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Seismic images of the source area of the 2004 Mid-Niigata prefecture earthquake in Northeast Japan
Earth and Planetary Science Letters 244 (2006) 16 – 31 www.elsevier.com/locate/epsl
Seismic images of the source area of the 2004 Mid-Niigata prefecture earthquake in Northeast Japan Zhi Wang ⁎, Dapeng Zhao Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 18 May 2005; received in revised form 8 February 2006; accepted 9 February 2006 Editor: Scott King
Abstract To better understand the generation mechanism of the 23 October 2004 Mid-Niigata prefecture earthquake (M 6.8), we used 120,352 P-wave and 95,391 S-wave high-quality arrival times from 5013 earthquakes to determine the three-dimensional seismic velocity (Vp, Vs) and Poisson's ratio (σ) structures in and around the source area. The hypocenter locations of the aftershocks are relocated accurately by using absolute and relative travel time (double-difference) data. Our results demonstrate that the seismic velocity and Poisson's ratio vary markedly around the source area. Most active faults are located along the margins of low-velocity and high-velocity zones. An obvious change in seismic velocity and Poisson's ratio occurs between the northwest and southeast parts of the study area, and this boundary zone reflects the spatial distribution of active faults, being well consistent with the surface geological features. Most of the large historic crustal earthquakes are located in or around the low seismic velocity zones. The mainshock hypocenter is located near the margin of low-velocity (high-Poisson's ratio) and high-velocity (low-Poisson's ratio) anomalies along the active fault zone. A zone with pronounced low-velocity and high-Poisson's ratio is clearly imaged in the lower crust under the source area, reflecting the existence of fluids that are released due to the dehydration of the subducting Pacific slab. Such fluids might have reduced the mechanical strength of the fault zone, and thus initiated the Mid-Niigata prefecture earthquake. © 2006 Elsevier B.V. All rights reserved. Keywords: Seismic tomography; 2004 Mid-Niigata prefecture earthquake; Crustal earthquake; Fluids; Slab dehydration
1. Introduction On 23 October 2004, a destructive earthquake of JMA (Japan Meteorological Agency) magnitude 6.8, the 2004 Mid-Niigata prefecture earthquake (hereafter we call it the 2004 Niigata earthquake), occurred in the
⁎ Corresponding author. Tel.: +81 89 927 8258; fax: +81 89 927 8167. E-mail addresses: [email protected] (Z. Wang), [email protected] (D. Zhao). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.02.015
southwestern portion of Northeast Japan (Fig. 1). More than 40 people were killed, 4000 were injured, about 100,000 people were forced into temporary shelters, and as many as 10,000 might be displaced from their upland homes for several years (EERI, 2005). Japanese authorities estimated the total damage resulting from the earthquake to be US$40 billions. It is the most significant earthquake to affect Japan since the 1995 Kobe earthquake (M 7.2). The mainshock occurred at a shallow depth near the Japan Sea in a zone of relatively high seismicity. Within 100 km of the source area there have been 5 large earthquakes with JMA magnitude
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larger than 6 since 1941, including the 1964 Niigata earthquake (M 7.5). The frequent occurrence of large crustal earthquakes close to the densely populated areas on the Japan Islands has caused significant damage and loss of human life, e.g. the 1964 Niigata earthquake (M 7.5), 1995 Kobe earthquake (M 7.2), 2000 Tottori earthquake (M 7.3), and this 2004 Niigata earthquake (M 6.8). The 2004 Niigata mainshock was followed by an aftershock sequence with four events of magnitude 6 or greater, and 12 events of magnitude 5 to 5.9, all of which were recorded by the Hi-net seismic network during the period from 23 October to 28 November, 2004. The hazards caused by these large crustal earthquakes demonstrated an urgent need for better knowledge of the nucleation mechanism of the earthquakes. The results of many seismological and geophysical studies indicate that the 2004 Niigata
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earthquake occurred in a SW–NE oriented reverse fault zone and might have been triggered by the presence of fluids in the crust [1,2]. However, no detailed studies have been undertaken to provide direct evidence for the source of the fluids. We consider that variations in seismic velocity in the source area of large earthquakes may reflect the physical properties of the crustal rocks. If the physical properties of the rocks have influenced rupture nucleation and the faulting process, then they can be imaged by seismic tomography. In addition, crustal heterogeneities in the hypocentral area of the mainshock might be related to the deep structure anomalies and the dehydration reactions of the subducting Pacific slab. Over the last 20 years, many studies have investigated seismic structures in the source areas of large crustal earthquakes [3–6]. In the present study we investigate seismic velocity and Poisson's ratio structures in and around the source
Fig. 1. (a) Tectonic background of the Japan Islands. The Niigata–Kobe Tectonic Zone (NKTZ) is indicated with the bold shaded line. Plate boundaries in and around Japan Islands are also shown. (b) Squares A and B indicate the large and small inversion areas, respectively (see text for details). Open circles denote the large historical crustal earthquakes (M N 5.0) occurred in the period from 869 to 1998 [57]. Open triangles indicate active volcanoes. The large star shows the epicenter of the 2004 Niigata earthquake. The NKTZ is shown with a shaded area. Active faults on the Japan Islands are indicated with curved solid lines. Magnitude scale of the earthquakes is shown above (b).
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area of the 2004 Niigata earthquake to understand the generation mechanism of this event. The double-difference technique [7,8] is a useful method for accurately locating earthquakes. Recent studies have demonstrated substantial improvements in determining hypocentral locations using this method, and the method has been successfully applied to many areas (e.g., [7–9]). In the present study, we use the double-difference technique to accurately relocate the hypocenters of the Niigata aftershocks; this in turn enables us to better determine the seismic velocity and Poisson's ratio structures in the seismic source area. To investigate the deep structure beneath the source area and its effect on the generation of the 2004 Niigata earthquake, we also used 1708 earthquakes at depths of 0–200 km to determine the seismic structure of the lower crust and upper mantle beneath the study area.
2. Tectonic setting and geology The 2004 Niigata earthquake occurred in the central part of the Niigata–Kobe tectonic zone (NKTZ) within the Shinano River seismic belt (SRSB), which extends from Matsumoto to Niigata along the Shinano River (Fig. 1b). The SRSB is known as an area of pronounced recent crustal deformation and seismicity [10]. The NKTZ is characterized by mutual interactions of three or four plates [11]: the Philippine Sea plate (PHS), the Pacific slab, the Eurasian plate (EUR) and the Okhotsk plate, as shown in Fig. 1a. On the eastern margin of the Japan Sea, the plate boundary between the EUR and Okhotsk plates is located at the coastline near Niigata and traverses the land area along the SRSB [12]. Beneath the NKTZ, the Pacific slab is subducting WNW to a depth greater than 150 km. In the south of the
Fig. 2. Yellow and blue dots show hypocenters of 5013 earthquakes, which consists of two groups of events. One group includes 3305 aftershocks (yellow dots) shallower than 30 km recorded by the dense Hi-net between 23 October and 28 November 2004. The other group includes 1708 earthquakes (M N 2.5; depths b 200 km) recorded by Hi-net and J-Array networks from June 1991 to September 2004 (blue dots). The red star shows the location of the mainshock. Red squares denote 126 seismic stations used in this study. Areas A and B are the same as the two regions in Fig. 1b.
