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Continuous Permeability Measurements Record Healing Inside the Wenchuan Earthquake Fault Zone

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Science  28 Jun 2013:
Vol. 340, Issue 6140, pp. 1555-1559
DOI: 10.1126/science.1237237

Water at the Bottom of a Well

Earthquakes generate numerous fractures as they propagate through an underground fault zone. These fractures strongly influence the way in which fluids flow in the subsurface, and the permeability of fault zones is often used as a proxy for the extent of fracturing. Following the 2008 Mw 7.9 Wenchuan earthquake in central China, several wells were drilled in and around the fault zone to understand the mechanics of the earthquake. Because the bottoms of these deep boreholes were open, the water levels in the wells were sensitive to tidal forces acting on the surrounding rock. Through continuous measurements of water levels over 1.5 years, Xue et al. (p. 1555) found that the rate at which water was pumped in and out of the borehole was proportional to the permeability of the fault zone, providing a direct way to measure the evolution of the hydrologic properties of a fault zone following a major earthquake. Permeability decreased ∼25% during that time, suggesting that fractures generated in fault zones heal relatively rapidly.

Abstract

Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquake. We used the tidal response of water level in a deep borehole to track permeability for 18 months in the damage zone of the causative fault of the 2008 moment magnitude 7.9 Wenchuan earthquake. The unusually high measured hydraulic diffusivity of 2.4 × 10−2 square meters per second implies a major role for water circulation in the fault zone. For most of the observation period, the permeability decreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increases attributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal a process of punctuated recovery as healing and damage interact in the aftermath of a major earthquake.

The initiation and propagation of earthquakes depend critically on the hydrogeologic properties of the fault zone, including the fracture-dominated damage zone (16). Fault zone permeability serves as a proxy for fracturing and healing, as the fault regains strength during one of the most unconstrained phases of the earthquake cycle (7). In addition, permeability and storage help to govern the pore pressure and effective stress on a fault. Because earthquakes generate fractures in a damage zone around a fault, it is reasonable to expect that after a large earthquake, the fault zone permeability transiently increases. Over time, the permeability may decrease as a result of a combination of chemical and mechanical processes (7). However, measuring in situ fault zone hydrogeologic properties requires post-earthquake rapid-response drilling, and appropriate data have not previously been recorded continuously immediately after a large earthquake.

The devastating moment magnitude 7.9 Wenchuan earthquake occurred on 12 May 2008 and was the largest seismic event in China in the past 50 years. Shortly afterward, the Wenchuan earthquake Fault Scientific Drilling Project (WFSD) constructed a series of boreholes penetrating the main rupture zone. The first borehole (WSFD-1; 31.1°N, 103.7°E) is 1201 m deep and nearly vertical, in a locale with 6 m of vertical displacement at the surface (8). The borehole is open to fluid flow in the formation below 800 m (Fig. 1) and provides a unique opportunity to directly measure fault zone permeability over time. The borehole intersects the likely principal slip zone at a depth of 590 m, which is a major lithological boundary between the upthrust Pre-Cambrian Pengguan granitic and volcanic complex and the underlying Triassic sediments (8, 9). The fault breccia extends to 760 m, and the fracture density remains high to the bottom of the borehole (8). Mature faults have damage zones extending at least ~100 m from the edge of the fault core (10). Therefore, the damage zone of this site is expected to extend into the open interval beginning at 800 m.

Fig. 1 Location and sketch of the WFSD-1 site.

Red lines in the inset indicate the main rupture zone; the red star is the epicenter of the Wenchuan earthquake. In the sketch, the black line is the fault core, which is surrounded by the damage zone. The borehole is 1201 m deep, and 800 to 1201 m is the open interval where water can flow into the hole from the formation (white arrows). The fault that was most likely active during the Wenchuan earthquake is the major lithological boundary between the pre-Cambrian Pengguan complex and the Triassic sediments at 590 m.

We measured the water level response to tidal forcing in WFSD-1 to constrain the average hydrogeologic properties of the damage zone between 800 and 1200 m below the ground surface [~200 to 600 m below the principal slip zone (8, 9)]. We used these measurements to infer the hydraulic diffusivity and permeability variations inside the Wenchuan earthquake fault zone from 1 January 2010 to 6 August 2011. The WFSD-1 pressure transducer recorded data with a sample rate of 2 min and at a resolution of 6 mm (Fig. 2). Data gaps occurred every month or two, when the instruments were removed from the well to retrieve the data and measure temperature profiles. The raw records show clear tidal oscillations superimposed on the long-term recharge trend (Fig. 2).

Fig. 2 Water levels from WFSD-1 recorded from 1 January 2010 to 6 August 2011.

The oscillations in the inset are generated by Earth tides. The precision of the water level measurement is 6 mm. Water level is assumed to be continuous across the data gaps. The measured water level is the height of water above the pressure transducer.

