Three-dimensional seismic structure of subducting lithospheric plates under the Japan islands

Physics of the Earth and Planetary Interiors, 21(1980)109—119 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands

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THREE-DIMENSIONAL SEISMIC STRUCTURE OF SUBDUCTING LITHOSPHERIC PLATES UNDER THE JAPAN ISLANDS KAZURO HIRAHARA and TAKESHI MIKUMO Disaster Prevention Research Institute, Kyoto University, U/i, Kyoto (Japan)

(Received December 12, 1978; revised and accepted March 31, 1979) Hirahara, K. and Mikumo, T., 1980. Three-dimensional seismic structure of subducting lithospheric plates under the Japan Islands. Phys. Earth Planet. Inter., 21: 109—119. The three-dimensional seismic structure of subducting lithospheric plates under the Japan Islands has been investigated in detail by using an inversion method developed by Aki and co-workers. The present analysis dearly reveals the subduction of the Pacific plate, with a gradual narrowing in breadth down to about 600 km beneath the Sea of Japan, and also gives some indication of the Philippine Sea plate in the uppermost mantle, less than 50 km under the southernmost part of Japan. The upper boundary of the descending Pacific plate shows sharp velocity contrasts with respect to the overlying low-velocity zone, while the lower boundary appears to have a transitional nature. Large positive Bouguer anomalies in northeastern Japan may be explained, in part, by the effects of the subducting Pacific plate, but those over the Sea of Japan cannot be accounted for by lateral heterogeneities in the upper mantle.

1. Introduction Lateral heterogeneities in the structure of the upper mantle under the Japan Islands were first hivestigated by Utsu (1967, 1971), and velocity contrasts in and around the downgoing lithosphere have since been discussed further by several Japanese geophysicists. These investigations were essentially twodimensional, and assumed the location and configuration of the descending lithosphere based on the hypocenter distribution of intermediate- and deep-focus earthquakes. In a previous paper, hereafter referred to as Paper 1, Hirahara (1977) made a successful estimate of the large-scale, three-dimensional seismic structure down to 650 km beneath the Japan Islands and the Sea of Japan, by applying a three-dimensional inversion technique developed by Aid et al. (1976, 1977) and by Aki and Lee (1976). This analysis revealed general features of the downgoing high-velocity Pacific plate and their relation to deep seismic activity. The main purposes of the present paper are twofold. One is to investigate, in more detail, the struc-

ture and physical properties of the subducting lithospheric plate, and in particular to estimate the sharpness of the upper and lower boundaries of plate margins, and the magnitude of any structural heterogeneities inside the plate. These features play an impor. tant role in understanding problems of mantle dynamics such as the interactions between the lithosphere and the underlying asthenosphere (possibly with mantle convection), and possibly in deducing relations between the stress distribution within the plate and the source mechanism of shallow- to deepfocus earthquakes within and just above the plate. Further three-dimensional analysis is made for this purpose. The structure obtained here is also tested by simple three-dimensional ray tracing. The other main reason for this work is to provide a basis for open discussions on the tectonics of the Japan Islands and the Sea of Japan, from the obtained lithospheric and asthenospheric structures. For this reason, we calculate the distribution of grayity anomalies based on the refined three-dimensional seismic structure, and compare them with the observed Bouguer anomalies.

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2. Data and method of analysis The data used here are 1286 P-wave travel times observed at Japanese stations, and 2292 arrival times at 232 worldwide stations from 20 intermediate- and deep-focus earthquakes around Japan with magnitudes greater than 4.5 and depths between 100 and 500 km, selected from the ISC Bulletins cited in Paper 1. Figure 1 shows the location of these earthquakes. The method of analysis using a three-dimensional inversion is the same as that employed in Paper 1, in which the method of Aki and Lee (1976) was slightly modified to incorporate data both from Japanese stations at short distances and from worldwide stations at teleseismic distances. Standard travel times are computed from the Jeffreys—Bullen velocity model, with simple corrections for ellipticity and station elevations. In Paper 1, the velocity anomalies in 317 blocks with a size of 2°X 2°X 100km (2°X 2°X 50km ~ •