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NKTZ, the PHS is sandwiched by the subducting Pacific slab and the overriding Japanese crust. The compressional strain rate in the NKTZ is up to 200 nanostrain/yr, according to the analyses of GPS data and qualitative modeling [13]. Across this zone, high WNW–ESE shortening rates of ∼10–7 strain/yr are observed, which are several times larger than those in the surrounding areas [13]. The existence of a zone with such a high strain rate is also indicated by triangulation data collected over the past 100 years [14,15]. Also within this zone are many active Quaternary faults exist, which generated a number of large historical earthquakes (Fig. 1b). Thus, the high strain rates observed by the GPS and triangulation surveys are not temporal but possibly reflect tectonic loading of the active faults in central Japan over a long time. 3. Data and methods For tomographic imaging, we used a large number of P- and S-wave arrival times from shallow and intermediate-depth earthquakes, as recorded by seismic networks in central and Northeast Japan. Fig. 2 shows the three-dimensional (3-D) hypocentral distribution of 5013 selected events that generated 120,352 P- and 95,391 S-wave arrival times used in this study. The 5013 events consist of two groups of earthquakes. One group includes 3305 aftershocks shallower than 30 km, as recorded by the dense Hi-net and other seismic networks in Japan between 23 October and 28 November, 2004. The other group includes 1708 earthquakes at depths of 0–200 km with JMA magnitude greater than 2.5, as recorded by the Hi-net and J-Array networks from June 1991 to September 2004. The Hi-net has been installed and operated by the National Research Institute for Earth Science and Disaster Prevention in Japan since 1999, and consists of more than 600 seismic stations that provide a dense and uniform coverage of the Japan Islands. Thus, a large number of arrival time data from the 2004 Niigata earthquake sequence are available, allowing us to study the detailed 3-D seismic structure in and around the source area. For the 3305 aftershocks, the picking accuracy varies from 0.05 to 0.1 s for P-waves and about 0.1 s for Swaves. All the aftershocks were recorded by more than 12 seismic stations, and they were selected carefully; the uncertainties of the epicenters of the aftershocks are less than 0.5 km while those of the focal depths are 0–2 km. The depth distribution of the events shows that most of the aftershocks occurred in the depth range of 0–18 km (Fig. 3).
Fig. 3. Vertical cross-sections of hypocentral distributions of the aftershocks relocated by using the double-difference method [7,16] along the lines A–A′, B–B′, C–C′, D–D′, E–E′, and F–F′. Magnitude scale of the aftershocks is shown below the insert map.
To better understand the generation mechanism of the 2004 Niigata earthquake, the two groups of data are used jointly (Fig. 2). The aftershock data are used for detailed tomographic imaging of the source area. The rays of this data set are entirely within the source area. The doubledifference method [7,16] was used to accurately relocate hypocenters of the aftershocks. The second group of events occurred in or around the source area at depths of 0–200 km, and these are used for imaging the structure below the source area in the lower crust and uppermost mantle. Therefore, a better ray coverage is achieved for the target source area (region B shown in Fig. 2). The number of rays intersecting the target source area is 88,636 for P-waves and 62,762 for S-waves. We used the double-difference location techniques [16], based on the tomographic method [17], to accurately relocate the hypocenters of the 3305 aftershocks. We used 155,998 event-pairs of the aftershocks to relocate the hypocenters. In the tomographic inversion, we have taken into account three discontinuities: the Conrad, the Moho, and the upper boundary of the subducting Pacific slab. The three discontinuities are not simple flat planes but have complicated geometries [17–19]. We adopted their depth distributions as determined by Zhao et al. [17,20]. In the present study, a total number of 126 seismic stations are used, which recorded arrival time data from July 1991 to November 2004 (Fig. 2). These stations belonged to the Japanese Universities, Japan Meteorological Agency (JMA), Hi-net, and J-Array.
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4. Resolution and results An optimal 1-D velocity model, which minimizes the overall misfit between the model predictions and observations, was determined by using the program of Zhao et al. [17]. The model was determined in two steps. First, a Vp model was calculated using only P-wave arrival times, with a starting model modified from Zhao et al. [17]. To obtain the 1-D P-wave velocity model to well reflect seismic velocities along the ray paths used, the earthquakes with over 10 P-wave data were selected for calculating. Secondly, a 1-D Vs model was determined by using only S-wave arrival times, with a starting model derived from the obtained 1-D Vp model and a mean Vp / Vs ratio of 1.73 for the study region. The earthquakes with more than 8 S-wave arrivals were used for determining the Vs model. We conducted checkerboard resolution tests (CRT) for evaluating the resolution of the 3-D velocity structure. Positive and negative velocity perturbations of ± 3%
were assigned to the grid nodes, and synthetic travel times were calculated for the checkerboard model. Then random errors with a standard deviation of 0.08 s for Pwaves and 0.15 s for S-waves were added to the synthetic data. The numbers of events, stations and ray paths are the same as those used in the final tomographic inversion. We adopted a grid spacing of 0.2° in the horizontal and 10–20 km in depth for the region surrounding the Niigata aftershock area (region A in Fig. 2). Fig. 4 shows the CRT results for six depth levels. The resolution is good over the entire study area. Then a smaller grid spacing of 0.05° in the horizontal and 5 km in depth was tested for the entire area. The inversion results show that the CRT pattern is well recovered in area B while the resolution is poor in area A (Fig. 5). Therefore, a grid spacing of 0.05° in the horizontal and 5 km in depth for area B and that of 0.2° in the horizontal and 10–20 km in depth for area A were used for the inversion of the real data. Fig. 6 shows the combined grid model adopted for the synthetic test and the tomographic inversions.
Fig. 4. Results of checkerboard resolution test (CRT) for P-wave (a) and S-wave (b) at six depths. The CRT results are obtained with a grid separation of 0.2° in N–S and E–W directions and 10–20 km in depth. The depth of each layer is shown at the upper left corner of each map. The scale of velocity perturbation is shown on the right.
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Fig. 5. The same as Fig. 4 but for a grid spacing 0.05° in N–S and E–W directions and 5 km in depth.
We also conducted the model-recovery synthetic tests to demonstrate the recovery of the amplitudes of Vp and Vs in the target area B (Fig. 7). We input 4% velocity anomalies in the grid nodes and then calculate the inversion value at each grid. Fig. 7a shows that the amplitudes of Vp and Vs are well recovered at the depths of 10, 15 and 20 km; the amplitudes of P-wave velocity are similar to those of S-wave velocity. The amplitudes of Vp and Vs are well recovered along the cross-section AA′ (Fig. 7b). Thus, the low-velocity anomalies and the Poisson's ratio perturbations derived from the inverted Vp and Vs are considered to be reliable features in the source area. The final inversion results were obtained after five iterations with the combined grid model (Fig. 6). When we carried out the checkerboard resolution test and the final inversion, an optimal damping value of 8 was used which was determined by a number of tests. The grid nodes with hit counts greater than 15 were included in the velocity inversion. The root-mean-square (RMS) travel time residuals, calculated from the optimal velocity model and the relocated hypocenters, are 0.229 s for P-waves and 0.263 s for S-waves, which
Fig. 6. Three-dimensional distribution of grid nodes adopted in this study. The Niigata mainshock location is indicated with a black star. Lines AA′ and BB′ show the locations of the cross-sections in Fig. 9a.
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Fig. 7. (a) Results of the amplitude-recovery tests at depths of 10, 15 and 20 km in the study region B (shown in Fig. 2a), determined by using a method similar to the previous study [57]. We input 4% velocity perturbations (the same as the inverted maximum velocity anomalies) to each grid nodes and then calculate the synthetic data. The recovery-amplitude scale is shown below (a). The degree of recovery is, in general, the same for Pand S-wave velocity structures for both the pattern and amplitude of the input velocity anomalies. (b) Synthetic recovery-test along the cross-section AA′ (shown in Fig. 9a). Scale of velocity perturbation is shown on the right of (b). The upper panel shows the input model and the lower two panels denote the inversion results of Vp and Vs. The low-velocity zone below the Niigata mainshock (red star) is recovered well for both Vp and Vs.
