Bulletin of the Seismological Society of America Manuscript Draft 
Manuscript Number: BSSA-D-10-00009 
Title: Intermediate Field Water Level Changes Observed From the Wenchuan Earthquake 
Article Type: Short Note 
Section/Category: Special Issue on the 2008 Wenchuan Earthquake 
Abstract: Water level changes at different monitoring stations are observed during the Wenchuan earthquake (Ms8.0) in the Chinese mainland. Our analysis of the data suggests that in the intermediate field, the size of the water level change is mostly related to the value of Skempton's coefficient B. Verification of this fact comes from analyzing the water level changes from the wells with constant epicentral distance. Therefore we conclude that, unlike other earthquakes for the intermediate field, the size of the water level change is not only related to the earthquake magnitude and epicentral distance, but also connected to the extent of the confinement of the aquifer (which is related to Skempton's coefficient B). Thus we also prove the applicability of poroelastic theory for the intermediate field coseismic water level changes from the Wenchuan earthquake. 
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Intermediate Field Water Level Changes Observed From the Wenchuan Earthquake 
Yan Zhang1 and Fuqiong Huang2 
1. Institution of Geophysics, China Earthquake Administration, No 5 Minzu Daxue Nan Rd., Haidian District, Beijing, China. 
eve_041744@163.com 
2. China Earthquake Networks Center, Beijing, China 

Abstract 
Water level changes at different monitoring stations are observed during the Wenchuan earthquake (Ms8.0) in the Chinese mainland. Our analysis of the data suggests that in the intermediate field, the size of the water level change is mostly related to the value of Skempton‘s coefficient B. Verification of this fact comes from analyzing the water-level changes from the wells with constant epicentral distance. Therefore W We conclude that, unlike other earthquakes for the intermediate field, the size of the water-level change is not only related not only to the earthquake magnitude and epicentral distance, but also to the extent of aquifer confinement. (which is related to Skempton‘s coefficient B). Thus we also prove twe contend that poroelastic theory can explain the intermediate field coseismic water-level changes from the Wenchuan earthquake. 

Introduction  
Several types of earthquake-induced groundwater level changes and corresponding mechanisms have been recognized for decades. In the near field (epicentral distance between 0-100 km), most documented water levels show abrupt (step-like) coseismic changes (Wakita 1975; Quilty and Roeloffs, 1997; Wang et al., 2001, 2004; Chia et al., 2001; Wang and Chia, 2008). Undrained dilatation and consolidation of the sediments may be responsible for the step-like water-level changes in the near field, and can often be quantitatively related to the poroelastic response to static strain. In the intermediate field (epicentral distance between 100-1000 km), most documented changes are gradual and can persist for days or weeks. These are coined by Roeloffs (1998) as ”sustained” water-changes, and an earthquake-enhanced permeability may be responsible for this intermediate field phenomenon (Wang and Chia, 2008). In the far field (epicentral distance greater than 1000 km), only transient oscillations of the water levels have been documented. There are several existing models for far-ﬁeld coseismic pore pressure changes: mobilization of gas bubbles, (Roeloffs, 1998), shaking-induced dilatancy (Bower and Heaton, 1978), fracture of an impermeable fault (King et al., 1999), fracture clearing (Brodsky et al., 2003), and shaking induced by surface waves (West et al., 2005; Sil and Freymueller, 2006). 
Investigation of coseismic water level changes has been of scientific interest for decades (Wang and Manga, 2010). Groundwater-level changes following earthquakes can affect water supply; seismic waves can affect oil well productionn,; and it has been suggested that in some cases, the induced seismicity can stimulate oil production (Beresnev and Johnson, 1994). 
Earthquake-induced fluid pressure changes are hypothesized to control the timing and/or location of the aftershocks and trigger seismicity (Hill et al., 1995; Gomberg, 1996). Finally, these groundwater level changes could also be related to  hydrologic earthquake precursors (Roeloffs, 1998). 
In this paper we calculate Skempton‘s coefficient B from the poroelastic relationship between water level changes and tidal strain using data prior to the earthquake. We then use water-level data from the Groundwater Monitoring Network (GMN) (see Data and Resources Section) to analyze the poroelastic response to the Wenchuan earthquake for the intermediate fields A relation between the amplitude of the water level and the earthquake magnitude and distance was developed by Roeloffs (1998) for the ―sustained‖ water level changes. To develop this relationship, different intermediate field earthquakes are used. Several authors have obtained similar empirical relations between water level change, epicentral distance, and the earthquake magnitude (Matsumoto et al., 2003; Yang et al., 2005; Sil and Freymueller, 2006). In addition to the above observation, we find that the size of the water level change at GMN stations in the intermediate field is not only related to the earthquake magnitude and epicentral distance, but also related to the value of Skempton‘s coefficient B. In the intermediate field, large B-values come with large changes of water level. This phenomenon is a supplement to the reported empirical equations referring to the relationships between water level, epicentral distance, and magnitude, and hints at the key role played by poroelsticity in water level changes. Therefore, we conclude that the undrained dilatation and consolidation of sediments may be responsible for the water level changes in the intermediate field for the Wenchuan earthquake. 