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blocks were used for only the uppermost portion) dividing the modelling space, and also the source corrections, have been estimated simultaneously in a least-squares sense. However, the above block size seems somewhat too large to trace three-dimensional seismic rays and to correlate the obtained velocity anomalies with seismicity and gravity anomalies. For this reason, in the present paper we have subdivided about 90 blocks, lying within, just above and just below the high-velocity lithosphere, into 367 blocks with a size of 10 X 10 X 100 km for a second trial, and 183 of these blocks are further subdivided into 366 blocks with the smaller size of 10 X 10 X 50km for the final trial. The velocity anomalies in other blocks, and the source locations, have been fixed as described in Paper 1. Since, in the subdivided smaller blocks, the standard errors and hence the resolutions would decrease due to fewer observations, we use here a damping parameter half as great as that employed in Paper 1, which, in principle, makes the resolution and standard errors larger. However, because of our complex procedure, including some fixed solutions and the use of different observational equations in each step, the resolution and covariance matrix cannot be directly compared with those in the previous case. Although our present trials are not complete because we have not computed the resolution and covariance matrix in this paper, our solution appears to have been somewhat improved, as wifi be described in later sections.

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The main results obtained here, together with some of the previous results, are described below. As in Paper 1, we classify the obtained velocity anomaliesintofivegrades(<—1,—1—’0,0—’+1,+l’~ +2.5, and >+2.5%, respectively) as shown in Fig. 2 (a—f); the earthquakes located by the NOAA are also plotted. Each of the divided blocks is referred to by its location coordinates (L, M, N) as in Paper 1, where L, M and N denote locations in the longitudinal latitudinal and depth directions, respectively. 3.1. Lateral velocity anomalies in each layer Layer 1 (0—50 km) indicates weak high-velocity anomalies along the Pacific coastal region, suggesting

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from inland areas towards the ocean. The highest velocities appear in the Kanto region (6, 3, 1) to the south, although this involveszones a small anomaly.Ku There are also high-velocity in low the southern Peninsula (4,2, 1), which might be a manifestation of the sub ducting Philippine Sea plate. This was not so clearly identified in the previous paper. It is noticed that low velocities cover inland areas, particularly in the central mountams (5 3, 1) and m the Kyushu regions (2, 1, 1), (2, 2, 1) and (1, 2, 1). These correspond to crustal thickening in the former area and suggest a low Pn velocity over southwest Japan Layer 2 (50—150 km ) and Layer 3 (150—250 km) m the uppermost mantle have remarkably high veloc ity anomalies along the Hokkaido corner to the northeastern Honshu arc [(9,7, 2)—(8, 6, 2)—(6, 2, 2); (8,7, 3)—(6, 6, 3)—(5, 3, 3)]. It should be noticed that these high velocities, indicating the subducting Pacific plate, are confined to long but rather limited zones, which are clearly bounded by low velocities just west of the Honshu arc. The breadths of these high-velocity zones have been much better defmed than in Paper 1. Another interesting feature is that earthquakes at these intermediate depths in northeastern Japan take place exactly along these highvelocity zones, which move by about one or a half block westwards from Layer 2 down to Layer 3. At these depths there are no clear indications of the subducting Philippine Sea plate except in the Ku Peninsula (4, 2, 2). This is not unreasonable, considering that the leading edge of the plate may only reach a depth of about 80 km in southwest Japan (see for example Shiono, 1974). In Layer 4 (250—350 km) and Layer 5 (350—450

km), the high-velocity zones appear under the northeastern part of the Sea of Japan and move towards its central part. The cross-section of the zones at these depths becomes shortened in the north—south direction as compared with that in Layers 2 and 3, which