were reduced by 50% of their initial values. Fig. 8a–c shows the plan views of P- and S-wave velocity and Poisson's ratio images obtained together with the seismicity around each layer. The velocity perturbations are from the average value of the inverted velocities in each layer. The relative Poisson's ratio images (Fig. 8c) are calculated from the P- and S-wave velocity perturbations (Fig. 8a,b). At depths of 0 and 5 km, the spatial distributions of P- and S-wave velocities are similar to each other, although the amplitudes of S-wave velocity anomalies are larger than those of P-wave anomalies. The seismic velocity changes rapidly at these depths around the Niigata aftershock area. In the third and fourth layers (10 and 15 km), we see that the locations of the mainshock and aftershocks are located in a zone where Vp and Vs are high and σ is low. The images of velocity and Poisson's ratio show that most of
the active faults are located along zones where the velocity and Poisson's ratio change rapidly over a short distance (Fig. 8). The images at 20–25 km depth show low-Vp, low-Vs and high-σ anomalies in or near the source area (Fig. 8). Though there are few events located in the lower crust and uppermost mantle, numerous rays from the intermediate-depth earthquakes (60–200 km) and rays of head waves (P*, S*, Pn, Sn) from the crustal events refracted at the Conrad and Moho discontinuities sampled the lower crust and uppermost mantle. The anomalies of seismic velocity and Poisson's ratio in the deeper crust are therefore well imaged. Fig. 9a shows vertical cross-sections of the velocity and Poisson's ratio together with mainshock and aftershock seismicity along the lines AA′ and BB′ shown in Fig. 6a. It is clear that the mainshock is located
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Fig. 8. (a) P-wave velocity, (b) S-wave velocity, and (c) Poisson's ratio perturbations (in percent) at six depths. Red color denotes low velocity and high Poisson's ratio while blue color indicates high velocity and low Poisson's ratio. The 2004 Niigata mainshock is indicated with a red star. White stars denote the large Niigata aftershocks (M ≥ 6.0). Open circles indicate the large crustal earthquakes (M ≥ 5.0) from 869 to 1998 [58]. Crosses show earthquakes occurred within a 5 km depth range of each horizontal section. Active faults are shown with curved solid lines. The depth of each layer is shown at the upper left corner of each map. The velocity or Poisson's ratio perturbation scale is shown on the right.
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Fig. 8 (continued).
near the edge portion where both seismic velocity and Poisson's ratio change rapidly over a short distance. The P-wave velocity is slightly high down to 16 km depth
and becomes higher at 20–27 km depths beneath the mainshock. S-wave velocity is higher in the source area and becomes lower below the source area. Poisson's
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Fig. 8 (continued).
ratio is lower from 3 to 16 km depth, and is higher at 18– 30 km. A zone with markedly low Vs and high Poisson's ratio is clearly imaged below the source
area. The CRT results and the amplitude-recovery tests show high resolution and good recovery in this area (Figs. 5 and 7), indicating that the images of Vp, Vs and
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σ anomalies in and below the source area are reliable features. Vertical cross-sections at depths of 0–150 km of the seismic velocity and Poisson's ratio through the source area are shown in Fig. 9b, which were determined by 3-D tomographic inversion using a large number of arrival times from 5123 intermediatedepth earthquakes [21]. For this inversion, a grid model with grid spacing of 0.2° in horizontal and of 10–20 km in depth was used. Low-velocity and high-Poisson's ratio anomalies are visible in the back-arc under the mainshock and the aftershock area. At the depth of 15 km, high-velocity and low-Poisson's ratio zones are visible in the source area, being well consistent with those shown in Fig. 9a. 5. Discussion Seismic velocity and Poisson's ratio in the crust are related to factors such as composition, crack density, temperature and fluid content. The low S-wave velocity and high Poisson's ratio anomaly below the source area might reflect the existence of fluids within fractured rock [5,22–24] that contributed to the initiation of the mainshock rupture (Figs. 8 and 9). This conjecture has been supported by evidence from seismological, geophysical, and hydrological investigations conducted in the Niigata earthquake area. These studies have revealed many anomalous features in and around the source area that provide useful information for the interpretation of our tomographic results in terms of the possible role of fluids in the faulting process. 5.1. Seismic observations Fig. 3 shows an example of hypocentral distribution relocated by using the double-difference method for the depth range of 0–25 km. Several event clusters are apparent along the fault zone. The spatial distribution of the relocated aftershocks may reflect a complicated fault zone that includes not only the dominant faults (at least two parallel faults) dipping to the northwest, but also conjugate faults (see cross-sections in Fig. 3). Similar features of the aftershock distribution have been revealed by previous studies but their resolutions are different from one to the other [2,25–27]. The Niigata aftershocks occurred within this fault system. The spatial distribution features of the aftershock sequence in the fault system may indicate the aftershockgenerating process, because the distribution of the aftershocks is closely linked to the geometry of the fault zone that ruptured to produce the mainshock, which in turn depends on the physical properties of the
fault zone such as seismic velocity and Poisson's ratio as well as the distribution of strength, stress and fluids [28]. High S-wave velocity and low-Poisson's ratio are imaged at depths of 10–18 km in the Niigata aftershock area (Fig. 9a), which may indicate the brittle seismogenic layer [29–32]. Below the source area, low S-wave velocity and high-Poisson's ratio zones are revealed at depths of 18–30 km (Fig. 9a). A conductive zone under the source area was imaged in the lower crust and uppermost mantle [1,33], being well consistent with the velocity and Poisson's ratio anomalies identified from our tomographic images (Fig. 9a), which may reflect the existence of fluids. The edge portion of the seismogenic layer above the brittle–ductile boundary can be weakened due to the penetration of fluids from the lower part. Large crustal earthquakes are likely to occur in the area where the cutoff depth of seismicity varies abruptly [34]. Many large inland earthquakes initiate at a depth of about 15 km; the cutoff depth of microseismic activity in the crust is also located around this depth [34,35]. Thus, the distribution of the relocated aftershocks in the high-velocity and low-Poisson's ratio zone at the depths of 8–18 km could be a reliable feature. 5.2. Geophysical evidence Strain rate distribution was estimated from daily coordinate data from GPS stations. These data show that deformation in the source area acts to increase stress in areas with abrupt lateral variations in the seismicity cutoff depth [34,35]. High conductivity anomalies are revealed under the source area [1]. It has been suggested that concentrated deformation in the source area results from the lower crust which is weakened by a high water content [36–38]. If a zone of high conductivity is found along the active fault, it may reflect the existence of fluids in fracture zones [37]. Low-Vp anomalies at the depths of 0–8 km and high-Vp heterogeneities at the depths of 8–15 km were revealed by a previous study [39], being well consistent with those determined by the present study (Fig. 9a). Similar features of the velocity anomalies were revealed only at the shallow depths of 0–15 km [2,40]. No detailed seismic structures were imaged by these studies below the source area because they did not use ray paths passing through the lower crust and uppermost mantle. Compared with the previous studies, the present work has advantages over them in the data selection and the grid model setting. Our tomographic results show high-Poisson's ratio and low Vs anomalies in the lower crust and uppermost mantle (Figs. 8 and 9) that may correspond to the high conductivity anomalies [1,36,37] and thus demonstrate
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Fig. 9. (a) Vertical cross-sections of P-wave and S-wave velocity and Poisson's ratio perturbations along the lines A–A′ and B–B′ shown in Fig. 6a. The mainshock is indicated with a red star. White dots denote the aftershocks relocated by using the double-difference method. (b) Vertical cross-sections of P- and S-wave velocity and Poisson's ratio perturbations along the line CC′ shown in the insert map determined by Wang and Zhao [21]. Small white dots show background seismicity along the section CC′. The Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab are indicated with three solid lines. Large white solid circles denote the large Niigata aftershocks (M ≥ 6.0). The location of the volcanic front is indicated with red triangles. The surface topography is shown on the top of each profile. Red color denotes low velocity and high Poisson's ratio and blue color represents high velocity and low Poisson's ratio. The velocity and Poisson's ratio perturbation scale is shown at the bottom.
