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New evidence for a low-velocity layer on the subducting Philippine Sea plate in southwest Japan
Tectonophysics 332 (2001) 347±358
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New evidence for a low-velocity layer on the subducting Philippine Sea plate in southwest Japan Hitoshi Oda*, Tomiwo Douzen Department of Earth Sciences, Faculty of Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan Received 18 October 1999; accepted 3 July 2000
Abstract A distinct precursory phase is sometimes found prior to S arrival, when deep-focus and intermediate-depth earthquakes are observed at seismic stations in the Chugoku district, southwest Japan. The phase from intermediate-depth earthquakes has apparent velocity close to that of S wave in the Philippine Sea (PHS) plate, but at epicentral distance of 100±250 km it arrives earlier by about 2.5 s than the S wave propagated as a head wave along the upper boundary of the HPS plate. Thus we interpreted the phase as Sp wave into which the S wave refracted upward after traveling in the high-velocity PHS plate was converted at a boundary while propagating on a ray path between the plate and seismic station. The precursory phase to S wave from deep-focus earthquakes was also identi®ed as Sp wave that was caused by conversion of the direct S wave into P wave, because the S±Sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter. The Sp conversion interface was located at the upper boundary of the PHS plate. Further, it was demonstrated that a thin low-velocity layer should overly just above the PHS plate in order to explain both the observed difference in polarity of the S and Sp waves and the amplitude ratio of the Sp wave to the direct S wave. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PHS plate; precursory phase; S-to-P conversion; polarity
1. Introduction Many seismic phases found on seismograms of deep-focus and intermediate-depth earthquakes have been available for the study of the seismic velocity structure of the Earth. For example, Oda et al. (1990) reported that a conspicuous phase was necessarily observed just after the arrival of a weak initial P phase from intermediate-depth earthquakes that occurred in the Chugoku and Shikoku districts, southwest Japan. They interpreted the phase as a guided wave trapped in a thin low-velocity layer lying just * Corresponding author. Fax: 181-86-251-7895. E-mail address: [email protected] (H. Oda).
above the Philippine Sea (PHS) plate. This interpretation leads to a conclusion that the PHS plate subducts with the thin low-velocity layer beneath the Chugoku and Shikoku districts. Similar phases were also found in other regions of the Japan Islands (Fukao et al., 1983; Hori et al., 1985; Hori, 1990). On the other hand, Nakanishi (1980) found a precursory phase to ScS wave on seismograms recorded at seismic stations in the Chugoku and Shikoku districts, and identi®ed it as ScSp wave converted at the upper boundary of the PHS plate. In addition, he emphasized that a negative velocity gradient with increasing depth across the ScS to ScSp conversion interface was required to explain the relative polarity between the ScS and ScSp phases. Although there exists a
0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-195 1(00)00157-8
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H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 1. Epicentral distribution of intermediate-depth (top panel) earthquakes and deep-focus earthquakes (bottom panel). The numbers correspond to the events listed in Table 1.
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
349
Table 1 Source parameters and seismic stations of earthquakes (data of hypocenter and earthquake magnitude are taken from the Seismological Bulletin of the Japan Meteorological Agency) Event No.
Date
Latitude (8N)
Longitude (8E)
Depth (km)
Magnitude
Station
#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 #17 #18 #19 #20
07/25/85 02/19/89 07/30/91 09/09/91 04/08/92 02/25/93 12/18/94 09/13/95 05/31/81 07/04/82 07/19/83 10/31/83 11/30/83 04/06/84 04/21/85 02/12/87 03/20/87 05/03/93 07/30/95 03/01/98
33.71 34.61 33.74 33.77 33.97 34.32 33.71 33.96 30.76 27.80 33.74 30.24 32.65 35.12 29.69 43.23 29.19 36.85 30.60 33.55
134.74 136.47 134.76 134.68 135.79 132.85 132.50 132.41 137.98 137.23 136.96 137.63 140.20 138.59 138.09 132.92 138.28 138.17 139.05 138.42
51 45 52 45 59 45 45 55 500 500 386 531 124 189 514 584 533 207 488 312
5.2 5.3 4.1 3.6 4.3 3.9 3.4 3.3 5.6 7.0 5.7 5.9 6.0 6.7 5.0 6.4 6.1 4.6 6.0 5.6
SBK SBK BSI BS1 BSI OKU BSI BSI SBK SBK SBK SBK SBK SBK SBK SBK SBK OKU OKU OKU
difference in seismic phases used in the above studies, a common conclusion that a low-velocity layer overlies just above the PHS plate is drawn. Since ScSp wave is generated when ScS phase is converted to P wave at the upper boundary of a subducting oceanic plate (Okada, 1979; Nakanishi, 1980; Nakanish et al., 1981), S-to-P converted wave or P-to-S converted wave due to the plate boundary may also be observed. Matsuzawa et al. (1986) reported that a clear phase was really observed between P and S arrivals from deep-focus earthquakes in the Tohoku district, and attributed the phase to P-toS conversion at the upper boundary of the Paci®c plate. However, there have been no reports on the observations of the converted waves in the Chugoku and Shikoku districts which are in a subduction zone of the PHS plate. In this study, we show that a conspicuous phase is observed at seismic stations in the Chugoku district as a precursor to the direct S wave from intermediate-depth and deep-focus earthquakes, and investigate the relationship between the phase and the PHS plate. In particular, we examine the possibility that the phase from deep-focus earthquakes is Sp wave converted at the PHS plate. Furthermore,
the problem of whether or not a thin low-velocity layer exists on the plate is solved by using the polarity of the direct S and converted waves. 2. Precursors to the direct S wave and their causes Fig. 1 shows the location of seismic stations, Shibukawa (SBK), Okayama University (OKU) and Bisei (BSI). At SBK and OKU, three-component seismometers with a natural period of 5 s are operated, and those of BSI have a natural period of 1 s. Source parameters and magnitudes of the earthquakes used in this study are listed in Table 1, and their epicenters are shown in Fig. 1. The ®rst eight events are intermediatedepth earthquakes with focal depths of 40±60 km, and the remaining events are deep-focus earthquakes that occurred in the vicinity of the Japan Islands. Two examples of the seismograms of the intermediatedepth and deep-focus earthquakes are shown in Fig. 2a and b, respectively. A clear phase (X1) appears prior to the arrival time of S wave from intermediate-depth earthquakes (see Fig. 2a). The travel time of the X1 phase is plotted
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H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 2. Seismograms of: (a) intermediate-depth earthquakes (#4 and #6); and (b) deep-focus earthquakes (#12 and #15). The X1 and X2 phases are clearly found prior to the direct S arrivals.
