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Seismic imaging of arc magma and fluids under the central part of northeastern Japan
Tectonophysics 341 (2001) 1 – 17 www.elsevier.com/locate/tecto
Seismic imaging of arc magma and fluids under the central part of northeastern Japan Junichi Nakajimaa,* , Toru Matsuzawaa, Akira Hasegawaa, Dapeng Zhaob a
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 21 November 2000; accepted 9 July 2001
Abstract A dense seismic network was temporarily deployed during 1997 – 1999 in the central part of northeastern Japan by Japanese university research groups. Using the local earthquake arrival time data recorded by the network, we have determined highresolution three-dimensional images of P-wave velocity (Vp), S-wave velocity (Vs) and Vp/Vs ratio in this area. Our results show that low Vp, low Vs and high Vp/Vs areas are extensively distributed in the uppermost mantle along the volcanic front. In the lower crust, low Vp, low Vs and high Vp/Vs areas are visible but they are confined to the individual volcanic areas. In contrast, the upper crust of volcanic areas shows low Vp, low Vs and low Vp/Vs rather than high Vp/Vs. These observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas. Low Vp, low Vs and low Vp/Vs in the upper crust of the volcanic areas suggest the presence of H2O (rather than melt) there. Deep low-frequency microearthquakes, perhaps caused by the rapid movement of fluids, are located mostly at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low Vp/Vs, suggesting that most of them are filled with H2O. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Northeastern Japan; Travel-time tomography; Seismic velocity structure; Active volcano; Fluids
1. Introduction In the northeastern (NE) Japan subduction zone, shallow crustal seismicity in the overriding plate is relatively more active along the central mountain range, the Ou backbone range, which is the largest
*
Corresponding author. Fax: +81-22-264-3292. E-mail address: [email protected] (J. Nakajima).
mountain chain in NE Japan and runs in parallel to the Japan Trench. The range contains many active volcanoes that are distributed at nearly regular intervals and form a long volcanic chain, the volcanic front, in parallel to the trench axis. In the central part of NE Japan, the Ou backbone range is bounded on both the east and west by reverse-type active faults (Fig. 1). The 1896 Rikuu earthquake (M7.2), shown by a open star in Fig. 1, occurred along Senya Fault on the western border of the range, and caused severe damage to the area. Since then, three earthquakes with a
0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 1 8 1 - 0
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Fig. 1. Map showing the study area, the central part of northeastern Japan. Black triangles and solid lines show active volcanoes and active faults (Active Fault Research Group, 1991), respectively. Topography is also shown by black and white scale.
magnitude of 6 have occurred along the active faults in this area including a recent M6.1 earthquake near Iwate volcano. Shallow seismicity is also relatively active near the coastline of the Japan Sea due to the reverse-type active faults striking in the north – south direction. These shallow earthquakes are confined to the upper 15 km of the brittle seismogenic zone, the portion below it being governed by creep or flows (Hasegawa et al., 2000). During October 1997 to June 1999, intensified seismic observation campaigns for both active and passive source experiments were carried out by Japa-
nese university research groups in the central part of NE Japan. The purpose of these experiments was to investigate the crustal structure in detail for a better understanding of seismotectonics and the deformation process of the overriding plate in this region (Hasegawa and Hirata, 1999). Many temporary stations were installed in the central part of NE Japan as a part of the Joint Seismic Observations. Seismic signals at each station are transmitted by a satellite communication telemetry system and collected at Research Center for Prediction of Earthquakes and Volcanic Eruptions (RCPEV) of Tohoku University. The continuously
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recorded waveform data from the temporary stations are processed by the automatic processing system at RCPEV together with those of the permanent stations of Tohoku University and Japan Meteorological Agency (JMA). Thus, we have 96 stations covering the central part of NE Japan with a station separation of 10 – 20 km. Fig. 2 shows the distribution of the temporary stations as well as the permanent network stations of Tohoku University and JMA. Recently, we have tried to determine more accurate three-dimensional (3D) P- and S-wave velocity structures for the entire NE Japan by using data recorded at 230 stations (Nakajima et al., 2001b). The spatial resolution is about 25 km in the horizontal direction and 10– 30 km in depth. In order to image the heterogeneous structure under the volcanic front and magma chambers under each of active volcano in
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detail, in this work, we have tried to determine the 3D velocity structure under the central portion of NE Japan with an even higher resolution ( 12 km) because a much denser seismic network is available there. The 3D velocity model for the whole NE Japan (Nakajima et al., 2001b) was adopted as the structure outside the present study area. This enabled us to use rays outside the study area in addition to those within it, which provided a much better ray coverage. The present study has the following advantages over the previous studies of Hasemi et al. (1984), Obara et al. (1986) and Zhao et al. (1992). First, we used travel time data recorded at a much denser seismic network. In the central part of NE Japan, 96 seismic stations had been installed during October 1997 to June 1999 in contrast with 23 stations in Zhao et al. (1992). Second, because the number of earthquakes used in this study is considerably increased and rays, particularly those for S-wave, cover the study area much more densely. This enabled us to discuss Vp/Vs structure in detail. Third, we adopted the three-dimensional velocity model for the whole NE Japan (Nakajima et al., 2001b) as the structure outside the present study area. This enabled us to use rays outside the study area as mentioned above. Because of these advantages, we could estimate 3D velocity structure in the central part of NE Japan with much higher resolution, particularly for S-wave.
2. Data and method
Fig. 2. Map showing the locations of seismic stations deployed in the study area. Black squares, gray circles and crosses denote permanent seismic stations of Tohoku University (THK), those of Japan Meteorological Agency (JMA) and temporary seismic stations installed by Japanese national research groups as a part of the Joint Seismic Observations (JSO), respectively.
