Analyzing the Frequency Dependence of SS Precursors to Understand the Nature of Transition Zone Discontinuities

分析 SS 前体的频率依赖性以了解过渡区不连续性的性质

• 批准号: 1114445
• 负责人: Christine Houser
• 金额: $ 7.46万
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-10-01 至 2013-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1114445&HistoricalAwards=false
• 关键词: Analyzing; Frequency; Dependence; SS; Precursors

The Earth's mantle consists of the rock layer lying between the crust (7-70 km deep) and the core (2886 km deep) that makes up over 80% of the Earth's volume. The "transition zone" is one of the least understood regions of the mantle even though it is one of the most important in terms of understanding how plate tectonics operates in the deep Earth. The transition zone is defined by two abrupt jumps in seismic velocity (i.e. discontinuities) at approximately 410 and 660 km depth that are referred to as the 410 and 660 km discontinuities. Earthquakes produce shear waves that reflect off of these seismic jumps, and these SS phases provide global discontinuity depth maps. These seismic jumps result from an abrupt change in atomic structure due to high pressures at depth, i.e. a phase transition. Variations in the depth of these discontinuities indicate regional changes in temperature and/or composition, which provide a means of locating the source of warm volcanic regions and the accumulation of oceanic crust in the mantle due to plate tectonics.A perplexing observation from global mapping of these seismic jumps is that the 410 km discontinuity is correlated with the 660 km discontinuity at very long wavelengths. This observation defies expectations since the phase changes responsible for these discontinuities have opposite responses to changes in temperature. It is well known that higher temperatures will move the 410 km seismic discontinuity to higher pressures (i.e. greater depth). Likewise, colder temperatures will move this phase change to lower pressures (i.e. shallower depths). This relationship is reversed for the 660 km discontinuity. Therefore, a simple thermal anomaly is expected to produce opposite topography of the 410 and 660 km discontinuities, not the observed positive correlation. The correlated topography could be due to a shift in dominance from olivine to the garnet system at high temperature. This study tests this hypothesis by examining whether the reflections change character at different frequencies. The current catalog of clean long-period SS arrivals will be used to establish a database of higher frequency arrivals and to compare the PI's recently expanded long-period stacks to their short-period equivalents. These observed long and short-period reflections will be compared to predictions for different 1D transition zone velocity models. The increased seismic activity in the Sumatran region along with the progress of the USArray TA network have resulted in a staggering increase in the number of SS traces available for analysis. These highest-quality stacks occur under the NW Pacific where slow shear wave velocities and depressed 410 km discontinuity topography indicate regions that are likely warmer than the average mantle. Thus, the highest quality data is co-located with the regions where it may be hot enough to change the dominant phase from the olivine system to garnet system.This work will provide the greater solid Earth community with the specific seismic characteristics needed to distinguish between the competing effects of temperature, composition, and mineralogy on the transition zone discontinuities. If the character of the reflections indicates that there are two phase changes occurring around 660 km depth, this would favor the explanation that there is a change in mineralogy due to high temperatures. Likewise, if there is no indication of a second phase transition, then chemical heterogeneity may better explain the observations, and this information is critical to arguments regarding the spatial distribution of chemical heterogeneities in Earth's mantle. The results of this work are important to mantle convection calculations and should drive further mineral physics experimentation at these depths and temperatures.

地幔由位于地壳（7-70公里深）和地核（2886公里深）之间的岩石层组成，地核占地球体积的80%以上。“过渡带”是地幔中最不为人所知的区域之一，尽管它对于理解板块构造如何在地球深处运作是最重要的。过渡带由大约410和660公里深度的地震速度（即不连续面）的两次突然跳跃来定义，称为410和660公里不连续面。地震产生的横波反射出这些地震跳变，这些SS相位提供了全球不连续深度图。这些地震跳变是由于深部高压引起的原子结构的突然变化，即相变。这些不连续面深度的变化表明温度和（或）成分的区域变化，这提供了一种确定温暖的火山区域来源和由于板块构造而在地幔中积累海洋地壳的方法。从这些地震跳变的全球测绘中得到的一个令人困惑的观察结果是，410公里的不连续面与660公里的不连续面在非常长的波长上是相关的。这一观察结果违背了预期，因为引起这些不连续的相变对温度变化有相反的反应。众所周知，更高的温度将使410公里的地震不连续面向更高的压力（即更深的深度）移动。同样，较冷的温度将使这一相变转向较低的压力（即较浅的深度）。在660公里的不连续层中，这种关系是相反的。因此，一个简单的热异常预计会产生410 km和660 km不连续面相反的地形，而不是观测到的正相关。相关的地形可能是由于在高温下优势从橄榄石到石榴石体系的转变。本研究通过检查反射是否在不同频率下改变特征来验证这一假设。目前的清洁长期SS到达目录将用于建立一个频率较高的数据库，并将PI最近扩大的长期库存与短期同类产品进行比较。这些观测到的长周期和短周期反射将与不同一维过渡带速度模型的预测结果进行比较。随着USArray TA网络的进步，苏门答腊地区地震活动的增加导致可用于分析的SS道数量惊人地增加。这些质量最高的地幔堆发生在西北太平洋下方，在那里缓慢的横波速度和凹陷的410公里不连续地形表明，这些地区可能比平均地幔温度高。因此，最高质量的数据与可能足够热的地区共存，这些地区可以将主要相从橄榄石系统转变为石榴石系统。这项工作将为更大的固体地球群落提供所需的具体地震特征，以区分温度、成分和矿物学对过渡带不连续的竞争性影响。如果反射的特征表明在660公里深度附近发生了两个相位变化，这将有利于由于高温而导致矿物学变化的解释。同样，如果没有第二阶段转变的迹象，那么化学非均质性可能更好地解释观测结果，而这一信息对于有关地幔化学非均质性空间分布的争论至关重要。这项工作的结果对地幔对流计算很重要，并将推动在这些深度和温度下进一步的矿物物理实验。