The interplate coupling and stress accumulation process of large earthquakes along the Nankai trough, southwest Japan, derived from geodetic and seismic data

The interplate coupling and stress accumulation process of large earthquakes along the Nankai trough, southwest Japan, derived from geodetic and seismic data

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INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS, NANKAI TROUGH

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216

subducting plates (Uyeda and Kanamori, 1979), back-arc spreading related to subduction (Uyeda and Kanamori, 1979; Ruff and Kanamori, 1980), the age of the subducting plate (Vlaar and Wortel, 1976; Ruff and Kanamori, 1980; Peterson and Seno, 1984), the asperity content of the interface (Lay and Kanamori, 1981), accretionary prism growth (Uyeda and Kanamori, 1979), seismic moment release rate (Peterson and Seno, 1984), cxpansion patterns of the aftershock area (Tajima and Kanamori, 1985; Singh and Suárez, 1988), time variations of the fault plane solutions in shallower and deeper regions associated with a large earthquake (Astiz and Kanamori, 1986; Astiz et al., 1988; Christensen and Ruff, 1988; Lay et al., 1989). In this paper, we investigate the interplate coupling between the overriding Eurasian and subducting Philippine Sea plates along the Nankai trough (Fig. 1). Historical documents show that large earthquakes have occurred there periodically at an average interval of about 120 years (e.g., Ando, 1975). As almost a half century has passed since the occurrence of the latest events (the 1944 Tonankai (M= 8.0) and the 1946 Nankaido (M = 8.1)), it is naturally considered that the stress accumulation for the next large events has already started. As extensive geodetic measurements, such as levelling, tide gauge and trilateration have been repeated over the Japanese islands during the last several decades, and considering the fact that the Nankai trough is located relatively close to the land area, the possibility exists of detecting vanous effects of interplate coupling using these geodetic data. If successful, in addition to the abovementioned controlling factors of interplate coupling, the geodetic data should also become an effective means of analysing the coupling properties. In order to elucidate some aspects of these mechanisms, two-dimensional finite element models (2-D FEM) (vertical cross-section) perpendicular to the trench axis have been constructed in northeast and southwest Japan (e.g. Bischke, 1974; Shimazaki, 1974; Smith, 1974; Kato, 1979; Seno, 1979a; Miyashita, 1987). Yamashina (1976) and Seno (1979b, c) also calculated the horizontal strain fields over the Japanese islands by means of

S. YOSHIOKA

an analytical method and a 2-D FEM, respectively, taking into account the convergence of the Pacific and Philippine Sea plates with the Eurasian plate. Two-dimensional modelling seems to be insufficient, however, because of the oblique subduetion of the Philippine Sea plate (Fitch, 1972; Shimazaki, 1976) and its three-dimensional complicated structure subducting beneath the Eurasian plate. Accordingly, it is necessary to apply three-dimensional analysis to obtain more precise information. The purpose of this paper is to estimate the amount and regional variations of interplate coupling associated with the subduction of the Philippine Sea plate, and to clarify their tectonic implications in southwest Japan.

2. Data Recently, Hashimoto (1990) obtained the horizontal strain rates over the Japanese islands on the basis of several sets of first-order trilateration surveys for the last 90 years by the Geographical Survey Institute (GSI) (Fig. 2). The solid and dashed lines shown in this figure denote the principal axes of extension and contraction, respectively. Since the crustal deformations associated with several large earthquakes are excluded, this pattern can be regarded as the stationary strain fields during an interseismic period. As evident from the figure, the NW—SE oriented compressive strain fields are dominant along the southern coastal regions in Tokai, Kinki, and Shikoku. In particular, a large amount of strain of about 5—6 1 can be seen in the southwestern X iO~ year part of Shikoku. In view of Seno’s results (1977) that the Philippine Sea plate moves in the N54°W to N50 W direction in these regions, it is conjectured that the plate motion of the Philippine Sea plate relative to the Eurasian plate may have some influence on these strain fields. It should also be noted that these effects do not appear to reach further north to the Chugoku region. Kato and Tsumura (1983) have obtained longterm vertical crustal movements in Japan, using tidal records from about 100 stations for the period 1951—1978 (Fig. 3a). The stippled regions in the °

217

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

figure denote areas of uplift, other regions being areas of subsidence. These tidal data are useful for evaluating the nearly absolute crustal movements (see Kato and Tsumura, 1979). The southernmost tips of the Ku peninsula and Shikoku are regions of subsidence which may be the result of the downward drag due to the subducting Philippine Sea plate. However, unlike the subsidence pattern along the eastern coastal region in northeast Japan, an E—W trending uplift region evident in the northern region areas adjacent to the isabove-mentioned southernmost of subsidence in southwest Japan. This regional difference will be discussed in more detail later, Although these tidal data are very useful for estimating the nearly absolute crustal movements along the coastal regions, they do not provide any information on the inland crustal movements. In order to overcome this and to obtain the nearly absolute crustal movements there, we combine the

tidal records with the levelling data, which show the crustal movements relative to a fixed point. Here, we estimate the correction C for the nearly absolute crustal movement by minimizing the following quantity in the least-square sense N —

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218

5. YOSHIOKA

levelling route, we can accomplish the above purpose. Here, we use the levelling data compiled by Thatcher (1984) and Miyashita (1989) for the

period 1960—1975. The obtained results are shown in Fig. 3c, and we regard them as the crustal movements during the interseismic period. The

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mm/yr (c) 10 INTERSEISMIC Fig. 3. (a) Vertical crustal movements along the coast of the Japanese islands during the period January 1951 to August 1982, deduced from tide gauge records (mm year~). Stippled area denotes regions of uplift, other areas are of subsidence (modified from Kato and Tsumura, 1983). (b) Locations of the tide gauge stations to obtain the nearly absolute crustal movements over the Ku peninsula and Shikoku. (c) The calculated vertical crustal movements for the interseismic period (mm year’). The barbed side of contour lines denotes the direction of decreasing movements.