We used P- and S-wave arrival time data from shallow and intermediate-depth earthquakes that occurred in NE Japan in the depth range of 0– 250 km during a period from October 1997 to July 1999. Fig. 3 shows the hypocenter distribution of these earthquakes located by the seismic network of Tohoku University. The accuracy in identifying P-wave first arrivals is estimated to be about 0.1 s. The uncertainty for S-wave first arrivals is slightly larger, being 0.1– 0.2 s. We used a total of 230 seismic stations in NE Japan, including those located outside the present study area (Nakajima et al., 2001b). We used the tomographic method developed by Zhao et al. (1992). A 3D grid net was set up in the model space to express the 3D velocity structure. Velocities at every grid nodes are taken as unknown
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Fig. 3. Hypocenter distribution of 4338 shallow and intermediate-depth earthquakes used in this study. Hypocenters are shown for areas of (a) 36°N – 37°N, (b) 37°N – 38°N, (c) 38°N – 39°N, (d) 39°N – 40°N, (e) 40°N – 41°N and (f ) 41°N – 42°N.
parameters. The velocity at any point in the model is calculated using a linear interpolation of the velocities at eight grid nodes surrounding that point. The tomographic method of Zhao et al. (1992) can deal with a general velocity model in which complexly shaped
velocity discontinuities exist. In this work, we have taken into account three seismic discontinuities, the Conrad, the Moho and the upper boundary of the subducted Pacific slab, in the model space. The three discontinuities are not simple flat planes but have
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complicated geometries (Horiuchi et al., 1982a,b; Hasegawa et al., 1983; Matsuzawa et al., 1986, 1990; Zhao et al., 1990, 1997; Nakajima et al., 2001a). We adopted the depth distributions of the Conrad discontinuity and the upper slab boundary estimated by Zhao et al. (1990) and Hasegawa et al. (1983), respectively, which were used also in the inversions by Zhao et al. (1992). For the Moho discontinuity, we adopted its depth distribution (Fig. 4) recently obtained by Nakajima et al. (2001a) using reflected waves at the Moho (PmP and SmS) from shallow earthquakes and S-to-P (SP) and P-to-S (PS)
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converted waves at the Moho from intermediate-depth earthquakes. We consider the newly determined Moho geometry is more reliable than that by Zhao et al. (1990) who used refracted waves at the Moho, though the two results are quite consistent with each other. In the present study, we have tried to use two sets of data. One contains the rays that are entirely within the study area (the central part of NE Japan). The other contains the rays having a part of the ray path located outside the study area but within NE Japan. Thus, a better ray coverage may be achieved for the target area. For the area surrounding the central part of
Fig. 4. Depth distribution of the Moho discontinuity estimated by Nakajima et al. (2001a) using travel time data of reflected waves at the Moho (PmP and SmS) from crustal earthquakes and those of converted waves at the Moho (PS and SP) from intermediate-depth events. The depth to the Moho is shown by contour lines at an interval of 1 km. Dashed lines are error contours with 0.5 km interval.
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NE Japan, we adopted the 3D P- and S-wave velocity models estimated for the whole NE Japan with a grid spacing of 0.25° in horizontal and 10 –30 km in depth
(Nakajima et al., 2001b). The second data set has a possibility to lead to errors inside the study area if the velocity structure for wider area has not been estimated precisely. In the present case, however, we consider that no serious errors propagate inside the study area, since earthquakes and stations we used are in the area of 36°N – 42°N, 138.5°E – 142.25°E and 0– 200 km in depth, for which both P- and S-wave velocity structures were estimated with high resolution by Nakajima et al. (2001b). Then we estimated the 3D P- and S-wave velocity structures for the central part of NE Japan with a smaller grid spacing. The whole NE Japan 3D model was used as the starting model for the inversion. The distribution of grid net adopted in this inversion is shown in Fig. 5. Grid spacing in the present inversion is 0.125° in horizontal and 5 – 15 km in depth. The present target area is 38.5°N –40.0°N, 140.0°E –141.5°E and depth range of 0 – 40 km (Fig. 5). The number of rays intersecting the study area is 78,686 for P-wave and 54,284 for S-wave.
3. Results
Fig. 5. Configuration of the grid net adopted in the tomographic inversions. (a) Map view, and (b) EW vertical cross-section. Solid lines in the vertical cross-section show seismic velocity discontinuities. The rectangles in (a) and (b) show the present study area.
A converged solution was obtained after six iterations. The root mean square (rms) residual was reduced from 0.214 to 0.206 s for P-wave and from 0.437 s to 0.426 s for S-wave. Figs. 6 and 7 show Pand S-wave velocity perturbations obtained together with active volcanoes. The velocity perturbation is from the average value of the inverted velocities in each layer (Nakajima et al., 2001b). The velocity images at 0 km depth mainly reflect the near-source features such as the sediments just beneath the seismic stations, and we do not discuss them here. We can see that the spatial distributions of P- and S-wave velocities are very similar to each other, although the amplitudes of S-wave velocity anomalies are generally larger than those of P-wave. The velocity images at the second layer (5 km depth) show that lowvelocity anomalies spread out along the Senya Fault, to the west of it, and beneath the southern volcanic area, while high-velocity anomalies are distributed beneath Kitakami Mountain range for both P- and S-waves. In the third layer (10 km depth), lowvelocity anomalies are distributed locally around the northern, southern and western volcanic areas. They
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Fig. 6. P-wave velocity perturbations at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. Circles and crosses represent low and high velocities, respectively. Solid triangles denote active volcanoes.
are more widely distributed around the volcanic areas in the lower crust (25-km depth). In the uppermost mantle (40-km depth), low-velocity anomalies are
extensively distributed along the volcanic front, especially in S-wave velocity images. These features are, on the whole, similar to the results for the whole NE
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Fig. 7. S-wave velocity perturbations at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. The symbols are the same as those in Fig. 6.
Japan by Nakajima et al. (2001b), but the images presently obtained have a much higher spatial resolution than those by Nakajima et al. (2001b).
Figs. 8 and 9 show the results of checkerboard resolution test (CRT) for P- and S-waves, respectively. We assigned positive and negative velocity perturba-
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Fig. 8. Results of checkerboard resolution test for P-wave velocity structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km.
tions of ± 3% alternately to the grid nodes, and calculated travel times for this model to make synthetic data. We added random noises corresponding to picking errors to the synthetic data: 0.1 s for P-wave and 0.2 s for S-wave. The synthetic data thus made were inverted with an initial model of zero velocity perturbations. The results of CRT for P-wave are excellent (Fig. 8). Fig. 9 shows that the resolution for S-wave is inferior to that for P-wave. However, the checkerboard patterns are recovered on the whole.
Especially, the resolution in the middle to the eastern part of the study area is fairly good. This improved resolution is due to the better ray coverage for this part. We also carried out a restoring resolution test (RRT) (Zhao et al., 1992). The results of RRT are very good for areas where CRT was well resolved. Fig. 10 shows the spatial distribution of Vp/Vs ratio, which is calculated from P- and S-wave velocity structures shown in Figs. 6 and 7. This result has almost the same spatial pattern as that for the whole
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Fig. 9. Results of checkerboard resolution test for S-wave velocity structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km.
