Gravity changes observed between 2004 and 2009 near the Tokai slow-slip area and prospects for detecting fluid flow during future slow-slip events
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 8 July 2010
Accepted: 10 November 2010
Published: 3 February 2011
Slow-slip events (SSEs) have been observed in many plate-boundary zones along the circum-Pacific seismic belt. Previous studies have revealed that high-pressure fluids supplied from the subducted oceanic plate can generate SSEs. However, the behavior of these fluids during an SSE has not been fully elucidated. This paper discusses possible fluid migration along the plate boundary on the basis of spatiotemporal gravity changes observed by absolute and relative gravimeters during a long-term SSE in the Tokai district, Japan. Relative-gravity data are sometimes unreliable because of limited observation accuracies and possible noise produced by groundwater. Nevertheless, the observed gravity changes show a systematic pattern of spatial changes over the slow-slip area. This pattern can be explained by a poroelastic model assuming fluid migration along the plate interface, for which an inversion indicates a permeability of about 10−15 m2. This lies within the range of permeability values inferred by other studies in slow-slip areas. Long-term SSEs have occurred repeatedly in the Tokai district. If the permeability remains greater than 10−15 m2 during a future SSE, it will be possible to detect fluid migration by improving the observation accuracy to the 1-μGal level and accurately evaluating groundwater-related noise.
Slow-slip events (SSEs) in plate-subduction zones are related to the presence of high-pressure pore fluids that are generated by the dehydration of hydrous minerals within the subducted oceanic plate (Schwartz and Rokosky, 2007; Audet et al.,2009; Song et al.,2009; Kato et al., 2010). Geodetic and seismological observations have revealed that a majority of SSEs occur repeatedly on the plate boundary at depths of around 20–40 km (Schwartz and Rokosky, 2007; Delahaye et al., 2009), just beneath the seismogenic zone for megathrust earthquakes. Seismic studies have detected high-Poisson-ratio anomalies that indicate the presence of pore fluids in the source areas of these events (Audet et al., 2009; Kato et al., 2010). Deep non-volcanic tremors have also been detected in some of these areas, and their rapid migrations can be explained by fluid flow along the subducting plate (Vidale et al., 2009). Moreover, the fact that tremors can apparently be triggered by very small stress perturbations suggests that the shear strength is small on the plate interface because of high-pressured fluids (Miyazawa and Mori, 2006). Laboratory experiments on rock friction have indicated that the injection of pore fluids effectively decreases shear strength (Kato et al., 2003). Simulations incorporating these experimental results imply that high-pressure pore fluids help to create a frictional regime on plate boundaries that could periodically trigger SSEs (Scholz, 1998; Liu and Rice, 2007).
While laboratory and numerical experiments suggest that high-pressure fluids can induce SSEs, little is known about the way fluids and shear-rupture processes interact on scales relevant to the natural environment.
Recently, terrestrial and satellite measurements have detected minute gravity changes, on the order of 1 μGal = 10 nm/s2, caused by earthquake-induced crustal deformation (Tanaka et al., 2001; Imanishi et al., 2004; Chen et al., 2007). There have also been reports of gravity changes induced by fluid flow within the crust, such as magma migration due to volcanic activities (Battaglia et al., 1999; Furuya et al., 2003), groundwater flow (Kazama and Okubo, 2009) and post-seismic mantle-water flow (Ogawa and Heki, 2007). Absolute gravity measurements in the Cascadia plate-subduction zone suggest a long-term increase in the mass anomaly of unknown origin (Mazzotti et al., 2007). Absolute and relative gravity measurements are also being performed on the island of Taiwan to investigate tectonic deformation and mass-transfer effects (Mouyen et al., 2009). A more precise observation method combining absolute and superconducting gravimeters has been developed to monitor slow vertical deformation (Van Camp et al., 2005). This article reports on gravity changes observed during a long-term SSE and discusses whether these changes can be explained by subsurface density changes caused by high-pressure-fluid flow along the plate interface.