219

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

barbed side of each contour line denotes the direction of decreasing vertical crustal movements. As seen from the figure, there are some remarkable features; the E—W oriented upheaved zones over the Ku peninsula and Shikoku, which are also seen in Fig. 3a; the northward trend of subsidence in the northern region of the Ku peninsula and Shikoku; a large amount of uplift of about 10 mm year’ in the southwestern part of Shikoku; a subsidence rate of —7.1 mm year’ at the Cape Muroto and —0.9 mm year~ at Shionomisaki.

3. Model and method Fitch (1972) pointed out that there are some developed faults parallel to the plate margin near the axis of an active volcanic chain on the continental side in western Pacific regions. The Median Tectonic Line (MTL) in southwest Japan is one of the remarkable examples, and may be

considered a result of transcurrent movements due to the oblique subduction of the Philippine Sea plate. Shimazaki (1976) also noted that the extension of the axes of maximum shortening in southwest Japan, mostly across the Nankai trough, lies at an angle of about 45 to its general trend. Seno (1977) calculated the convergence rate and direction of the Philippine Sea plate relative to the Eurasian plate. The results show that the direction changes from N54°W to N50°W in the area from the east coast of the Ku peninsula to the western part of Shikoku (Fig. 4a). From these results, it is evident that the Philippine Sea plate subducts obliquely beneath the Eurasian plate in southwest Japan. In addition to this, unlike usual subducting plates in other regions, the configuration of the subducting Philippine Sea plate is very cornplicated. Its upper surface, which is inferred from the distribution of subcrustal earthquakes (e.g., Mizoue, 1976), shows a contorted structure with a °

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220

5. YOSHLOKA

dip varying from 8 to 400 from region to region (Fig. 6a). The leading edge of the subducting Philippine Sea plate is not parallel to the Nankai trough and its depth ranges between 40 and 100 km. Considering the oblique subduction and spatially contorted structure of the Philippine Sea plate, we need to introduce three-dimensional analysis into southwest Japan. According to this requirement, we constructed a three-dimensional

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finite element model, taking into account the intncacies mentioned above. The model space, the mesh design, boundary conditions and the structural model used here are almost the same as those in Yoshioka et al. (1989) and Yoshioka and Hashimoto (1989a, b), except that the mesh is expanded westwards so as to include the entire Kyushu region (Fig. 5a and b). In our model, a three-dimensional configuration of the subducting Philippine Sea plate was taken from Mizoue (1976)

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for the region west of the central part of the Ku peninsula, and from Yamazaki and Ooida (1985) for its eastern part (Fig. 6a and b).

same subducting rate as the coupled region. In contrast to this, weak coupling means that the overriding plate is dragged at a rate much less than the actual subducting rate.

4. Results and discussion

4.1. A strongly coupled model over the extensive regions during the interseismic period (model I)

In this section we estimate the coupled region and degree of coupling to fit the observations. Through our study, we define the degree of coupling by the ratio of annual movement of the overriding plate to that of the subducting one at their interface over the extensive regions. Thus, strong coupling means that the overriding plate is dragged with the subducting plate at nearly the

Here, we discuss the patterns of horizontal strains and vertical displacements expected from a strongly coupled model. First of all, we constructed the strongly coupled model from the Ku peninsula to the eastern side of Kyushu. We assigned an amount of slip corresponding to the convergence rate of the Philippine Sea plate rela-

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Fig. 7. (a) Horizontal projection of the coupled region (stippled) for the calculations in model I. The convergence rate between the subducting Philippine Sea plate and overriding Eurasian plate is assigned on the coupled region along the plate boundary. (b) Horizontal strain rates calculated from model I (year~). (c) Vertical displacements calculated from model I (mm year~). (d) Horizontal displacements calculated from model I (cm year~).

