Determining Stress State of Source Media with Identified Difference between Groundwater Level during Loading and Unloading Induced by Earth Tides

Huaizhong Yu,Yulong Chang,Chen Yu,Chong Yue,Rong Gao,Binbin Zhao,Yuchuan Ma

doi:10.3390/w13202843

Abstract

The groundwater level might be adopted as a useful tool to explore pre-seismic stress change in the earth crust, because it circulates in the deep crust and should be altered by the processes associated with the preparation of earthquakes. This work makes a new attempt that applies the load/unload response ratio (LURR) technique to study the stress state of the source media before the large earthquakes by calculating the ratio between the water levels during the loading and unloading phases. The change of Coulomb failure stress induced by earth tides in the tectonically preferred slip direction on the fault surface of the mainshock is adopted for differentiating the loading and unloading periods. Using this approach, we tested the groundwater level in the wells near the epicenters of some large earthquakes that occurred in the Sichuan-Yunnan region of southwest China. Results show that the LURR time series fluctuated narrowly around 1.0 for many years and reached anomalously high peaks 2~8 months prior to the mainshocks. For the earthquakes with multiple observation wells, the magnitude of the maximum values decreases with the distance from the epicenter. The underlying physics of these changes should be caused by the pre-seismic dilatancy. The corresponding volume variations in the crust could be observed in the geodetic time series in the same neighborhoods and during the same period.

Highlights

The instrumentally recorded data of groundwater level changes have been widely applied to study processes associated with the occurrence of earthquakes, and reported in many studies [1,2,3]
In our latest studies [14], we find that the notion of Load/unload response ratio (LURR) may be applied to explore physical processes associated with the preparation of earthquakes via groundwater level
Comparing the load/unload response ratio (LURR) time series derived from the groundwater level at different wells, we find that significant anomalies can be observed prior to the ensuing large earthquakes

Summary

Introduction

The instrumentally recorded data of groundwater level changes have been widely applied to study processes associated with the occurrence of earthquakes, and reported in many studies [1,2,3]. Roeloffs [4] revealed the response of water level fluctuations in a well near the Parkfield to distant and local earthquakes. Brodsky et al [6] suggested the mechanism of groundwater level changes and pressure induced by distant earthquakes. The results seem to support the notion that observation of groundwater level changes helps us understand the responses of the hydrogeologic system to earthquakes [7]. The current indexes of the anomalies for detection of the time, location, and magnitude of future large earthquakes are mostly obtained by using the statistical techniques [8], e.g., the groundwater level anomalies preceding the large earthquakes in China and Japan [9], the short-term pre-seismic changes in groundwater level in Turkmenistan [10], and the precursory changes before moderate local earthquakes and some large distant earthquakes in the well water level near the Tono mine in central Japan [11]. Due to the lack of understanding of the physical mechanism of earthquake preparation, less information can be extracted from the ambiguous anomalies and their uncertain time scales for the studies of earthquake potential [12,13]

Methods

Results

Discussion

Conclusion