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Possibility of a major earthquake in the tokai district, Japan and its pre-estimated seismotectonic effects
25 (1975)
Tectonophysics, 0 Elsevier
69-85 Publishing Company,
Scientific
Amsterdam
-
Printed
in The Netherlands
POSSIBILITY OF A MAJOR EARTHQUAKE IN THE TOKAI DISTRICT, JAPAN AND ITS PRE-ESTIMATED SEISMOTECTONIC EFFECTS
MASATAKA
ANDO*
Enrihquake
Research
(Submitted
for publication
Institute,
Tokyo
January
University,
24, 1974;
Tokyo
revised
(Japan)
version
accepted
August
16, 1974)
ABSTRACT Ando, M., 1975. pre-estimated
Possibility of a major earthquake in the Tokai district, seismotectonic effects. Tectonophysics, 25: 69-85.
Japan
and its
Crustal deformations, tsunamis and seismic intensity are pre-estimated for a large hypothetical earthquake, which it is feared may occur in the Tokai district along the Nankai trough. The long-term seismic quiescence since 1854, as well as the high rate of the present crustal movements in the district, form the principal evidence for the risk of the approaching catastrophe. The location and the mode of faulting in this earthquake are hypothesized in reference to the source mechanisms of the recent and historical earthquakes there. The fault parameters thus assumed are as follows: dip direction: N30” W; dip angle: 25” ; fault dimension : 100 km X 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The following are the principal conclusions: (a) the eastern part of the epicentral region including the Point Omaezaki will rise up about 100 cm, whereas the western part covering Ise and Mikawa bays will subside about lo-30 cm; (b) the coast extending from Omaezaki to the Shima peninsula will receive tsunami waves as high as 3 m in maximum, which may be locally amplified by the factor 2 or more on the rias coast along the Shima peninsula; (c) the Tokai coastal region with thick alluvium layers may suffer seismic damages as severe as those experienced in the 1854 Ansei I earthquake. INTRODUCTION
Seismic disasters may be prevented or reduced significantly, if we can foretell the location and the mode of the faulting to occur, together with the associated crustal deformations, tsunamis and seismic waves. For example, if we can estimate, in advance, the tsunamis’ arrival time and the amplitude of a future earthquake, we may apply the data usefully to the plan of refuge or the design of tide banks. This sort of pre-estimate is also useful in improving a scientific operation in the prediction project. Optimum distribution and adjustment of instruments thus achieved will provide us with the most reasonable network for monitoring purposes.
* Now at: Disaster
Prevention
Research
Institute,
Kyoto
University,
Uji, Kyoto,
Japan.
The recent developments in seismology have enabled us to interpret the overall aspects of an earthquake consistently by a simple elastic dislocation model of faulting. Kanamori (1974) has synthesized long-period ground motions to be generated from three hypothetical great earthquakes in the consuming-plate boundaries. This paper presents a detailed investigation of a hypothetical earthquake off the Tokai district, which seems likely to occur in the near future. In the first place, we examine a possibility of earthquake occurrence there. Then we estimate quantitatively the crustal deformations, tsunamis, and seismic intensity to be associated with this earthquake. POSSIBILITY
Seismicity
OF A GREAT
EARTHQUAKE
OFF THE TOKAI
DISTRICT
gap
Historical records show that great earthquakes have occurred repeatedly along the Nankai trough. They are all explained as thrustings at the boundary between the continental and the oceanic lithospheres (Fitch and Scholz, 1971; Kanamori, 1972a; Ando, 1974a). Based on the source-mechanism studies of these recent and historical earthquakes, Ando (1974a) proposed to divide the entire fault region (530 km long) into four parts, A, B, C and D from west to east, which are more or less decoupled mechanically from each other. His conclusion is that the major earthquakes which occurred there can be reasonably explained by proper combination of these unit planes, and that three principal cycles are recognized in the seismic sequence since the 18th century. They are the Hoei, Ansei and recent cycles, with a recurrence time of approximately 100 years. It is notable that the recent cycle since 1944 is not completed, the D-plane being aseismic since 1854 (see Fig. 1). Studies on seismicity and plate tectonics suggest that the risk of future earthquakes is very high in such quiescent sites. In other words, the crust in such sites is close to the critical conditions due to the strain-energy accumulation during a long period. This idea has been applied to predict the location of future major earthquakes by many seismologists, e.g., Imamura (1928), Fedotov (1965), Mogi (1968), Sykes (1971), Kelleher (1972), Utsu (1972), Kanamori (197213) and Kelleher et al. (1973). Prediction based on the seismicity gaps has been used particularly successfully by Mogi (1968) and Utsu (1972), who correctly predicted the location of the Hokkaido-Toho-Oki earthquake of August 11,1969 (M, = 7.8) and the Nemuro-Oki earthquake of June 17,1973 (M, = 7.7), respectively. Thus we can say that the risk of a major earthquake is very high off the Tokai district. Creep-like do w slip Kanamori (1972b) suggested that a creep-like slow deformation may take place at depth preceding the great earthquakes. This idea is supported in-
71
B
A
ii-07
Ansel
,D
Hoe]
B
A 1854
,c
/,
C 1854
I
/
D
Ansel
/ II
1900A
.m-
1946
B Nankaido
,
C 1944
Tononkat
,
D
, , I
Fig. 1. Sequence of major earthquakes along the Nankai trough since the 18th century (Ando, 1974a). A, B, C and D represent the unit fault planes, respectively. The seismicity gap in D since the last cycle suggests the high potentiality of a future earthquake.
