Characteristic And Mechanism Of Co-Seismic Hydrological Response Induced By Earthquakes

Earthquake-induced groundwater level, temperature, chemistry composition changes have been documented for thousands of years, studying of these phenomena provide a way for us to understand the interaction between tectonic activities and hydrological processes in spatial and temporal scale otherwise can not be studied. Thus, the research on co-seismic hydrological response caused by earthquake is one of the hot topics in earthquake hydrology. Many mechanisms have been proposed in the previous studies although there are debates among them. Based on the records of groundwater monitoring networks all around China, we collected several large earthquake occurred in recent years(the September 12 th, 2007 Sumatra earthquake, the May 12 th, 2008 Wenchuan earthquake, the March 11 th, 2011 Tohoku earthquake, the April 11 th, 2012 Sumatra earthquake and the April 20 th, 2013 Lushan earthquake). Using data from single well in response to multiple earthquakes, from a set of wells in response to single earthquake, from large-scale responses with dense network of wells in response to multiple earthquakes, we systematically studied the response characteristic and mechanism of co-seismic groundwater and temperature changes.At continental-scale, the co-seismic water level responses are complex over large spatial scales, and there is great variability both in the sign and amplitude of water level responses in the data set. The static volumetric strain only have small impact on the co-seismic water level changes while tectonic setting and hydrogeoloical condition of the well-aquifer system played dominant role in determining the co-seismic response. Permeability changes which caused by stress(both static and dynamic stress) may explain the complicated and great variability of co-seismic responses. This suggests that earthquake can cause the widespread permeability changes in the shallow crust although the magnitude of permeability change is uncertain.For the well-aquifer system located in the near-, intermediate field, most co-seismic responses can not be explained by the co-seismic static strain model, and larger earthquakes can trigger co-seismic hydrological response much more easily than those with smaller magnitude earthquakes. Earthquake-induced permeability changes is the mechanism to explain the water level changes.Based on the tidal response of three well-aquifer systems located in Huayingshan fault zone, continual permeability evolution were inferred and we find that permeability increased in all of the three well-aquifer systems following Wenchuan earthquake but with different recovery processes, which indicate that the permeability of the fault zone is under a dynamic system and is controlled by the local hydrogeological conditions.According to the co-seismic water level and temperature changes in response to multiple great earthquakes, we find that co-seismic temperature changes following the changes of water level and there exist good relationship between water level, temperature and the transmissivity and storage coefficient. Thus we consider that water level and temperature have common mechanism, that is permeability changes caused by earthquake.Based on the multiple wells in response to multiple earthquakes, we again found that permeability changes is the dominant mechanism to explain different complicated water level response. However, the permeability changes occurred in different parts of the well-aquifer system, only one third of these wells that showed sustained post-seismic response can be explained by earthquake-induced permeability change in aquifers, while the others that did not show sustained changes are more likely affected by permeability changes near the wellbore, such as in the backfill layer.Combining above conclusions, we can summary that permeability change following earthquakes is the mechanism to explain the different co-seismic responses. Meanwhile, tectonic and hydrogeological settings have significant impact on controlling the co-seismic responses. Further studies on the relationship between tectonic, hydrogeological settings and co-seismic responses is important in providing lights on determining the location of monitoring wells and understanding the processes between tectonic activities and hydrological processes.