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 2. (continued)
351
352
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 3. Travel time diagram for eight intermediate-depth earthquakes. Solid line shows the linear regression curve obtained by a least-squares method.
against epicentral distance in Fig. 3. From the slope of the regression line determined by a least-squares calculation, the apparent velocity of the X1 phase is estimated to be 5.0 (1/Vs 0.20 ^ 0.07) km/s, which is close to the apparent velocity (4.9 km/s) of the S wave that propagates into the high-velocity PHS plate (Oda et al., 1990). Six ray paths are examined on propagation of the X1 phase in order to investigate where the X1 phase is generated (see Fig. 4). With the exception of case (6), each case shows that the X1 phase travels once as P wave on a part of the ray path. It should be noted that, in case (6), S wave travels as a head wave along the upper boundary of the high-velocity PHS plate. We calculated the travel time curves of the X1 phase in the six cases, assuming a horizontally layered velocity model which was made of the upper crust, the lower crust, the upper mantle and the high-velocity PHS plate. The assumption of the horizontal PHS plate is because the source±receiver direction is almost parallel to the Table 2 Seismic velocity model (Vp, Vs and H are P wave and S wave velocities and layer thickness, respectively)
Fig. 4. Possible ray paths of the X1 phase from an earthquakes source located at a depth of 48 km. Solid and dashed lines represent P and S wave ray paths, respectively. UC, LC, UM and PHS means the upper crust, the lower crust, the upper mantle and the PHS plate, respectively.
Layer
Vp (km/s)
Vs (km/s)
H (km)
Upper crust Lower crust Upper mantle PHS plate
6.09 6.70 7.80 8.10
3.46 3.87 4.50 4.90
15 20 15 ±
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
353
Fig. 5. Plot of the observed S±X1 time (cross) against epicentral distance. Theoretical S±Sp time is shown by dashed line. Solid line is drawn by shifting the dashed line by about 2.5 s.
strike of the Nankai trough (see Fig. 1). Layer parameters of the velocity model are listed in Table 2. The velocity model explains well the observed travel times of the P and S waves of the eight events selected here. The apparent velocity of the X1 phase in each case was determined by applying a least-squares method to the travel times calculated at epicentral distances of 100±250 km. The result is shown in Table 3. The apparent velocity of case (6) is nearly equal to the observed value (5.0 km/s), but those of other cases are largely different from the observed one. In addition, the ray path in case (6) satis®es the condition that the X1 phase must arrive earlier at epicentral distance of 100±250 km than the direct S wave. The travel time difference between the direct S wave and X1 phase is plotted against epicentral distance and compared with the theoretical curve (dashed line) calculated for case (6) (see Fig. 5). The observed data do not fall on the theoretical curve, but they are Table 3 Apparent S wave velocity for six ray paths Apparent velocity (km/s) Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
8.06 6.17 8.06 8.00 6.25 4.90
Fig. 6. Horizontal particle motion of the X2 phase for deep-focus earthquakes (#12 and #15). Arrow shows the azimuth direction of the epicenter.
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H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 7. Diagrams of the S±Sp time against epicentral distance (top) and focal depth (bottom).
in good agreement with the straight line (solid line) drawn by shifting the theoretical curve by 2.5 s. This result means that the X1 phase travels once as a head wave with the S wave velocity along the upper boundary of the high-velocity PHS plate but arrives earlier by about 2.5 s than the travel time of S wave calculated for case (6). In order to demonstrate the early arrival of the X1 phase, S-to-P conversion is needed on a ray path between the PHS plate and station because the converted wave travels with P wave velocity higher than S wave velocity. Thus we interpret the X1 phase to be an Sp wave that is generated when the up-going S wave refracted upward from the plate boundary after traveling as a head wave in the PHS plate is converted into P wave at a conversion interface existing between the plate and seismic station. In this case, the Sp wave may be an `inhomogeneous' wave associated with phase distortion (cf. Aki and Richards, 1980) because the
S-to-P conversion at the interface may violate Snell's law. Fig. 2b shows examples of seismograms of deepfocus earthquakes observed at SBK. A clear phase (X2) is found as a precursor to S wave. Since the direction of the horizontal particle motion of the X2 phase is nearly identical with the azimuth direction of the epicenter (see Fig. 6), we identi®ed the X2 phase as P wave. The travel time difference between the X2 phase and direct S waves is plotted in Fig. 7 against focal depth and epicentral distance. The S±X2 time data scatter between 4 and 6 s, and no systematic change is found in both the diagrams. Thus the X2 phase is interpreted as Sp wave which is caused by Sto-P conversion at a boundary existing between the seismic station and hypocenter. The travel time differences are not so large that the conversion interface may be located in the vicinity of seismic stations.