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tive to the Eurasian plate along the plate boundary (indicated as a stippled region in Fig. 7a). The southern edge of the coupled region corresponds to a depth of about 4 km and the northern edge to the leading edge of the subducting Philippine Sea plate. The assigned convergence rate is 3.5 cm year~ on the eastern edge of the Ku peninsula, gradually increasing westwards to 4.7 cm year1 on the southern edge of Kyushu (Seno, 1977). This model is hereafter referred to as model I. We show the calculated horizontal strain fields and the vertical and horizontal displacements on the Earth’s surface in Fig. 7(b—d). Comparing Fig. 7b with the observations (Fig. 2), we recognize some notable discrepancies between them. Around the Ku channel, where the Philippine Sea plate is subducting with a higher angle of dip, the observed strain fields represent the NW—SE compressive strains, while some rotation of the cornpressive axes can be identified there in model I. In the Kyushu region, the N—S oriented tensile strain fields can be observed, as pointed out by Tada (1984), while the calculation yields the weak NW—

YOSHIOKA

SE compressive strains. This suggests that the tectonic force, presumably accompanied by the spreading of the Okinawa trough in a back-arc basin of the Ryukyu trench (Fig. 1) (Tada, 1984), is more predominant than that owing to the interplate coupling. Based on numerical modelling, Hashimoto (1985) interpreted that a N—S oriented tensile stress field may be generated by the interaction between the slab pull force, crustal buoyancy and flows in the asthenosphere. The effects of the interplate coupling appear too weak to be identified in the Kyushu region, because the distance from the nearest coupled zone to the Earth’s surface in Kyushu is much longer than that to other regions. In other words, it would be difficult to detect by our method whether the interplate coupling is strong or not on the eastern side of Kyushu. We therefore do not refer to the observed strain fields there in our following discussion. With regard to the vertical movements (Fig. 7c), the calculation yields considerable amounts of subsidence over the southern Ku peninsula and

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Fig. 8. (a) Horizontal projection of the coupled region for the calculations in model II. More than 70% of the convergence rate between the subducting Philippine Sea plate and overriding Eurasian plate is assigned on the coupled region along the plate boundary. See text for explanation of regions K (including hatched region 0), L, M, N and 0. (b) Horizontal strain rates calculated from model II (year~1)• (c) Vertical displacements calculated from model II (mm year~). (d) Horizontal displacements calculated from model II (cm year~).

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

Shikoku in contrast to the E—W trending upheaved zones shown in the observations (Fig. 3c). In particular, a large amount of subsidence of

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about —15 mm yeart is identified near the Ku channel in model I. Consequently, the strongly coupled model beneath the Ku channel does not

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fit both the observations in the horizontal strain fields and the vertical movements, suggesting much weaker interplate coupling there, In conclusion, it seems to be rather difficult to explain by model I the above two differences in the observed data for the extensive regions. This means that we should take account of some regional differences in the location and strength of interplate coupling even on the same plate boundary. 4.2. A partially coupled model during the interseismic period (model II) Figure 8a shows a partially coupled region to explain the observed strain fields and the vertical crustal movements. We determined the location of this region by trial-and-error method, mainly noticing the southward subsidence pattern in the vertical movements. For the westernmost rectangular-like coupled region (K in Fig. 8a), we assumed a constant rate of 3 cm year1, which is about 70—75% of the actual convergence rate, and a dip of 200, being a little higher than the actual angle of about 100. Here, we assigned the higher dip angle to explain the relatively large amount of upheaval and its limited E—W trending zone in Shikoku. For the other parts of the coupled region (L, M and N in Fig. 8a), we assumed strong coupling on the plate boundary, giving the actual convergence rate and direction obtained by Seno (1977). The region N was introduced to fit the observed northward subsidence in the Ku peninsula (Fig. 3c). This model is hereafter referred to as model II. Comparing this with model I, the location of the coupled region is limited to the relatively shallow region. We show the calculated horizontal strain fields and vertical and horizontal displacements in Fig. 8(b—d). Comparing Fig. 8b with the observed strain fields (Fig. 2), there are some remarkable agreements to be noted. In the Xii peninsula, the NW—SE and NNW—SSE oriented compressive strain fields observed in its central and western regions, respectively, are well explained by model II. In Shikoku, the agreements are more pronounced in the NW—SE oriented compressive strain fields, with the relatively large NE—SW

S. YOSHIOKA

oriented tensile fields located in the southern region, in the compressive axes gradually rotated from NW—SE to NWN—SES directions in the southwestern region, and in the change of the direction of the compressive strain axes from NW—SE to NNW—SSE toward north. In addition to these, the results indicating that the effects of interplate coupling do not reach the further inland, Chugoku region, are also consistent with the observations. The amounts of the calculated strains are, however, smaller by a factor of two to three as a whole than those observed. Considering the facts that the observed strain fields show relatively large standard deviations in southwest Japan (Hashimoto, 1990), our results may explain the observations within a permissible extent. The vertical movements indicated by the calculations (Fig. 8c) and the observations (Fig. 3c) also show fairly good agreement in the amount of subsidence at Shionomisaki (—0.9 mm year1 calculated) and at Cape Muroto (— 8.3 mm year1 calculated), and in the amount of uplift of about 5 mm year~1near Shirahama on the west coast of the Xii peninsula (Fig. 8c). On the other hand, some distinctive discrepancies are noticed in the northern Ku peninsula, northern Shikoku and southwestern Shikoku regions. In the northern region of the Xii peninsula and Shikoku, a noticeable amount of subsidence towards the north can be recognized. Even if we constructed a variety of models, these observed subsidences cannot be explained. These discrepancies provide important information, as follows: with regard to vertical movements the effects of interplate coupling are limited to only the southern regions of the Ku peninsula and Shikoku, and the land subsidence in their northern regions may be the result of some other reasons. One reason could be the pumping of ground-water to meet the demand of heavy industry, which, in the late 1960s, developed rapidly in Japan. This may have had some effect on the ground level in the northern part of the Xii peninsula, which is close to two major industrial areas (Chukyo and Hanshin), and also in the northern region of Shikoku, which is close to the Setouchi industrial area (Fig. ib). Accordingly, there is a possibility that the levelling data are affected by these artifi-

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

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228

cial noises in these regions. If this is the case, the deeper coupled region (N in Fig. 8a) in the northeastern part of the Xii peninsula, which was introduced to explain the observed data there, would be spurious. For these reasons, we do not discuss hereafter the crustal movements in these northern regions.