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the presence of fluids. In addition, a dense gravity survey detected negative gravity anomalies southwest of the source area and positive anomalies to the northeast, as determined by the Gravity Research Group in Southwest Japan. The hypocenter of the 2004 Niigata mainshock is located in an area where the gravity varies markedly, which is consistent with the feature of velocity and Poisson's ratio anomalies in the source area. 5.3. Origin of the fluids in the lower crust Many researchers have suggested that fluids occur widely in the crust and uppermost mantle in subduction zones [41–44]. The existence of fluids beneath the seismogenic layer may affect the compositional evolution of a fault zone, change the strength of a fault zone, and alter the local stress regime [45,46]. It is possible that the lower crust in the Niigata aftershock area is weakened by dehydration of the Pacific slab, and that its viscosity is much lower than that of the surrounding lower crust [36]. The previous studies also suggested that the subducting Pacific slab significantly affects inland deformation [36]. There are two possible origins of fluids in the lower crust along the NKTZ: shallow fluids such as meteoric water, pore fluids and mineral dehydration in the crust [47], and fluids of deep origin such as dehydration of the subducting oceanic plate [44,48]. Our tomographic images show that the lowvelocity and high-Poisson's ratio anomalies below the source area are closely related to the subducting Pacific slab (Fig. 9b). We, thus, conclude that these anomalous features of the low-velocity and high-Poisson's ratio and the high conductivity in and below the source area are mainly caused by fluids associated with dehydration of the subducting Pacific slab. 5.4. Interpretations An obvious change in Vp and Vs is visible across the faults oriented from SW to NE (Fig. 7); this may reflect lithological changes across the fault zone. Surface geological structures above the source region are different across the Shibata–Koide Tectonic Line (SKTL; Fig. 1b). Southeast of the tectonic line, indurated preNeogene rocks outcrop, while on the northwest side there are deep sedimentary basins (N 6 km in thickness) that might have formed over half grabens related to crustal extension. The tomographic images (Fig. 8) show high-velocity and low-Poisson's ratio on the southeast side of the SKTL, and low-velocity and highPoisson's ratio northwest of SKTL; this is consistent with the surface geological features [49]. Similar fea-
tures are delineated by a previous study [2], showing good consistence with our tomographic images at the depths of 5–15 (Fig. 8). Previous tomographic studies show that many large crustal earthquakes occur in or around zones of low seismic velocity, including the 1995 Kobe earthquake (M 7.2) [5] and the 2000 Tottori earthquake (M 7.3) [48]. Hauksson and Haase [50] showed that four earthquakes (M N 5.9) occurred in or close to highvelocity zones in the Los Angeles basin area, although the earthquake ruptures were actually located in or near the boundary between high- and low-velocity zones, similar to our tomographic results (Figs. 8 and 9). Fluids in the lower crust below the source area perhaps reduce the seismic velocity of surrounding crust and cause the overlying brittle seismogenic layer to become locally thinner and weaker. The previous studies [5,51] reported that main rupture zones and aftershocks are mostly located in high velocity (i.e. high rigidity) regions. Our tomographic results (Figs. 8 and 9) also demonstrate that most of the aftershocks were located in the high-velocity and low-Poisson's ratio zone in the source area. According to Kumar et al. [52], shear stress in the source region is larger than in the surrounding areas. We consider that the aftershock sequence might be triggered by shear stress accumulated in the low-velocity, highPoisson's ratio and high conductivity zones around the source area that contains fluids. Our tomographic results (Fig. 9b) indicate that the crustal weakening is closely related to the dehydration of the subducting slab in this region. Spatial and temporal variations in the crustal stress field have been reported for the source areas of the Kobe earthquake (M 7.2) [53] and the 1994 Northridge earthquake (M 6.7) in southern California [54], both of which are interpreted to be associated with fluids in the fault zones. The regional east– west compressional stress field causes strong crustal deformation in the Niigata sedimentary basin [55]. The large deformation rate may result from aseismic slip of many ductile faults in the lower crust. In addition, plastic lower crust can be deformed by ductile flow due to regional tectonic stress resulting from plate motions. Deformation in the lower crust acts to increase stress in areas with abrupt lateral variations in the mechanical strength of the seismogenic crust [45]. The mainshock was located in or near the junction between low-velocity and high-velocity zones (Figs. 8 and 9) where the mechanical strength of materials is much weaker than that in normal sections of the seismogenic layer due to the penetration of fluids from the lower crust. Thus the margin of a crustal block is an ideal location to generate large earthquakes that produce faults that reach to the
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Earth's surface or blind faults within the brittle upper crust [37,44]. The strong heterogeneities in the lower crust and uppermost mantle in and around the source area (Figs. 8 and 9a) might have influenced the nucleation and rupture process of the 2004 Niigata earthquake. Over-pressurized fluid-filled fractures may play a role in the process of fault-valve behavior and eventually in the occurrence of large earthquakes [45]. The low-Vs and high-σ zone, coinciding with the high conductive zone [1], below the mainshock hypocenter might reflect highly fractured rock and fluid intrusion from the deep crust and uppermost mantle. Such an anomaly in velocity, Poisson's ratio and conductivity might potentially have initiated the mainshock rupture. The fluids reduced the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2004 Niigata mainshock. 6. Conclusions Detailed 3-D seismic velocity (Vp, Vs) and Poisson's ratio (σ) images in and around the source area of the 2004 Niigata earthquake were determined by using a larger number of high-quality P- and S-wave arrival times. Most of the aftershocks occurred in areas with low-σ that may indicate brittle and competent patches of the fault zone. An obvious change in seismic velocity and Poisson's ratio is clearly imaged in the shallow crust, consistent with surface geological features. Lateral heterogeneities are revealed in the lower crust and uppermost mantle in the source area. A strong low-Vs and high-σ anomaly zone is revealed in the lower crust under the mainshock hypocenter, which may reflect the presence of fluids in the source area. The anomalous zone in and below the seismogenic layer might be caused by the presence of fluids, which acted to reduce the strength of the crust and decrease the seismic velocity. Our tomographic results indicate that the fluids are closely related to the dehydration of the subducting Pacific slab. Such fluids may have reduced the mechanical strength of the fractured rock matrix and thus triggered the 2004 Niigata earthquake. Acknowledgements We thank Prof. S. King and an anonymous reviewer for their constructive comments and suggestions. We thank the Hi-net and J-Array data center for providing us P and S arrival time data and waveform data via Internet. Some figures were made by using GMT (Generic Mapping Tools) software [56]. This work was partially
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supported by research grants (Kiban-B 11440134, and Kiban-A 17204037) from Japan Society for the Promotion of Science to D. Zhao. References [1] M. Uyestima, Y. Ogawa, Y. Honkura, S. Koyama, N. Ujihara, T. Mogi, Y. Yamaya, M. Harada, S. Yamaguchi, I. Shiozaki, T. Noguchi, Y. Kuwaba, Y. Tanaka, Y. Mochido, N. Manabe, M. Nishihara, M. Saka, M. Serizawa, Resistivity imaging across the source region of the 2004 Mid-Niigata prefecture earthquake (M6.8), central Japan, Earth Planets Space 57 (2005) 441–446. [2] A. Kato, E. Kurashimo, N. Hirata, T. Iwasaki, T. Kanazawa, Imaging the source region of the 2004 Mid-Niigata prefecture earthquake and the evolution of the seismogenic thrustrelated fold, Geophys. Res. Lett. 32 (2005) L07307, doi:10. 1029/2005GL022366. [3] D. Zhao, H. Kanamori, The 1992 Landers earthquake sequence: earthquake occurrence and structural heterogeneities, Geophys. Res. Lett. 20 (1993) 1083–1086. [4] J. Lees, C. Nicholson, Three-dimensional tomography of the 1992 southern California earthquake sequence: constraints on dynamic earthquake rupture? Geology 21 (1993) 387–390. [5] D. Zhao, H. Kanamori, H. Negishi, Tomography of the source area of the 1995 Kobe earthquake: evidence for fluids at the hypocenter? Science 274 (1996) 1891–1894. [6] A. Nakamura, A. Hasegawa, A. Ito, B. Ucer, S. Baris, Y. Honkura, P-wave velocity structure of the crust and its relationship to the occurrence of the 1999 Izmit, Turkey, earthquake and aftershocks, Bull. Seismol. Soc. Am. 92 (2002) 330–338. [7] F. Waldhauser, W.L. Ellsworth, A double-difference earthquake location algorithm: method and application to the Northern Hayward Fault, California, Bull. Seismol. Soc. Am. 90 (2000) 1353–1368. [8] H. Zhang, C.H. Thurber, Double-difference tomography: the method and its application to the Hayward fault, California, Bull. Seismol. Soc. Am. 93 (2003) 1875–1889. [9] P.M. Shearer, Improving local earthquake locations using the L1 norm and waveform cross correlation: application to the Whittier Narrows, California, aftershock sequence, J. Geophys. Res. 102 (1997) 8269–8283. [10] H. Xue, Y. Oki, O. Sato, H. Watanabe, Geochemistry of groundwaters in the Utsunomata landslide area, Maki Village, Higashikubiki District, Niigata Prefecture, J. Jpn. Landslide Soc. 34 (1997) 25–34. [11] S. Yoshioka, T. Yabuki, M. Matsuʻura, T. Sagiya, T. Tada, Interplate coupling and its tectonic implications in the Kanto district, central Japan as deduced from geodetic data inversion, Tectonophysics 229 (1994) 165–180. [12] M. Ohkake, A seismic gap in the eastern margin of the Sea of Japan as inferred from the time-Pacific slab distribution of past seismicity, Isl. Arc 4 (1995) 156–165. [13] T. Sagiya, S. Miyazaki, T. Tada, Continuous GPS array and present-day crustal deformation of Japan, Pure Appl. Geophys. 157 (2000) 2303–2322. [14] M. Hashimoto, Average horizontal crustal strain rates in Japan during interseismic period deduced from geodetic surveys (Part 1), Zisin (J. Seismol. Soc. Japan), Ser. 2–41 (1990) 13–26. [15] M. Hashimoto, D. Jackson, Plate tectonics and crustal deformation around the Japanese islands, J. Geophys. Res. 98 (1993) 16149–16166.
