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。地震确定的泊松比是在地球成分调查中补充岩石学和地球化学研究的有力工具。在这个项目中，研究人员将使用Lin和Sheeller(2007)的技术来估计与泊松比直接相关的纵横波速度比(Vp/vs)。该方法将稳健的最小二乘方法应用于由波形互相关得到的P波和S波的到达时间差。它提供了高精度的结果，因为互相关可以测量到几毫秒以内的差分时间，并且可以达到估计的VP/VS比的0.001的精度，对应于泊松比的大约0.0004。所提出的工作包括进行波形互相关并将原位方法应用于多个区域。应用的重点领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、洛马普里埃塔断裂带和帕克菲尔德的SAFOD目标事件区域。由此得到的Vp/VS比值的空间和时间变化将有助于跟踪火山内岩浆和挥发分的运动，并估计活动断裂带中破裂和流体含量的空间变化。震源区高精度的波速比也将被用来提高地震定位精度。研究人员将采用的方法实施简单，执行速度快，因为它使用稳健的最小二乘法来拟合差分时间。这种方法的另一个优点是，通过简单地按空间和/或时间选择事件的子集来研究Vp/VS比率的空间和时间变化将是直接的，这是基于层析成像难以实现的，因为获得可靠的Vp/VS模型的难度和问题的非唯一性。即使没有S波目录拾取，也可以得到波速比，因为只需要差分时间，根据地震位置和给定的速度模型就可以估计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.