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Fig. 8. Depth distribution of the Sp conversion points for different focal depths of 400, 500 and 600 km. The depth of the conversion point is shown as a function of horizontal distance from a station.
The possible Sp conversion interface is one of the Conrad discontinuity, the Moho discontinuity and the upper boundary of the PHS plate. In order to examine which of them is preferable as the Sp conversion interface, the conversion points were
located on the assumption that the S±Sp time is 5 s. We adopted the seismic velocity model of Ichikawa and Mochizuki (1971) for the travel time calculations. The depth of conversion point is shown in Fig. 8 as a function of horizontal distance from
Fig. 9. Distribution of the Sp conversion points (cross) projected on the ground surface. Triangles show the seismic stations, OKU and SBK.
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H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 10. Schematic illustration of the ray path of the Sp phase. A low-velocity layer with width, W, is sandwiched between the PHS plate and upper mantle.
seismic station for different focal depths. Since no conversion point is found at a depth shallower than 30 km, either the Moho discontinuity or the upper boundary of the PHS plate is the Sp conversion interface. However, the Moho discontinuity is not appropriate, because the observed polarity relationship between the direct S and Sp waves cannot be explained if the Sp conversion is attributed to the Moho discontinuity (the detailed discussion on the polarity of Sp and S waves is given below). Thus the upper boundary of the high-velocity PHS plate is appropriate for the Sp conversion interface. Sacks and Snake (1977) proposed to compare polarity between Sp and S waves in order to clarify ®ne velocity structure in the vicinity of a conversion interface. It is demonstrated, by amplitude calculation of seismic waves traveling through two semi-in®nite media in
contact with a plane boundary, that the polarity of vertical component of S wave is opposite to that of Sp wave when the S wave propagates through the boundary from the lower layer with low velocity to the upper layer with high velocity (e.g. Aki and Richards, 1980). Our vertical component seismograms show that the polarity of S wave is really opposite to that of Sp wave (see Fig. 2b). This observation proves that the Sp wave is generated by a conversion interface across which the seismic velocities decrease with increasing depth. Thus we concluded that the upper boundary of the PHS plate is preferable as the Sp conversion interface and the Moho discontinuity is inappropriate because the Moho discontinuity has ordinarily a positive velocity jump with increasing depth. Since the Sp wave has been interpreted to be caused by S-to-P conversion at the upper boundary of the PHS plate, the negative velocity gradient required to explain the opposite polarity suggests that the seismic wave velocities of the lower layer (PHS plate) are lower than those of the upper layer (upper mantle). But the result is inconsistent with a common knowledge that the seismic velocities in the oceanic plate are higher than those in the mantle surrounding the plate. To solve the discrepancy, we put a low-velocity layer between the upper mantle and the high-velocity PHS plate. In the velocity model having the lowvelocity layer on the PHS plate, the Sp wave is generated at the upper boundary of the low-velocity layer. 3. Discussion
Fig. 11. Synthetic seismograms. The top synthetic is the radial component seismogram of the SV wave. The vertical component synthetics of the Sp wave are shown for W 0 and W 5 km.