directly by the sea-level change noticed precursorily to several Japanese earthquakes (Fujita, 1971). In this case, a creep-like deformation provides us with a powerful tool for earthquake prediction. At the 1944 Tonankai earthquake, we observed this sort of slow deformation by levelling at a site, far from the major epicentral region. The Tonankai earthquake of December 7,1944 occurred off the eastern coast of the Kii peninsula. The location and geometry of the fault plane are set for C-plane (Fig. 2B) by consistently referring to its distribution patterns of crustal deformations, tsunamis and aftershocks. This fault plane is in good harmony with Kanamori’s model deduced from seismological data (1972a). No distinct crustal deformations are observed beyond the eastern fault extremity, as witnessed by several tide-gages in the Mikawa bay area (Omote, 1946a). Referring to the precise levelling by the Land Survey Department, however, a very local crustal movement is noticed in the eastern Tokal district, which is located further east of the Mikawa area (see Fig. 2; Imamura, 1945; Sato, 1970). Fortunately, the levelling data are available for the period immediately before and after the earthquake. The preseismic work was started on November 25 and completed in the morning of December 7, the very day of the earthquake occurrence. From this data, it is found that Kakegawa subsided 4 cm relatively to Mikura for the period 1934-1944. In other words, this area was tilting down to the south (Fig. 2A). Immediately
Observed
1944
Nov
-
34 hior
Fig. 2. A. Pre- and postseismic vertical deformations along the routes from Mikura to Kakegawa, and from Mikura trr Omaezaki, respectively (Imamura, 1945; Sato, 1970). B. Index map of the epicentral regions along the eastern Nankai trough and the levelling routes.
after the completion of the 1944 (December) survey, the Tonankai earthquake occurred. The next survey was repeated promptly along the same levelling route. This postseismic survey discovered a very surprising movement, namely that Kakegawa rose up as much as 11 cm relatively to Mikura and Omaezaki. It is hard to tell the precise time of this local deformation, by levelling data only. However, it is sure that the movement was not associated with a rapid fracturing, because no local earthquakes were reported from any of the three seismological stations (J.M.A.) which are only 40 km or less distant from this area. One possible explanation for this anomalous movement may be provided by hypothesizing a creep-like slow slip at depth. This seems very likely, because the fault plane of the Nankai trough thrusting is considered to extend eastward as far as this area. Then, the anticlinal movement (Fig. 2A) can be well understood by a partial slip along this buried fault, which occupies the bottom part of the D-plane (see Fig. 3). Referring to Ando (1974a), its fault parameters are supposed to be as follows: dip direction: N20”W; dip angle:
73 I38’E I
I
1
Fig. 3. Synthetic vertical displacement to explain the observed vertical movement in Fig. 2. Solid and dashed contours are for uplift and subsidence, respectively. The hatched small rectangle illustrates the part of the fault plane of the
25”; fault dimensions: 20 km X 15 km; depth to the top side: 20 km; dislocation: 1 m (reverse dip-slip/right-lateral slip: 3/l). Though the location and geometry of this slip plane is not precisely determined owing to scanty data, the writer presumes that this slip occurred in the D-plane, not extending further to the east. This assumption is supported by the levelling data at Mikura, Kakegawa and Numazu (Fig. 4). The step-like movement (uplift) is noticed at Kakegawa, but it is not seen at Numazu. From these considerations, it is concluded that the 1944 Tonankai earthquake occurred by a faulting in the C-plane including a small part of the D-plane. The slow slip at depth, as previously discussed, may be evidence for triggering of the shallower faulting. Thus, we must say that the risk of a large earthquake is very high there. Present crustal deformations Preseismic strain accumulation associated with the underthrusting lithosphere have been discussed by many authors (Plafker, 1965; Mogi, 1970; Fitch and Scholz, 1971; Shimazaki, 1974). Although their work does not necessarily refer to plate tectonics, this hypothesis seems to work very well to explain their results, basically. Fig. 5 shows a schematic model for vertical and horizontal movements. In the preseismic stage, the continental lithosphere is dragged downward by the descending oceanic lithosphere, resulting in horizontal compression. The typical premonitory effect had been observed in the eastern part of Hokkaido (G.S.I., 1972;
Fig. 4. Vertical Mikura relative
movement at Kakegawa to Numazu (after G.S.I.,
relative 1970).