The tidal oscillations serve as probes of the fault’s hydrogeologic properties. The tidal forcing imposes a dilatational strain on the surrounding rock formation that pumps water cyclically in and out of the well through the rock around the uncased portion of the borehole below 800 m depth. The clear oscillations indicate that the aquifer is well-confined. The transmissivity and storage coefficient determine the phase and amplitude response of the water level to the tidal loading (11). To first order, phase lag is inversely related to transmissivity, and amplitude response is proportional to storage coefficient. Using tidal response to measure hydrogeologic properties has two distinct advantages: (i) Tidal response is passive and records the in situ properties undisturbed by repeated pump tests or water injections, and (ii) tidal response provides a continuous record of temporal changes of hydrogeologic properties in the rocks below the main rupture zone.

We translated the phase and amplitude responses into transmissivity and storage coefficient values on the basis of the analytical solution for a two-dimensional isotropic, homogeneous, and laterally extensive aquifer (12, 13). The observed phase lag ranged from –20° to –30°, where negative values indicate that the water level oscillations lag behind the imposed dilatational strain; the amplitude response ranged from 5.5 × 10−7 to 6.3 × 10−7 m−1 (Fig. 3). The corresponding transmissivity T varied systematically over a range of 3.6 × 10−6 to 6.8 × 10−6 m2 s−1, with an average value of 5.1 × 10−6 m2 s−1 (Fig. 4). In contrast, the storage coefficient S did not evolve systematically, having an average value of 2.2 × 10−4 with small fluctuations about this mean (standard deviation = 5.7 × 10−6). The different behavior for storage and transmissivity is the result of the weak sensitivity of the solution to variations in storage coefficient (12) and little real variation of the storage coefficient. Accordingly, we fixed S to the average value to more robustly solve for T and found that the resulting values of transmissivity were unchanged from the original inversion (Fig. 4). Because of the geometrical idealizations of the model, the absolute values are lower bounds (13), although relative variations over time are more robust.

Fig. 3 Water level response relative to semidiurnal tidal dilatation strain.

(A) Phase lag; (B) amplitude response. Values were calculated using a Bayesian Monte Carlo Markov chain inversion method in the time domain (13). The inversion was applied by 29.6-day segments overlapping by 80%, respectively. The error bars represent the 95% confidence interval.

Fig. 4 Hydrogeologic properties of the well-aquifer system over time.

(A) Permeability and transmissivity; (B) storage coefficient. Values were inverted from the phase and amplitude of each 29.6-day segment based on the analytical model (9). Segments that overlap the remote earthquakes (vertical dashed lines) were not inverted [see (13) for inversion results including these times]. The black dots denote an unconstrained inversion; the red dots are the results of inversion with the storage coefficient fixed to a single value. Because the two separate inversions have identical results for transmissivity, the red dots cover the black dots in (A). The vertical dashed lines show the time of the selected teleseismic events, which correspond to sudden increases in permeability. The best-fit linear trends between each set of permeability increases are shown as light gray dashed lines. Permeability errors are estimated by propagating the range of phase errors.

For the average values of T and S over the observation period, the average hydraulic diffusivity, D = T/S, is 2.4 × 10−2 m2 s−1. On the basis of this observed hydraulic diffusivity, the tidal observations sense a zone extending ~40 m from the well and thus are sensitive to mesoscale fractures (13). The most transmissive units in the damage zone control the tidally driven flow. The estimate of D is two orders of magnitude larger than that of the most directly comparable post-earthquake fault zone study [7 × 10−5 m2 s−1 on the Chelungpu fault slip zone after the 1999 Chi-Chi earthquake from a cross-hole experiment (14)]. Our diffusivity value implies that as soon as the currently observed level of damage developed during the earthquake, coseismic drainage was important. However, the diffusivity may be small enough that advective flow through the fault zone does not have a major impact on the postseismic temperature measurements that are another objective of post-earthquake studies (15). Previous modeling work has shown that hydraulic diffusivities comparable to the observed effective D could suppress the temperature anomaly by at most a factor of 2 relative to a conductively cooled model (13, 16), and therefore thermal anomalies in the fault zone could be observable if the fault friction is comparable to laboratory values.

The effective permeability k is related to transmissivity T byk=μρgdT (1)where μ is the fluid dynamic viscosity, d is the thickness of the open interval of the well, and ρ is the density of fluid. Using μ = 10−3 Pa·s at 20°C, ρ = 103 kg m−3, g = 9.8 m s−2, and d = 400 m, the average value of the effective permeability is 1.4 × 10−15 m2 (Fig. 4). Permeability errors are estimated by propagating the range of phase errors. This approach is appropriate for measuring the precision of the inversion, and these errors are useful for assessing time variability. The absolute value of the permeability is more strongly affected by the limitations of the flow model (13). The observation constrains the effective permeability averaged over the entire open interval and is therefore a lower bound for the effective permeability of the highly fractured regions.