Theory and Methods 
Skempton‘s coefficient, B, is a significant pore-fluid parameter in poroelastic theory. A poroelastic material consists of an elastic matrix containing interconnected fluid saturated pores. Fluid saturated crust behaves as a poroelastic material to a good degree of approximation. 
Rice and Cleary (1976) summarized the following equations for a linearly elastic isotropic porous medium, which are the building blocks of the poroelastic theory: 

According to equation 3, Skempton‘s coefficient B can be qualitatively defined: In the ―undrained‖ condition, B is the ratio of the induced pore pressure divided by the change in mean stress (Wang, 2000). B governs the magnitude of water level changes due to an applied stress since pore pressure is directly proportional to water level. The value of B is always between 0 and 1. When B is 1, the applied stress is completely transferred into changing pore pressure. B equals 0 indicates no change in pore pressure after applying the stress. When an aquifer is not confined, an applied stress can be easily transferred outside the aquifer system without increasing the pore pressure. Thus a low value of B indicates a poorly confined aquifer system (Sil, 2006). Laboratory studies indicate the value of B depends upon the fluid saturated pore volume of the sample (Wang, 2000). 
Equation 3 can be expressed in terms of tidal strain as well (Roeloffs, 1996): 
Equation 4 shows that water level changes proportionally in a poroelastic material under the influence of tidal strain (εt). Here 
From equation 4 we obtain: 
 (5) 
Calculation of the Skempton‘s coefficient, B, according to the relationship between water level and tidal strain (Equation 5) is a reasonable method in a relatively stable state (without the incident of earthquake). With this equation we can get the value of B without the occurrence of an earthquake and without any laboratory experiments (Zhang et. al, 2009). To apply this method (Zhang et. al, 2009), we obtain the frequency of the M2 wave from the water level and the tidal strain by using band-pass filter (the frequency of the M2 wave is 0.080511 4 h1). Disposing the obtained frequency parts of the M2 wave by IFFT (inverse fast Fourier transform) and adjusting their phase, through the least square fit and putting the results into equation (5), we can finally derive B. 
We apply the above method to the wells where earthquake induced water level changes are observed. Pre-earthquake analysis is carried out using data from May 2, 2008 to May 10, 2008 to obtain the pre-earthquake B values. Calculation is performed using =1000 kg/m3, g=9.8 m/s2, 
u=0.29, and G=6 GPa. After that our coseismic analysis shows that for the wells with similar epicentral distances (within a range of less than 0.15 degrees or 16.68 km), large B values correspond to large magnitude water level changes, this phenomenon mainly exists in the intermediate field. Note that B values are calculated using pre-earthquake data. We only find 24 wells which can form groups that have the similar epicentral distance in the intermediate field of mainland China. We divided those wells into eleven groups; each group has a specific range of epicentral distance and contains two to three wells (Table 1). Table 1 helps us to get the relationship between water level change, epicentral distance, and Skempton‘s coefficient B (Figure 1). Separation of the wells in eleven groups is shown in Figure 2 along with their relation between coseismic water level changes and pre-earthquake B values. 

