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Deep crustal structure around the Atotsugawa fault system, central Japan: A weak zone below the seismogenic zone and its role in earthquake generation
© 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: 7 July 2009
Accepted: 25 June 2010
Published: 31 August 2010
We carried out travel-time tomography around the Atotsugawa fault in central Japan, using arrival-time data obtained from a dense temporary seismograph network in joint observations by the Japanese University group. The observed velocities beneath the fault are 10–13% lower than those of the assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro) in the lower crust. Because the seismogenic layer is thickest in the central part of the fault, reaching a depth of ∼15 km, we infer that the low-velocity anomaly is caused by aqueous fluids. Fluid fractions in the lower crust are estimated to be 2–3% and ∼10%, assuming pyroxene amphibolite and hornblende-pyroxene gabbro, respectively. A distinct low-velocity anomaly is imaged in the central part of the Atotsugawa fault at a depth of 10 km, where seismic activity is very low at the upper 7 km and creeplike movement is observed at the surface. This anomaly is horizontally isolated but vertically connected to the low-velocity anomaly in the lower crust. We interpreted that abundant aqueous fluids supplied from the lower crust are responsible for this anomaly. High pore fluid pressure may enhance the stability of factional slip, resulting in aseismic or episodic slip along the fault.
The recurrence interval of large earthquakes along the Atotsugawa fault system is estimated to be 2,500 years (Takeuchi et al., 2003). In 1858, the Hietsu earthquake (∼M 7) occurred along the Atotsugawa and Mozumi-Sukenobu faults. Repeated electronic distance measurement observations on short baselines across the fault have detected creeplike movement of 1.5 mm/yr in the central section of the Atotsugawa fault (Geographical Survey Institute, 1997), where seismic activity is very low in the upper 7 km. Hirahara et al. (2003) also suggested the possibility of creeplike movement in the central section using an across-fault GPS array.
The Atotsugawa fault system is located in a zone of high strain rate, identified by GPS observations (Sagiya et al., 2000), called the Niigata-Kobe Tectonic Zone (NKTZ) (Fig. 1(a)). The NKTZ, which is ∼500 km long in the NE-SW direction and ∼100 km wide, undergoes contraction in the WNW-ESE direction at a rate several times greater than in surrounding regions. The observed horizontal displacement rates in the NKTZ have been discussed in terms of interplate deformation (e.g., Shimazaki and Zhao, 2000) and intraplate deformation (e.g., Iio et al., 2002; Yamasaki and Seno, 2005). Nakajima and Hasegawa (2007a) described a low-velocity zone in the lower crust along the NKTZ, and suggested that anelastic deformation in the lower crust contributes to the large contraction at the surface. Jin and Aki (2005) reported that a low-Q zone appears beneath the NKTZ at frequency bands of 1–2 and 2–4 Hz.
We apply travel-time tomography to the high-quality arrival-time data recorded by the temporary network and the nationwide seismograph network to obtain a detailed seismic velocity structure of the crust and uppermost mantle around the Atotsugawa fault system. This paper explores the relationship between structural heterogeneity, seismic activity, and geodetic movement, then discusses a possible strain-accumulation process for the Atotsugawa fault.
2. Data and Methods
We established the study area as latitudes of 34.4°– 38.0°N, longitudes of 135.4°–139.6°E, and depths of 0– 400 km. All of the earthquakes and stations were located in this area. However, only the seismic velocities at grid nodes within latitudes of 35.6°–37.0°N, longitudes of 136.4°–138.4°E, and depths of 0–60 km (hereafter referred to as the inversion area; see Fig. 2) were regarded as unknown parameters. In the inversion area, horizontal grid nodes were set at a spacing of 0.1° and vertical nodes were set at 0, 5, 10, 15, 25, 40, and 60 km (Fig. 4(c)). When a ray penetrated the inversion area, a weighting factorproportional to the ray length within that area relative to the entire ray length was assigned to each arrival time. If a ray did not intersect the inversion area at all, its weighting factor was zero, and it was not involved in the inversion. We adopted the 3D seismic velocity model of Nakajima and Hasegawa (2007a) as the initial model, and estimated velocity structures in the inversion area. The number of rays intersecting the inversion area was 142,537 for P waves and 89,112 for S waves. Final results were obtained after four iterations. The RMS values of the travel-time residuals, which were 0.14 s for the P waves and 0.27 s for the S waves in the initial model, were reduced to 0.11 s and 0.25 s, respectively.