S. YOSHLOKA

4.3. An anomalously strong coupling on the western coupled region off Shikoku (model III) and its tectonic implications A large amount of observed uplift of about 10 mm year1 is identified in the southwestern part of Shikoku. Even if we introduced the actual convergence rate into the limited region in model II (K in Fig. 8a), instead of an assumed value of 3 cm year’, the calculated uplift would reach only 4 mm year1 at most. In order to fit the observed uplift, in addition to the large amount of horizontal strains of about 6 x iO~ year~ there, we applied an additional NW oriented horizontal force, passing through the western half (0 in Fig. 8a) of the westernmost rectangular-like coupled region off Shikoku. In this case, the effective force corresponding to a convergence rate of 7—8 cm year~ is exerted on the continental side of this region. This model is hereafter referred to as model III. The calculated strain fields and the vertical and horizontal displacements based on this model are shown in Fig. 9. Although this kind of local stress concentration on the plate boundary is not necessarily a priori justifiable, we will show some possible evidence to support this idea, together with the geomorphological peculiarity there, As mentioned previously, large earthquakes have occurred periodically at an average recurrence of about 120 years along the Nankai trough. For a couple of recent events, fault parameters have been estimated by Ando (1975). We recognize from his results that the amount of slip on fault A has always been significantly larger than that on the adjacent faults for each event (Fig. 4). Incidentally, if we divide the estimated displacement of 6 m on fault A for the 1946 Nankaido earthquake by the average recurrence time of 120 years, the result gives 5 cm year ~, which is slightly larger than the convergence rate 4 cm year~

,

there. If we consider that the effective stress is relatively large on the western half off Shikoku, this apparently contradictory estimate might be explained by the occurrence of large stress accumulation and release there. Using a linear multivariate regression, Ruff and Kanamori (1980) pointed out that the strength of coupling is correlated well with two variables: the convergence rate and age of a subducting slab. In general, as the convergence rate increases and the age of the subducting plate becomes younger, stronger interplate coupling would be expected. On the other hand, in regions with a low convergence rate and an older plate age, the force acting normal to the interface between the overlying and subducting plates would decrease. It is considered that the Shikoku basin was formed about 15—30 Ma B.P. by spreading along the NNW—SSE trending axis of a back-arc basin of the previous Bonin—Mariana arc (e.g. Kobayashi and Nakada, 1978). Combining these variations of the age of subducting plates with the convergence rate of the Philippine Sea plate relative to the Eurasian plate, we plotted these estimates on the diagram by Ruff and Kanamori (1980) (Fig. 10). The right upper portion in the figure corresponds to strongly coupled regions, and vice versa. Letters A—E correspond to the locations shown in Fig. 5a. This figure shows that there also appears to be appreciable discrepancies of interplate coupling even along the same plate boundary, being consistent with the strong coupling assumed in the western part of Shikoku. These two analyses may support the idea that the western part of Shikoku is a strongly coupled region. Geographically this region is located between the NNW—SSE trending Xinan Seamount Chain and the Kyushu—Palau Ridge (Fig. la). Recently, based on swelling features on the bathymetric profile and on magnetic anomalies, Yamazaki and Okamura (1989) indicated the possible existence of a subducting seamount located in the south of the Xii channel. They also suggested that a subducting broad seamount exists in the same manner at the northern extension of the Kyushu—Palau Ridge, under the fore-arc wedge. The large negative free-air anomaly of about —140 p.gal (Kono

229

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAL TROUGH

and Furuse, 1989) centring around the Hyuganada region may also be a manifestation of the remnant of a broad seamount. As discussed in detail in the next section, from analogy to the mode of subduetion between the Xii channel and the eastern side of the Kyushu region, it may be conjectured that the Philippine Sea plate subducts easily, with weak coupling from the eastern side of the Kyushu region. This effect would cause a resistant force against the subduction on the western part off Shikoku. Furthermore, the Philippine Sea plate penetrates northwards with a low dip angle beneath Shikoku (Fig. 6a). Taking account of the NW oriented movement of the Philippine Sea plate relative to the Eurasian plate, the western part of Shikoku might substantially support the compressive stress due to the plate motion. This suggests that the large amount of stress accumulation occurs easily, resulting in further strong coupling in

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addition to the strong coupling associated with the drag of the subducting plate there. We show the calculated horizontal displacement fields based on the above assumption in Fig. 9c. In the western part of Shikoku, a large displacement reaching 3 cm year1 should be expected, gradually decreasing towards the north. It should be noted that direction changes from NW to NNW towards the north both in Shikoku and in the Xii peninsula, as in the case of the strain fields. 4.4. A close correlation between the strength of interplate coupling and other seismological data in southwest Japan Figure 11 shows the epicentral distribution of earthquakes that occurred for the period from 1960 to 1988 in southwest Japan, together with the isodepth contours of the upper surface of the