to Mikura
and Numazu,
and that at
Shimazaki, 1974), where we had a strong earthquake (M, = 7.7) recently (June 17,1973), as mentioned before. This type of premonitory deformation is also seen in the Tokai district. Mogi (1970) stressed the tectonic significance of the landward compression here, referring to the first-order triangulation data by Harada and Isawa
Force
Oceanic Plate
A Fig. 5. A. Cross-sectional view of the island arc being strained by the sinking oceanic plate: upper, preseismic stage; lower, postseismic stage. B. Typical pattern of the horizontal displacement in the island arc: upper, pattern of strain accumulation; lower, pattern of strain release by a large earthquake (modified from Mogi’s (1970) figure). Direction of the force vectors is considered here to represent the slip direction of the oceanic plate.
75
I
-*
Im
l ”
Fig. 6. Horizontal displacements of triangulation points for the period from 1883-1904 to 1948-1964 (after Harada and Isawa, 1969). Note the seaward vectors in the source areas of the 1944 Tokai and 1946 Nankaido earthquakes, and the landward ones in the assumed source area.
(1969) (Fig. 6). This movement can be understood reasonably as a preseismic effect by the previous model (Fig. 5B). That is, if no great shallow earthquake occurs during the resurvey period (recent 50-60 years, in the present example), the island is compressed into the inner direction relative to the coast of the Japan Sea (Fig. 5, upper part), and with the occurrence of a great shallow earthquake along the coast, a reverse trend appears in the corresponding region of the land (Fig. 5, lower part). From these considerations, we may say that the Tokai district has a potentiality for a large earthquake in the future. This prediction is indirectly supported by the postseismic horizontal movements in the epicentral regions of the 1944 Tonankai and 1946 Nankaido earthquakes, all rebounding oceanward (Fig. 6). It must be remarked, however, that the horizontal displacements thus deduced are not free from some uncertainty due to the arbitrariness in the net adjustment (e.g. Fujita, 1973). But these horizontal displacements seem too large to be explained by the factor mentioned above. Contrary to Fujita (1973), it should not be denied after all that the Tokai district is now being increasingly compressed. Patterns of the vertical movements are not so simple as to harmonize with the model (Fig. 7). Subsidences in the Suruga bay area and uplifts in the Akaishi range are notable features in the figure. Profiles A-B and A-C in Fig. 7 show that the sea side is sinking relatively to the land side. They are typical preseismic vertical movements as inferred from the above-mentioned model. However, a question may arise as to why Omaezaki and its environs have not subsided so much. That is, if the oceanic plate is under-thrusting
76
3P
Fig. 7. Vertical land displacements during 1889-1967 (after dashed contours are for uplift and subsidence, respectively.
G.S.I.,
1974).
Solid and
against the continental plate to drag it down, Omaezaki, which presumably represents the movement of the hypothetical epicentral region, would have subsided much more. One possible answer to the question is that where the thrusting of the oceanic plate against the continental plate is not deep enough to drag the continental plate down, the preseismic sinking does not necessarily occur on the land. Actually, there is evidence for the fact that horizontal compression predominates here in comparison with other places along the Nankai trough (Ando, 1974b). Thus the inactive vertical movement at Omaezaki is not a serious contradiction to the above-stated idea. Furthermore, we may positively attribute the uplifting of the Akaishi range and its environs to the crustal buckling caused by this predominant horizontal stress. SEISMOTECTONIC EARTHQUAKE
EFFECTS
ASSOCIATED
WITH THE HYPOTHETICAL
Fault model
The hypothesis can be advanced that the future earthquake, if it occurs in the D-plane, will be similar to the past events in the same area with respect to their fault location and parameters (Ando, 1974a) (Fig. 2B). The case of the 1854 Ansei I earthquake may be notable from this point of view. Thus, the fault strike was taken parallel to the local strike of the Nankai trough, N60” W. The assumed fault parameters are as follows: dip direction: N20” W;
dip angle: 25” ; fault dimension 100 km X 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The eastern extremity of this fault is not located definitely, but it is sure that the fault plane does not extend deep under Suruga bay, judging from the distribution of crustal deformations in the 1854 previous event. Crustal deformation
In computing the crustal deformations we employ Voltera’s dislocation model in a semi-infinite homogeneous medium (Maruyama, 1964). Practically, we take the simple exact expressions by Manshinha and Smylie (1971) for the computation. For simplicity’s sake, it is assumed here that the fault plane is a rectangle and that the dislocation is uniform both in amplitude and direction over the whole fault plane. Under these conditions, the static horizontal and vertical displacements are computed as shown in Fig. 8. Uplift in and near Omaezaki and subsidence over the Ise bay, Mikawa bay and Atsumi peninsula characterize the deformation pattern. A similar aspect was also observed at the Ansei I earthquake of 1854 (Ando, 1974a). This mode of the displacement pattern has been observed not only in this district but also in many other earthquakes of a low-angle thrusting type: the 1923 Kanto earthquake (Ando, 1971); the 1944 Tonankai earthquake (Ando, 1974a), the 1946 Nankaido earthquake (Fitch and Scholz, 1971), the 1960 Chile earthquake (Plafker and Savage, 1970), the 1964 Alaska earthquake
35’N
50 \
‘\\
1
.