Our observed fault zone permeability is much larger than core-scale laboratory measurements of permeability from active-fault core samples (17), which range from 10−19 to 10−18 m2; it is also larger than the previously measured average permeability of 1.9 × 10−16 m2 for the intact upper Triassic rock near the Wenchuan drilling site (18). The difference is likely due to mesoscale fractures and highlights the importance of damage in determining the field-scale behavior (6).

There are also substantial temporal changes in transmissivity, which we interpret as permeability changes because the formation thickness and fluid properties are unlikely to vary during the observation period. During most of the study period, the permeability trends downward and is most easily interpreted as a reduction in fracture aperture and connectivity during the continuous evolution since the original earthquake. Seismic studies in Wenchuan suggest that damage healed over a protracted time after the earthquake (19), and permeability does not generally evolve in time in the absence of a disturbance (20). The only candidate perturbation besides the earthquake is the drilling itself, which could potentially produce transient damage. However, drilling-induced factures are expected to extend at most a few borehole radii away from the hole (21) (i.e., <0.3 m) and cannot account for the phase change of the long-period tidal response that senses average properties up to ~40 m from the borehole.

This decreasing permeability may reflect the healing process of the fault zone after the Wenchuan earthquake due to a combination of fracture closure, sealing, precipitation, biogenic growth, and pressure solution (7). The healing rates range from 4.1 × 10−16 m2 year−1 to 2.1 × 10−17 m2 year−1, using linear fits to each interval between perturbation events (Fig. 4) (13). Previous work (22, 23) modeled fault zone healing as an exponential recovery process with decay times on the order of decades or longer. However, our data are best fit with much shorter exponential decay times of 0.6 to 2.5 years, indicating a much more rapid process than anticipated (table S2). The short exponential decay times might indicate a fast healing process, such as removal of props trapped in fractures, or crack sealing with a strongly disequilibrated fluid to allow mass transfer with the observed characteristic times.

Fault zone healing has been documented in seismic velocity changes (24, 25) and has been suspected on the basis of discrete repeated active formation tests (26). After the 1995 Kobe (Hyogoken-Nanbu) earthquake, water injection experiments in 1997 and 2000 tracked fluid flow in the hanging wall 50 m from the Nojima fault core and found that the permeability in 2000 had decreased to 50% of the value in 1997 (26). Seismic studies document seismic velocity decreases around the fault after an earthquake continuing for years, which can also be interpreted as a consequence of fracture closure (19, 24, 25). In Wenchuan, the repeated seismic velocity measurements made in the first year are consistent with such healing (19).

The sudden increases in permeability result in an overall rate of decrease that is more gradual than the short-term trends by a factor of 1.5 to 7.5. Previous work suggests that permeability might be enhanced by remote or regional earthquakes (20, 27). Plausible mechanisms include fracture unclogging due to the rapid, oscillatory flow driven by the seismic waves as they pass through the fault zone (27, 28). The times of the four permeability increases in Fig. 4 are correlated with the four teleseismic earthquakes that produced the largest integrated seismic shaking at the drilling site during the observation period (table S2). However, like many hydrogeologic observations, the magnitudes of the perturbations are not simply proportional to that of the peak amplitude of the seismic wave (27). Most important, our observations imply that any physical modeling of precipitation, fracture closure, or any other healing process of a fault zone needs to match the much more rapid healing rate that is only visible in the continuously recorded data.

An interplay between permeability evolution and fault strength has previously been suggested on geological and theoretical grounds (29). The Wenchuan earthquake Fault Scientific Drilling Project captured the permeability evolution in the critical post-earthquake period, when damage heals and the stage is set for the next earthquake. The unexpectedly high average hydraulic diffusivity (2.4 × 10−2 m2 s−1) measured here also implies substantial fluid circulation in the evolving fault zone. If this value represents the hydrogeologic properties during the earthquake, fluid flow should take place during the earthquake rupture.

Supplementary Materials

www.sciencemag.org/cgi/content/full/340/6140/1555/DC1

Materials and Methods

Supplementary Text

Figs. S1 and S2

Tables S1 and S2

References (3134)

References and Notes

  1. Transmissivity is a measure of the rate of volumetric flow through a unit width of aquifer under a unit hydraulic gradient and is directly proportional to permeability. Storage coefficient is the volume of water released from storage per unit surface area of aquifer per unit imposed head (30).
  2. See supplementary materials on Science Online.
  3. Acknowledgments: Supported by the National Science and Technology Planning Project in China (H.-B.L.) and NSF grant EAR1220642 (E.E.B.). Seismic data from the Chinese national network are archived and distributed by Incorporated Research Institutions for Seismology (IRIS) Data Management System.
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