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subducting Philippinb Sea plate and the estimated coupled region. For investigation of the seismicity data, we used the SEIS-PC program (Ishikawa et al., 1985), which includes all events obtained by the network of the Japan Meteorological Agency (JMA) since 1926. Here, the lower limit of magnitudes is taken to be 4.0, and events shallower than 50 km are plotted, Comparing the seismic activity with the configuration of the Philippine Sea plate, the degree of coupling and the location of seamounts, there seems to be a notable correlation among them: the region where seismicity is high (e.g. Xii channel) corresponds well to the region with the possible existence of seamounts, a high dip angle of the subducting slab and weak coupling, while the low seismicity region (e.g. off Shikoku and southeastem side of the Xii peninsula) correlates with the region with no seamounts, low dip angle subduetion and strong coupling, These suggest that stress accumulation over an extensive area is difficult in the high dip angle subducting region owing to stationary stress release as a result of the occurrence of moderatesized earthquakes on the corrugated interface likely to occur where seamounts are being subducted. In this case, areas in contact between overriding and subducting plates would be localized near seamounts, and it would be difficult for the overriding plate to be dragged extensively with the subducting plate, leading to weak interplate coupling there. On the other hand, on the strongly coupled region, the stress is likely to accumulate effectively because of the low seismicity and the smooth plate boundary with an extensive area of contact, releasing the stress in a single large earthquake. Taking account of these considerations for the eastern side of the Kyushu region, where it was not clear whether the coupling is strong or not, through the comparison between the observations and our calculations, we may say that the coupling is not strong because of the high dip angle subduetion, the possible existence of a buoyant ridge and regular pattern of moderate-sized earthquakes. This indicates that the features in two extreme subducting modes, such as ‘Chilean’ and ‘Mariana’ types (Uyeda and Kanamori, 1979) could apply to

231

those in small-scale regions. However, we should note some clear differences, such as the pattern of seismic activity. On the western part of the strongly coupled region L (Fig. 8a), however, it should be noted that the Kinan Seamount Chain is situated in this area and that the frequency of seismic activity is not necessarily low, thus indicating the possibility of weak coupling. In order to obtain the extent of the degree of coupling there, we further tested a variety of values and compared the calculated strain fields and vertical displacements with those for the strongly coupled case. As the degree of coupling is lowered, the NE—SW oriented tensile strains become dominant and the amount of vertical deformation tends to decrease just to the north of the coupled region. Taking into account this tendency in the calculated patterns, the possible extent of the degree of coupling in the western part of the region L is estimated in the range 30—100%. This uncertainty in the degree of coupling is owing to the lack of comparable observed data on the land around the Xii channel (see Fig. 2). In conclusion, it would be difficult at present to specify the exact degree of coupling there based only on the above-mentioned data. 4.5. Comparison of the interplate coupling between southwest and northeast Japan As pointed out earlier, there is a considerable discrepancy in the patterns of vertical displacement rate, on the Pacific coastal regions, between southwest Japan and northeast Japan (Fig. 3a). This is probably due to the difference in coupling strength related to the coupling of oceanic and continental plates, and it may reflect various tectonic processes related to island-arc subduetion. Considering that the Philippine Sea plate subducts beneath the Eurasian plate in southwest Japan in contrast to the subduction of the Pacific plate beneath the Eurasian plate in northeast Japan, the discrepancy may be the result of the difference in the nature of the contacting plate boundary. However, the large 1944 Tonankai and 1946 Nankaido earthquakes in southwest Japan occurred as a result of the underthrusting of the Philippine Sea plate beneath the Eurasian plate

232

(e.g. Fitch and Scholz, 1971; Kanamori, 1972; Ando, 1975). This might explain the difference in the strength and location of the coupling before and after these events. In order to make clearer the influence of the spatial and temporal differences on the discrepancy, we further investigated the crustal movements preceding these two large earthquakes in southwest Japan. We show in Fig. 12a the obtained vertical movement rates during the 40 years (pre-seismic) preceding these events, which are based on the levelling data compiled by Thatcher (1984) and Miyashita (1989). The same method for the case of the interseismic period was applied to obtain the nearly absolute annual rate during this period. However, there are a few points to be noted. The only continuous tide gauge records available during this period are those recorded at Kushimoto (—5.6 mm year ~) (Fig. 3b), and these observations were employed to estimate the nearly absolute level over the Xii peninsula. In the Shikoku region, since there were no continuous tide gauge records for this period, we calculated the nearly absolute level over the Shikoku region so as to relate the movements at Muroto for the pre-seismic period with the interseismic period (— 7.1 mm year~). In contrast to the interseismic movement rate (Fig. 3c), some important features may be noticed in the pre-seismic record. In the northern part of the Ku peninsula and Shikoku, the northward subsidence, which was remarkably noticeable for the interseismic period, is not observed. In the southeastern part of the Xii peninsula, southeastward subsidence is seen for the pre-seismic period. In Fig. 12b, we show the most appropriate coupled region for this period. In order to fit the observations, we arbitrarily assumed rates ranging from 70 to 100% of the convergence rate on the major part of the coupled region, with slightly lower rates just beneath the southwest coast in the Xii peninsula. For the eastern half of the westernmost rectangular-like coupled region off Shikoku (P in Fig. 12b), we further assumed a dip of 13°, instead of the actual angle of about 100. In order to explain the southeastward subsidence in the Xii peninsula, it is necessary to introduce strong coupling in the deeper portion on the plate boundary.