The X2 phase was interpreted as Sp wave converted at the upper boundary of a thin low-velocity layer overlying the high-velocity PHS plate. We located
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
the Sp conversion points so that the theoretical S±Sp times agree with the observed ones, using the velocity model in Table 2, where the depth of the upper boundary of the PHS plate is assumed to be 50 km. In locating the conversion points, the low-velocity layer on the PHS plate was not taken into account because its effect on the S±Sp times is small enough. The incident angle of the direct S wave on the upper boundary of the PHS plate is estimated by travel time calculation using the velocity model of Ichikawa and Mochizuki (1971). The conversion points projected on the ground surface are concentrated in the southern part of the Chugoku district (see Fig. 9). This result supports the view that the leading edge of the PHS plate subducting at the Nankai trough may reach the upper mantle beneath the southern Chugoku district, and is consistent with a common conclusion obtained in the studies of ScSp waves (Nakanishi, 1980; Nakanishi et al., 1981), seismic tomography (Tanaka and Oda, 1997; Ochi et al., 1999) and seismicity of intermediate-depth earthquakes (Kinoshita and Nakanishi, 1997). The ratio of vertical component amplitude of the Sp wave to radial component amplitude of the direct S wave is between 0.1 and 0.2. In order to explain the amplitude ratio, we calculated synthetic seismograms by means of the Thomson±Haskell matrix method (Haskell, 1962) using a simple velocity model where a low-velocity layer with thickness, W, is sandwiched between the upper mantle and the PHS plate (see Fig. 10). The synthetic seismograms, which were calculated for SV-wave incidence on the low-velocity layer from the PHS plate, are shown in Fig. 11. The wavepacket in the case of W 5 km is formed by superposition of Sp waves converted at the upper and lower boundaries of the low-velocity layer, because the lowvelocity layer is too thin to split the wavepacket into the Sp waves converted at both the boundaries. In this case, the amplitude ratio of vertical component of the Sp wave to the radial component of the SV wave falls between 0.1 and 0.2, whereas in the case of W 0 the ratio is much smaller than 0.1. Thus, a thin low-velocity layer just above the PHS plate is needed to explain the amplitude ratio as well as the polarity relationship between the Sp wave and direct S wave. In order to explain magma genesis in the subduction zone of an oceanic plate, Ringwood (1975) proposed a model in which a major part of the oceanic
357
crust subducts into the mantle beneath the continents as a part of the oceanic lithosphere. In the seismological aspect, the subducting oceanic crust may be detected as a low-velocity layer in the upper mantle because its seismic velocities are lower than those of the upper mantle and the oceanic plate. Thus, the thin low-velocity layer which was found just above the PHS plate by this study, as well as the studies of the ScSp wave (Nakanishi, 1980) and guided waves (Oda et al., 1990), strongly supports the subduction of the oceanic crust in the Ringwood's model. 4. Conclusions At seismic stations in the Chugoku district, southwest Japan, a clear precursory phase was recorded a few seconds prior to the S arrival from intermediate-depth and deep-focus earthquakes. We investigated the origin for which the seismic phase was generated. The results are summarized as follows: (1) The precursory phase from intermediate-depth earthquakes is interpreted as Sp wave into which the S wave traveled as head wave in the high-velocity PHS plate is converted at a boundary on a ray path from the plate to seismic station. (2) The precursory phase from the deep-focus earthquakes is identi®ed as Sp wave that is caused by S-to-P conversion at the upper boundary of the PHS plate. (3) A thin low-velocity layer should overly just above the high-velocity PHS plate in order to explain both the amplitude ratio and the polarity relationship between the Sp wave and the direct S wave. (4) The leading edge of the PHS plate reaches the upper mantle beneath the southern part of the Chugoku district. Acknowledgements We thank D. Zhao and an anonymous reviewer for giving critical comments to revise the manuscript. References Aki, K., Richards, P.G., 1980. Quantitative Seismology, Theory and Methods, 1. Freeman, San Francisco. Fukao, Y., Hori, S., Ukawa, M., 1983. A seismological constraint on
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the depth of basalt±eclogite transition in a subducting oceanic crust. Nature 303, 413±415. Haskell, N.A., 1962. Crustal re¯ection of plane P and SV waves. J. Geophys. Res. 67, 4751±4767. Hori, S., 1990. Seismic waves guided by untransformed oceanic crust subducting into the mantle: the case of the Kanto district, central Japan. Tectonophysics 176, 355±376. Hori, S., Inoue, H., Fukao, Y., Ukawa, M., 1985. Seismic detection of the untransformed ªbasalticº oceanic crust subducting into the mantle. Geophys. J. R. Astron. Soc. 83, 169±197. Ichikawa, S., Mochizuki, E., 1971. On traveltime tables for local earthquakes. Pap. Meteorol. Geophys. 22, 229±290 (in Japanese with English abstract). Kinoshita, Y., Nakanishi, I., 1997. Subcrustal seismicity beneath the Southern part of the Chugoku region, Japan. J. Phys. Earth 45, 307±312. Ochi, F., Asamori, K., Zhao, D., 1999. Deep structure of the Japan Islands determined with J-array's teleseismic data, (2) Southwest Japan, Programme and Abstract, Fall Meeting, Seismological Society of Japan, vol. B30 (in Japanese). Oda, H., Tanaka, T., Seya, K., 1990. Subducting oceanic crust on the Philippine Sea plate in southwest Japan. Tectonophysics 172, 175±189.
Okada, H., 1979. New evidence of the discontinuous structure of the descending lithosphere as revealed by ScSp phase. J. Phys. Earth 27, S53±S56. Matsuzawa, T., Umino, N., Hasegawa, A., Takagim, A., 1986. Upper mantle velocity structure estimated from PS-converted wave beneath the north-eastern Japan Arc. Geophys. J. R. Astron. Soc. 85, 767±787. Nakanishi, I., 1980. Precursors to ScS phases and dipping interface in the upper mantle beneath southwest Japan. Tectonophysics 69, 1±35. Nakanishi, I., Suyehiro, K., Yokota, T., 1981. Regional variations of amplitudes of ScSp phases observed in the Japanese Islands. Geophys. J. R. Astron. Soc. 67, 615±634. Tanaka, T., Oda, H., 1997. Three-dimensional structure of P wave velocity in the crust and upper mantle beneath southwest Japan. Okayama Univ. Earth Sci. Rep. 4, 1±20 (in Japanese with English abstract). Ringwood, A.E., 1975. Composition and Petrology of the Earth's Mantle. McGraw-Hill, New York. Sacks, I.S., Snake, J.A., 1977. The use of converted phases to infer the depth of the lithosphere±asthenosphere boundary beneath South America. J. Geophys. Res. 82, 2011±2017.
