Identification of Postseismic Transients in PBO GPS Time-Series

成功的PBO的使命，推断长期稳定状态的表面速度，关键取决于大地测量师的能力，以确定和消除所有非稳态的GPS时间序列的贡献。如果没有这种能力，稳态速度将被错误识别，导致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，下地壳和上地幔的震后弛豫对最广泛的区域具有最大的长期影响潜力。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位移时间序列中删除，震后瞬变与所有大（M大于或等于7）地震在过去的几百年中，在美国西南部和阿拉斯加。这种分析的最重要的方面是找到应变率和应力之间的本构关系，可以适当地表征作为构造环境（应力，温度，水）的函数的非线性粘弹性强度。考虑到与应力相关的粘性的非线性变化，这一点尤其正确，这导致地震后时空变化的粘性结构，以及对应力变化的初始瞬态响应比稳态流动弱得多。为此，一个新的瞬态/稳态功率法被引入，结合了双粘性特性的Burgers模型与温度和应力依赖性的实验室推导的功率法。这项工作的主要目标是：（1）校准这种新的瞬态/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）利用流动定律计算了与各大地震带有关的PBO GPS位移时间序列的震后弛豫分量（M ≥ 7）地震活动。之所以选择这些地区，是因为它们包含了大部分的PBO网络，并且它们经历了相当数量的M大于或等于7的地震。次要目标包括确定是否可以使用单一流动定律来描述美国西部和阿拉斯加边缘的流变强度，这将告诉我们一些关于板块边缘流变特性的均匀性与非均匀性的信息，以及对震后弛豫的空间和时间影响的一般理解的发展，作为地震震级的函数，这将提供对哪些历史地震继续影响当代速度场以及最近事件影响的可能持续时间的洞察。分析正在进行使用有限元程序ABAQUS，这使得所需的复杂网格的发展和新的瞬态/稳态功率法的结合。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，它有可能影响整个地球观测界的许多研究项目-任何利用全球定位系统时间序列的项目。

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.