NE Japan by Nakajima et al. (2001b). In the present results, however, we have imaged smaller-scale heterogeneities in the crust and uppermost mantle. At 5km depth, low Vp/Vs areas are extensively distributed in regions including the volcanic front, particularly beneath the northern and southern volcanic areas, while high Vp/Vs areas are distributed in the western and eastern parts of the study area. At 10-km depth, high Vp/Vs areas are distributed in more confined areas, and the northern and southern volcanic areas
have low Vp/Vs values. A high Vp/Vs area spreads out beneath Mt. Hayachine, whose location is shown in Fig. 1, and to the south of it at 5-km depth. At 10-km depth, this high Vp/Vs area is located 20 km to the south of it. The center of this area at 0 km depth lies on Mt. Hayachine. Ultrabasic rocks mostly serpentinized are exposed at the surface in an area of 4 20 km centering at Mt. Hayachine (Geological Survey of Japan, 1992). This spatial relationship suggest that the high Vp/Vs area is perhaps caused by rocks composed
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Fig. 10. Vp/Vs structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. Crosses and circles represent low and high Vp/Vs ratios, respectively. Solid triangles denote active volcanoes.
of serpentine which is known to have low Vs and high Vp/Vs (e.g., Christensen, 1996). We suppose from Fig. 10 that the root of serpentinized rocks is inclined toward the south in the upper crust. In the lower crust (25-km depth), high Vp/Vs areas are distributed in and around the volcanic areas. They are distributed extensively along the volcanic front in the uppermost mantle (40-km depth). Similar features of Vp/Vs ratio to the present study are also seen in the result recently
obtained by Sato (2000) based on the block inversion method of Aki and Lee (1976).
4. Discussion Fig. 11 shows vertical cross-sections of Vp, Vs and Vp/Vs images we obtained. P- and S-wave velocity perturbations and Vp/Vs ratio are shown in the upper,
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Fig. 11. Vertical cross-sections of (top) P-wave, (middle) S-wave velocity perturbations and (bottom) Vp/Vs ratio along lines A – A0, B – B0, C – C0, D – D0, E – E0 and F – F0 in the inserted map. Red colors show low velocity and high Vp/Vs; blue colors show high velocity and low Vp/Vs. Red triangles and black bars on the top of each figure show active volcanoes and active faults (Active Fault Research Group, 1991), respectively. Dots and red circles show shallow microearthquakes and low-frequency microearthquakes (Okada and Hasegawa, 2000), respectively. Red lines denote distinct S-wave reflectors detected by Hori et al. (1999). Black lines denote seismic velocity discontinuities.
middle and lower figures, respectively. Distinct Swave reflectors detected in the midcrust (Hori et al., 1999), deep low-frequency microearthquakes detected in the lowermost crust and the uppermost mantle (Okada and Hasegawa, 2000), and ordinary shallow microearthquakes in the upper crust are also shown. We can see from these figures that remarkable lowvelocity anomalies for both Vp and Vs are distributed beneath active volcanoes continuously from the uppermost mantle to the upper crust in the cross-sections along lines A – A0, C –C0 and D – D0, which pass through the volcanic areas (Fig. 11a,c,d). On the contrary, continuously distributed low-velocity anomalies are not visible in the cross-section along line B –B0, which does not intersect the volcanic areas (Fig. 11b). These characteristic features can be clearly seen in the NS vertical cross-sections along the volcanic front (lines E – E0 and F– F0; Fig. 11e,f).
The upper crust exhibits, on the whole, low Vp/Vs values as mentioned in the previous section. In all the vertical cross-sections shown, high Vp/Vs areas exist in the uppermost mantle along the volcanic front. However, high Vp/Vs areas in the lower crust are distributed only beneath active volcanoes. We investigated the effects of fluid-filled inclusions in rocks on seismic velocities using the model of Yamamoto et al. (1981) and found that the low Vp, low Vs and high Vp/Vs anomalies were perhaps caused by inclusions containing melt with a small aspect ratio (Nakajima et al., 2001b). The present results suggest that partial melting materials are distributed extensively in the uppermost mantle just beneath the Moho along the volcanic front and they rise up locally to the lower crust just beneath active volcanoes. The low Vp, low Vs and low Vp/Vs anomalies in the upper crust can be explained by the presence of inclusions filled with H2O with a large
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Fig. 11 (continued ).
aspect ratio (Nakajima et al., 2001b). Partial melting areas may not exist in the upper crust beneath active volcanoes, but areas with much H2O may exist there. A schematic vertical cross-section of the structure beneath the active volcanoes is shown in Fig. 12. Note that smaller-scale partial melting areas may exist but may not be imaged because of the limited spatial resolution. North – south vertical cross-sections of Vp, Vs and Vp/Vs along the volcanic front (Fig. 11e,f ) clearly show low Vp, very low Vs and very high Vp/Vs in the lower crust and the uppermost mantle beneath active volcanoes. The upper crust beneath active volcanoes has low Vp, low Vs and low Vp/Vs instead of high Vp/Vs. Shallow earthquakes are distributed in low Vp and low Vs anomalies as well as in high Vp and high Vs anomalies. Comparison of their hypocenter locations with the Vp/Vs structure shows that most of them occur in low Vp/Vs areas. This is mainly because shallow earthquakes occur at 10 km depth or so, close to the bottom of the seismogenic upper crust, and Vp/Vs values are low in most of the
areas at this depth (Fig. 10b). Some clustered earthquakes are seen in low Vp, low Vs and low Vp/Vs anomalies in the upper crust (Fig. 11). Low Vp, low Vs and low Vp /Vs anomalies are perhaps caused by inclusions of H2O, as mentioned above. This may suggest that some clustered earthquakes in such anomalous areas may be triggered by the migration or diffusion of H2O. Anomalously low-frequency microearthquakes have been detected in and around low Vp areas of the lowermost crust and the uppermost mantle beneath active volcanoes (Hasegawa et al., 1991). They occur at depths where rheological properties of rocks are ductile, well below the brittle seismogenic zone of the upper crust. They have extremely low predominant frequencies for both P- and S-waves compared with ordinary microearthquakes in the upper crust, and do not have simple double-couple focal mechanisms. Hasegawa and Yamamoto (1994) suggested that these anomalous events were generated by the magmatic activity of mantle diapirs in the mantle wedge. Fig. 11 shows that they are distributed in and around low Vp
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Fig. 11 (continued ).