‘:.
i
km
I
Fig. 8. Vertical and horizontal displacements synthesized for the hypothetical earthquake. Solid and dashed contours are for uplift and subsidence, respectively. Arrows indicate the horizontal displacements, vectorially. The rectangle illustrates the surface projection of the hypothetical fault plane, dipping to the northwest. The upper edge of the fault is set on the earth’s surface.
78
Height
20'
IO' I
137% I
of the unit
Fukue
Terrace
(0
I my)
L
: m
-34’4dN
1
I
Fig. 9. Tectonic tilting of the Fukue terrace (1.2-1.3 peninsula (from Ishikawa and Ota, 1967). Contours shorelines above the present sea-level.
lo4 years of age) in the Atsumi represent the height of the former
(Plafker, 1965; Savage and Hastie, 1966) and the 1968 Tokachi-Oki and 1969 Hokkaido-Toho-Oki eartquakes (Abe, 1973). This hypothetical mode of the crustal deformation is also consistent with the long-term tectonic data, i.e. the distribution of the coastal terraces. That is, if the coseismic land uplifts have not recovered fully during the subsequent intra-seismic period, the residual part would have been accumulated over a long period to form a series of marine terraces. Ishikawa and Ota (1967) discovered that the Pleistocene marine terraces (0.1 million years of age) in the Atsumi peninsula tilt NNW as shown in Fig. 9. It is clear from Figs. 8 and 9 that the pattern of the hypothetical coseismic vertical movement in the future earthquake is in positive correlation with these terraces. On the other hand, no positive correlation between the coseismic and the tectonic aspects is seen with respect to the horizontal movements. This is not strange, because a thrusting fault at the plate boundary will accumulate little horizontal displacement. Tsunamis
Tsunamis are most efficiently excited if the submarine vertical movement takes place abruptly. Let us suppose that the submarine topographic change takes place in a manner as computed from the model in Fig. 8. Programs for the numerical calculation of tsunamis generation due to sea-bottom movement are referred to by Aida (1969). Fig. 10 shows the region and the sea depths used for computation, where the region is bounded by, two kinds of lines, namely, the coast line (thick line in the figure) and the outer border of
79
Fig. 10. Region for the numerical experiments and water depths (in meters) there. This region is specified by the coast lines (thick solid line) and the outer c;eaward borders (thick dashed line). Scale on the borders represents the grid lines (120 x 66) with separation of 2 km. Circles with numbers affixed are the points of tsunami observations. The rectangle shows the surface projection of the hypothetical fault plane.
the test area (thick dashed line). The water transportation normal to the coast is fixed zero at the coast line. At the outer boundary, on the other hand, the water level and the water transportation are related by a progressive long wave. The numerical region is divided into 120 X 66 meshes with unit side length As, 2 km. Then, the disturbances are traced stepwise with time interval At, 6 sec. The rise time of the crustal deformation is assumed as three minutes. Under these conditions, the theoretical tsunami records are computed at the twenty-seven points as given in Fig. 10. Fig. 11 shows the tsunamis for 3m-I hours after the origin time. The accuracy of this technique has been tested successfully for the 1964 Niigata and 1968 Tokachi-Oki e~thquakes by Aida (1969) by comparing the computed tsunamis with the observed tide records. However, it should be remarked that the numerical data tend to underestimate the tsunami effect by two factors. Firstly, the theoretical sites of stations cannot be set at the actual shore precisely, tending to be shifted offshore, by two kilometers (the grid separation) in the worst case. And, secondly, the run-up effect at the coast is ignored. Nevertheless, the reduction factor for these sorts of errors may not be larger than 2, unless the local amplification effect is not exceptionally large. Therefore, we approximately reduce the theoretical values by 1.5 for the shoaling beach of depth 10 m or less at more than 2 km offshore and by 2.0 for the other places. Based on this assumption, Fig. 12 shows the maximum height of tsunamis thus estimated for the twenty-seven observation sites in Fig. 10.
80 Ise
Bay
Mlkowa
Bay
22 Suruga
Boy.
