A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations
A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations
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DOI：
10.1029/2010jb008156
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发表时间：
2011-04
影响因子：
--
通讯作者：
V. Tsai
中科院分区：
文献类型：
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作者：
V. Tsai
众所周知，GPS时间序列包含一个不是由于构造运动造成的季节变化，最近的研究表明，地壳地震速度也可能随季节变化。为了解释这些变化，提出了许多假设，其中热弹性和水文诱导的应力和应变是主要的候选。然而，不幸的是，由于不存在理解这种季节变化的一般框架，目前不可能快速评估这些假设的合理性。为了填补这一文献空白，我推广了一个二维热弹性应变模型，为热弹性应力或水文载荷引起的位移和波速变化提供了解析解，该模型由孔弹性应力和纯弹性应力组成。热弹性模型假定地表温度是周期性的，水文模型同样假定近地表水负荷是周期性的。由于这三个模型都是二维和周期性的，它们只能近似于任何现实情况；但是，这些模型仍然为估计热弹性和水文变化的影响提供了一个定量框架。由于一些相关参数存在很大的不确定性，使模式与观测之间的定量比较变得更加复杂。尽管存在这种不确定性，但我发现，最大实际热弹性效应不太可能解释典型GPS位移时间序列中观测到的大部分年变化，也不太可能解释南加州观测到的地震波速度的年变化。另一方面，水文荷载可能能够解释位移和地震波速度的年变化的更大一部分。这两个模型都不太可能解释从观测中推断出的所有地震波速度变化。然而，在模型参数得到更好的约束之前，无法得出更明确的结论。
It is known that GPS time series contain a seasonal variation that is not due to tectonic motions, and it has recently been shown that crustal seismic velocities may also vary seasonally. In order to explain these changes, a number of hypotheses have been given, among which thermoelastic and hydrology-induced stresses and strains are leading candidates. Unfortunately, though, since a general framework does not exist for understanding such seasonal variations, it is currently not possible to quickly evaluate the plausibility of these hypotheses. To fill this gap in the literature, I generalize a two-dimensional thermoelastic strain model to provide an analytic solution for the displacements and wave speed changes due to either thermoelastic stresses or hydrologic loading, which consists of poroelastic stresses and purely elastic stresses. The thermoelastic model assumes a periodic surface temperature, and the hydrologic models similarly assume a periodic near-surface water load. Since all three models are two-dimensional and periodic, they are expected to only approximate any realistic scenario; but the models nonetheless provide a quantitative framework for estimating the effects of thermoelastic and hydrologic variations. Quantitative comparison between the models and observations is further complicated by the large uncertainty in some of the relevant parameters. Despite this uncertainty, though, I find that maximum realistic thermoelastic effects are unlikely to explain a large fraction of the observed annual variation in a typical GPS displacement time series or of the observed annual variations in seismic wave speeds in southern California. Hydrologic loading, on the other hand, may be able to explain a larger fraction of both the annual variations in displacements and seismic wave speeds. Neither model is likely to explain all of the seismic wave speed variations inferred from observations. However, more definitive conclusions cannot be made until the model parameters are better constrained.