Gravity measurements have been conducted in the slow-slip area at a rate of once or twice every year after the reference site A2 was set up in 2004 (Fig. 2). Gravity observations have also been in operation near the tip of the peninsula since 1996. Gravity values at A1 and A2 were determined with an accuracy of 2 μGal using FG5 absolute gravimeters (Okubo et al., 1997). Differences in gravity at these reference sites and at control stations of the GPS network (R1–R9) were measured with an accuracy of approximately 10 μGal using calibrated LaCoste and Romberg (L&R) Model G relative gravimeters (Torge, 1989). The relative measurements were performed using two of the three available gravimeters (denoted S/N 581, 705, and 876) during each campaign with a view to eliminating instrumental differences. Combining robust absolute gravity measurements with less robust but easier relative measurements using portable L&R gravimeters helped to enhance the spatial resolution of the long-term gravity change estimates. The same approach was previously adopted to detect co-seismic and magma-migration-induced gravity changes (Tanaka et al., 2001; Furuya et al., 2003).
2.2 Absolute gravity data
At A1, there seemed to be a gravity change between 1996 and 2000 that could not be attributed to the vertical movement of the site (Geospatial Information Authority of Japan, 2009b). However, no clear change was discerned around the time the SSE started in the fall of 2000. In the following, we focus on gravity changes that took place during the SSE and do not discuss changes prior to the SSE.
2.3 Relative gravity data
The relative gravimeters were calibrated as follows. The scale factors were determined from single-day observations in Miyazaki, Hokkaido, and Tokyo and at Mt. Fuji, Japan, where reference gravity values had been determined precisely by absolute gravity measurements. The effect of magnetic orientation was measured in a laboratory at the Earthquake Research Institute (ERI). Periodic errors due to dial turns were corrected for by using data measured in an ERI building and all available data from previous observations. An automated Burris gravimeter equipped with a digital feedback system was used to correct for dial rotations worth less than ten turns. These errors remained smaller than 10 μGal.
Observations in the Tokai area were conducted following a conventional procedure in which all measurements were repeated twice within a single day to evaluate drift errors. The precision of each gravity measurement was calculated using a least-squares method incorporating a model of solid-Earth tides to remove linear drifts. At this stage, data with measurement precisions poorer than approximately 10 μGal were rejected because they were considered to contain human-induced measurement errors. Oceanic tides and vertical crustal movements at the measurement sites were corrected for by following the same method used for the absolute gravity data. The GPS stations used for these corrections are listed in column (d) in Table 1. Air pressure was corrected for by using in-situ data obtained during each measurement.
Trend estimates by absolute and relative gravity data.
(c) Lat. (deg.)
(d) GPS site
(e) Trend & σ (μGal/yr)
On the basis of the above analysis, trends in the time-series data were determined using the minimum L1-norm condition (Menke, 1989). All data were assigned an equal weight of 1/σ2 = 1/(16 μGal)2. The result is shown in Table 1 and Fig. 4 with prediction bands corresponding to 1σ.
2.4 Other potential sources of gravity change
We did not correct the absolute and relative gravity data for changes arising from the following sources because their effects were estimated to be smaller than the measurement accuracy.
2.4.1 Variations in sea-surface height
We have evaluated long-term gravity changes due to variations in sea-surface height for sites A1 and R3, which are located only a few hundred meters from the coastline, using local tide-gauge data (available on the Web site of the Coastal Movements Data Center, Geospatial Information Authority of Japan, http://cais.gsi.go.jp/cmdc/center/annualgra.html). The linear trends in sea-surface height at tide stations T1 and T2 (Fig. 2) between 2004 and 2009 were −5.6±4.7 and − 9.2±5.1 mm/yr, respectively, with vertical crustal movements removed by the method of Kato and Tsumura (1979) (Geospatial Information Authority of Japan, 2009c). These are expected to have effects of approximately only 0.01 and 0.1 μGal/yr at A1 and R3, respectively.
We have also evaluated gravity changes due to the fluctuation of the Kuroshio axis using data published by the Hydrographic and Oceanographic Department of the Japan Coast Guard (http://www1.kaiho.mlit.go.jp/KANKYO/KAIYO/qboc/index.html; in Japanese). Its effect was estimated to be less than 0.01 μGal for site A1.
