Carlson G, Shirzaei M, Werth S, Zhai G, Ojha C
School of Earth and Space Exploration Arizona State University Tempe AZ USA.
School of Geographical Sciences and Urban Planning Arizona State University Tempe AZ USA.
J Geophys Res Solid Earth. 2020 Jan;125(1):e2019JB018490. doi: 10.1029/2019JB018490. Epub 2020 Jan 20.
Changes in terrestrial water content cause elastic deformation of the Earth's crust. This deformation is thought to play a role in modulating crustal stress and seismicity in regions where large water storage fluctuations occur. Groundwater is an important component of total water storage change in California, helping to drive annual water storage fluctuations and loss during periods of drought. Here we use direct estimates of groundwater volume loss during the 2007-2010 drought in California's Central Valley obtained from high resolution Interferometric Synthetic Aperture Radar-based vertical land motion data to investigate the effect of groundwater volume change on the evolution of the stress field. We show that GPS-derived elastic load models may not capture the contribution of groundwater to terrestrial water loading, resulting in an underestimation of nontectonic crustal stress change. We find that groundwater unloading during the drought causes Coulomb stress change of up to 5.5 kPa and seasonal fluctuations of up to 2.6 kPa at seismogenic depth. We find that faults near the Valley show the largest stress change and the San Andreas fault experiences only ~40 Pa of Coulomb stress change over the course of a year from groundwater storage change. Annual Coulomb stress change peaks dominantly in the fall, when the groundwater level is low; however, some faults experience peak stress in the spring when groundwater levels are higher. Additionally, we find that periods of increased stress correlate with higher than average seismic moment release but are not correlated with an increase in the number of earthquakes. This indicates groundwater loading likely contributes to nontectonic loading of faults, especially near the Valley edge, but is not a dominant factor in modulation of seismicity in California because the amplitude of stress change declines rapidly with distance from the Valley. By carefully quantifying and spatially locating groundwater fluctuations, we will improve our understanding of what drives nontectonic stress and forces that modulate seismicity in California.
陆地水含量的变化会导致地壳发生弹性形变。这种形变被认为在调节储水量波动较大地区的地壳应力和地震活动中发挥着作用。地下水是加利福尼亚州总储水量变化的一个重要组成部分，在干旱时期有助于推动年度储水量波动和损失。在此，我们利用基于高分辨率干涉合成孔径雷达的垂直地面运动数据获得的2007 - 2010年加利福尼亚州中央谷地干旱期间地下水量损失的直接估计值，来研究地下水量变化对应力场演化的影响。我们表明，全球定位系统（GPS）得出的弹性负荷模型可能无法捕捉到地下水对陆地水负荷的贡献，从而导致对非构造地壳应力变化的低估。我们发现，干旱期间的地下水卸载在地震发生深度处会导致高达5.5千帕的库仑应力变化以及高达2.6千帕的季节性波动。我们发现谷地附近的断层显示出最大的应力变化，而圣安德烈亚斯断层在一年中因地下水储量变化仅经历约40帕的库仑应力变化。年度库仑应力变化峰值主要出现在秋季，此时地下水位较低；然而，一些断层在春季地下水位较高时经历应力峰值。此外，我们发现应力增加的时期与高于平均水平的地震矩释放相关，但与地震数量的增加无关。这表明地下水负荷可能有助于断层的非构造负荷，特别是在谷地边缘附近，但不是加利福尼亚州地震活动调制的主导因素，因为应力变化的幅度随着与谷地距离的增加而迅速下降。通过仔细量化和在空间上定位地下水波动，我们将增进对驱动加利福尼亚州非构造应力和调节地震活动的力的理解。