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narrower in this deeper mantle. It is also interesting to note that the high-velocity Pacific plate appears to be detached to some extent at these depths just west of Hokkaido (6,6,4), (6,7,4), (5,6,5) and (5,7,5). Deep seismicity has also some gaps around there, and almost continues with minor interruptions along the central high-velocity zones to the south. The high seismicity down to the southeast does not correspond to high-velocity regions. Layer 6 (450-550 km) still indicates high-velocity anomalies corresponding to the bottom part of the subducting plate in the northwestern part ofthe Sea of JapanC&5,6), (X6,6), (3,5,6) and (3,6,6), where a small number of the deepest earthquakes take place. This appears to be the leading edge of the Pacific plate. For the bottom layer, Layer 7 (not shown here), we do not think that the calculated results, which involve some strange high- and lowvelocity anomaly patterns, represent real heterogeneities at this depth: these results may indicate that the distribution of earthquakes and stations is not adequate to resolve the structure. 3.2. Velocity anomalies in vertical cross-sections The obtained velocity anomalies in two vertical cross-sections along profiles across the Tohoku region to Vladivostok and along the 38ONline are shown in Fig. 3 (a and b). Since we are particularly interested, in the present paper, in the upper and lower boundaries between the downgoing high-velocity lithosphere and the low-velocity zones and also in velocity contrasts within the plate, the present calculations are restricted to blocks within and around the lithosphere, with the velocity anomalies in other blocks fixed as described in Paper 1. The results shown for the above two profiles clearly indicate that the highvelocity Pacific plate penetrates down to a depth of about 600 km with a dip of about 30”, but there could be a gap or interruption of the plate at a depth of around 500 km. It should also be noted that the upper boundary of the high-velocity plate becomes more clearly defined than was the case in Paper 1. The lower boundary, however, still appears somewhat vague because of weaker velocity contrasts. It is possible that the lower boundary has not been resolved to a sufficient degree because of the relatively limited number of seismic rays travelling across the boun-

dary, except, of course, for the rays for the teleseismic stations. If however this is not the case, the more transitional velocity structure around the lower boundary suggests that the temperature gradient there may be smaller than that along the upper boundary. This interpretation appears consistent with thermal models of regions around the subducting lithosphere, where frictional heating is an order of magnitude greater on the top half of the slab than on the lower half (see for example Toksijz et al., 1973; Sleep, 1973). There is, however, another possibility, namely that the transitional nature of the boundary might be somehow associated with a partial melting zone which was originally inthe upper portion of the asthenosphere and which has now been dragged into the deeper mantle just below the slab. A key to solving this problem in the future would be to use more seismic rays which travel across the boundary from deep-focus earthquakes in the Izu-Bonin-Mariana arc. It may be difficult, therefore, to estimate correctly the thickness of the descending lithosphere, which has a gradually changing nature, but it may be of the order of 100 km or more. It should be noted that intermediate- and deep-focus earthquakes take place along and in the upper half of the plate but not in the lower half of the plate. 3.3. Three-dimensional ray tracing In this section, we calculate theoretical travel-time residuals by using three-dimensional ray tracing (Jacob, 1970) on the above refined structure, although some iterative inversion processes have been suggested (Aki and Lee, 1976; Ellsworth, 1978). Here, as a first step, we simply compare the residuals with the observed residuals at Japanese stations, This comparison provides some visual insight into the appropriateness of our solution, in order to examine more closely the goodness of fit and the distribution of the calculated travel-time residuals, and also the effects of ray distortions on both the horizontal and vertical sections. To establish which seismic rays could reach various sites in the Japan Islands, we made a number of calculations for different take-off angles and azimuths, with appropriate increments, emitted from the source. Figure 4 (a and1b) gives some examples of the distribution of the travel-time residuals observed at

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Japanese stations. (right-hand-side) and of the calculated residuals (left-hand-side), for two deep-focus earthquakes near Vladivostok and on the north coast of southwest Japan. It can be seen that the general pattern of the calculated residuals agrees well with that of the observations; there are negative residuals or early arrivals of P-waves over northeasternPacific coastal regions, and positive residuals or late arrivals over southwest Japan and the Japan Sea coast. There is, however, some disagreement around the Kyushu region, in the extreme west of Japan, which appears to indicate that the structure under this region is not well resolved. Figure 5 illustrates some examples of

the ray paths from the source to the surface, which are projected onto the vertical section along profile A inserted in Fig. 4a, although these rays are subjected to lateral distortions due to the three-dimensional plate structure (Jacob, 1972; Davies and Julian, 1972; Sleep, 1973; Engdahl et al., 1977) and hence deviate slightly from this section. This illustration shows how positive or negative travel-time anomalies are produced for the different rays and also the effects of ray distortions. We notice that the ray with positive residuals to the Japan Sea coast travels through the upper part of high-velocity zones and penetrates rather steeply up to the uppermost mantle with low-

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velocity anomalies, whereas negative travel-time residuals are produced for the rays which travel less steeply, mainly through the subducting lithosphere,

These ray distortions should be taken into account when estimating Q-structures around the lithospheric

plate from seismic wave amplitudes, It may be concluded from the results above that

