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

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.

Evolution Characteristics and Mechanism of the Load/Unload Response Ratio Based on Strain Observation Before the Jiuzhaigou MS7.0 Earthquake

“…Then, the LURR value begins to increase above 1.0 gradually from 6 months to 1 year before the occurrence of JZGEQ, reaching the maximum value in 1 to 3 months before the earthquake and subsequently returning to a low level prior to the earthquake. The trend of the LURR time series is similar to that of the previous calculation of LURR using small earthquakes and groundwater level (Yin et al, 2000;Yu et al, 2021), which indicates that the LURR method based on strain observation can better extract the pre-seismic anomalies. On the other hand, under the influence of station distribution, epicentral distance, and observation instruments, there are still noticeable differences between the LURR curves.…”

“…et al (2000) used the Benioff strain of small earthquakes as the loading/unloading response in the LURR calculations in different tectonic settings (California, United States, and Kanto region, Japan). Yu et al (2021) proposed a new attempt by using water levels as calculation data and achieved a good application effect. The results of the studies prove that LURR anomalies are relatively significant prior to different earthquakes, that is, the stress accumulation before an earthquake in the seismogenic region will lead to the change of the Benioff strain and water level.…”

“…where H i is strain observation at the ith record, and N+ or N− represents the numbers of records during the loading phase and unloading phase, respectively. The time window used to calculate the LURR value typically contains multiple loading-unloading cycles (Yu et al, 2021).…”

“…We calculated the LURR time series results of 81 items and obtained the spatial distribution results of different stations by further interpolation, as shown in Figure 3. The calculation time window length is 1 month, the sliding step length is 1 month, and the friction coefficient is 0.4 (Yu et al, 2020;Yu et al, 2021). The selection of time window should not only consider the stability of calculation results but also identify anomalies before an earthquake.…”

Study of the electromechanical coupling process before the 2020 Ms 6.4 Yutian, China earthquake

“…In recent years, Yu et al 11 have found that the Load/unloading response ratio (LURR) might be applied to explore nonlinear mechanical process associated with earthquake preparation via groundwater level and the pre-seismic changes were explained by dilatancy. This may provide an opportunity for detect pre-seismic electric field changes.…”