zones, as Hasegawa et al. (1991) pointed out. Moreover, the comparison with the Vp/Vs structure in Fig. 11 shows that many low-frequency microearthquakes occur near the upper surface of high Vp/Vs areas where both P- and S-wave velocities are low. Based on these observations, we believe that the low-frequency events are most possibly caused by H2O migration originated from low Vp, low Vs and high Vp/Vs partial melting areas (Fig. 12). In NE Japan, distinct S-wave reflectors (bright spots) with very large reflection coefficients have been detected in the midcrust beneath active volcanoes (Mizoue et al., 1982; Horiuchi et al., 1988; Hasegawa et al., 1991; Hori and Hasegawa, 1991). These reflectors have the same features as the magma body detected in the Rio Grand rift near Socorro, NM (Sanford et al., 1973, 1977; Reinhart et al., 1979; Brown et al., 1980; Balch et al., 1997). Recently, similar S-wave reflectors in the midcrust have been detected not only beneath active volcanoes but also in many other regions where active volcanoes do not exist (Hori et al., 1999). Anomalously large reflection coefficients for S-waves indicate that the reflector bodies are filled with fluids. Matsumoto and Hasegawa (1996) studied in detail an S-wave reflector in the midcrust beneath Nikko-Shirane volcano located
in the south part of NE Japan. Based on spectral ratio analyses of the direct and reflected S-waves, they found that the midcrustal S-wave reflector body had an area extent of 15 15 km and had a thickness of only about 100 m and a very low rigidity. These observations show that the reflector is a thin magma body and perhaps acts as a temporary storage of magma at the midcrustal level. Most of the S-wave reflectors detected by Hori et al. (1999) in the central part of NE Japan (red lines in Fig. 11) are distributed in low Vp/Vs areas, some of which are low Vp and low Vs and others not. Moreover, their dimensions are not so large as that beneath Nikko-Shirane volcano. We infer, based on these observations, that most of the reflector bodies detected by Hori et al. (1999) in the present study area are not filled with melt but with H2O. The S-wave reflectors detected are distributed mainly along the volcanic front (Hori et al., 1999). Considering the presence of extensive partial melting areas (low Vp, low Vs and high Vp/Vs) in the uppermost mantle along the volcanic front, we further infer that H2O forming the Swave reflectors in the midcrust are originated from these partial melting materials. The close relationship between the seismic velocity structure presently obtained and other seismolog-
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Fig. 12. Schematic illustration of the anomalous structure in the crust and the uppermost mantle beneath active volcanoes. (a) Across-arc vertical cross-section of NE Japan from Hasegawa et al. (1994, Fig. 12). (b) Cross-section updated by this study but for the portion enclosed by broken lines in (a).
ical observations as described in this section indicates that extensive melting areas are distributed in the uppermost mantle along the volcanic front beneath the central part of NE Japan, and a portion of partial melting materials rises up and intrudes locally into the lower crust in and around active volcanoes. Partial melting materials attained the lower crustal level and H2O expelled from them due to the fall in an ambient temperature would form fluid chambers, some of which are detected as S-wave reflectors. H2O generated from the dehydration of the partial melting materials at the lower crustal level could trigger the low-frequency microearthquakes. A schematic illustration of the anoma-
lous structure beneath active volcanoes is shown in Fig. 12.
5. Conclusions We estimated high-resolution, three-dimensional Vp, Vs and Vp/Vs images in the central part of northeastern Japan, where a very dense seismic network was temporarily deployed. Our results have revealed an anomalous structure in the crust and uppermost mantle beneath the active volcanoes in this area. Low Vp and low Vs anomalies are extensively distributed in the uppermost mantle along the volcanic front and
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spread out up to the crust beneath active volcanoes. They attain to the surface where active volcanoes are located. These low-velocity anomalies have high Vp/Vs values in the uppermost mantle and lower crust. However, those in the upper crust beneath active volcanoes show low Vp/Vs rather than high Vp/Vs. These characteristic features of the velocity structures are probably caused by the concentrated distribution of fluids, such as melts and/or H2O, in the crust beneath active volcanoes and the uppermost mantle along the volcanic front. Ordinary shallow earthquakes are distributed mainly in the areas with low Vp/Vs. Deep low-frequency microearthquakes are located at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low Vp /Vs, suggesting that most of them are filled with H2O. Acknowledgements We appreciate the fruitful discussions with T. Yoshida, M. Wyss, I.S. Sacks and N. Umino and all the members at Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University. We are also grateful to the members who have participated in the 1997 –1999 Joint Seismic Observations in Tohoku for their efforts in the field work. We also thank the staffs of JMA for allowing us to use some of the data recorded by their seismic network. We benefited from the comments of two anonymous referees provided thoughtful reviews, which improved the manuscript. All the figures in this paper are plotted using the GMT (Wessel and Smith, 1995). This research was partially supported by a grant from the Ministry of Education, Science and Culture of Japan.
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J. Nakajima et al. / Tectonophysics 341 (2001) 1–17 Matsuzawa, T., Umino, N., Hasegawa, A., Takagi, A., 1986. Upper mantle velocity structure estimated from PS-converted wave beneath the north-eastern Japan arc. Geophys. J. R. Astron. Soc. 86, 767 – 787. Matsuzawa, T., Kono, T., Hasegawa, A., Takagi, A., 1990. Subducting plate boundary beneath the northeastern Japan arc estimated from SP converted waves. Tectonophysics 181, 123 – 133. Mizoue, M., Nakamura, I., Yokota, T., 1982. Mapping of an unusual crustal discontinuity by microearthquake reflections in the earthquake swarm area near Ashio, northwestern part of Tochigi Prefecture, central Japan. Bull. Earthquake Res. Inst., Univ. Tokyo 57, 653 – 686. Nakajima, J., Matsuzawa, T., Hasegawa, A., 2001a. Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Phys. Earth Planet. Inter. submitted. Nakajima, J., Matsuzawa, T., Hasegawa, A., Zhao, D., 2001b. Three-dimensional structure of Vp, Vs and Vp/Vs beneath the northeastern Japan arc: implications for arc magmatism and fluids. J. Geophys. Res., in press. Obara, K., Hasegawa, A., Takagi, A., 1986. Three-dimensional P and S wave velocity structure beneath the northeastern Japan arc (in Japanese with English abstract). J. Seismol. Soc. Jpn. 39, 201 – 215. Okada, T., Hasegawa, A., 2000. Activity of deep low-frequency microearthquakes and their moment tensor in northeastern Japan (in Japanese with English abstract). Bull. Volcanol. Soc. Jpn. 45, 47 – 63. Rinehart, E.J., Sanford, A.R., Ward, R.M., 1979. Geographic extent and shape of an extensive magma body at mid-crustal depths in
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the Rio Grande rift near Socorro, New Mexico. In: Riecher, R. (Ed.), Rio Grande Rift: Tectonics and Magmatism. AGU, Washington, DC, pp. 237 – 251. Sanford, A.R., Alptekin, O., Toppozada, T.R., 1973. Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande Rift. Bull. Seismol. Soc. Am. 63, 2021 – 2034. Sanford, A.R., Mott Jr., R.P., Shuleski, P.J., Rinehart, E.J., Caravella, F.J., Ward, R.M., Wallace, T.C., 1977. Geophysical evidence for a magma body in the crust in the vicinity of Socorro, New Mexico. In: Heacock, J.G. (Ed.), The Earth’s Crust. Geophys. Monogr. vol. 20. AGU, Washington, DC, pp. 385 – 403. Sato, S., 2000. Seismic velocity structures in the central part of the Tohoku District inferred from travel time data recorded at dense seismic network (in Japanese). MS thesis, Hirosaki Univ., Hirosaki, Japan, 70 pp. Wessel, P., Smith, W.H.F., 1995. New version of the generic mapping tools released. EOS Trans. AGU 76, 329. Yamamoto, K., Kosuga, M., Hirasawa, T., 1981. A theoretical method for determination of effective elastic constants of isotorophic composites. Sci. Rep. Tohoku Univ. 28, 47 – 67. Zhao, D., Horiuchi, S., Hasegawa, A., 1990. 3-D seismic velocity structure of the crust and the uppermost mantle in the northeastern Japan arc. Tectonophysics 181, 135 – 149. Zhao, D., Hasegawa, A., Horiuchi, S., 1992. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909 – 19928. Zhao, D., Matsuzawa, T., Hasegawa, A., 1997. Morphology of the subducting slab boundary in the northeastern Japan arc. Phys. Earth Planet Inter. 102, 89 – 104.