2.4.2 Variations in groundwater levels
A number of groundwater observation wells, operated by the Geological Survey of Japan to monitor crustal deformation (depths 150–340 m), are located near gravity measurement stations A1, R1, and A2. Their locations are marked as W1–W4 in Fig. 2. Time-series groundwater level data from these wells are illustrated in Reports of the Coordinating Committee for Earthquake Prediction (Geological Survey of Japan, 2002, 2003, 2004, 2006, 2009, 2010) (http://cais.gsi.go.jp/YOCHIREN/report.e.html). Annual variations dominate in all these data sets. At W1, the annual variation was approximately 20 cm peak-to-peak. A secular increase of 10 cm took place between January 2005 and January 2007, but otherwise, no clear secular variation was discerned. When a porosity of 10% is assumed (Torge, 1989), an increase of 10 cm is expected to bring about a gravity change of +0.4 μGal (+1 m of water height = +40 μGal), which could not have significantly affected the long-term gravity change observed at A1. At W2 and W3, annual variations were less than 20 cm, and no significant secular variations were observed. At W3, a shallower well, with a depth of 35 m, was attached to the deep well. The shallower well had an annual variation of 40 cm, although no obvious secular variation was discerned. This annual variation corresponds to a gravity change of ±0.8 μGal, which should be negligible compared to the accuracy of the relative gravity measurements at R1. At W4, the amplitude of the annual variation was approximately 50 cm. A secular increase of 50 cm took place between January 2004 and September 2004, but otherwise, no remarkable trend was seen. The annual variation corresponds to a gravity change of ±1 μGal. This amplitude is comparable to that of the seasonal changes that can be discerned in the gravity records at A2 (Fig. 3). However, the gravity records demonstrate no obvious change that could be associated with the secular increase between January 2004 and September 2004. To evaluate the effect of groundwater more accurately, it would be necessary to conduct continuous measurements of near-surface groundwater levels and gravity and determine the admittance between these two quantities.
The effects of groundwater remain unknown for the other gravity measurement sites. Relative gravity data indicate that gravity decreased by 10–20 μGal during a 5-year period at stations R3–R7. To account for these changes by groundwater alone, changes of 2.5–5 m would be required at all these stations if a porosity of 10% is assumed. Such a broad variation was never observed in the shallower well at W3, neither in the secular nor in the seasonal component.
To develop a method to correct for gravity changes due to groundwater disturbances, continuous observations were carried out, using a superconducting gravimeter, at the National Astronomical Observatory of Japan, Mizusawa, which is located on flat terrain. Secular and seasonal variations in gravity were at most several μGal (Kazama, 2010). On the other hand, it is known that gravity changes due to groundwater sometimes amount to 20 μGal near an active volcano in Japan (Kazama and Okubo, 2009). Elevations at the gravity measurement sites in the Tokai area ranged between 6 and 110 m, indicating that the terrain apparently has more in common with the former case than with the latter case.
2.4.3 Volumetric changes in the crust due to the slow slip event
We used the dislocation theory of Okubo (1992) to evaluate the effects of elastic volumetric changes in the crust due to the SSE. The computed gravity changes, however, were at least one order of magnitude smaller than the corrections for vertical movement shown in Figs. 3 and 4. This result indicates that the effects of near-surface groundwater and pore-fluid flow driven by elastic deformation (e.g. Jóonsson et al., 2003) would be too small to explain the observed trends because changes in fluid density are at most of the same order of magnitude as the elastic volumetric changes. The gravity changes estimated by the model of Ogawa and Heki (2007), which is based on elastic deformation, also turned out to be too small. Both of these elastic deformation models failed to explain the observed gravity changes because the moment magnitude of the SSE (M w ~7) was smaller and/or because the source depth (~25 km) was larger than those in previously studied cases.
2.5 Spatial pattern of the trends
Although the gravity trends estimated at the relative measurement sites had large standard deviations, they demonstrated the following systematic features of spatial distribution: (1) the gravity decreased on the shallower side of the areas of maximum slow slip (R3–R7); (2) the gravity did not decrease at R2 located to the southeast and, in contrast, increased at R1 and A1 farther to the southeast; (3) no significant changes were observed at R8, A2, and R9 located to the north and west. Such a systematic correlation of the observed gravity changes with the slow-slip area suggests that the gravity changes may be related to the SSE, although the possibility that this pattern was generated by chance by short-term or long-term groundwater disturbances cannot be ruled out.