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Seismic images of the source area of the 2004 Mid-Niigata prefecture earthquake in Northeast Japan Zhi Wang ⁎, Dapeng Zhao Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 18 May 2005; received in revised form 8 February 2006; accepted 9 February 2006 Editor: Scott King
Abstract To better understand the generation mechanism of the 23 October 2004 Mid-Niigata prefecture earthquake (M 6.8), we used 120,352 P-wave and 95,391 S-wave high-quality arrival times from 5013 earthquakes to determine the three-dimensional seismic velocity (Vp, Vs) and Poisson's ratio (σ) structures in and around the source area. The hypocenter locations of the aftershocks are relocated accurately by using absolute and relative travel time (double-difference) data. Our results demonstrate that the seismic velocity and Poisson's ratio vary markedly around the source area. Most active faults are located along the margins of low-velocity and high-velocity zones. An obvious change in seismic velocity and Poisson's ratio occurs between the northwest and southeast parts of the study area, and this boundary zone reflects the spatial distribution of active faults, being well consistent with the surface geological features. Most of the large historic crustal earthquakes are located in or around the low seismic velocity zones. The mainshock hypocenter is located near the margin of low-velocity (high-Poisson's ratio) and high-velocity (low-Poisson's ratio) anomalies along the active fault zone. A zone with pronounced low-velocity and high-Poisson's ratio is clearly imaged in the lower crust under the source area, reflecting the existence of fluids that are released due to the dehydration of the subducting Pacific slab. Such fluids might have reduced the mechanical strength of the fault zone, and thus initiated the Mid-Niigata prefecture earthquake. © 2006 Elsevier B.V. All rights reserved. Keywords: Seismic tomography; 2004 Mid-Niigata prefecture earthquake; Crustal earthquake; Fluids; Slab dehydration
1. Introduction On 23 October 2004, a destructive earthquake of JMA (Japan Meteorological Agency) magnitude 6.8, the 2004 Mid-Niigata prefecture earthquake (hereafter we call it the 2004 Niigata earthquake), occurred in the
⁎ Corresponding author. Tel.: +81 89 927 8258; fax: +81 89 927 8167. E-mail addresses: [email protected] (Z. Wang), [email protected] (D. Zhao). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.02.015
southwestern portion of Northeast Japan (Fig. 1). More than 40 people were killed, 4000 were injured, about 100,000 people were forced into temporary shelters, and as many as 10,000 might be displaced from their upland homes for several years (EERI, 2005). Japanese authorities estimated the total damage resulting from the earthquake to be US$40 billions. It is the most significant earthquake to affect Japan since the 1995 Kobe earthquake (M 7.2). The mainshock occurred at a shallow depth near the Japan Sea in a zone of relatively high seismicity. Within 100 km of the source area there have been 5 large earthquakes with JMA magnitude
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larger than 6 since 1941, including the 1964 Niigata earthquake (M 7.5). The frequent occurrence of large crustal earthquakes close to the densely populated areas on the Japan Islands has caused significant damage and loss of human life, e.g. the 1964 Niigata earthquake (M 7.5), 1995 Kobe earthquake (M 7.2), 2000 Tottori earthquake (M 7.3), and this 2004 Niigata earthquake (M 6.8). The 2004 Niigata mainshock was followed by an aftershock sequence with four events of magnitude 6 or greater, and 12 events of magnitude 5 to 5.9, all of which were recorded by the Hi-net seismic network during the period from 23 October to 28 November, 2004. The hazards caused by these large crustal earthquakes demonstrated an urgent need for better knowledge of the nucleation mechanism of the earthquakes. The results of many seismological and geophysical studies indicate that the 2004 Niigata
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earthquake occurred in a SW–NE oriented reverse fault zone and might have been triggered by the presence of fluids in the crust [1,2]. However, no detailed studies have been undertaken to provide direct evidence for the source of the fluids. We consider that variations in seismic velocity in the source area of large earthquakes may reflect the physical properties of the crustal rocks. If the physical properties of the rocks have influenced rupture nucleation and the faulting process, then they can be imaged by seismic tomography. In addition, crustal heterogeneities in the hypocentral area of the mainshock might be related to the deep structure anomalies and the dehydration reactions of the subducting Pacific slab. Over the last 20 years, many studies have investigated seismic structures in the source areas of large crustal earthquakes [3–6]. In the present study we investigate seismic velocity and Poisson's ratio structures in and around the source
Fig. 1. (a) Tectonic background of the Japan Islands. The Niigata–Kobe Tectonic Zone (NKTZ) is indicated with the bold shaded line. Plate boundaries in and around Japan Islands are also shown. (b) Squares A and B indicate the large and small inversion areas, respectively (see text for details). Open circles denote the large historical crustal earthquakes (M N 5.0) occurred in the period from 869 to 1998 [57]. Open triangles indicate active volcanoes. The large star shows the epicenter of the 2004 Niigata earthquake. The NKTZ is shown with a shaded area. Active faults on the Japan Islands are indicated with curved solid lines. Magnitude scale of the earthquakes is shown above (b).
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area of the 2004 Niigata earthquake to understand the generation mechanism of this event. The double-difference technique [7,8] is a useful method for accurately locating earthquakes. Recent studies have demonstrated substantial improvements in determining hypocentral locations using this method, and the method has been successfully applied to many areas (e.g., [7–9]). In the present study, we use the double-difference technique to accurately relocate the hypocenters of the Niigata aftershocks; this in turn enables us to better determine the seismic velocity and Poisson's ratio structures in the seismic source area. To investigate the deep structure beneath the source area and its effect on the generation of the 2004 Niigata earthquake, we also used 1708 earthquakes at depths of 0–200 km to determine the seismic structure of the lower crust and upper mantle beneath the study area.