S. YOSHIOKA

A good agreement between the calculated vertical displacements (Fig. 12c) and the observations (Fig. 12a), in their magnitude and pattern, can be seen over the southern part of the Xii peninsula and Shikoku. Comparing Fig. 12b with Fig. 8a, as expected from the difference in the vertical crustal movements in the southeastern part of the Xii peninsula, the coupled region during the pre-seismic period appears to have penetrated deeper than that during the interseismic period. The degree of coupling is 80% of the convergence rate there. This might suggest that the extent of the coupled region varies with time, gradually spreading from a shallower to deeper portion with the transition from interseismic to pre-seismic stage. If this is true, longterm observations of vertical crustal movements might become one of the important clues in understanding the stress accumulation process of an interplate large earthquake in subducting regions, and further, to predict it. On the other hand, a variety of co-seismic fault models of the 1944 Tonankai and the 1946 Nankaido earthquakes have been proposed by many investigators (e.g. Fitch and Scholz, 1971; Kanamori, 1972; Ando, 1975, 1982; Inouchi and Sato, 1975; Ishibashi, 1976; Aida, 1979; Fujii, 1980). We represent some of them for each event in Fig. 13. Comparing these models with the coupled region for the pre-seismic stage, it appears that the coupled region described above coincides well with the co-seismic faulting region except beneath the southeastern part of the Xii peninsula. However, if we consider the aseismic slip model (Thatcher and Rundle, 1979), which proposes the downdip extensions of co-seismic fault planes to explain the large vertical movements for the several years after the two events, the gap between the co-seismic models and the pre-seismic coupled region could be filled. This suggests that the strains accumulated on the coupled region until the occurrence of the events may have been released by the co-seismic and post-seismic faulting. Although the nearly absolute vertical movements over the Shikoku region are not clear for the pre-seismic stage, the pattern of vertical movements might also show southward subsidence over the entire southern region of Shikoku, as in the case in the

233

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

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234

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INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS, NANKAI TROUGH

Xii peninsula. This case would also be explained by the above-mentioned aseismic fault model, Here, in order to compare our results with those in other regions, we consider the case in northeast Japan. Making a comparison between the seismic slip and the rate of plate motion, Kanamori (1977) suggested that the seismic slip constitutes a very small portion, approximately one-quarter, of the total slip in northern Japan. Analysing the geodetic data in eastern Hokkaido (Fig. 1) preceding the 1973 Nemuro-oki earthquake by means of a 2-D FEM, Shimazaki (1974) estimated the amount of coupling along the plate boundary between the Eurasian and Pacific plates, resulting in almost the same conclusion mentioned above. He also concluded that the coupling would occur to a depth of about 100 km. Two similar analyses have been made in central Tohoku (Fig. 1), showing almost the same amount of coupling and range as in the case of Hokkaido (Kato, 1979; Seno, 1979a). On the basis of Shimazaki’s results, Kasahara and Kato (1980—1981) (by using the tide gauge records of the succeeding 5 years) suggested the possibility that the stress accumulated preceding the 1973 earthquake in the deeper portion in eastern Hokkaido has not been released even after the event, From a petrological point of view, Shimamoto (1985) interpreted Shimazaki’s analysis as follows: shallow subducting plate boundaries above depths of -20—25 km are characterized by a low frequency of seismic events, low tectonic stress, interplate decoupling, ductile deformation, and the presence of abundant H20. Contrary to this, our analysis for southwest Japan requires strong coupling at this depth range, suggesting different modes of subduction and earthquake generating mechanisms in southwest Japan and northeast Japan. Frpm these results, we conclude as follows. In southwest Japan, in general, the interplate coupling is strong and its region is limited to a relatively shallow portion (< 30 km for interseismic; <50 km for pre-seismic). The pre-seismic coupled region coincides with the co-seismic and post-seismic fault zone, suggesting that the accumulated stress is released by a single large earthquake associated with low angled thrust faulting. In contrast to this, in northeast Japan, the cou-