Hawaii (Ellsworth and Koyanagi, 1977). Gravity anomalies due to the two-dimensional upper-mantle

heterogeneities including a subducting lithosphere have also been calculated (see for example Yoshii, l972;Grow and Bowin, 1975). The most serious problem involved in these calculations is to choose the correct form for the seismic velocity—density rela-

the three-dimensional structure obtained for north-

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1957;Woollard, 1959) and also for laboratory experiments (see for example Birch, 1961; Thompson and Taiwani, l966;Anderson, 1967). Furthermore, the relationship in the upper mantle should, in principle, be subject to the combined effects of temperature— pressure increase, partial melting and changes in mineral composition (see for example Yoshii, 1972). The gradient dp/dVp in these studies ranges from 0.31 to 0.10. In the present paper we refer to Birch’s relation (dp/dVp = 0.302) to estimate density anomalies in Layer 1 (0—50 km) from the obtained velocity anomalies in the crust and uppermost mantle, but use a tentative conversion factor of dp/dVp = 0.15 for the structure from Layer 2 down to Layer 7, i.e. in the depth range 50—650 km, since Birch’s factor does not seem appropriate for the deeper mantle. To compute gravity anomalies due to three-dimensional structure, we have mainly used the method of Nagy (1966) and Banerjee and Gupta (1977), in which the gravitational

4. Gravity anomalies due to three-dimensional structure In order to discuss the general tectonic features of the Japan Islands in relation to the subducting lithospheric plates, we calculate the gravity anomalies expected from the refined three-dimensional seismic structure described above, and then compare them with the distribution of the observed Bouguer anomalies in the same region. Although the Bouguer anomalies should strongly reflect the crustal structure, including sedimentary layers rather than the uppermantle structure, it is still expected that large-scale heterogeneities will cause gravity anomalies with long wavelengths. Similar gravity calculations based on a three-dimensional seismic structure have been made for California and Montana (Zandt, 1975) and for

117

attraction due to a right-rectangular prism can be cglculated analytically. We apply this method to 6 X 6 X 2 divided blocks around the observation site, and integrate contributions from each of the divided blocks. For other blocks far from the site, we simply sum up the gravitational force by multiplying each density difference by the volume. Figure 6 (a and b) shows the gravity anomalies thus computed from Layer 1 (Fig. 6a) and from the deeper mantle, Layers 2—7 (Fig. 6b), to allow us to see their contributions separately. The observed Bouguer-anomaly distribution, compiled by Tomoda (1973) and reproduced in Fig. 7 with some simplifications, covers not only the land area of the Japan Islands but also the adjacent oceanic regions. A comparison of Fig. 6a with Fig. 7 shows good correlations in concentrated negative anomalies in southern Hok kaido and in some parts of central Japan, and in highly positive anomalies in the Kanto region, on the southeastern coast of Japan. Small positive anomalies that appeared at the head of the Ku Peninsula are partly consistent with the observations. These indicate that the Bouguer anomalies in these regions mainly reflect velocity anomalies in the crust and uppermost mantle at depths of less than 50 km. On the other hand, the deeper mantle heterogeneities,

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ticularly from Tohoku to the’off-shore regions, are made up of contributions from the subduction of the Pacific plate. However, much larger Bouguer anomalies, reaching 300 mgal towards the Japan trench,

may not be explained by this effect alone, but are also partly due to steep crustal thmning (see for example Ludwig et al., 1.966; Yoshii, 1972). The Bouguer anomalies in the Sea of Japan reach 200 mgal, whereas the computed anomalies show significantly negative values. This situation is the same as in the two-dimensional calculations made by Yoshii (1972). The disagreement could be partly reduced if a smaller density contrast than we assumed here is adopted. The remaining discrepancy might be

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reconciled if we take into consideration the very thin crust of this marginal sea (Yoshii and Asano, 1972) together with the possible existence of a high-velocity

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boundary appears to have a somewhat transitional nature, which might be explained by lower temperature gradients than expected from existing thermal models. The thickness of the plate cannot be accurately estimated, but it may be of the order of 100 km or a little more. A number of intermediate- and deep-focus earthquakes take place in the upper half

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breadth of the downgoing plate becomes remarkably narrow as it penetrates down to the mantle under the Sea of Japan. The upper plate boundary can be clearly defined, with sharp velocity contrasts with respect to the inner low-velocity zone. The lower

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Fig. 7. Distribution of Bouguer anomalies [after Tomoda (1973) with some simplifications].

lid (Yoshii, 1972). Excessively large negative computed anomalies in southwest Japan suggest that the density contrast in this region might be somewhat weaker, due to possible thermal effects and partial melting. Although this is still open to question, the present results raise a new problem with regard to the tectonics of southwest Japan including the Sea of Japan.