Enshyu-No
Kumano-Nada
I
-
Lp&+M/q::
(hour)
Fig. 11. Synthetic records of tsunamis the vertical land movements.
at the 13 points
in Fig. 10. Heights
corrected
for
Seismic intensity Seismic intensity depends seriously on the high-frequency components in the ground motion. Such high-frequency waves are probably excited by the irregular developments of the fault slippage, reflecting the irregularities in the stress conditions at the growing fault edge. In addition they are also generated or modified by the soft alluvial layers and the lateral heterogeneity in the structures. Therefore, it is extremely difficult to forecast the seismic intensity deterministically by the present simple faulting model. In the following, therefore, we shall examine the seismic intensity only empirically, referring to the previous two earthquakes, the 1944 Tonankai and 1854 Ansei I earthquakes. Seismic intensity and damages of the 1944 event are presented by J.M.A. (1945), Minakami and Utibori (1946), Omote (1946b) and Miyamura (1946). Those of the 1854 event by Omori (1913) and Hagiwara (1971). With these data, we estimate the hypothetical intensity of the future earthquake as shown in Fig. 13. The intensity is represented in the J.M.A.
81
Fig. 12. Heights of tsunami waves along the coast. Solid and dashed lines represent, respectively, the maximum height corrected for the vertical land movements and the inundation height corrected for both vertical land movements and coastal effects. Fine solid line denotes the arrival time of the first waves, when we observe the water surface moving above the post-seismic sea-level (corrected for land movements) by 10 cm.
I 134-E
I 136’
_
I 13.9’
I 139.
36%
Fig.
13. Seismic
intensity
(J.M.A.
scale)
of the hypothetical
earthquake
(empirical).
82
intensity scale. The pattern is very similar to that of the 1854 event. In the Tokai district, Miyamura’s (1946) detailed survey shows that the seismic intensity is seriously affected by the local geological conditions. In fact, the coastal region of the Tokai district, far from the epicentral region, was severely damaged by the 1944 shock, in contrast with relatively slight damages in the eastern part of the Kii peninsula, adjacent to the fault plane. Such an anom~ous pattern was also observed at the 1854 event. Seismic intensities as high as 6 and ‘7 (equivalent to the M.M. scale of IX, and X-XII, respectively) were recorded in the eastern district including Suruga bay and its environments. These anomalous features of seismic intensity distribution may be attributed to the thick alluvium covering the coast of the Tokai district and the Tenryu river. It is reasonable to conclude that this area will receive relatively high intensities from the future earthquake. DISCUSSIONS
AND CONCLUSIONS
The writer has pointed out a possibility of a large earthquake in the Tokai district in the near future and has discussed its seismotectonic effects from various viewpoints. The seismicity gap and the local strain accumulation of crustal deformations are the primary data of concern. A creep-like slow slip in the fault plane at depth was hypothesized in the present study as a basic precursory phase of a large earthquake. This idea is in good harmony with laboratory data for the frictional characteristics by Scholz et al. (1972). They show that a small amount of stable slip always precedes every stick-slip, amounting to 2-5% of the final slip amplitude. Their experimental results for the frictional characteristics is in good harmony with the present precursor model. This model is of course speculative, but it is given in order to underscore the possible significance of this effect. In these circumstances, it seems worthwhile to monitor crustal deformations for the purpose of prediction. High-sensitivity strain meters and tilt meters will be useful for this purpose. Fortunately crustal-movement observatories with these instruments are available at three sites, Mikawa, Inuyama and Fujigawa, around the hypothetical source region, which are all expected to detect preseismic and precursory movements if they appear. If a creep-like slip occurs at depth (dip direction: N20”W; dip angle: 25”; fault dimensions: 20 km X 15 km; depth to the top side: 25 km; dislocation: 1 m, see Fig. 14), the cooperative work of these three stations will be particularly useful for detecting its location. In the present study, the hypothetical fault plane is assumed not to extend into Suruga bay. In other words, it is assumed that no major earthquakes related to the plate boundaries will occur there in the future. This assumption is made by considering the fact that the occurrence of crustal deformations from the historical earthquakes were not reported here, and that this area has no geomorphological evidence for the unde~hrusting. This scrt of information suggests that the principal faulting does not reach the
x3
Fig. 14. Maximum strain change on the earth’s surface synthesized for the hypothetical slow slip. Small hatched and large rectangles illustrate the part of the hypothetical fault plane of the slow slip and that of the major earthquake, respectively (surface projection).