4. Analysis of the results and discussion 
For intermediate distance earthquakes, several authors previously obtained similar empirical equations (shown below) relating water level change, epicentral distance, and magnitude of the earthquakes (Roeloffs, 1998; Matsumoto et al., 2003; Yang et al., 2005; Sil and Freymueller, 2006). They attribute the magnitude of the water level change to two major impact factors: earthquake magnitude and epicentral distance. The empirical relation found by them can be written as: 

. . . is the well-hypocenter distance in kilometers (Roeloffs, 1998). The importance of equation 6 is that, for intermediate distances, it can explain earthquake induced water level changes, where poroelastic theory generally is not applicable. 
But in our case, from Figure 1 and Figure 2, we can see that, for the intermediate field for wells with similar epicentral distances, the size of the water level change depends on the value of Skempton‘s coefficient B. In the same group, large B values lead to large magnitude water level changes. Thus, we can conclude the size of the water level change is not only related to the earthquake magnitude and epicentral distance but also connected with the confinement of the aquifer, which is related to Skempton‘s coefficient B (Sil, 2006). This is a supplement to the reported empirical equations referring to the relationships of water level, hypocentral distance, and earthquake magnitude. 
In the intermediate field, large B-values come with large changes in water level. This phenomenon is in accordance with the poroelastic theory. When the aquifer is confined (B-values are high), the applied stress is mostly transferred into changing pore pressure, which leads to relatively large changes in water level. When an aquifer is unconfined (B-values are low), an applied stress can be easily transferred outside the aquifer system without increasing the pore pressure resulting in small water level changes (Sil, 2006). From that, we can see undrained dilatation and consolidation of sediments may be responsible for the water level changes in the intermediate field, and those results can prove that the poroelastic theory is appropriate for the 
intermediate field at least for this earthquake of relatively high magnitude. 
We could not find data from near field wells with the similar epicentral distance during the Wenchuan earthquake. While, in the relatively far field, we obtain seven groups of wells recorded water level changes that have the similar epicentral distance. However, among those seven groups, three groups of wells do not show any relationship between B values and water level changes. Perhaps, this is because the poroelastic theory can not be applied to the far field. The exact mechanism for these far field events may be amongst the previously mentioned assumptions: shaking induced dilatancy, fracture of an impermeable fault, fracture clearing and shaking induced by surface waves. Further investigation is required to understand these far field effects. Note that, the ―undrained‖ condition can hardly last for a long time. Whenever the fluid flow starts, the undrained condition disrupts and is replaced by the drained condition. Besides, each well has its specific geology, and it is difficult to decide the in-situ shear modulus accurately for every well. In this paper we assume G=6 GPa. Discrepancy in the far filed observations may be attributed to these difficulties. 
The magnitude of the Wenchuan earthquake is relatively large (Ms 8). Therefore, even without computing, we can expect that the static strain field from the earthquake will affect a relatively large area. Thus we assume that our observation is not contradicting any existing theory of earthquake induced water level changes. 

Conclusions 
The present study indicates that the magnitude of the water level changes at GMN stations in the intermediate field is not only related to the earthquake magnitude and epicentral distance, but also connected to Skempton‘s coefficient, B. This phenomenon is a supplement to the reported empirical equations referring to the relationships between water level, epicentral distance, and magnitude. Therefore, we conclude that the poroelastic theory may play an important role in the intermediate field water level changes, at least for the high magnitude Wenchuan earthquake. 

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