Seismic imaging of arc magma and fluids under the central part of northeastern Japan Junichi Nakajimaa,* , Toru Matsuzawaa, Akira Hasegawaa, Dapeng Zhaob a
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 21 November 2000; accepted 9 July 2001
Abstract A dense seismic network was temporarily deployed during 1997 – 1999 in the central part of northeastern Japan by Japanese university research groups. Using the local earthquake arrival time data recorded by the network, we have determined highresolution three-dimensional images of P-wave velocity (Vp), S-wave velocity (Vs) and Vp/Vs ratio in this area. Our results show that low Vp, low Vs and high Vp/Vs areas are extensively distributed in the uppermost mantle along the volcanic front. In the lower crust, low Vp, low Vs and high Vp/Vs areas are visible but they are confined to the individual volcanic areas. In contrast, the upper crust of volcanic areas shows low Vp, low Vs and low Vp/Vs rather than high Vp/Vs. These observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas. Low Vp, low Vs and low Vp/Vs in the upper crust of the volcanic areas suggest the presence of H2O (rather than melt) there. Deep low-frequency microearthquakes, perhaps caused by the rapid movement of fluids, are located mostly at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low Vp/Vs, suggesting that most of them are filled with H2O. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Northeastern Japan; Travel-time tomography; Seismic velocity structure; Active volcano; Fluids
1. Introduction In the northeastern (NE) Japan subduction zone, shallow crustal seismicity in the overriding plate is relatively more active along the central mountain range, the Ou backbone range, which is the largest
*
Corresponding author. Fax: +81-22-264-3292. E-mail address: [email protected] (J. Nakajima).
mountain chain in NE Japan and runs in parallel to the Japan Trench. The range contains many active volcanoes that are distributed at nearly regular intervals and form a long volcanic chain, the volcanic front, in parallel to the trench axis. In the central part of NE Japan, the Ou backbone range is bounded on both the east and west by reverse-type active faults (Fig. 1). The 1896 Rikuu earthquake (M7.2), shown by a open star in Fig. 1, occurred along Senya Fault on the western border of the range, and caused severe damage to the area. Since then, three earthquakes with a
0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 1 ) 0 0 1 8 1 - 0
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Fig. 1. Map showing the study area, the central part of northeastern Japan. Black triangles and solid lines show active volcanoes and active faults (Active Fault Research Group, 1991), respectively. Topography is also shown by black and white scale.
magnitude of 6 have occurred along the active faults in this area including a recent M6.1 earthquake near Iwate volcano. Shallow seismicity is also relatively active near the coastline of the Japan Sea due to the reverse-type active faults striking in the north – south direction. These shallow earthquakes are confined to the upper 15 km of the brittle seismogenic zone, the portion below it being governed by creep or flows (Hasegawa et al., 2000). During October 1997 to June 1999, intensified seismic observation campaigns for both active and passive source experiments were carried out by Japa-
nese university research groups in the central part of NE Japan. The purpose of these experiments was to investigate the crustal structure in detail for a better understanding of seismotectonics and the deformation process of the overriding plate in this region (Hasegawa and Hirata, 1999). Many temporary stations were installed in the central part of NE Japan as a part of the Joint Seismic Observations. Seismic signals at each station are transmitted by a satellite communication telemetry system and collected at Research Center for Prediction of Earthquakes and Volcanic Eruptions (RCPEV) of Tohoku University. The continuously
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recorded waveform data from the temporary stations are processed by the automatic processing system at RCPEV together with those of the permanent stations of Tohoku University and Japan Meteorological Agency (JMA). Thus, we have 96 stations covering the central part of NE Japan with a station separation of 10 – 20 km. Fig. 2 shows the distribution of the temporary stations as well as the permanent network stations of Tohoku University and JMA. Recently, we have tried to determine more accurate three-dimensional (3D) P- and S-wave velocity structures for the entire NE Japan by using data recorded at 230 stations (Nakajima et al., 2001b). The spatial resolution is about 25 km in the horizontal direction and 10– 30 km in depth. In order to image the heterogeneous structure under the volcanic front and magma chambers under each of active volcano in
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detail, in this work, we have tried to determine the 3D velocity structure under the central portion of NE Japan with an even higher resolution ( 12 km) because a much denser seismic network is available there. The 3D velocity model for the whole NE Japan (Nakajima et al., 2001b) was adopted as the structure outside the present study area. This enabled us to use rays outside the study area in addition to those within it, which provided a much better ray coverage. The present study has the following advantages over the previous studies of Hasemi et al. (1984), Obara et al. (1986) and Zhao et al. (1992). First, we used travel time data recorded at a much denser seismic network. In the central part of NE Japan, 96 seismic stations had been installed during October 1997 to June 1999 in contrast with 23 stations in Zhao et al. (1992). Second, because the number of earthquakes used in this study is considerably increased and rays, particularly those for S-wave, cover the study area much more densely. This enabled us to discuss Vp/Vs structure in detail. Third, we adopted the three-dimensional velocity model for the whole NE Japan (Nakajima et al., 2001b) as the structure outside the present study area. This enabled us to use rays outside the study area as mentioned above. Because of these advantages, we could estimate 3D velocity structure in the central part of NE Japan with much higher resolution, particularly for S-wave.