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New evidence for a low-velocity layer on the subducting Philippine Sea plate in southwest Japan Hitoshi Oda*, Tomiwo Douzen Department of Earth Sciences, Faculty of Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan Received 18 October 1999; accepted 3 July 2000
Abstract A distinct precursory phase is sometimes found prior to S arrival, when deep-focus and intermediate-depth earthquakes are observed at seismic stations in the Chugoku district, southwest Japan. The phase from intermediate-depth earthquakes has apparent velocity close to that of S wave in the Philippine Sea (PHS) plate, but at epicentral distance of 100±250 km it arrives earlier by about 2.5 s than the S wave propagated as a head wave along the upper boundary of the HPS plate. Thus we interpreted the phase as Sp wave into which the S wave refracted upward after traveling in the high-velocity PHS plate was converted at a boundary while propagating on a ray path between the plate and seismic station. The precursory phase to S wave from deep-focus earthquakes was also identi®ed as Sp wave that was caused by conversion of the direct S wave into P wave, because the S±Sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter. The Sp conversion interface was located at the upper boundary of the PHS plate. Further, it was demonstrated that a thin low-velocity layer should overly just above the PHS plate in order to explain both the observed difference in polarity of the S and Sp waves and the amplitude ratio of the Sp wave to the direct S wave. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PHS plate; precursory phase; S-to-P conversion; polarity
1. Introduction Many seismic phases found on seismograms of deep-focus and intermediate-depth earthquakes have been available for the study of the seismic velocity structure of the Earth. For example, Oda et al. (1990) reported that a conspicuous phase was necessarily observed just after the arrival of a weak initial P phase from intermediate-depth earthquakes that occurred in the Chugoku and Shikoku districts, southwest Japan. They interpreted the phase as a guided wave trapped in a thin low-velocity layer lying just * Corresponding author. Fax: 181-86-251-7895. E-mail address: [email protected] (H. Oda).
above the Philippine Sea (PHS) plate. This interpretation leads to a conclusion that the PHS plate subducts with the thin low-velocity layer beneath the Chugoku and Shikoku districts. Similar phases were also found in other regions of the Japan Islands (Fukao et al., 1983; Hori et al., 1985; Hori, 1990). On the other hand, Nakanishi (1980) found a precursory phase to ScS wave on seismograms recorded at seismic stations in the Chugoku and Shikoku districts, and identi®ed it as ScSp wave converted at the upper boundary of the PHS plate. In addition, he emphasized that a negative velocity gradient with increasing depth across the ScS to ScSp conversion interface was required to explain the relative polarity between the ScS and ScSp phases. Although there exists a
0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-195 1(00)00157-8
348
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 1. Epicentral distribution of intermediate-depth (top panel) earthquakes and deep-focus earthquakes (bottom panel). The numbers correspond to the events listed in Table 1.
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
349
Table 1 Source parameters and seismic stations of earthquakes (data of hypocenter and earthquake magnitude are taken from the Seismological Bulletin of the Japan Meteorological Agency) Event No.
Date
Latitude (8N)
Longitude (8E)
Depth (km)
Magnitude
Station
#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 #17 #18 #19 #20
07/25/85 02/19/89 07/30/91 09/09/91 04/08/92 02/25/93 12/18/94 09/13/95 05/31/81 07/04/82 07/19/83 10/31/83 11/30/83 04/06/84 04/21/85 02/12/87 03/20/87 05/03/93 07/30/95 03/01/98
33.71 34.61 33.74 33.77 33.97 34.32 33.71 33.96 30.76 27.80 33.74 30.24 32.65 35.12 29.69 43.23 29.19 36.85 30.60 33.55
134.74 136.47 134.76 134.68 135.79 132.85 132.50 132.41 137.98 137.23 136.96 137.63 140.20 138.59 138.09 132.92 138.28 138.17 139.05 138.42
51 45 52 45 59 45 45 55 500 500 386 531 124 189 514 584 533 207 488 312
5.2 5.3 4.1 3.6 4.3 3.9 3.4 3.3 5.6 7.0 5.7 5.9 6.0 6.7 5.0 6.4 6.1 4.6 6.0 5.6
SBK SBK BSI BS1 BSI OKU BSI BSI SBK SBK SBK SBK SBK SBK SBK SBK SBK OKU OKU OKU
difference in seismic phases used in the above studies, a common conclusion that a low-velocity layer overlies just above the PHS plate is drawn. Since ScSp wave is generated when ScS phase is converted to P wave at the upper boundary of a subducting oceanic plate (Okada, 1979; Nakanishi, 1980; Nakanish et al., 1981), S-to-P converted wave or P-to-S converted wave due to the plate boundary may also be observed. Matsuzawa et al. (1986) reported that a clear phase was really observed between P and S arrivals from deep-focus earthquakes in the Tohoku district, and attributed the phase to P-toS conversion at the upper boundary of the Paci®c plate. However, there have been no reports on the observations of the converted waves in the Chugoku and Shikoku districts which are in a subduction zone of the PHS plate. In this study, we show that a conspicuous phase is observed at seismic stations in the Chugoku district as a precursor to the direct S wave from intermediate-depth and deep-focus earthquakes, and investigate the relationship between the phase and the PHS plate. In particular, we examine the possibility that the phase from deep-focus earthquakes is Sp wave converted at the PHS plate. Furthermore,
the problem of whether or not a thin low-velocity layer exists on the plate is solved by using the polarity of the direct S and converted waves. 2. Precursors to the direct S wave and their causes Fig. 1 shows the location of seismic stations, Shibukawa (SBK), Okayama University (OKU) and Bisei (BSI). At SBK and OKU, three-component seismometers with a natural period of 5 s are operated, and those of BSI have a natural period of 1 s. Source parameters and magnitudes of the earthquakes used in this study are listed in Table 1, and their epicenters are shown in Fig. 1. The ®rst eight events are intermediatedepth earthquakes with focal depths of 40±60 km, and the remaining events are deep-focus earthquakes that occurred in the vicinity of the Japan Islands. Two examples of the seismograms of the intermediatedepth and deep-focus earthquakes are shown in Fig. 2a and b, respectively. A clear phase (X1) appears prior to the arrival time of S wave from intermediate-depth earthquakes (see Fig. 2a). The travel time of the X1 phase is plotted
350
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 2. Seismograms of: (a) intermediate-depth earthquakes (#4 and #6); and (b) deep-focus earthquakes (#12 and #15). The X1 and X2 phases are clearly found prior to the direct S arrivals.
