Identification of Postseismic Transients in PBO GPS Time-Series

PBO GPS 时间序列中震后瞬变的识别

• 批准号: 0952234
• 负责人: Andrew Freed
• 金额: $ 15.12万
• 依托单位: Purdue University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-04-01 至 2014-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0952234&HistoricalAwards=false
• 关键词: Identification; Postseismic; Transients; PBO; GPS

The success of PBO's mission to infer long-term steady-state surface velocities depends critically on the ability of geodesists to identify and remove all non-steady-state contributions to GPS time series. Without this ability, steady-state velocities will be misidentified, leading to a misinterpretation of long-term strain rates and stress accumulation and an undermining of our understanding of the tectonics of North America. Of all non-steady-state contributions to GPS time series, postseismic relaxation of the lower crust and upper mantle has the potential for the greatest long-term influence on the broadest regions. In this project, viscoelastic finite element models are being developed to identify for removal from PBO GPS displacement time series, postseismic transients associated with all large (M greater than or equal to 7) earthquakes in the past several hundred years in the southwestern US and Alaska. The most important aspect of this analysis is finding a constitutive relationship between strain rate and stress that can appropriately characterize nonlinear viscoelastic strength as a function of tectonic environment (stress, temperature, water). This is especially true in consideration of nonlinear changes in viscosity associated with stress, which leads to a viscosity structure that varies spatially and temporally after an earthquake, and an initial transient response to stress changes that are much weaker than steady-state flow. To this end a new transient/steady-state power law is being introduced that combines the biviscous characterization of a Burgers model with the temperature and stress-dependent nature of a laboratory derived power law. The main objectives of this work are to: (1) calibrate this new transient/steady-state power-law to characterize the response of lower crustal and upper mantle flow through the earthquake cycle in the western US and mainland Alaska, and (2) use the flow law to calculate postseismic relaxation components in PBO GPS displacement time series associated with all large (M greater than or equal to 7) earthquakes in the past 100-200 years in these regions. These regions were chosen because they contain the bulk of the PBO network and they have experienced a significant number of M greater than or equal to 7 earthquakes. Secondary objectives include the determination of whether a single flow law can be used to describe the rheological strength of both the western US and Alaska margins, which will tell us something about the homogeneity versus heterogeneity of rheological properties at plate margins, and the development of a general understanding of the spatial and temporal influence of postseismic relaxation as a function of earthquake magnitude, which will provide insight into which historical earthquakes are continuing to influence the contemporary velocity field, and the likely duration of influence from more recent events. The analysis is being conducted using the finite element program ABAQUS, which enables the development of the required complex meshes and the incorporation of the new transient/steady-state power law. This project has the potential to improve the accuracy of one of the central missions of PBO: to infer long-term steady-state surface velocities and strain rates. Thus, it has the potential to influence a very broad number of research projects throughout the Earthscope community -- any project that utilizes GPS time series.

PBO能否成功推断长期稳态地表速度，关键取决于测地线师识别和消除GPS时间序列中所有非稳态贡献的能力。如果没有这种能力，稳态速度将被错误识别，导致对长期应变率和应力积累的误解，并破坏我们对北美构造的理解。在对GPS时间序列的所有非稳态贡献中，下地壳和上地幔的地震后松弛对最广泛的地区具有最大的长期影响的潜力。在这个项目中，粘弹性有限元模型正在开发中，以确定从PBO GPS位移时间序列中移除，与过去几百年来美国西南部和阿拉斯加所有大地震（M大于或等于7）相关的震后瞬变。该分析最重要的方面是找到应变率和应力之间的本构关系，可以适当地表征非线性粘弹性强度作为构造环境（应力，温度，水）的函数。考虑到与应力相关的粘度的非线性变化，这一点尤其正确，这导致地震后粘度结构在空间和时间上发生变化，并且对应力变化的初始瞬态响应比稳态流弱得多。为此，引入了一种新的瞬态/稳态幂律，将Burgers模型的双粘性特性与实验室导出的幂律的温度和应力依赖特性结合起来。本工作的主要目的是：(1)校准这一新的瞬态/稳态幂律，以表征美国西部和阿拉斯加大陆地震周期中下地壳和上地幔流动的响应；(2)利用该流动律计算这些地区过去100-200年来所有大地震（M大于或等于7）的PBO GPS位移时间序列中的震后松弛分量。之所以选择这些地区，是因为它们包含了PBO网络的大部分，并且经历了相当多的大于或等于7次地震。次要目标包括确定是否可以使用单一流动规律来描述美国西部和阿拉斯加边缘的流变强度，这将告诉我们关于板块边缘流变特性的均匀性与非均匀性的一些情况，以及发展对地震后松弛的时空影响作为地震震级函数的一般理解。这将使我们深入了解历史上哪些地震正在继续影响当代的速度场，以及最近发生的地震影响的可能持续时间。分析使用ABAQUS有限元程序进行，该程序可以开发所需的复杂网格，并结合新的瞬态/稳态幂律。该项目有可能提高PBO中心任务之一的准确性：推断长期稳态表面速度和应变率。因此，它有可能影响整个Earthscope社区的大量研究项目——任何利用GPS时间序列的项目。