2. Data and method
Fig. 2. Map showing the locations of seismic stations deployed in the study area. Black squares, gray circles and crosses denote permanent seismic stations of Tohoku University (THK), those of Japan Meteorological Agency (JMA) and temporary seismic stations installed by Japanese national research groups as a part of the Joint Seismic Observations (JSO), respectively.
We used P- and S-wave arrival time data from shallow and intermediate-depth earthquakes that occurred in NE Japan in the depth range of 0– 250 km during a period from October 1997 to July 1999. Fig. 3 shows the hypocenter distribution of these earthquakes located by the seismic network of Tohoku University. The accuracy in identifying P-wave first arrivals is estimated to be about 0.1 s. The uncertainty for S-wave first arrivals is slightly larger, being 0.1– 0.2 s. We used a total of 230 seismic stations in NE Japan, including those located outside the present study area (Nakajima et al., 2001b). We used the tomographic method developed by Zhao et al. (1992). A 3D grid net was set up in the model space to express the 3D velocity structure. Velocities at every grid nodes are taken as unknown
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Fig. 3. Hypocenter distribution of 4338 shallow and intermediate-depth earthquakes used in this study. Hypocenters are shown for areas of (a) 36°N – 37°N, (b) 37°N – 38°N, (c) 38°N – 39°N, (d) 39°N – 40°N, (e) 40°N – 41°N and (f ) 41°N – 42°N.
parameters. The velocity at any point in the model is calculated using a linear interpolation of the velocities at eight grid nodes surrounding that point. The tomographic method of Zhao et al. (1992) can deal with a general velocity model in which complexly shaped
velocity discontinuities exist. In this work, we have taken into account three seismic discontinuities, the Conrad, the Moho and the upper boundary of the subducted Pacific slab, in the model space. The three discontinuities are not simple flat planes but have
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complicated geometries (Horiuchi et al., 1982a,b; Hasegawa et al., 1983; Matsuzawa et al., 1986, 1990; Zhao et al., 1990, 1997; Nakajima et al., 2001a). We adopted the depth distributions of the Conrad discontinuity and the upper slab boundary estimated by Zhao et al. (1990) and Hasegawa et al. (1983), respectively, which were used also in the inversions by Zhao et al. (1992). For the Moho discontinuity, we adopted its depth distribution (Fig. 4) recently obtained by Nakajima et al. (2001a) using reflected waves at the Moho (PmP and SmS) from shallow earthquakes and S-to-P (SP) and P-to-S (PS)
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converted waves at the Moho from intermediate-depth earthquakes. We consider the newly determined Moho geometry is more reliable than that by Zhao et al. (1990) who used refracted waves at the Moho, though the two results are quite consistent with each other. In the present study, we have tried to use two sets of data. One contains the rays that are entirely within the study area (the central part of NE Japan). The other contains the rays having a part of the ray path located outside the study area but within NE Japan. Thus, a better ray coverage may be achieved for the target area. For the area surrounding the central part of
Fig. 4. Depth distribution of the Moho discontinuity estimated by Nakajima et al. (2001a) using travel time data of reflected waves at the Moho (PmP and SmS) from crustal earthquakes and those of converted waves at the Moho (PS and SP) from intermediate-depth events. The depth to the Moho is shown by contour lines at an interval of 1 km. Dashed lines are error contours with 0.5 km interval.
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NE Japan, we adopted the 3D P- and S-wave velocity models estimated for the whole NE Japan with a grid spacing of 0.25° in horizontal and 10 –30 km in depth
(Nakajima et al., 2001b). The second data set has a possibility to lead to errors inside the study area if the velocity structure for wider area has not been estimated precisely. In the present case, however, we consider that no serious errors propagate inside the study area, since earthquakes and stations we used are in the area of 36°N – 42°N, 138.5°E – 142.25°E and 0– 200 km in depth, for which both P- and S-wave velocity structures were estimated with high resolution by Nakajima et al. (2001b). Then we estimated the 3D P- and S-wave velocity structures for the central part of NE Japan with a smaller grid spacing. The whole NE Japan 3D model was used as the starting model for the inversion. The distribution of grid net adopted in this inversion is shown in Fig. 5. Grid spacing in the present inversion is 0.125° in horizontal and 5 – 15 km in depth. The present target area is 38.5°N –40.0°N, 140.0°E –141.5°E and depth range of 0 – 40 km (Fig. 5). The number of rays intersecting the study area is 78,686 for P-wave and 54,284 for S-wave.
3. Results
Fig. 5. Configuration of the grid net adopted in the tomographic inversions. (a) Map view, and (b) EW vertical cross-section. Solid lines in the vertical cross-section show seismic velocity discontinuities. The rectangles in (a) and (b) show the present study area.
A converged solution was obtained after six iterations. The root mean square (rms) residual was reduced from 0.214 to 0.206 s for P-wave and from 0.437 s to 0.426 s for S-wave. Figs. 6 and 7 show Pand S-wave velocity perturbations obtained together with active volcanoes. The velocity perturbation is from the average value of the inverted velocities in each layer (Nakajima et al., 2001b). The velocity images at 0 km depth mainly reflect the near-source features such as the sediments just beneath the seismic stations, and we do not discuss them here. We can see that the spatial distributions of P- and S-wave velocities are very similar to each other, although the amplitudes of S-wave velocity anomalies are generally larger than those of P-wave. The velocity images at the second layer (5 km depth) show that lowvelocity anomalies spread out along the Senya Fault, to the west of it, and beneath the southern volcanic area, while high-velocity anomalies are distributed beneath Kitakami Mountain range for both P- and S-waves. In the third layer (10 km depth), lowvelocity anomalies are distributed locally around the northern, southern and western volcanic areas. They
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Fig. 6. P-wave velocity perturbations at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. Circles and crosses represent low and high velocities, respectively. Solid triangles denote active volcanoes.
are more widely distributed around the volcanic areas in the lower crust (25-km depth). In the uppermost mantle (40-km depth), low-velocity anomalies are
extensively distributed along the volcanic front, especially in S-wave velocity images. These features are, on the whole, similar to the results for the whole NE
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Fig. 7. S-wave velocity perturbations at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. The symbols are the same as those in Fig. 6.