3. Results and Resolution Tests
3.1 Checkerboard resolution tests
We carried out checkerboard resolution tests (CRTs) to evaluate the adequacy of ray coverage and the reliability of the obtained images. In the CRTs, positive and negative velocity perturbations of 10% were assigned to alternating grid nodes along both the horizontal and vertical directions, and travel times were calculated to generate synthetic data for this model. Synthetic data were constructed using the same source-receiver geometry as the observations, with random noises corresponding to phase-picking errors (a standard deviation of 0.1 s for P waves and 0.2 s for S waves). We then inverted the calculated synthetic travel-time data using the initial model without any velocity anomalies.
To evaluate the relationship between seismic activity and structural heterogeneity, we selected 53,521 ordinary and 587 low-frequency earthquakes with phase picks at 10 or more stations from the JMA-unified catalogue in 2001–2007, and relocated them with the 3D velocity model obtained in this study. The relocated hypocenters are used for discussion below. Vertical errors for the relocated hypocenters were generally less than 1–2 km.
4.1 Velocity heterogeneity in the lower crust: Evidence for fluid concentration beneath the Atotsugawa fault
P- and S-wave velocities for lower crustal rocks have been determined in the laboratory (e.g., Kern et al., 1999; Kono et al., 2006; Nishimoto et al., 2008). Nishimoto et al. (2008) simultaneously measured P- and S-wave velocities of five mafic xenoliths from northeastern Japan under sub-solidus conditions up to 1.0 GPa and 800°C, and estimated P- and S-wave velocities as functions of temperature and pressure under dry conditions. Seismic velocities for these five rocks at 0.8 GPa and 740°C are shown by orange and gray symbols in Fig. 10. The temperature of 740°C was estimated from the heat flow of 80 mW/m2 and heat transfer coefficient of 2.7 W/K m, following Kagami and Watanabe (2009). We here compare these laboratory-derived seismic velocities with those we observed in the lower crust beneath the Atotsugawa fault (red crosses in Fig. 10) and beneath the NKTZ (blue crosses in Fig. 10), and discuss quantitatively the cause of the low-velocity anomalies. To exclude low-velocity anomalies related to magmatic activity, we considered seismic velocities only at grid nodes more than 20 km away from active volcanoes.
Temperature derivatives of the rocks measured by Nishimoto et al. (2008) are less than 0.09 km s−1/100°C for P waves and 0.05 km s−1/100°C for S waves, which requires unrealistically high temperatures of up to 1300–1500°C to explain the observed seismic velocities if only thermal variation is considered. However, solidus temperatures for lower-crustal rocks are considered to be ∼800°C (e.g., Nishimoto et al., 2008), and the resultant generation of partial melts yields high V p /V s values (e.g., Takei, 2002) rather than the low V p /V s values observed in this study. Moreover, as discussed below, high-temperature conditions would not be expected in the central section based on the bowl-shaped distribution of seismicity along the Atotsugawa fault. Therefore, we propose that the presence of aqueous fluids (H2O) has a predominant effect on the low-velocity anomaly in the lower crust, and estimate aspect ratios and volume fractions of H2O from the observed seismic velocities.
Observed velocity reduction and aspect ratio and fluid fraction of H2O-filled pores in the lower crust.
Atotsugawa fault system
Niigata-Kobe Tectonic Zone
dlnV s /dlnV p
Mizuno et al. (2005) showed that the angle between the strike of the Atotsugawa fault and the direction of the maximum horizontal stress inferred from shear-wave splitting analysis ranges from 55° to 80° at stations 2–8 km away from the fault and approaches ∼45° at stations within 1 km of the fault. From their finite-element modeling of the stress field around the fault, they interpreted this feature as caused by local stress accumulation due to localized flow or slip along the deep extension of the fault. Iio et al. (20022004) proposed a model in which a weak zone with low viscosity in the lower crust beneath the NKTZ is responsible for concentrated deformation at the surface. The presence of abundant fluids beneath the Atotsugawa fault system could reduce the strength of the lower crust, which could help localize deformation above it under a regional stress regime. Our results suggest that a water-weakened lower crust plays an important role in stress accumulation along the Atotsugawa fault system.
4.2 Low-velocity zones beneath volcanoes on the both ends of the Atotsugawa fault
We observe prominent low-velocity anomalies around the volcanoes at the ends of the Atotsugawa fault. Average velocities immediately below Tateyama and Hakusan volcanoes are 5.22 km/s for P waves and 3.02 km/s for S waves at a depth of 10 km, and 5.93 km/s for P waves and 3.17 km/s for S waves at a depth of 25 km, yielding moderate V p /V s values of ∼1.73 in the upper crust and high values of ∼1.87 in the lower crust. Similar features have also been observed in volcanic areas of northeastern Japan (Nakajima et al., 2001).