H. Oda, T. Douzen / Tectonophysics 332 (2001) 347±358
Fig. 2. (continued)
351
352
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Fig. 3. Travel time diagram for eight intermediate-depth earthquakes. Solid line shows the linear regression curve obtained by a least-squares method.
against epicentral distance in Fig. 3. From the slope of the regression line determined by a least-squares calculation, the apparent velocity of the X1 phase is estimated to be 5.0 (1/Vs 0.20 ^ 0.07) km/s, which is close to the apparent velocity (4.9 km/s) of the S wave that propagates into the high-velocity PHS plate (Oda et al., 1990). Six ray paths are examined on propagation of the X1 phase in order to investigate where the X1 phase is generated (see Fig. 4). With the exception of case (6), each case shows that the X1 phase travels once as P wave on a part of the ray path. It should be noted that, in case (6), S wave travels as a head wave along the upper boundary of the high-velocity PHS plate. We calculated the travel time curves of the X1 phase in the six cases, assuming a horizontally layered velocity model which was made of the upper crust, the lower crust, the upper mantle and the high-velocity PHS plate. The assumption of the horizontal PHS plate is because the source±receiver direction is almost parallel to the Table 2 Seismic velocity model (Vp, Vs and H are P wave and S wave velocities and layer thickness, respectively)
Fig. 4. Possible ray paths of the X1 phase from an earthquakes source located at a depth of 48 km. Solid and dashed lines represent P and S wave ray paths, respectively. UC, LC, UM and PHS means the upper crust, the lower crust, the upper mantle and the PHS plate, respectively.
Layer
Vp (km/s)
Vs (km/s)
H (km)
Upper crust Lower crust Upper mantle PHS plate
6.09 6.70 7.80 8.10
3.46 3.87 4.50 4.90
15 20 15 ±
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Fig. 5. Plot of the observed S±X1 time (cross) against epicentral distance. Theoretical S±Sp time is shown by dashed line. Solid line is drawn by shifting the dashed line by about 2.5 s.
strike of the Nankai trough (see Fig. 1). Layer parameters of the velocity model are listed in Table 2. The velocity model explains well the observed travel times of the P and S waves of the eight events selected here. The apparent velocity of the X1 phase in each case was determined by applying a least-squares method to the travel times calculated at epicentral distances of 100±250 km. The result is shown in Table 3. The apparent velocity of case (6) is nearly equal to the observed value (5.0 km/s), but those of other cases are largely different from the observed one. In addition, the ray path in case (6) satis®es the condition that the X1 phase must arrive earlier at epicentral distance of 100±250 km than the direct S wave. The travel time difference between the direct S wave and X1 phase is plotted against epicentral distance and compared with the theoretical curve (dashed line) calculated for case (6) (see Fig. 5). The observed data do not fall on the theoretical curve, but they are Table 3 Apparent S wave velocity for six ray paths Apparent velocity (km/s) Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
8.06 6.17 8.06 8.00 6.25 4.90
Fig. 6. Horizontal particle motion of the X2 phase for deep-focus earthquakes (#12 and #15). Arrow shows the azimuth direction of the epicenter.
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Fig. 7. Diagrams of the S±Sp time against epicentral distance (top) and focal depth (bottom).
in good agreement with the straight line (solid line) drawn by shifting the theoretical curve by 2.5 s. This result means that the X1 phase travels once as a head wave with the S wave velocity along the upper boundary of the high-velocity PHS plate but arrives earlier by about 2.5 s than the travel time of S wave calculated for case (6). In order to demonstrate the early arrival of the X1 phase, S-to-P conversion is needed on a ray path between the PHS plate and station because the converted wave travels with P wave velocity higher than S wave velocity. Thus we interpret the X1 phase to be an Sp wave that is generated when the up-going S wave refracted upward from the plate boundary after traveling as a head wave in the PHS plate is converted into P wave at a conversion interface existing between the plate and seismic station. In this case, the Sp wave may be an `inhomogeneous' wave associated with phase distortion (cf. Aki and Richards, 1980) because the
S-to-P conversion at the interface may violate Snell's law. Fig. 2b shows examples of seismograms of deepfocus earthquakes observed at SBK. A clear phase (X2) is found as a precursor to S wave. Since the direction of the horizontal particle motion of the X2 phase is nearly identical with the azimuth direction of the epicenter (see Fig. 6), we identi®ed the X2 phase as P wave. The travel time difference between the X2 phase and direct S waves is plotted in Fig. 7 against focal depth and epicentral distance. The S±X2 time data scatter between 4 and 6 s, and no systematic change is found in both the diagrams. Thus the X2 phase is interpreted as Sp wave which is caused by Sto-P conversion at a boundary existing between the seismic station and hypocenter. The travel time differences are not so large that the conversion interface may be located in the vicinity of seismic stations.