Suruga bay area. Rather, Suruga bay is considered as a marginal region where the two sea-plate units are now colliding (Ando, 197413). The tsunami height inshore is subject to the complex topographic effect. Thus some uncertainty is unavoidable, particularly in predicting the tsunamis at the rias-type valleys in the Shima peninsula. Referring to t,he past tsunamis in this region, it seems possible that they are amplified inshore to a height 2 -4 times as large as the present prediction. Thus, in the worst case, there is fear of large tsunamis reaching 4-6 m in the Shima peninsula. The southwestern part of the Izu peninsula experienced large tsunamis in the 1854 Ansei I earthquake (Hatori, 1974). But this tsunami pattern is not deduced from the present model. The coastal topographic effect is too small to explain this discrepancy. Consequently the fault plane, as hypothesized above, might better extend for about 20 km to the east or accompany a small secondary fault in Suruga bay. The hypothetical earthquake will complete the present migration cycle since 1944, so that, after its occurrence, the whole Nankai trough system will become quiescent for a long time until the next cycle begins. ACKNOWLEDGEMENT
I am grateful to Prof. Keichi Kasahara for critically reading the manuscript and giving many suggestions. I benefited from discussions with Prof. Takashi Mikumo and Drs. Isamu Aida, Katsuyuki Abe, Yoichiro Fujii and Tokutaro Hatori. Dr. Isamu Aida generously allowed me to use his computer program for synthetic tsunami wave forms.
84
REFERENCES Abe, K., 1973. Tsunamis and mechanism of great earthquakes. Phys. Earth Planet. Inter., 7: 143-153. Aida, I., 1969. Numerical experiments for the tsunami propagation -the 1964 Niigata tsunami and the 1968 Tokachi-Oki tsunami. Bull. Earthquake Res. Inst. Tokyo Univ., 47: 673-700. Ando, M., 1971. A fault-origin model of the great Kanto earthquake of 1923 as deduced from geodetic data. Bull. Earthquake Res. Inst. Tokyo Univ., 49: 19-32. Ando, M., 1974a. Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan (in preparation). Ando, M., 1974b. Northward migration of the Izu peninsula: a small scaled collision between oceanic and continental plates (in preparation). Fedotov, S.A., 1965. Regularities of the distribution of strong earthquakes in Kamchatka, the Kurile Islands and northeastern Japan. Acad. Sci. U.S.S.R., Tr. Inst. Phys. Earth, 36 (203): 66-93 (in Russian). Fitch, T.J. and Scholz, C.H., 1971. Mechanism of underthrusting in southwest Japan: a model of convergent plate interactions. J. Geophys. Res., 76: 7260--7292. Fujita, N., 1971. Tidal observation and earthquake prediction. Mar. Sci., 562-566 (in Japanese). Fujita, N., 1973. Horizontal displacement vectors in Kanto and Chubu districts. Rep. Coord. Comm. Earthquake Predict., 10: 64-67 (in Japanese). Geographical Survey Institute, 1970. Vertical crustal deformation in the Tokai district, Rep. Coord. Comm. Earthquake Predict., 2: 49-53 (in Japanese). Geographical Survey Institute, 1972. Crustal movement in eastern part of Hokkaido district. Rep. Coord. Comm. Earthquake Predict., 7: l-3 (in Japanese). Geographical Survey Institute, 1974. Vertical movements in Tokai district, 2. Rep. Coord. Comm. Earthquake Predict., 11: 102-104. Hagiwara, T., 1971. Distribution of the seismic intensity associated with the 1854 Tokai earthquake. Rep. Coord. Comm. Earthquake Predict., 3: 51-52 (in Japanese). Harada, T. and Isawa, N., 1969. Horizontal deformation of the crust in Japan - Result obtained by multiple fixed stations. J. Geodet. Sot. Jap., 14: 101-105 (in Japanese). Hatori, T., 1974. Sources of large tsunamis in southwest Japan, Zishin, 27: 10-24 (in Japanese). Imamura, A., 1928. On the seismic activity of central Japan. Jap. J. Astron. Geophys., 6: 119-137. Imamura, A.. 1945. Land deformations associated with the recent Tokaido earthquake, Proc. Imp. Acad. Japan, 19: 193-196. Ishikawa, K. and Ota, Y., 1967. Upwarping movement during the Quaternary in the Atsumi peninsula, Central Japan. Quaternary Res., 6: 89-92 (in Japanese). Japan Meteorological Agency, 1945. Report on surveys of the 1944 Tonankai earthquake. Japan Meteorological Agency, Tokyo, pp. l-94 (in Japanese). Kanamori, H., 1972a. Tectonic implications of the 1944 Tonankai and the 1946 Nankaido earthquakes. Phys. Earth Planet. Inter., 5: 129-136. Kanamori, H., 1972b. Relation between tectonic stress, great earthquakes and earthquake swarms. Tectonophysics, 14: l-12. Kanamori, H. 1974. Long-period ground motion in the epicentral area of major earthquakes. Tectonophysics, 24: 341-356. Kelleher, J.A., 1972. Rupture zones of large South American earthquakes and source predictions. J. Geophys. Res., 77: 2087-2103. Kelleher, J., Sykes, L. and Oliver, J., 1973. Possible criteria for predicting earthquake locations and their application to major plate boundaries of the Pacific and the Caribbean. J. Geophys. Res., 78: 2547--2585.