Japan by Nakajima et al. (2001b), but the images presently obtained have a much higher spatial resolution than those by Nakajima et al. (2001b).
Figs. 8 and 9 show the results of checkerboard resolution test (CRT) for P- and S-waves, respectively. We assigned positive and negative velocity perturba-
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Fig. 8. Results of checkerboard resolution test for P-wave velocity structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km.
tions of ± 3% alternately to the grid nodes, and calculated travel times for this model to make synthetic data. We added random noises corresponding to picking errors to the synthetic data: 0.1 s for P-wave and 0.2 s for S-wave. The synthetic data thus made were inverted with an initial model of zero velocity perturbations. The results of CRT for P-wave are excellent (Fig. 8). Fig. 9 shows that the resolution for S-wave is inferior to that for P-wave. However, the checkerboard patterns are recovered on the whole.
Especially, the resolution in the middle to the eastern part of the study area is fairly good. This improved resolution is due to the better ray coverage for this part. We also carried out a restoring resolution test (RRT) (Zhao et al., 1992). The results of RRT are very good for areas where CRT was well resolved. Fig. 10 shows the spatial distribution of Vp/Vs ratio, which is calculated from P- and S-wave velocity structures shown in Figs. 6 and 7. This result has almost the same spatial pattern as that for the whole
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Fig. 9. Results of checkerboard resolution test for S-wave velocity structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km.
NE Japan by Nakajima et al. (2001b). In the present results, however, we have imaged smaller-scale heterogeneities in the crust and uppermost mantle. At 5km depth, low Vp/Vs areas are extensively distributed in regions including the volcanic front, particularly beneath the northern and southern volcanic areas, while high Vp/Vs areas are distributed in the western and eastern parts of the study area. At 10-km depth, high Vp/Vs areas are distributed in more confined areas, and the northern and southern volcanic areas
have low Vp/Vs values. A high Vp/Vs area spreads out beneath Mt. Hayachine, whose location is shown in Fig. 1, and to the south of it at 5-km depth. At 10-km depth, this high Vp/Vs area is located 20 km to the south of it. The center of this area at 0 km depth lies on Mt. Hayachine. Ultrabasic rocks mostly serpentinized are exposed at the surface in an area of 4 20 km centering at Mt. Hayachine (Geological Survey of Japan, 1992). This spatial relationship suggest that the high Vp/Vs area is perhaps caused by rocks composed
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Fig. 10. Vp/Vs structure at depths of (a) 5 km, (b) 10 km, (c) 25 km and (d) 40 km. Crosses and circles represent low and high Vp/Vs ratios, respectively. Solid triangles denote active volcanoes.
of serpentine which is known to have low Vs and high Vp/Vs (e.g., Christensen, 1996). We suppose from Fig. 10 that the root of serpentinized rocks is inclined toward the south in the upper crust. In the lower crust (25-km depth), high Vp/Vs areas are distributed in and around the volcanic areas. They are distributed extensively along the volcanic front in the uppermost mantle (40-km depth). Similar features of Vp/Vs ratio to the present study are also seen in the result recently
obtained by Sato (2000) based on the block inversion method of Aki and Lee (1976).
4. Discussion Fig. 11 shows vertical cross-sections of Vp, Vs and Vp/Vs images we obtained. P- and S-wave velocity perturbations and Vp/Vs ratio are shown in the upper,
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Fig. 11. Vertical cross-sections of (top) P-wave, (middle) S-wave velocity perturbations and (bottom) Vp/Vs ratio along lines A – A0, B – B0, C – C0, D – D0, E – E0 and F – F0 in the inserted map. Red colors show low velocity and high Vp/Vs; blue colors show high velocity and low Vp/Vs. Red triangles and black bars on the top of each figure show active volcanoes and active faults (Active Fault Research Group, 1991), respectively. Dots and red circles show shallow microearthquakes and low-frequency microearthquakes (Okada and Hasegawa, 2000), respectively. Red lines denote distinct S-wave reflectors detected by Hori et al. (1999). Black lines denote seismic velocity discontinuities.
middle and lower figures, respectively. Distinct Swave reflectors detected in the midcrust (Hori et al., 1999), deep low-frequency microearthquakes detected in the lowermost crust and the uppermost mantle (Okada and Hasegawa, 2000), and ordinary shallow microearthquakes in the upper crust are also shown. We can see from these figures that remarkable lowvelocity anomalies for both Vp and Vs are distributed beneath active volcanoes continuously from the uppermost mantle to the upper crust in the cross-sections along lines A – A0, C –C0 and D – D0, which pass through the volcanic areas (Fig. 11a,c,d). On the contrary, continuously distributed low-velocity anomalies are not visible in the cross-section along line B –B0, which does not intersect the volcanic areas (Fig. 11b). These characteristic features can be clearly seen in the NS vertical cross-sections along the volcanic front (lines E – E0 and F– F0; Fig. 11e,f).
The upper crust exhibits, on the whole, low Vp/Vs values as mentioned in the previous section. In all the vertical cross-sections shown, high Vp/Vs areas exist in the uppermost mantle along the volcanic front. However, high Vp/Vs areas in the lower crust are distributed only beneath active volcanoes. We investigated the effects of fluid-filled inclusions in rocks on seismic velocities using the model of Yamamoto et al. (1981) and found that the low Vp, low Vs and high Vp/Vs anomalies were perhaps caused by inclusions containing melt with a small aspect ratio (Nakajima et al., 2001b). The present results suggest that partial melting materials are distributed extensively in the uppermost mantle just beneath the Moho along the volcanic front and they rise up locally to the lower crust just beneath active volcanoes. The low Vp, low Vs and low Vp/Vs anomalies in the upper crust can be explained by the presence of inclusions filled with H2O with a large
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Fig. 11 (continued ).