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Fig. 8. Depth distribution of the Sp conversion points for different focal depths of 400, 500 and 600 km. The depth of the conversion point is shown as a function of horizontal distance from a station.
The possible Sp conversion interface is one of the Conrad discontinuity, the Moho discontinuity and the upper boundary of the PHS plate. In order to examine which of them is preferable as the Sp conversion interface, the conversion points were
located on the assumption that the S±Sp time is 5 s. We adopted the seismic velocity model of Ichikawa and Mochizuki (1971) for the travel time calculations. The depth of conversion point is shown in Fig. 8 as a function of horizontal distance from
Fig. 9. Distribution of the Sp conversion points (cross) projected on the ground surface. Triangles show the seismic stations, OKU and SBK.
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Fig. 10. Schematic illustration of the ray path of the Sp phase. A low-velocity layer with width, W, is sandwiched between the PHS plate and upper mantle.
seismic station for different focal depths. Since no conversion point is found at a depth shallower than 30 km, either the Moho discontinuity or the upper boundary of the PHS plate is the Sp conversion interface. However, the Moho discontinuity is not appropriate, because the observed polarity relationship between the direct S and Sp waves cannot be explained if the Sp conversion is attributed to the Moho discontinuity (the detailed discussion on the polarity of Sp and S waves is given below). Thus the upper boundary of the high-velocity PHS plate is appropriate for the Sp conversion interface. Sacks and Snake (1977) proposed to compare polarity between Sp and S waves in order to clarify ®ne velocity structure in the vicinity of a conversion interface. It is demonstrated, by amplitude calculation of seismic waves traveling through two semi-in®nite media in
contact with a plane boundary, that the polarity of vertical component of S wave is opposite to that of Sp wave when the S wave propagates through the boundary from the lower layer with low velocity to the upper layer with high velocity (e.g. Aki and Richards, 1980). Our vertical component seismograms show that the polarity of S wave is really opposite to that of Sp wave (see Fig. 2b). This observation proves that the Sp wave is generated by a conversion interface across which the seismic velocities decrease with increasing depth. Thus we concluded that the upper boundary of the PHS plate is preferable as the Sp conversion interface and the Moho discontinuity is inappropriate because the Moho discontinuity has ordinarily a positive velocity jump with increasing depth. Since the Sp wave has been interpreted to be caused by S-to-P conversion at the upper boundary of the PHS plate, the negative velocity gradient required to explain the opposite polarity suggests that the seismic wave velocities of the lower layer (PHS plate) are lower than those of the upper layer (upper mantle). But the result is inconsistent with a common knowledge that the seismic velocities in the oceanic plate are higher than those in the mantle surrounding the plate. To solve the discrepancy, we put a low-velocity layer between the upper mantle and the high-velocity PHS plate. In the velocity model having the lowvelocity layer on the PHS plate, the Sp wave is generated at the upper boundary of the low-velocity layer. 3. Discussion
Fig. 11. Synthetic seismograms. The top synthetic is the radial component seismogram of the SV wave. The vertical component synthetics of the Sp wave are shown for W 0 and W 5 km.