85 Manshinha, L. and Smylie, D.E., 1971. The displacement fields of inclined faults. Bull. Seismol. Sot. Am., 61: 1433-1440. Maruyama, T., 1964. Statical elastic dislocations in an infinite and semi-infinite medium. Bull. Earthquake Res. Inst. Tokyo Univ., 42: 289-368. Minakami, T. and Utibori, S., 1946. The To-Nan-Kai earthquake damage and aftershocks. Bull. Earthquake Res. Inst. Tokyo Univ., 24: 19-30 (in Japanese). Miyamura, S., 1946. Geographic distribution of seismic damages of the 1944 Tokai earthquake, I. Bull. Earthquake Res. Inst. Tokyo Univ., 24: 99m-134 (in Japanese). Mogi, K., 1968. Some features of recent seismic activity in and near Japan, 1. Bull. Earthquake Res. Inst. Tokyo Univ., 46: 1225-1236. Mogi, K., 1970. Recent horizontal deformation of the earth’s crust and tectonic activity in Japan, 1. Bull. Earthquake Res. Inst. Tokyo Univ., 48: 413-430. Omori, F., 1913. Generai remarks on major earthquakes in this country. Pubi. Earthquake Invest. Comm., 68: l-179 (in Japanese). Omote, S., 1946a. The tsunami, the earthquake sea waves that accompanied the great earthquake of Dec. 7, 1944. Bull. Earthquake Res. Inst. Tokyo Univ., 24: 31-58 (in Japanese). Omote, S., 1946b. Comparison of the vulnerability rates of the ground revealed in Tonankai and Mikawa earthquakes. Bull. Earthquake Res. Inst. Tokyo Univ., 24: 87-98 (in Japanese). Plafker, G., 1965. Tectonic deformation associated with the 1964 Alaska earthquake. Science, 148: 1675-1687. Plafker, G. and Savage, J.C., 1970. Mechanism of the Chilian earthquakes of May 21 and 22, 1960. Geol. Sot. Am. Bull., 81: 1001-1030. Sato, H., 1970. Crustal deformation associated with the Tonankai earthquake of 1944. J. Geodet. Sot. Japan, 15: 177-180 (in Japanese). Savage, J.C. and Hastie, L.M., 1966. Surface deformation associated with dip-slip faulting. J. Geophys. Res., 71: 4897-4904. Scholtz, C., Molnar, P. and Johnson, T. 1972. Detailed studies of frictional sliding of granite and implications for the earthquake mechanism. J. Geophys. Res., 77 : 6392-6406. Shimazaki, K., 1974. Pre-seismic crustal deformation caused by an underthrusting oceanic plate, in eastern Hokkaido, Japan. Phys. Earth Planet. Inter., 8: 148-157. Sykes, L.R., 1971. Aftershock zones of great earthquakes, seismicity gaps, and earthquake prediction for Alaska and the Aleutians. J. Geophys. Res., 76: 8021-8031. Ut.su, T., 1972. Large earthquakes near Hokkaido and the expectancy of the occurrence of a large earthquake off Nemuro. Rep. Coord. Comm. Earthquake Predict., 7: 7-m13 (in Japanese).
Tectonophysics, 0 Elsevier
69-85 Publishing Company,
Scientific
Amsterdam
-
Printed
in The Netherlands
POSSIBILITY OF A MAJOR EARTHQUAKE IN THE TOKAI DISTRICT, JAPAN AND ITS PRE-ESTIMATED SEISMOTECTONIC EFFECTS
MASATAKA
ANDO*
Enrihquake
Research
(Submitted
for publication
Institute,
Tokyo
January
University,
24, 1974;
Tokyo
revised
(Japan)
version
accepted
August
16, 1974)
ABSTRACT Ando, M., 1975. pre-estimated
Possibility of a major earthquake in the Tokai district, seismotectonic effects. Tectonophysics, 25: 69-85.
Japan
and its
Crustal deformations, tsunamis and seismic intensity are pre-estimated for a large hypothetical earthquake, which it is feared may occur in the Tokai district along the Nankai trough. The long-term seismic quiescence since 1854, as well as the high rate of the present crustal movements in the district, form the principal evidence for the risk of the approaching catastrophe. The location and the mode of faulting in this earthquake are hypothesized in reference to the source mechanisms of the recent and historical earthquakes there. The fault parameters thus assumed are as follows: dip direction: N30” W; dip angle: 25” ; fault dimension : 100 km X 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The following are the principal conclusions: (a) the eastern part of the epicentral region including the Point Omaezaki will rise up about 100 cm, whereas the western part covering Ise and Mikawa bays will subside about lo-30 cm; (b) the coast extending from Omaezaki to the Shima peninsula will receive tsunami waves as high as 3 m in maximum, which may be locally amplified by the factor 2 or more on the rias coast along the Shima peninsula; (c) the Tokai coastal region with thick alluvium layers may suffer seismic damages as severe as those experienced in the 1854 Ansei I earthquake. INTRODUCTION
Seismic disasters may be prevented or reduced significantly, if we can foretell the location and the mode of the faulting to occur, together with the associated crustal deformations, tsunamis and seismic waves. For example, if we can estimate, in advance, the tsunamis’ arrival time and the amplitude of a future earthquake, we may apply the data usefully to the plan of refuge or the design of tide banks. This sort of pre-estimate is also useful in improving a scientific operation in the prediction project. Optimum distribution and adjustment of instruments thus achieved will provide us with the most reasonable network for monitoring purposes.
