In Situ Estimation of Poisson's Ratio

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。地震学测定的泊松比是研究地球组成的岩石学和地球化学研究的有力补充。在本项目中，研究人员将使用Lin和Shearer（2007）的技术估算与泊松比直接相关的纵波速度与横波速度之比（Vp/Vs）。该方法采用鲁棒最小二乘法对波形互相关得到的纵波和横波到达时间差进行处理。它提供了高度精确的结果，因为互相关可以在几毫秒内测量差分时间，并且可以在估计的Vp/Vs比中达到0.001的精度，对应于泊松比约0.0004。建议的工作包括进行波形互相关和将原位方法应用于几个区域。应用的重点区域是火山和活动断裂带，包括黄石公园、意大利的埃特纳火山、长谷火山口、洛马普里塔断裂带和帕克菲尔德的SAFOD目标事件区。所得Vp/Vs比值的时空变化将有助于跟踪火山内岩浆和挥发物的运动，并估计活动断裂带中压裂和流体含量的空间变化。震源区的高精度Vp/Vs比值也将用于提高地震定位精度。研究人员将采用的方法实现简单，执行速度快，因为它使用鲁棒最小二乘来拟合微分时间。该方法的另一个优点是，通过简单地按空间和/或时间选择事件子集，可以直接研究Vp/Vs比率的时空变化，这是基于层析成像难以实现的，因为难以获得可靠的Vp/Vs模型以及问题的非唯一性。即使没有S波目录选择，也可以获得Vp/Vs比，因为只需要差分时间，并且可以从地震位置和给定的速度模型估计S互相关的时间窗口。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Seismologically determined Poisson's ratio is a powerful tool to complement the petrological and geochemical studies in the investigation of the composition of the Earth. In this project, the researchers will estimate the ratio of compressional- to shear-wave velocity (Vp/Vs), which is directly related to Poisson's ratio, using the technique of Lin and Shearer (2007). This approach applies a robust least-squares method to arrival time differences for P and S waves obtained from waveform cross-correlation. It provides highly precise results because cross-correlation can measure differential times to within a few milliseconds and can achieve a precision of 0.001 in estimated Vp/Vs ratio, corresponding to about 0.0004 in Poisson's ratio. The proposed work includes performing waveform cross-correlation and applying the in-situ method to several regions. The focus areas of application are volcanoes and active fault zones, including Yellowstone, Mount Etna Volcano in Italy, Long Valley Caldera, the Loma Prieta rupture zone, and the SAFOD target event region for Parkfield. The spatial and temporal variations in the resulting Vp/Vs ratios will help to track movement of magma and volatiles within a volcano and to estimate spatial variations in fracturing and fluid content in active fault zones. The high-precision Vp/Vs ratio in the source region will also be used to improve earthquake location accuracy.The method the researchers will apply is simple to implement and fast to execute since it uses a robust least-squares to fit the differential times. Another advantage of this method is that it will be straightforward to study the spatial and temporal variations of Vp/Vs ratios by simply selecting a subset of events by space and/or time, which would be difficult to achieve based on tomography because of the difficulties of obtaining reliable Vp/Vs models and the non-uniqueness of the problem. The Vp/Vs ratios can be obtained even without S-wave catalog picks, because only the differential times are needed and the time window for S cross-correlation can be estimated from the earthquake locations and a given velocity model.