5. Conclusions We have investigated the three-dimensional seismic structure under the Japan Islands and the Sea of Japan, by applying the inversion method of Aki and co-workers, and in particular have estimated detailed velocity structures within the subducting lithospheric plates and around their upper and lower boundaries. The final model was examined by three-dimensional ray tracing, and also compared with Bouguer gravity anomalies. The main conclusions drawn are as follows. The sub duction of the high-velocity Pacific plate has been clearly identified down to 600 km. The

of the subducting plate but none occur in the lower

portion An indication of the subducting Philippine Sea plate can only be recognized in the uppermost mantle less than 50 km beneath the southern Ku Peninsula. The distribution of travel-time residuals calculated from three-dimensional ray tracing agrees well with that of the observed residuals, except for the

Kyushu region. The gravity anomalies calculated from the threedimensional seismic structure indicate that a consider-

able part of the large positive Bouguer anomalies in northwestern Japan results from the subduction of the Pacific plate. The large positive anomalies over the Sea of Japan cannot be explained by lateral heterogeneities in the deep mantle.

Acknowledgements We wish to thank Prof. K. Aki and Dr. W.L. Ellsworth for several suggestions and comments on the present study, and also the staff members of our laboratory for helpful discussions. Computations were made at the Data Processing Center of Kyoto University and the Computer Center of the Institute for Chemical Research, Kyoto University, Uji. References Aki, K. and Lee, H.K., 1976. Determination of three-dimensional velocity anomalies under a seismic array using first P arrival times from local earthquakes. 1. A homogeneous initial model. J. Geophys. Res., 81: 4381—4399. Aki, K., Christoffersson, A. and Husebye, E.S., 1976. Threedimensional seismic structure of the lithosphere under Montana LASA. Bull. Seismol. Soc. Am., 66: 501—524.

119 Aki, K., Christoffersson, A. and Husebye, E.S., 1977. Determination of the three-dimensional seismic structure of the lithosphere. J. Geophys. Res., 82: 277—296. Anderson, D.L., 1967. A seismic equation of state. Geophys. J. R. Astron. Soc., 13: 9—30. Banerjee, B. and Gupta, S.P.D., 1977. Gravitational attraction of a rectangular parallelepiped. Geophysics, 42: 1053—1055. Birch, F., 1961. Composition of the Earth’s mantle. Geophys. J. R. Astron. Soc., 4: 295—311. Davies, D. and Julian, B.R., 1972. A study of short-period P wave signals from Longshot. Geophys. J. R. Astron. Soc., 29: 185—202. . . Ellsworth, W.L., 1978. Iterative determination of three-dimensional velocity structure using distant sources. Symp. Mathematical Geophysics. 12th, Caracas, August 1978. Abstracts. Ellsworth, W.L. and Koyanagi, R.Y., 1977. Three-dimensional crust and mantle of Kilauea Volcano, Hawaii. J. Geophys. Res., 82: 5379—5393. Engdahl, E.R., Sleep, N.H. and Liii, M., 1977. Plate effects in north Pacific subduction zones. Tectonophysics, 37: 95— 116. Grow,J.A. and Bowin, C.O., 1975. Evidence for high-density crust and mantle beneath the Chile trench due to the descending lithosphere. J. Geophys. Res., 80: 1449—1458. Hirahara, K., 1977. A large-scale three-dimensional seismic structure under the Japan Islandsand the Sea of Japan. J. Phys. Earth, 25: 393—4 17. Jacob, K.H., 1970. Three-dimensional seismic ray tracing in a laterally heterogeneous spherical Earth. J. Geophys. Res., 75: 6675—6689. Jacob, K.H., 1972. Global tectonic implications of anomabus seismic P travel-times from the nuclear explosion Longshot. J. Geophys. Res., 77: 25562573. Ludwig, W.J., Ewing, 1.1., Murauchi, S., Den, N., Asano, ~,, Hayakawa, M., Asanuma, T., Ichikawa, K. and Noguchi, I., 1966. Sediments and structure of the Japan trench. J.

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