The X2 phase was interpreted as Sp wave converted at the upper boundary of a thin low-velocity layer overlying the high-velocity PHS plate. We located
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the Sp conversion points so that the theoretical S±Sp times agree with the observed ones, using the velocity model in Table 2, where the depth of the upper boundary of the PHS plate is assumed to be 50 km. In locating the conversion points, the low-velocity layer on the PHS plate was not taken into account because its effect on the S±Sp times is small enough. The incident angle of the direct S wave on the upper boundary of the PHS plate is estimated by travel time calculation using the velocity model of Ichikawa and Mochizuki (1971). The conversion points projected on the ground surface are concentrated in the southern part of the Chugoku district (see Fig. 9). This result supports the view that the leading edge of the PHS plate subducting at the Nankai trough may reach the upper mantle beneath the southern Chugoku district, and is consistent with a common conclusion obtained in the studies of ScSp waves (Nakanishi, 1980; Nakanishi et al., 1981), seismic tomography (Tanaka and Oda, 1997; Ochi et al., 1999) and seismicity of intermediate-depth earthquakes (Kinoshita and Nakanishi, 1997). The ratio of vertical component amplitude of the Sp wave to radial component amplitude of the direct S wave is between 0.1 and 0.2. In order to explain the amplitude ratio, we calculated synthetic seismograms by means of the Thomson±Haskell matrix method (Haskell, 1962) using a simple velocity model where a low-velocity layer with thickness, W, is sandwiched between the upper mantle and the PHS plate (see Fig. 10). The synthetic seismograms, which were calculated for SV-wave incidence on the low-velocity layer from the PHS plate, are shown in Fig. 11. The wavepacket in the case of W 5 km is formed by superposition of Sp waves converted at the upper and lower boundaries of the low-velocity layer, because the lowvelocity layer is too thin to split the wavepacket into the Sp waves converted at both the boundaries. In this case, the amplitude ratio of vertical component of the Sp wave to the radial component of the SV wave falls between 0.1 and 0.2, whereas in the case of W 0 the ratio is much smaller than 0.1. Thus, a thin low-velocity layer just above the PHS plate is needed to explain the amplitude ratio as well as the polarity relationship between the Sp wave and direct S wave. In order to explain magma genesis in the subduction zone of an oceanic plate, Ringwood (1975) proposed a model in which a major part of the oceanic
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crust subducts into the mantle beneath the continents as a part of the oceanic lithosphere. In the seismological aspect, the subducting oceanic crust may be detected as a low-velocity layer in the upper mantle because its seismic velocities are lower than those of the upper mantle and the oceanic plate. Thus, the thin low-velocity layer which was found just above the PHS plate by this study, as well as the studies of the ScSp wave (Nakanishi, 1980) and guided waves (Oda et al., 1990), strongly supports the subduction of the oceanic crust in the Ringwood's model. 4. Conclusions At seismic stations in the Chugoku district, southwest Japan, a clear precursory phase was recorded a few seconds prior to the S arrival from intermediate-depth and deep-focus earthquakes. We investigated the origin for which the seismic phase was generated. The results are summarized as follows: (1) The precursory phase from intermediate-depth earthquakes is interpreted as Sp wave into which the S wave traveled as head wave in the high-velocity PHS plate is converted at a boundary on a ray path from the plate to seismic station. (2) The precursory phase from the deep-focus earthquakes is identi®ed as Sp wave that is caused by S-to-P conversion at the upper boundary of the PHS plate. (3) A thin low-velocity layer should overly just above the high-velocity PHS plate in order to explain both the amplitude ratio and the polarity relationship between the Sp wave and the direct S wave. (4) The leading edge of the PHS plate reaches the upper mantle beneath the southern part of the Chugoku district. Acknowledgements We thank D. Zhao and an anonymous reviewer for giving critical comments to revise the manuscript. References Aki, K., Richards, P.G., 1980. Quantitative Seismology, Theory and Methods, 1. Freeman, San Francisco. Fukao, Y., Hori, S., Ukawa, M., 1983. A seismological constraint on
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the depth of basalt±eclogite transition in a subducting oceanic crust. Nature 303, 413±415. Haskell, N.A., 1962. Crustal re¯ection of plane P and SV waves. J. Geophys. Res. 67, 4751±4767. Hori, S., 1990. Seismic waves guided by untransformed oceanic crust subducting into the mantle: the case of the Kanto district, central Japan. Tectonophysics 176, 355±376. Hori, S., Inoue, H., Fukao, Y., Ukawa, M., 1985. Seismic detection of the untransformed ªbasalticº oceanic crust subducting into the mantle. Geophys. J. R. Astron. Soc. 83, 169±197. Ichikawa, S., Mochizuki, E., 1971. On traveltime tables for local earthquakes. Pap. Meteorol. Geophys. 22, 229±290 (in Japanese with English abstract). Kinoshita, Y., Nakanishi, I., 1997. Subcrustal seismicity beneath the Southern part of the Chugoku region, Japan. J. Phys. Earth 45, 307±312. Ochi, F., Asamori, K., Zhao, D., 1999. Deep structure of the Japan Islands determined with J-array's teleseismic data, (2) Southwest Japan, Programme and Abstract, Fall Meeting, Seismological Society of Japan, vol. B30 (in Japanese). Oda, H., Tanaka, T., Seya, K., 1990. Subducting oceanic crust on the Philippine Sea plate in southwest Japan. Tectonophysics 172, 175±189.
Okada, H., 1979. New evidence of the discontinuous structure of the descending lithosphere as revealed by ScSp phase. J. Phys. Earth 27, S53±S56. Matsuzawa, T., Umino, N., Hasegawa, A., Takagim, A., 1986. Upper mantle velocity structure estimated from PS-converted wave beneath the north-eastern Japan Arc. Geophys. J. R. Astron. Soc. 85, 767±787. Nakanishi, I., 1980. Precursors to ScS phases and dipping interface in the upper mantle beneath southwest Japan. Tectonophysics 69, 1±35. Nakanishi, I., Suyehiro, K., Yokota, T., 1981. Regional variations of amplitudes of ScSp phases observed in the Japanese Islands. Geophys. J. R. Astron. Soc. 67, 615±634. Tanaka, T., Oda, H., 1997. Three-dimensional structure of P wave velocity in the crust and upper mantle beneath southwest Japan. Okayama Univ. Earth Sci. Rep. 4, 1±20 (in Japanese with English abstract). Ringwood, A.E., 1975. Composition and Petrology of the Earth's Mantle. McGraw-Hill, New York. Sacks, I.S., Snake, J.A., 1977. The use of converted phases to infer the depth of the lithosphere±asthenosphere boundary beneath South America. J. Geophys. Res. 82, 2011±2017.