2. Tectonic setting and geology The 2004 Niigata earthquake occurred in the central part of the Niigata–Kobe tectonic zone (NKTZ) within the Shinano River seismic belt (SRSB), which extends from Matsumoto to Niigata along the Shinano River (Fig. 1b). The SRSB is known as an area of pronounced recent crustal deformation and seismicity [10]. The NKTZ is characterized by mutual interactions of three or four plates [11]: the Philippine Sea plate (PHS), the Pacific slab, the Eurasian plate (EUR) and the Okhotsk plate, as shown in Fig. 1a. On the eastern margin of the Japan Sea, the plate boundary between the EUR and Okhotsk plates is located at the coastline near Niigata and traverses the land area along the SRSB [12]. Beneath the NKTZ, the Pacific slab is subducting WNW to a depth greater than 150 km. In the south of the
Fig. 2. Yellow and blue dots show hypocenters of 5013 earthquakes, which consists of two groups of events. One group includes 3305 aftershocks (yellow dots) shallower than 30 km recorded by the dense Hi-net between 23 October and 28 November 2004. The other group includes 1708 earthquakes (M N 2.5; depths b 200 km) recorded by Hi-net and J-Array networks from June 1991 to September 2004 (blue dots). The red star shows the location of the mainshock. Red squares denote 126 seismic stations used in this study. Areas A and B are the same as the two regions in Fig. 1b.
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NKTZ, the PHS is sandwiched by the subducting Pacific slab and the overriding Japanese crust. The compressional strain rate in the NKTZ is up to 200 nanostrain/yr, according to the analyses of GPS data and qualitative modeling [13]. Across this zone, high WNW–ESE shortening rates of ∼10–7 strain/yr are observed, which are several times larger than those in the surrounding areas [13]. The existence of a zone with such a high strain rate is also indicated by triangulation data collected over the past 100 years [14,15]. Also within this zone are many active Quaternary faults exist, which generated a number of large historical earthquakes (Fig. 1b). Thus, the high strain rates observed by the GPS and triangulation surveys are not temporal but possibly reflect tectonic loading of the active faults in central Japan over a long time. 3. Data and methods For tomographic imaging, we used a large number of P- and S-wave arrival times from shallow and intermediate-depth earthquakes, as recorded by seismic networks in central and Northeast Japan. Fig. 2 shows the three-dimensional (3-D) hypocentral distribution of 5013 selected events that generated 120,352 P- and 95,391 S-wave arrival times used in this study. The 5013 events consist of two groups of earthquakes. One group includes 3305 aftershocks shallower than 30 km, as recorded by the dense Hi-net and other seismic networks in Japan between 23 October and 28 November, 2004. The other group includes 1708 earthquakes at depths of 0–200 km with JMA magnitude greater than 2.5, as recorded by the Hi-net and J-Array networks from June 1991 to September 2004. The Hi-net has been installed and operated by the National Research Institute for Earth Science and Disaster Prevention in Japan since 1999, and consists of more than 600 seismic stations that provide a dense and uniform coverage of the Japan Islands. Thus, a large number of arrival time data from the 2004 Niigata earthquake sequence are available, allowing us to study the detailed 3-D seismic structure in and around the source area. For the 3305 aftershocks, the picking accuracy varies from 0.05 to 0.1 s for P-waves and about 0.1 s for Swaves. All the aftershocks were recorded by more than 12 seismic stations, and they were selected carefully; the uncertainties of the epicenters of the aftershocks are less than 0.5 km while those of the focal depths are 0–2 km. The depth distribution of the events shows that most of the aftershocks occurred in the depth range of 0–18 km (Fig. 3).
Fig. 3. Vertical cross-sections of hypocentral distributions of the aftershocks relocated by using the double-difference method [7,16] along the lines A–A′, B–B′, C–C′, D–D′, E–E′, and F–F′. Magnitude scale of the aftershocks is shown below the insert map.
To better understand the generation mechanism of the 2004 Niigata earthquake, the two groups of data are used jointly (Fig. 2). The aftershock data are used for detailed tomographic imaging of the source area. The rays of this data set are entirely within the source area. The doubledifference method [7,16] was used to accurately relocate hypocenters of the aftershocks. The second group of events occurred in or around the source area at depths of 0–200 km, and these are used for imaging the structure below the source area in the lower crust and uppermost mantle. Therefore, a better ray coverage is achieved for the target source area (region B shown in Fig. 2). The number of rays intersecting the target source area is 88,636 for P-waves and 62,762 for S-waves. We used the double-difference location techniques [16], based on the tomographic method [17], to accurately relocate the hypocenters of the 3305 aftershocks. We used 155,998 event-pairs of the aftershocks to relocate the hypocenters. In the tomographic inversion, we have taken into account three discontinuities: the Conrad, the Moho, and the upper boundary of the subducting Pacific slab. The three discontinuities are not simple flat planes but have complicated geometries [17–19]. We adopted their depth distributions as determined by Zhao et al. [17,20]. In the present study, a total number of 126 seismic stations are used, which recorded arrival time data from July 1991 to November 2004 (Fig. 2). These stations belonged to the Japanese Universities, Japan Meteorological Agency (JMA), Hi-net, and J-Array.
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4. Resolution and results An optimal 1-D velocity model, which minimizes the overall misfit between the model predictions and observations, was determined by using the program of Zhao et al. [17]. The model was determined in two steps. First, a Vp model was calculated using only P-wave arrival times, with a starting model modified from Zhao et al. [17]. To obtain the 1-D P-wave velocity model to well reflect seismic velocities along the ray paths used, the earthquakes with over 10 P-wave data were selected for calculating. Secondly, a 1-D Vs model was determined by using only S-wave arrival times, with a starting model derived from the obtained 1-D Vp model and a mean Vp / Vs ratio of 1.73 for the study region. The earthquakes with more than 8 S-wave arrivals were used for determining the Vs model. We conducted checkerboard resolution tests (CRT) for evaluating the resolution of the 3-D velocity structure. Positive and negative velocity perturbations of ± 3%
were assigned to the grid nodes, and synthetic travel times were calculated for the checkerboard model. Then random errors with a standard deviation of 0.08 s for Pwaves and 0.15 s for S-waves were added to the synthetic data. The numbers of events, stations and ray paths are the same as those used in the final tomographic inversion. We adopted a grid spacing of 0.2° in the horizontal and 10–20 km in depth for the region surrounding the Niigata aftershock area (region A in Fig. 2). Fig. 4 shows the CRT results for six depth levels. The resolution is good over the entire study area. Then a smaller grid spacing of 0.05° in the horizontal and 5 km in depth was tested for the entire area. The inversion results show that the CRT pattern is well recovered in area B while the resolution is poor in area A (Fig. 5). Therefore, a grid spacing of 0.05° in the horizontal and 5 km in depth for area B and that of 0.2° in the horizontal and 10–20 km in depth for area A were used for the inversion of the real data. Fig. 6 shows the combined grid model adopted for the synthetic test and the tomographic inversions.
Fig. 4. Results of checkerboard resolution test (CRT) for P-wave (a) and S-wave (b) at six depths. The CRT results are obtained with a grid separation of 0.2° in N–S and E–W directions and 10–20 km in depth. The depth of each layer is shown at the upper left corner of each map. The scale of velocity perturbation is shown on the right.
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Fig. 5. The same as Fig. 4 but for a grid spacing 0.05° in N–S and E–W directions and 5 km in depth.
We also conducted the model-recovery synthetic tests to demonstrate the recovery of the amplitudes of Vp and Vs in the target area B (Fig. 7). We input 4% velocity anomalies in the grid nodes and then calculate the inversion value at each grid. Fig. 7a shows that the amplitudes of Vp and Vs are well recovered at the depths of 10, 15 and 20 km; the amplitudes of P-wave velocity are similar to those of S-wave velocity. The amplitudes of Vp and Vs are well recovered along the cross-section AA′ (Fig. 7b). Thus, the low-velocity anomalies and the Poisson's ratio perturbations derived from the inverted Vp and Vs are considered to be reliable features in the source area. The final inversion results were obtained after five iterations with the combined grid model (Fig. 6). When we carried out the checkerboard resolution test and the final inversion, an optimal damping value of 8 was used which was determined by a number of tests. The grid nodes with hit counts greater than 15 were included in the velocity inversion. The root-mean-square (RMS) travel time residuals, calculated from the optimal velocity model and the relocated hypocenters, are 0.229 s for P-waves and 0.263 s for S-waves, which
Fig. 6. Three-dimensional distribution of grid nodes adopted in this study. The Niigata mainshock location is indicated with a black star. Lines AA′ and BB′ show the locations of the cross-sections in Fig. 9a.
