a grid-search algorithm and a simple velocity model parameterization; model uncertainties will be assessed by calculating the 1- and 2-dimensional model parameter marginal probability density functions. Our hypothesis - that the low velocity anomalies above the 410 km discontinuity manifest upwelling diapirs from a 410 melt layer - will be tested by correlating the 410 melt layer occurrence/thickness with P/S body wave tomogram(s). We will use the tomograms of Schmandt and Humphreys (in review) as this model has travel-time data from all the PASSCAL arrays we intend to analyze, in addition to the Transportable Array data.The western United States is a region currently experiencing significant volcanic activity, uplifting mountain ranges, and is actively being extended (e.g., to make the Basin and Range). Some portion of this tectonic activity is mostly likely driven by deep mantle processes such as upwelling mantle plumes and subducting oceanic lithosphere. This proposal intends to test a new hypothesis for how deep mantle process maybe affecting the tectonic activity of the western United States: specifically, that some of this tectonic activity maybe driven by upwelling diapirs that are shed from a partially molten melt layer that resides atop the 410 km depth mantle velocity discontinuity. This velocity discontinuity manifest the solid-state phase transition of the olivine component of the mantle, and due to differences in the olivine component water solubility above and below this phase transition, a partially molten melt layer (few percent melt porosity) maybe produced by an upward flux of sufficiently hydrate mantle from below the 410 km discontinuity. To test this hypothesis, the spatially distribution of the 410 km melt layer will be mapped using seismic data from high density PASSCAL and Flexible array seismic deployments in the western United States. Comparison of this new 410 melt layer map with new tomographic images of the velocity structure beneath this region will quantify the correlation between the numerous upper mantle low velocity ?pipes? and the 410 km melt layer occurence. The societal benefit to discovering whether the upper mantle low velocity anomalies manifest 410 melt diapirs would be an improved understanding of the origin of volcanic activity in the western US.