# Responses of Water Temperature and Level to Large Earthquakes in Tibet
## 4 Discussion  
### Co-Seismic Responses of the Water Temperature and Water Level to Large Earthquakes  
In this paper, we carried out an analysis of two M8.0 earthquakes for which co-seismic responses were recorded (Figure 7), which indicates the seismic wave travel time from the moment of seismogenesis to the moment of sudden energy change. Since the attenuation of seismic waves is more obvious due to the thick crust (>60 km) unique to the Tibetan Plateau, the corresponding co-seismic effects of water level and water temperature tend to be more obvious for large and deep earthquakes. The epicenter of the Wenchuan M8.0 earthquake that occurred on 12 May 2008, was 1189 km away from the observation well, and the source depth was 14 km. Through calculation, we found that the shock wave propagated westward and that the water temperature continued to heat up from 40 min to 920 min post-earthquake. On 25 April 2015, an M8.1 earthquake occurred in Nepal. The epicenter of this earthquake was 648 km away from the observation well, and the source depth was 20 km. Due to the close distance, the water temperature rapidly decreased in the early stage and then quickly returned to the pre-earthquake value. Figure 7: Response to occurrence (red line) of large earthquakes. (**a**) The M8.0 Wenchuan great earthquake on 12 May 2008. (**b**) M8.1 earthquake that occurred in Nepal on April 25 th, 2015. We further involved a comparison of the Lhasa station data set with the 400 wells in the China groundwater monitoring network (China Earthquake Network Center, Beijing, China) (e.g., [48]) for their different co-seismic responses in the water level (rise \(>\) 1 m, drop \(>\) 1 m, or oscillation) to the 2008 M8.0 Wenchuan earthquake (Figure S1). We noticed that the Lhasa station water level during the 2008 Wenchuan earthquake ranged from 16.35 m to 16.42 m, with an amplitude of approximately 0.07 m, while most of the adjacent wells in the China groundwater monitoring network exhibited a co-seismic rise (or drop) of as large as \(>\)1 m [48]. Wells in Northeast China even showed the largest amplitude of water-level oscillations (as large as 1.1 m at an epicenter distance of 1400 km) despite the large well-epicenter distance [48]. Although the Lhasa well is closer (1189 km) to the epicenter, the water level oscillation is nearly ten times smaller than that of the well in Northeast China. This is likely due to the larger attenuation of seismic waves in the thick crust (\(>\)60 km) unique to the Tibetan Plateau. It is also noteworthy that 2015 M8.1 Nepal earthquake results in a similar small-scale water level oscillation with the largest amplitude of 0.045 m, despite a well-epicenter distance of merely 648 km. In particular, corresponding to the 2011 M9 Tohoku earthquake, the co-seismic water level varied from 1.79 m to 1.89 m (in Figure 6(a1)), which implied that the far-field giant earthquake caused an amplitude of only 0.1 m in the water level of the Lhasa well, comparable to the oscillation amplitude caused by adjacent M8 earthquakes. As the only fluid monitoring well above high-altitude (\(>\)3000 m) in China, it is evident that the groundwater temperature response is significantly correlated with water level changes, especially the co-seismic response to large earthquakes [44]. Previous studies have concluded that the amplitudes of changes in water level and temperature are related to the earthquake magnitude and the distance from the epicenter [49]. Earthquakes with larger magnitudes tend to cause a rise or drop with greater amplitude, while earthquakes further from the epicenter tend to cause oscillations of smaller amplitude. The plateau fluid wells in the Tibetan region recorded the water temperature change of the Wenchuan earthquake as rising by 0.0065 \({}^{\circ}\)C and then dropping to 0.0079 \({}^{\circ}\)C, while the water temperature change after the Nepal earthquake was a rapid drop followed by a rapid rise finally oscillating to normal values (Figure 7a,b).