3.1 Mechanism driving high-pressure-fluid flow
We have seen that mass redistribution due to fluid flow driven by ordinary poroelastic deformation mechanisms is insufficient to account for the observed gravity changes. We therefore hypothesize that a larger mass redistribution took place due to the creation of new flow paths—that is, because the permeability structure in and around the slow-slip area was altered by a physical shock due to the initiation of shear-rupture processes involved in the SSE. A similar mechanism was proposed by Husen and Kissling (2001).
3.2 Mass redistribution indicated by the observed gravity changes
The spatial pattern of gravity changes mentioned in Section 2.5 suggests the ascension of mass anomalies along the plate interface. This could be generated if fluids flowed upward along the fault-fracture zone according to the above mechanism. The decrease in distance between the mass centroid of the fluid and the observation sites can explain the increase in gravity to the southeast. Downward flow along the plate boundary is also conceivable, but the gravity data indicate the dominance of upward flow. Fluid flow into the shallower part of the plate interface may have been easier because the permeability generally decreases with depth due to the increasing stress and pressure.
3.3 Mathematical formulation
Figure 6(a) shows the configuration of the model. High-pressure fluids are initially confined between X = 0and a (= 40 km). The horizontal projection of this area is shown by the red rectangle in Fig. 6(b). The initial pressure increment, Δ P, was set at 500 MPa, which represents an average lithostatic stress at depths of around 25 km, as indicated by previous studies (Kodaira et al., 2004; Kato et al., 2010). At t = 0, the seals shown in yellow are broken. For t > 0, the boundary condition Δ P = 0 was imposed at X = 0 and b, which means that fluid flow is constrained within the area delimited by the black rectangle in Fig. 6(b). The upper conduits (a < X < b) were added to account for the ascension of the mass centroid. The value of b (= 65 km) was determined by trial and error.
In this diffusion model, fluids flow out of the area delimited by the black rectangle when the boundary condition ΔP = 0 is imposed. Therefore, the total fluid mass in the area gradually decreases over time. However, at sites to the southeast, the effects of advection were larger than the effects of diffusion, which caused the gravity to increase. When the upper conduits were not added, the gravity increases did not occur at R1 and A1. However, the gravity decreases at the other sites could still be reproduced by slightly modifying the poroelastic parameters and the boundaries of the model.
3.4 Inversion for the fracture-zone width and permeability
4. Discussion and Conclusions
In the model presented here, we assumed a lithostatic pressure before the occurrence of the SSE. However, the model could hardly explain the observed gravity data when the initial pressure increment was more than one order of magnitude smaller than the lithostatic value (i.e., when ΔP < 50 MPa). In this case, either the fluid volume had to be larger or a higher permeability had to be assumed. The former possibility is restricted by the assumption that fluids are present only within the fault-fracture zone in the slow-slip area. When the latter was assumed, the time-scale of diffusion became smaller than the duration of the SSE, and the pressure reached equilibrium soon after the initiation of the SSE, resulting in poorer agreement between the model and the observations.
Husen and Kissling (2001) modeled post-seismic fluid flow due to a large subduction earthquake in Chile. They proposed that the low-permeability barrier was ruptured by the event and that fluids flowed out into the overlying continental plate. For the present SSE, the observed gravity changes did not indicate rupture of the barrier because vertical fluid conduits would have increased surface gravity throughout the slow-slip area.
The estimated permeability can be compared to that of oceanic plates at depths where SSEs occur. Sibson and Rowland (2003), who considered variations in the stress state due to fluid overpressuring, stated that the permeability of the subduction zone in New Zealand was less than 10−17 m2. Spinelli and Wang (2009), who used surface heat-flux data, determined that the upper limit on the permeability along the Nankai margin in Japan was 10−10 m2. A numerical simulation of dynamic rupture growth induced by slow slip revealed that SSEs did not easily occur when the permeability of a fault zone was less than 10−18 m2 (Suzuki and Yamashita, 2009). Because permeability can vary by many orders of magnitude due to small differences in fluid pathways and composition, our estimate can be regarded as lying within a reasonable range. The fault-fracture-zone width of 500 m is consistent with a scaling relationship between fault-process-zone width and fault length for strike-slip faults (Vermilye and Scholz, 1998), although no similar relationship has previously been reported for subduction zones.