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Fig. 7. (a) Results of the amplitude-recovery tests at depths of 10, 15 and 20 km in the study region B (shown in Fig. 2a), determined by using a method similar to the previous study [57]. We input 4% velocity perturbations (the same as the inverted maximum velocity anomalies) to each grid nodes and then calculate the synthetic data. The recovery-amplitude scale is shown below (a). The degree of recovery is, in general, the same for Pand S-wave velocity structures for both the pattern and amplitude of the input velocity anomalies. (b) Synthetic recovery-test along the cross-section AA′ (shown in Fig. 9a). Scale of velocity perturbation is shown on the right of (b). The upper panel shows the input model and the lower two panels denote the inversion results of Vp and Vs. The low-velocity zone below the Niigata mainshock (red star) is recovered well for both Vp and Vs.
were reduced by 50% of their initial values. Fig. 8a–c shows the plan views of P- and S-wave velocity and Poisson's ratio images obtained together with the seismicity around each layer. The velocity perturbations are from the average value of the inverted velocities in each layer. The relative Poisson's ratio images (Fig. 8c) are calculated from the P- and S-wave velocity perturbations (Fig. 8a,b). At depths of 0 and 5 km, the spatial distributions of P- and S-wave velocities are similar to each other, although the amplitudes of S-wave velocity anomalies are larger than those of P-wave anomalies. The seismic velocity changes rapidly at these depths around the Niigata aftershock area. In the third and fourth layers (10 and 15 km), we see that the locations of the mainshock and aftershocks are located in a zone where Vp and Vs are high and σ is low. The images of velocity and Poisson's ratio show that most of
the active faults are located along zones where the velocity and Poisson's ratio change rapidly over a short distance (Fig. 8). The images at 20–25 km depth show low-Vp, low-Vs and high-σ anomalies in or near the source area (Fig. 8). Though there are few events located in the lower crust and uppermost mantle, numerous rays from the intermediate-depth earthquakes (60–200 km) and rays of head waves (P*, S*, Pn, Sn) from the crustal events refracted at the Conrad and Moho discontinuities sampled the lower crust and uppermost mantle. The anomalies of seismic velocity and Poisson's ratio in the deeper crust are therefore well imaged. Fig. 9a shows vertical cross-sections of the velocity and Poisson's ratio together with mainshock and aftershock seismicity along the lines AA′ and BB′ shown in Fig. 6a. It is clear that the mainshock is located
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Fig. 8. (a) P-wave velocity, (b) S-wave velocity, and (c) Poisson's ratio perturbations (in percent) at six depths. Red color denotes low velocity and high Poisson's ratio while blue color indicates high velocity and low Poisson's ratio. The 2004 Niigata mainshock is indicated with a red star. White stars denote the large Niigata aftershocks (M ≥ 6.0). Open circles indicate the large crustal earthquakes (M ≥ 5.0) from 869 to 1998 [58]. Crosses show earthquakes occurred within a 5 km depth range of each horizontal section. Active faults are shown with curved solid lines. The depth of each layer is shown at the upper left corner of each map. The velocity or Poisson's ratio perturbation scale is shown on the right.
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Fig. 8 (continued).
near the edge portion where both seismic velocity and Poisson's ratio change rapidly over a short distance. The P-wave velocity is slightly high down to 16 km depth
and becomes higher at 20–27 km depths beneath the mainshock. S-wave velocity is higher in the source area and becomes lower below the source area. Poisson's
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Fig. 8 (continued).
ratio is lower from 3 to 16 km depth, and is higher at 18– 30 km. A zone with markedly low Vs and high Poisson's ratio is clearly imaged below the source
area. The CRT results and the amplitude-recovery tests show high resolution and good recovery in this area (Figs. 5 and 7), indicating that the images of Vp, Vs and
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σ anomalies in and below the source area are reliable features. Vertical cross-sections at depths of 0–150 km of the seismic velocity and Poisson's ratio through the source area are shown in Fig. 9b, which were determined by 3-D tomographic inversion using a large number of arrival times from 5123 intermediatedepth earthquakes [21]. For this inversion, a grid model with grid spacing of 0.2° in horizontal and of 10–20 km in depth was used. Low-velocity and high-Poisson's ratio anomalies are visible in the back-arc under the mainshock and the aftershock area. At the depth of 15 km, high-velocity and low-Poisson's ratio zones are visible in the source area, being well consistent with those shown in Fig. 9a. 5. Discussion Seismic velocity and Poisson's ratio in the crust are related to factors such as composition, crack density, temperature and fluid content. The low S-wave velocity and high Poisson's ratio anomaly below the source area might reflect the existence of fluids within fractured rock [5,22–24] that contributed to the initiation of the mainshock rupture (Figs. 8 and 9). This conjecture has been supported by evidence from seismological, geophysical, and hydrological investigations conducted in the Niigata earthquake area. These studies have revealed many anomalous features in and around the source area that provide useful information for the interpretation of our tomographic results in terms of the possible role of fluids in the faulting process. 5.1. Seismic observations Fig. 3 shows an example of hypocentral distribution relocated by using the double-difference method for the depth range of 0–25 km. Several event clusters are apparent along the fault zone. The spatial distribution of the relocated aftershocks may reflect a complicated fault zone that includes not only the dominant faults (at least two parallel faults) dipping to the northwest, but also conjugate faults (see cross-sections in Fig. 3). Similar features of the aftershock distribution have been revealed by previous studies but their resolutions are different from one to the other [2,25–27]. The Niigata aftershocks occurred within this fault system. The spatial distribution features of the aftershock sequence in the fault system may indicate the aftershockgenerating process, because the distribution of the aftershocks is closely linked to the geometry of the fault zone that ruptured to produce the mainshock, which in turn depends on the physical properties of the
fault zone such as seismic velocity and Poisson's ratio as well as the distribution of strength, stress and fluids [28]. High S-wave velocity and low-Poisson's ratio are imaged at depths of 10–18 km in the Niigata aftershock area (Fig. 9a), which may indicate the brittle seismogenic layer [29–32]. Below the source area, low S-wave velocity and high-Poisson's ratio zones are revealed at depths of 18–30 km (Fig. 9a). A conductive zone under the source area was imaged in the lower crust and uppermost mantle [1,33], being well consistent with the velocity and Poisson's ratio anomalies identified from our tomographic images (Fig. 9a), which may reflect the existence of fluids. The edge portion of the seismogenic layer above the brittle–ductile boundary can be weakened due to the penetration of fluids from the lower part. Large crustal earthquakes are likely to occur in the area where the cutoff depth of seismicity varies abruptly [34]. Many large inland earthquakes initiate at a depth of about 15 km; the cutoff depth of microseismic activity in the crust is also located around this depth [34,35]. Thus, the distribution of the relocated aftershocks in the high-velocity and low-Poisson's ratio zone at the depths of 8–18 km could be a reliable feature. 5.2. Geophysical evidence Strain rate distribution was estimated from daily coordinate data from GPS stations. These data show that deformation in the source area acts to increase stress in areas with abrupt lateral variations in the seismicity cutoff depth [34,35]. High conductivity anomalies are revealed under the source area [1]. It has been suggested that concentrated deformation in the source area results from the lower crust which is weakened by a high water content [36–38]. If a zone of high conductivity is found along the active fault, it may reflect the existence of fluids in fracture zones [37]. Low-Vp anomalies at the depths of 0–8 km and high-Vp heterogeneities at the depths of 8–15 km were revealed by a previous study [39], being well consistent with those determined by the present study (Fig. 9a). Similar features of the velocity anomalies were revealed only at the shallow depths of 0–15 km [2,40]. No detailed seismic structures were imaged by these studies below the source area because they did not use ray paths passing through the lower crust and uppermost mantle. Compared with the previous studies, the present work has advantages over them in the data selection and the grid model setting. Our tomographic results show high-Poisson's ratio and low Vs anomalies in the lower crust and uppermost mantle (Figs. 8 and 9) that may correspond to the high conductivity anomalies [1,36,37] and thus demonstrate
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Fig. 9. (a) Vertical cross-sections of P-wave and S-wave velocity and Poisson's ratio perturbations along the lines A–A′ and B–B′ shown in Fig. 6a. The mainshock is indicated with a red star. White dots denote the aftershocks relocated by using the double-difference method. (b) Vertical cross-sections of P- and S-wave velocity and Poisson's ratio perturbations along the line CC′ shown in the insert map determined by Wang and Zhao [21]. Small white dots show background seismicity along the section CC′. The Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab are indicated with three solid lines. Large white solid circles denote the large Niigata aftershocks (M ≥ 6.0). The location of the volcanic front is indicated with red triangles. The surface topography is shown on the top of each profile. Red color denotes low velocity and high Poisson's ratio and blue color represents high velocity and low Poisson's ratio. The velocity and Poisson's ratio perturbation scale is shown at the bottom.