235

pling is weak and its region of significance extends down to a depth of about 100 km. The co-seismic and post-seismic regions are limited to the upper half of the coupled region. Although the source of these differences is still open to question, it might be related to the difference in the tectonic history of subduction for different plates. 4.6. The stress fields expected from the interplate coupling in southwest Japan and their tectonic implication Investigating the focal mechanisms of earthquakes would be an effective way to obtain useful information on the tectonic stress fields. In southwest Japan, these have been well determined for a range of magnitudes, from major to microearthquakes that have occurred in the crust and uppermost mantle (e.g. Shiono, 1970a, b, 1977; Ichikawa, 1971; Nishida, 1973; Ooida and Ito, 1974; Shiono et al., 1980; Ukawa, 1982; Mizoue et al., 1983; Ohkura, 1988). The results show that the P-axes are oriented generally in an E—W to ESE—WNW direction in the crust beneath southwest Japan, except in the Kyushu region (Shiono, 1977). One of the possible sources for this stress field may be the nearly E—W oriented convergence between the Eurasian and Pacific plates (e.g. Huzita, 1980). Earthquakes in the uppermost mantle, on the other hand, show substantially different focal mechanisms compared with those in the crust. Earthquakes beneath the Xii channel and the eastem part of Shikoku below 20 km have P-axes oriented in the N—S direction (e.g., Shiono, 1970b; Sawamura and Ximura, 1971) (Fig. 14b). Okano et al. (1979) suggested that the deeper earthquakes with N—S compression, rather than the shallower earthquakes with E—W compression, represent the dominant stress field beneath Shikoku, since the earthquake energy released by the former is much greater than that by the latter. They also noted that it is rather difficult to explain the N—S onented compressive stress field by the plate motion because the movement of the Philippine Sea plate is oriented in the N50 °W direction. In addition, most of the fault plane solutions of the deeper earthquakes show strike-slip faulting with N—S compressive axes in spite of the fact that they are

236

S. YOSHIOKA

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considered to occur on the plate boundary (Fig. 14c). However, for earthquakes off Shikoku, their P-axes appear to coincide with the direction of the plate motion (Fig. 14b). Earthquakes that occur beneath the southern part of the Xii peninsula also show complicated features. They are classified into three different seismiccrustal, upper zones in transitional terms of and theirsubcrustal hypocentres; seismic the

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For deeper earthquakes in the transitional seismic zone (25—35 km), their fault plane solutions are more complicated (Fig. 15b), which appear to change from strike-slip type to normal faulting toward north. Most of the fault plane solutions of subcrustal earthquakes (>35 km) represent normal faulting with the T-axes in the NE—SW direcIn order to understand these complicated stress tion. fields in southwest Japan, many seismologists have investigated them in different ways. Shiono (1977) studied the fault plane solutions of subcrustal earthquakes that occur just above the slab, and showed that the tensile force parallel to the leading edge of the subducting Philippine Sea plate is dominant there (parallel extension). The subcrustal earthquakes that occur beneath the southern part of the Xii peninsula (>35 km) probably belong to this group. In terms of the orthogonality

40

Q42 Fig. 14. (a) Distribution of maximum compressional axes for earthquakes (M> 2) that occurred in the crust beneath the eastern part of Shilcoku (after Okano, 1988). (b) Distribution of maximum compressional axes for earthquakes (M> 3) that occurred in the uppermost mantle beneath the Ku channel and the eastern part of Shikoku (after Okano, 1988). (c) Fault plane solutions for subcrustal earthquakes (M> 4) that occurred beneath the Ku channel and the eastern part of Shikoku (projected onto the lower hemisphere). Numerals denote the focal depth (km) of each event (modified from Okano, 1988).

of the P-axes between shallower and deeper earthquakes and the above-mentioned large energy release for deeper events beneath Shikoku, Okano et al. (1979) suggested that the two different forces are not operating independently at two depths, but that the stress generating shallower events is secondary. For the subcrustal earthquakes, in terms of a geometric condition (Frank, 1968), Ukawa (1982) interpreted that the downwarping of the subducting plate produces a laterally extensional stress field. By using numerical modelling, Hashimoto (1982) showed that the westward moving Pacific plate may generate the E—W oriented compressive stress in the crust, and that the parallel extension may be caused by the negative

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAI TROUGH

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238

buoyancy of the subducting Philippine Sea plate, while the northwestward compressive force of the Philippine Sea plate may not play an important role in the stress field in southwest Japan. Taking account of the downwarping of the subducting plate beneath the Ku channel and the direction of plate motion at the eastern side of Kyushu, Ida (1984) suggested that the lateral stretching parallel to the Nankai trough occurs because the plate is fixed by frictional resistance beneath the Ku channel and is dragged westwards by the gravitational pull of the subducting plate beneath Kyushu. He also suggested that the E—W compressive stress field in the continental crust would be generated in relation to the E—W tensile stress field at depth. However, we also have to take into account the strain fields with the NW—SE oriented principal contraction axes at the Earth’s surface in order to elucidate the overall stress field in southwest Japan. Taking account of the existence of the right-lateral strike-slip component along the ENE—WSW trending Median Tectonic Line, which is one of the most active Quaternary faults in Japan, it is clear that the long-term NW—SE compressive stress has been dominant there (Hashimoto, 1990). In Fig. 16 we show the stress field calculated from the interplate coupling model II. The expression of the stress field follows Yoshioka and Hashimoto (1989a). In the figure, the radius of a circle is drawn in proportion to the maximum shear stress, together with the two nodal planes expected from the calculated directions of P- and T-axes. At a depth of about 10 km, the calculated stress field (Fig. 16a) represents strike-slip and reverse faulting with the NW—SE oriented P-axes, reflecting the convergence of the Philippine Sea plate relative to the Eurasian plate. Thus, we conclude that the observed stress field with the P-axes in the E—W direction cannot be explained by the interplate coupling. Figure 16b shows the calculated stress field near the interface between the overriding Eurasian plate and the subducting Philippine Sea plate. We can see from the figure that there are some significant features to be noted. From off the Ku peninsula to Shikoku, a stress field indicating

s.