aspect ratio (Nakajima et al., 2001b). Partial melting areas may not exist in the upper crust beneath active volcanoes, but areas with much H2O may exist there. A schematic vertical cross-section of the structure beneath the active volcanoes is shown in Fig. 12. Note that smaller-scale partial melting areas may exist but may not be imaged because of the limited spatial resolution. North – south vertical cross-sections of Vp, Vs and Vp/Vs along the volcanic front (Fig. 11e,f ) clearly show low Vp, very low Vs and very high Vp/Vs in the lower crust and the uppermost mantle beneath active volcanoes. The upper crust beneath active volcanoes has low Vp, low Vs and low Vp/Vs instead of high Vp/Vs. Shallow earthquakes are distributed in low Vp and low Vs anomalies as well as in high Vp and high Vs anomalies. Comparison of their hypocenter locations with the Vp/Vs structure shows that most of them occur in low Vp/Vs areas. This is mainly because shallow earthquakes occur at 10 km depth or so, close to the bottom of the seismogenic upper crust, and Vp/Vs values are low in most of the
areas at this depth (Fig. 10b). Some clustered earthquakes are seen in low Vp, low Vs and low Vp/Vs anomalies in the upper crust (Fig. 11). Low Vp, low Vs and low Vp /Vs anomalies are perhaps caused by inclusions of H2O, as mentioned above. This may suggest that some clustered earthquakes in such anomalous areas may be triggered by the migration or diffusion of H2O. Anomalously low-frequency microearthquakes have been detected in and around low Vp areas of the lowermost crust and the uppermost mantle beneath active volcanoes (Hasegawa et al., 1991). They occur at depths where rheological properties of rocks are ductile, well below the brittle seismogenic zone of the upper crust. They have extremely low predominant frequencies for both P- and S-waves compared with ordinary microearthquakes in the upper crust, and do not have simple double-couple focal mechanisms. Hasegawa and Yamamoto (1994) suggested that these anomalous events were generated by the magmatic activity of mantle diapirs in the mantle wedge. Fig. 11 shows that they are distributed in and around low Vp
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Fig. 11 (continued ).
zones, as Hasegawa et al. (1991) pointed out. Moreover, the comparison with the Vp/Vs structure in Fig. 11 shows that many low-frequency microearthquakes occur near the upper surface of high Vp/Vs areas where both P- and S-wave velocities are low. Based on these observations, we believe that the low-frequency events are most possibly caused by H2O migration originated from low Vp, low Vs and high Vp/Vs partial melting areas (Fig. 12). In NE Japan, distinct S-wave reflectors (bright spots) with very large reflection coefficients have been detected in the midcrust beneath active volcanoes (Mizoue et al., 1982; Horiuchi et al., 1988; Hasegawa et al., 1991; Hori and Hasegawa, 1991). These reflectors have the same features as the magma body detected in the Rio Grand rift near Socorro, NM (Sanford et al., 1973, 1977; Reinhart et al., 1979; Brown et al., 1980; Balch et al., 1997). Recently, similar S-wave reflectors in the midcrust have been detected not only beneath active volcanoes but also in many other regions where active volcanoes do not exist (Hori et al., 1999). Anomalously large reflection coefficients for S-waves indicate that the reflector bodies are filled with fluids. Matsumoto and Hasegawa (1996) studied in detail an S-wave reflector in the midcrust beneath Nikko-Shirane volcano located
in the south part of NE Japan. Based on spectral ratio analyses of the direct and reflected S-waves, they found that the midcrustal S-wave reflector body had an area extent of 15 15 km and had a thickness of only about 100 m and a very low rigidity. These observations show that the reflector is a thin magma body and perhaps acts as a temporary storage of magma at the midcrustal level. Most of the S-wave reflectors detected by Hori et al. (1999) in the central part of NE Japan (red lines in Fig. 11) are distributed in low Vp/Vs areas, some of which are low Vp and low Vs and others not. Moreover, their dimensions are not so large as that beneath Nikko-Shirane volcano. We infer, based on these observations, that most of the reflector bodies detected by Hori et al. (1999) in the present study area are not filled with melt but with H2O. The S-wave reflectors detected are distributed mainly along the volcanic front (Hori et al., 1999). Considering the presence of extensive partial melting areas (low Vp, low Vs and high Vp/Vs) in the uppermost mantle along the volcanic front, we further infer that H2O forming the Swave reflectors in the midcrust are originated from these partial melting materials. The close relationship between the seismic velocity structure presently obtained and other seismolog-
J. Nakajima et al. / Tectonophysics 341 (2001) 1–17
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Fig. 12. Schematic illustration of the anomalous structure in the crust and the uppermost mantle beneath active volcanoes. (a) Across-arc vertical cross-section of NE Japan from Hasegawa et al. (1994, Fig. 12). (b) Cross-section updated by this study but for the portion enclosed by broken lines in (a).
ical observations as described in this section indicates that extensive melting areas are distributed in the uppermost mantle along the volcanic front beneath the central part of NE Japan, and a portion of partial melting materials rises up and intrudes locally into the lower crust in and around active volcanoes. Partial melting materials attained the lower crustal level and H2O expelled from them due to the fall in an ambient temperature would form fluid chambers, some of which are detected as S-wave reflectors. H2O generated from the dehydration of the partial melting materials at the lower crustal level could trigger the low-frequency microearthquakes. A schematic illustration of the anoma-
lous structure beneath active volcanoes is shown in Fig. 12.
5. Conclusions We estimated high-resolution, three-dimensional Vp, Vs and Vp/Vs images in the central part of northeastern Japan, where a very dense seismic network was temporarily deployed. Our results have revealed an anomalous structure in the crust and uppermost mantle beneath the active volcanoes in this area. Low Vp and low Vs anomalies are extensively distributed in the uppermost mantle along the volcanic front and
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spread out up to the crust beneath active volcanoes. They attain to the surface where active volcanoes are located. These low-velocity anomalies have high Vp/Vs values in the uppermost mantle and lower crust. However, those in the upper crust beneath active volcanoes show low Vp/Vs rather than high Vp/Vs. These characteristic features of the velocity structures are probably caused by the concentrated distribution of fluids, such as melts and/or H2O, in the crust beneath active volcanoes and the uppermost mantle along the volcanic front. Ordinary shallow earthquakes are distributed mainly in the areas with low Vp/Vs. Deep low-frequency microearthquakes are located at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low Vp /Vs, suggesting that most of them are filled with H2O. Acknowledgements We appreciate the fruitful discussions with T. Yoshida, M. Wyss, I.S. Sacks and N. Umino and all the members at Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University. We are also grateful to the members who have participated in the 1997 –1999 Joint Seismic Observations in Tohoku for their efforts in the field work. We also thank the staffs of JMA for allowing us to use some of the data recorded by their seismic network. We benefited from the comments of two anonymous referees provided thoughtful reviews, which improved the manuscript. All the figures in this paper are plotted using the GMT (Wessel and Smith, 1995). This research was partially supported by a grant from the Ministry of Education, Science and Culture of Japan.
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