Geochemical analyses of rock samples have suggested lower crustal melting beneath Hakusan (Ujike and Mashimo, 2009) and the existence of a basaltic magma chamber in the lower crust beneath Ontake volcano, located south of Tateyama (Kimura and Yoshida, 1999). Therefore, under the assumption that partial melting occurs in the lower crust, we evaluate aspect ratios and volume fractions of melts beneath the two volcanoes. Values of d ln V p and d ln V s are calculated to be −0.13 for P waves and −0.20 for S waves for hornblende-pyroxene gabbro, and −0.10 for P waves and −0.19 for S waves for pyroxene amphibolite. The corresponding aspect ratios and fractions of melt-filled pores are estimated to be 0.03–0.04 and 4–5%, respectively, for hornblende-pyroxene gabbro and ∼0.01 and 1–2%, respectively, for pyroxene amphibolite (Fig. 12).
The observed moderate values of V p /V s in the upper crust suggest the absence of volumes of partial melting comparable to the spatial resolution (∼10 km) of seismic tomography, because the existence of partial melts causes high V p /V s values regardless of pore shapes (Takei, 2002). However, Matsubara et al. (2000) detected a small (—5 km) conduit-like zone of low P-wave velocity (4.1–5.5 km/s) and high V p /V s (1.8–2.7) in the upper crust immediately below Tateyama, and interpreted it caused by partially molten material with a fluid fraction of 5–15%. Thus, low-velocity and moderate V p /V s values observed in this study prefer to a large volume of aqueous fluids possibly with smaller-scale partial melting.
4.3 Deep origin of fluids in the lower crust
Iidaka et al. (2009) investigated shear-wave splitting in central Japan and demonstrated the existence of mantle anisotropy in the N-S direction beneath the Atotsugawa fault. They attributed their observations to a preferred orientation of olivine caused by mantle return flow induced by the subduction of the Philippine Sea slab or to the alignment of melt-filled cracks in the return flow, supporting the existence of upwelling flow in the mantle. Nakamura et al. (2008) pointed out that a relatively large amount of fluids from the Philippine Sea plate contributes to magmatism at Hakusan. Ujike and Mashimo (2009) also suggested that adakitic magmas erupting from Hakusan are affected by fluids from the oceanic crust of the subducting Philippine Sea plate. These results suggest that aqueous fluids originating from the subducting Philippine Sea slab migrate through the upwelling flow and are consequently supplied to the lower crust around the Atotsugawa fault as well as the NKTZ.
4.4 Velocity heterogeneity in the upper crust along the Atotsugawa fault: Implications for fault creep
A notable point is the presence of a low-velocity anomaly beneath the central section of the Atotsugawa fault at a depth of 10 km, with higher-velocity regions on both sides (Fig. 7(b)). This region has P-wave velocities of 5.8–6.0 km/s and S-wave velocities of 3.3–3.4 km/s. Kato et al. (2007) used arrival-time data from a dense temporary seismic network with a station separation of ∼1 km, and revealed high-velocity bodies in the western and eastern sections and a low-velocity body in the central section at depths of 2–7 km. Our results are consistent with the results of Kato et al. (2007) and suggest that such features extend at least to a depth of ∼ 10 km.
The structural heterogeneity revealed in this study appears to correlate spatially with seismic activity along the Atotsugawa fault. Seismic activity is very high in the highvelocity anomalies in the western and eastern sections with V p > 6.0 km/s and V s > 3.5 km/s, whereas in the central section seismic velocities are relatively low and creeplike movement has been observed at the surface (Geographical Survey Institute, 1997). We consider high-velocity anomalies to be associated with more brittle and competent parts of the upper crust that can sustain seismogenic stress, possibly acting as an asperity on the fault plane. Kato et al. (2006, 2007) argued that the high-velocity anomaly in the western section, where a high-resistivity body (Goto et al., 2005) and a low-scattering feature (Nishigami, 2007) have been observed, acted as a high-slip asperity in the 1858 Hietsu earthquake (Takeuchi et al., 2003). The eastern section shows a slight high-velocity anomaly compared to the central section, but its lateral extent is limited to 10–15 km. This implies that an earthquake potentially occurring in the eastern section would be much smaller than in the western section, unless a rupture propagates across the low-velocity central section. Frictional and physical properties in the isolated low-velocity zone of the central section are therefore very important in terms of seismic hazard along the fault.