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the presence of fluids. In addition, a dense gravity survey detected negative gravity anomalies southwest of the source area and positive anomalies to the northeast, as determined by the Gravity Research Group in Southwest Japan. The hypocenter of the 2004 Niigata mainshock is located in an area where the gravity varies markedly, which is consistent with the feature of velocity and Poisson's ratio anomalies in the source area. 5.3. Origin of the fluids in the lower crust Many researchers have suggested that fluids occur widely in the crust and uppermost mantle in subduction zones [41–44]. The existence of fluids beneath the seismogenic layer may affect the compositional evolution of a fault zone, change the strength of a fault zone, and alter the local stress regime [45,46]. It is possible that the lower crust in the Niigata aftershock area is weakened by dehydration of the Pacific slab, and that its viscosity is much lower than that of the surrounding lower crust [36]. The previous studies also suggested that the subducting Pacific slab significantly affects inland deformation [36]. There are two possible origins of fluids in the lower crust along the NKTZ: shallow fluids such as meteoric water, pore fluids and mineral dehydration in the crust [47], and fluids of deep origin such as dehydration of the subducting oceanic plate [44,48]. Our tomographic images show that the lowvelocity and high-Poisson's ratio anomalies below the source area are closely related to the subducting Pacific slab (Fig. 9b). We, thus, conclude that these anomalous features of the low-velocity and high-Poisson's ratio and the high conductivity in and below the source area are mainly caused by fluids associated with dehydration of the subducting Pacific slab. 5.4. Interpretations An obvious change in Vp and Vs is visible across the faults oriented from SW to NE (Fig. 7); this may reflect lithological changes across the fault zone. Surface geological structures above the source region are different across the Shibata–Koide Tectonic Line (SKTL; Fig. 1b). Southeast of the tectonic line, indurated preNeogene rocks outcrop, while on the northwest side there are deep sedimentary basins (N 6 km in thickness) that might have formed over half grabens related to crustal extension. The tomographic images (Fig. 8) show high-velocity and low-Poisson's ratio on the southeast side of the SKTL, and low-velocity and highPoisson's ratio northwest of SKTL; this is consistent with the surface geological features [49]. Similar fea-
tures are delineated by a previous study [2], showing good consistence with our tomographic images at the depths of 5–15 (Fig. 8). Previous tomographic studies show that many large crustal earthquakes occur in or around zones of low seismic velocity, including the 1995 Kobe earthquake (M 7.2) [5] and the 2000 Tottori earthquake (M 7.3) [48]. Hauksson and Haase [50] showed that four earthquakes (M N 5.9) occurred in or close to highvelocity zones in the Los Angeles basin area, although the earthquake ruptures were actually located in or near the boundary between high- and low-velocity zones, similar to our tomographic results (Figs. 8 and 9). Fluids in the lower crust below the source area perhaps reduce the seismic velocity of surrounding crust and cause the overlying brittle seismogenic layer to become locally thinner and weaker. The previous studies [5,51] reported that main rupture zones and aftershocks are mostly located in high velocity (i.e. high rigidity) regions. Our tomographic results (Figs. 8 and 9) also demonstrate that most of the aftershocks were located in the high-velocity and low-Poisson's ratio zone in the source area. According to Kumar et al. [52], shear stress in the source region is larger than in the surrounding areas. We consider that the aftershock sequence might be triggered by shear stress accumulated in the low-velocity, highPoisson's ratio and high conductivity zones around the source area that contains fluids. Our tomographic results (Fig. 9b) indicate that the crustal weakening is closely related to the dehydration of the subducting slab in this region. Spatial and temporal variations in the crustal stress field have been reported for the source areas of the Kobe earthquake (M 7.2) [53] and the 1994 Northridge earthquake (M 6.7) in southern California [54], both of which are interpreted to be associated with fluids in the fault zones. The regional east– west compressional stress field causes strong crustal deformation in the Niigata sedimentary basin [55]. The large deformation rate may result from aseismic slip of many ductile faults in the lower crust. In addition, plastic lower crust can be deformed by ductile flow due to regional tectonic stress resulting from plate motions. Deformation in the lower crust acts to increase stress in areas with abrupt lateral variations in the mechanical strength of the seismogenic crust [45]. The mainshock was located in or near the junction between low-velocity and high-velocity zones (Figs. 8 and 9) where the mechanical strength of materials is much weaker than that in normal sections of the seismogenic layer due to the penetration of fluids from the lower crust. Thus the margin of a crustal block is an ideal location to generate large earthquakes that produce faults that reach to the
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Earth's surface or blind faults within the brittle upper crust [37,44]. The strong heterogeneities in the lower crust and uppermost mantle in and around the source area (Figs. 8 and 9a) might have influenced the nucleation and rupture process of the 2004 Niigata earthquake. Over-pressurized fluid-filled fractures may play a role in the process of fault-valve behavior and eventually in the occurrence of large earthquakes [45]. The low-Vs and high-σ zone, coinciding with the high conductive zone [1], below the mainshock hypocenter might reflect highly fractured rock and fluid intrusion from the deep crust and uppermost mantle. Such an anomaly in velocity, Poisson's ratio and conductivity might potentially have initiated the mainshock rupture. The fluids reduced the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2004 Niigata mainshock. 6. Conclusions Detailed 3-D seismic velocity (Vp, Vs) and Poisson's ratio (σ) images in and around the source area of the 2004 Niigata earthquake were determined by using a larger number of high-quality P- and S-wave arrival times. Most of the aftershocks occurred in areas with low-σ that may indicate brittle and competent patches of the fault zone. An obvious change in seismic velocity and Poisson's ratio is clearly imaged in the shallow crust, consistent with surface geological features. Lateral heterogeneities are revealed in the lower crust and uppermost mantle in the source area. A strong low-Vs and high-σ anomaly zone is revealed in the lower crust under the mainshock hypocenter, which may reflect the presence of fluids in the source area. The anomalous zone in and below the seismogenic layer might be caused by the presence of fluids, which acted to reduce the strength of the crust and decrease the seismic velocity. Our tomographic results indicate that the fluids are closely related to the dehydration of the subducting Pacific slab. Such fluids may have reduced the mechanical strength of the fractured rock matrix and thus triggered the 2004 Niigata earthquake. Acknowledgements We thank Prof. S. King and an anonymous reviewer for their constructive comments and suggestions. We thank the Hi-net and J-Array data center for providing us P and S arrival time data and waveform data via Internet. Some figures were made by using GMT (Generic Mapping Tools) software [56]. This work was partially
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