Energy Geoscience Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA, 94720, USA.
Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720, USA.
Nat Commun. 2023 Apr 26;14(1):2395. doi: 10.1038/s41467-023-37784-3.
Fractures are integral to the hydrology and geochemistry of watersheds, but our understanding of fracture dynamics is very limited because of the challenge of monitoring the subsurface. Here we provide evidence that long-term, high-frequency measurements of the river concentration of the ultra-trace element thorium (Th) can provide a signature of bedrock fracture processes spanning neighboring watersheds in Colorado. River Th concentrations show abrupt (subdaily) excursions and biexponential decay with approximately 1-day and 1-week time constants, concentration patterns that are distinct from all other solutes except beryllium and arsenic. The patterns are uncorrelated with daily precipitation records or seasonal trends in atmospheric deposition. Groundwater Th analyses are consistent with bedrock release and dilution upon mixing with river water. Most Th excursions have no seismic signatures that are detectable 50 km from the site, suggesting the Th concentrations can reveal aseismic fracture or fault events. We find, however, a weak statistical correlation between Th and seismic motion caused by distant earthquakes, possibly the first chemical signature of dynamic earthquake triggering, a phenomenon previously identified only through geophysical methods.
断裂是流域水文学和地球化学的组成部分，但由于监测地下的挑战，我们对断裂动力学的理解非常有限。在这里，我们提供的证据表明，对超微量元素钍（Th）的河流浓度进行长期、高频测量，可以为科罗拉多州相邻流域的基岩断裂过程提供特征。河流 Th 浓度显示出突然的（亚日）偏移和双指数衰减，具有约 1 天和 1 周的时间常数，浓度模式与除铍和砷以外的所有其他溶质明显不同。这些模式与每日降水记录或大气沉积的季节性趋势无关。地下水 Th 分析与基岩释放和与河水混合时的稀释一致。大多数 Th 偏移都没有可在距站点 50 公里处检测到的地震特征，这表明 Th 浓度可以揭示无震断裂或断层事件。然而，我们发现 Th 与由远距离地震引起的地震运动之间存在微弱的统计相关性，这可能是动态地震触发的第一个化学特征，这一现象以前仅通过地球物理方法确定。