We therefore conclude that the observed gravity changes were possibly due to the migration of high-pressure fluids along the fracture zone during the SSE. This conclusion also implies that the fluids may have lubricated the plate interface and thus promoted the shear-rupture process of the SSE.
In our model of the Tokai SSE, the maximum pressure loss by 2009 in the slip area is about 300 MPa (Fig. 9). It is therefore suggested that variations of that magnitude may have taken place in fluid pressures during the long-term SSE. Geodetic survey results (Yamamoto et al., 2005) indicate that at least three SSEs of different magnitudes have taken place in the Tokai region during the past 27 years since 1983, including the event studied here. We expect that before the next SSE begins, fluid pressures will return to a lithostatic level, reversing the trends of gravity changes due to the recovery of the seals. The temporal variability of fluid pressures within a fault zone during the seismic cycle is supported by a geological model describing fault-valve behavior (Sibson, 1992). The evaluation of the recovery and the decreasing rates of fluid pressure using real observations is interesting because these rates may govern the fluctuating recurrence periods of the Tokai SSEs.
Gravity changes on the order of 1–10 μGal should be expected in the slow-slip area during the next SSE if the event magnitude and the permeability of the plate boundary remain the same. In that case, if the accuracy of gravity measurements improves to the 1-μGal/yr level, it should be possible to clearly detect fluid flow as described above. If the permeability falls below 10−15 m2, however, it may be difficult to detect fluid flow due to the proposed mechanism. The use of superconducting gravimeters and rigorous corrections for groundwater disturbances must be considered.
The Meteorological Research Institute (2009) points out that deep low-frequency tremor activity increased between 2000 and 2005 along the plate boundary, at depths around 30 km, on the northern side of the area of maximum slow slip. The mechanism of the change in activity has not been revealed. Using our model, we speculate that leakage of high-pressure fluids toward deeper portions increased pore-fluid pressures there to cause tremors. The Institute also reports that the change in activity may have begun in 1999 prior to the SSE. Our model cannot interpret this phenomenon because no gravity observations were in operation in the slow-slip area before the 2000 Tokai SSE. To help reveal its mechanism, it would be useful to find out if gravity changes are going to be observed, prior to a future SSE, synchronously with a change in tremor activity.
Gravity increase was seen only at two stations lying the farthest to the east. In this respect, the results of previous surveys (Kusumoto et al., 2008) could be used in partial support of this spatial pattern. These researchers reported a long-term gravity increase near the tip of the peninsula using relative observation data obtained in 1970 and 2007. The gravity increase averaged over the peninsula was about 30 μGal with respect to a site located near W3, and the increase was the largest near R1. The effects of crustal movement were removed by Bouguer correction. In our model, the gravity change was nearly zero at W3, and the location of the maximum increase almost agreed with their result (Fig. 6(b)). Three consecutive SSEs occurring at 10-year intervals, with each event continuing over the full 10-year cycle, are expected to bring about a total gravity change of about 30 μGal. This estimate is in rough agreement with the results of Kusumoto et al. (2008) and Yamamoto et al. (2005). Therefore, the long-term gravity changes observed by Kusumoto et al. (2008) may be associated with fluid migrations caused by SSEs. In 2008, we established a new absolute gravity measurement site between stations W2 and W3. It is important to determine whether upward migration of high-pressure fluids really happens because upward fluid flow could weaken the effective normal stress in the seismogenic zone.
We have been able to conduct the gravity observations by courtesy of Drs. Tsuneo Yamaguchi, Toshiki Watanabe of Nagoya University and Ryoya Ikuta of Shizuoka University. We used the F3 solution of the Geospatial Information Authority of Japan for the GPS data. We thank Drs. Yasuko Takei, Teruo Yamashita, and Kevin Fleming for reviewing an early version of the manuscript. We also thank Dr. John Townend and an anonymous reviewer for valuable comments which helped to improve the paper significantly.
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