YOSHIOKA

low-angled thrust faulting with NW—SE oriented P-axes is identified. This can be understood as a manifestation of the stress accumulation for the next large earthquake there. In the southern region of Shikoku, the stress field gradually changes to that of strike-slip faulting with the P-axes in the same direction. In particular, in the southeastern part of Shikoku, the NW—SE oriented P-axes also appear to change to a N—S direction towards the north. These changes of the P-axes and the fault plane solutions towards the north are consistent with the observations (Fig. 14b and c). However, the effects of the interplate coupling do not appear to reach most of the region between eastern Shikoku and Xii channel, where many earthquakes with P-axes in the N—S direction have occurred. Therefore, although the stress associated with the interplate coupling may induce these earthquakes, other earthquake generating stresses must be dominant in this region. As has been suggested by several previous studies (Shiono, 1977; Ukawa, 1982; Ida, 1984), a lateral stretching force in the E—W direction seems to be dominant in addition to the N—S compressive stress in and around the subducting plate at depths of 30—40 km. On the other hand, the observed ENE—WSW oriented large tensile strains are relatively large in the E—W trending alignment in the central part of Shikoku (triangles enclosed by solid lines in Fig. 2). For these reasons, one of the possible mechanisms to explain the complicated stress fields between eastern Shikoku and Ku channel may be attributed to a two-layered double-buckling structure: the upper layer, which includes the Earth’s surface and crust, is upwarping, causing the E—W oriented compression in the crust. On the other hand, for the lower layer, which includes the crust and subducting plate, downwarping is expected, leading to the same effect in the crust. From the southern Xii channel to the Xii peninsula, the calculated stress field is more complicated. It gradually changes from low-angled thrust to strike-slip, and also to normal fault types towards the north, reflecting subduction with a higher dip angle. This tendency appears similar to that in the observations in the southern part of the Ku peninsula (Fig. 15b). Thus, the deeper earthquakes in the transitional zone there may be re-

239

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240

lated to the subduction of the Philippine Sea plate. The only remaining problem in our model was the disagreement between the observed stress field with the E—W oriented P-axes and the calculated one with the P-axes in the NW—SE direction in the crust beneath the southern region of the Xii peninsula. Considering that the calculated strain field at the Earth’s surface and the stress field near the plate boundary fit the observations well, it is rather difficult to consider another earthquake generating force selectively lying between these two depths. Therefore, we conclude that the stress field with the E—W compression beneath the southern region of the Xii peninsula is not generated by the convergence of the Pacific plate relative to the Eurasian plate, but by the secondary stress associated with the subduction of the Philippine Sea plate. This may be supported also by the fact that the calculated maximum shear stresses at a depth of 10 km are considerably smaller than those near the plate boundary, and are easily affected by some additional stress, 5. Conclusions We have investigated the interplate coupling and the stress accumulation process of large earthquakes occurring between the overriding Eurasian and subducting Philippine Sea plates in southwest Japan, by using a three-dimensional finite element method. Several significant results have been obtamed, (1) A strongly coupled model from shallow to deep regions (model I) does not explain the observed horizontal strain field and vertical surface displacements. In particular, the strong coupling beneath the Xii channel, where the plate is subducting with a high dip angle, does not fit both of the observations, suggesting much weaker interplate coupling there. (2) A partially coupled model on a shallower plate boundary (model II) explains the observations reasonably well in the southern part of the Xii peninsula and Shikoku, though not in the southwestern part of Shikoku. This also indicates that the effects of interplate coupling do not reach regions further inland.

~ YOSHIOKA

(3) An anomalously strong coupling on the western coupled region off Shikoku (model III), corresponding to the convergence rate of 7—8 cm year1, was required to explain the geodetic data in the southwestern part of Shikoku. This may be caused by the geomorphological properties, such as the low dip angle of the subducting plate and smooth plate boundary, resulting in a large amount of stress concentration there. (4) A close correlation between the strength of interplate coupling and other seismological data was identified. The weakly coupled region corresponds well to regions with a high frequency of seismic activity, high dip angle of subduction and the possible existence of seamounts, while the strongly coupled region correlated with regions with a low frequency of seismic activity, low dip angle subduction and no seamounts. (5) Comparing the coupled region during the interseismic period with that during the pre-seismic period, the possibility of a temporal change in the coupled region was proposed. It might gradually spread from a shallower to deeper portion, with the transition from the interseismic to pre-seismic stage. (6) The presumed differences in coupling properties between southwest Japan and northeast Japan are as follows. In southwest Japan, in general, the coupling is strong and its region is limited to a relatively shallow portion. The pre-seismic coupled region coincides with the co-seismic and post-seismic fault zone, suggesting that the accumulated stress is released by a single large earthquake. In contrast to this, in northeast Japan, the coupling is weak and its extent appears to reach a depth of about 100 km. The co-seismic and postseismic regions are limited to the upper half of the coupled region. (7) The calculated stress field near the plate boundary agrees well with the overall patterns of the observed stress field beneath the southern Xii peninsula and Shikoku. Although the effects of the interplate coupling may induce strike-slip faulting with the N—S compressive axes beneath Shikoku, other earthquake generating stresses, such as two-layered double-buckling, would be dominant there.

INTERPLATE COUPLING AND STRESS ACCUMULATION PROCESS. NANKAL TROUGH

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