Kato et al. (2007) suggested that the low-velocity zone imaged in the upper crust in the central section reflects velocity reduction of basement rocks by crustal fluids. We observe velocity reductions from average velocities of 1–4% for P waves and 3–6% for S waves in the low-velocity anomaly in the central section, which can be explained by the existence of H2O-filled pores with aspect ratio of ∼0.01 and a volume fraction of 0.2–0.3% (Takei, 2002). The low-velocity anomaly at a depth of 10 km appears to be connected with the low-velocity anomaly below the seismogenic layer (Fig. 7), suggesting that fluids are supplied continuously from the lower crust and that effective pore pressure in that region becomes relatively high. Because the seismogenic layer is thickest in the central section, we infer that temperatures of aqueous fluids supplied to the central section are not so high (Fig. 12).
High pore-fluid pressure in the low-velocity zone of the central section may enhance the stability of frictional slip (Scholz, 1998), resulting in aseismic or episodic slip along the fault. Consequently, creeplike movement and low seismicity are associated with high pore-fluid pressures in the central section, as discussed by Kato et al. (2007). Jin et al. (2000) investigated the source parameters of 102 small earthquakes along the Atotsugawa fault and found that earthquakes at depths of 11–13 km in the creeping section have a lower stress drop (<0.1 MPa) than those in other portions of the Atotsugawa fault. They suggested that the friction along the fault may be relatively low and these earthquakes may have a character intermediate between regular earthquakes and creep events. These results are consistent with our observations that the low-velocity anomaly, probably caused by the presence of aqueous fluids, exist at a depth of ∼10 km in the central section.
In contrast, Mizoguchi et al. (2007) carried out friction experiments using fault gouge samples collected from the Atotsugawa fault and found an increase in the frictional coefficient from 0.50 to 0.58 with depth from 1 to 9 km. They proposed a model in which depth-dependent frictional strength can generate creeplike movements at shallower depths, because stress on the fault can result in low frictional strength at shallow depths earlier than at greater depths. However, a concentration of crustal fluids at a depth of 10 km discussed in this study could lower the effective frictional coefficient there. We need to determine structural heterogeneity along the Atotsugawa fault in much greater detail to develop a better model for explaining creeplike movement in the central section.
A very low-velocity anomaly is imaged locally along the Atotsugawa fault system at a depth of 25 km. The lateral extent of the anomaly is 20–30 km across the fault, and seismic velocities are 10–13% lower than those of assumed host rocks (pyroxene amphibolite and hornblende-pyroxene gabbro). Considering the existence of a low-velocity anomaly of 6–9% with a width of ∼100 km beneath the NKTZ, we propose that a hierarchical structure of low-velocity anomalies has developed around the Atotsugawa fault system. Volume fractions of H2O beneath the Atotsugawa fault are estimated to be 3–4 times larger than beneath the NKTZ. A water-weakened lower crust probably plays an important role in the stress accumulation process along the fault plane.
At both ends of the Atotsugawa fault system where active volcanoes are formed, seismic velocities are very low in the entire crust. The low-velocity anomaly immediately beneath active volcanoes is explained by the existence of partial melting with fractions of 1–5% in the lower crust, and can be attributed to aqueous fluids with smaller-scale partial melting in the upper crust.
Seismic activity along the Atotsugawa fault is very high in the western and eastern sections where P-and S-wave velocities are higher than 6.0 km/s and 3.5 km/s, respectively. We reveal a laterally-isolated low-velocity zone at a depth of 10 km in the central section of the Atotsugawa fault where seismic activity is very low and creeplike movements are observed at the surface. Because the seismic activity is deepest in the central section, we infer that aqueous fluids with lower temperatures are concentrated there and, consequently, that deformation there is partially anelastic.
This study illustrates major features of the complex velocity structure around the Atotsugawa fault and discusses their relationship to seismic activity along the fault. Our results allow the development of a better model of the deep crustal structure of the fault system. Further work could involve quantitative interpretation of the obtained velocity structures in detail together with other geophysical observations such as seismic attenuation, shear-wave splitting, and electrical conductivity to provide more realistic constraints on numerical simulations of stress accumulation of the Atotsugawa fault system as well as the NKTZ.
We thank D. Zhao for providing us with tomographic codes, and A. Hasegawa and T. Matsuzawa for fruitful discussions. We used the unified earthquake catalog compiled by the Japan Meteorological Agency (JMA) and seismic data from the National Research Institute for Earth Science and Disaster Prevention, Hokkaido University, Hirosaki University, Tohoku University, University of Tokyo, Nagoya University, Kyoto University, Kochi University, Kyushu University, Kagoshima University, National Institute of Advanced Industrial Science and Technology, the Tokyo Metropolitan Government, the Shizuoka Prefectural Government, the Kanagawa Prefectural Government, the City of Yokohama, the Japan Marine Science and Technology Center, and the JMA. All of the figures in this paper were plotted using GMT (Wessel and Smith, 1998). Constructive reviews by Martin Reyners and an anonymous reviewer improved the manuscript. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic Eruptions.
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