* Now at: Disaster
Prevention
Research
Institute,
Kyoto
University,
Uji, Kyoto,
Japan.
The recent developments in seismology have enabled us to interpret the overall aspects of an earthquake consistently by a simple elastic dislocation model of faulting. Kanamori (1974) has synthesized long-period ground motions to be generated from three hypothetical great earthquakes in the consuming-plate boundaries. This paper presents a detailed investigation of a hypothetical earthquake off the Tokai district, which seems likely to occur in the near future. In the first place, we examine a possibility of earthquake occurrence there. Then we estimate quantitatively the crustal deformations, tsunamis, and seismic intensity to be associated with this earthquake. POSSIBILITY
Seismicity
OF A GREAT
EARTHQUAKE
OFF THE TOKAI
DISTRICT
gap
Historical records show that great earthquakes have occurred repeatedly along the Nankai trough. They are all explained as thrustings at the boundary between the continental and the oceanic lithospheres (Fitch and Scholz, 1971; Kanamori, 1972a; Ando, 1974a). Based on the source-mechanism studies of these recent and historical earthquakes, Ando (1974a) proposed to divide the entire fault region (530 km long) into four parts, A, B, C and D from west to east, which are more or less decoupled mechanically from each other. His conclusion is that the major earthquakes which occurred there can be reasonably explained by proper combination of these unit planes, and that three principal cycles are recognized in the seismic sequence since the 18th century. They are the Hoei, Ansei and recent cycles, with a recurrence time of approximately 100 years. It is notable that the recent cycle since 1944 is not completed, the D-plane being aseismic since 1854 (see Fig. 1). Studies on seismicity and plate tectonics suggest that the risk of future earthquakes is very high in such quiescent sites. In other words, the crust in such sites is close to the critical conditions due to the strain-energy accumulation during a long period. This idea has been applied to predict the location of future major earthquakes by many seismologists, e.g., Imamura (1928), Fedotov (1965), Mogi (1968), Sykes (1971), Kelleher (1972), Utsu (1972), Kanamori (197213) and Kelleher et al. (1973). Prediction based on the seismicity gaps has been used particularly successfully by Mogi (1968) and Utsu (1972), who correctly predicted the location of the Hokkaido-Toho-Oki earthquake of August 11,1969 (M, = 7.8) and the Nemuro-Oki earthquake of June 17,1973 (M, = 7.7), respectively. Thus we can say that the risk of a major earthquake is very high off the Tokai district. Creep-like do w slip Kanamori (1972b) suggested that a creep-like slow deformation may take place at depth preceding the great earthquakes. This idea is supported in-
71
B
A
ii-07
Ansel
,D
Hoe]
B
A 1854
,c
/,
C 1854
I
/
D
Ansel
/ II
1900A
.m-
1946
B Nankaido
,
C 1944
Tononkat
,
D
, , I
Fig. 1. Sequence of major earthquakes along the Nankai trough since the 18th century (Ando, 1974a). A, B, C and D represent the unit fault planes, respectively. The seismicity gap in D since the last cycle suggests the high potentiality of a future earthquake.
directly by the sea-level change noticed precursorily to several Japanese earthquakes (Fujita, 1971). In this case, a creep-like deformation provides us with a powerful tool for earthquake prediction. At the 1944 Tonankai earthquake, we observed this sort of slow deformation by levelling at a site, far from the major epicentral region. The Tonankai earthquake of December 7,1944 occurred off the eastern coast of the Kii peninsula. The location and geometry of the fault plane are set for C-plane (Fig. 2B) by consistently referring to its distribution patterns of crustal deformations, tsunamis and aftershocks. This fault plane is in good harmony with Kanamori’s model deduced from seismological data (1972a). No distinct crustal deformations are observed beyond the eastern fault extremity, as witnessed by several tide-gages in the Mikawa bay area (Omote, 1946a). Referring to the precise levelling by the Land Survey Department, however, a very local crustal movement is noticed in the eastern Tokal district, which is located further east of the Mikawa area (see Fig. 2; Imamura, 1945; Sato, 1970). Fortunately, the levelling data are available for the period immediately before and after the earthquake. The preseismic work was started on November 25 and completed in the morning of December 7, the very day of the earthquake occurrence. From this data, it is found that Kakegawa subsided 4 cm relatively to Mikura for the period 1934-1944. In other words, this area was tilting down to the south (Fig. 2A). Immediately
Observed
1944
Nov
-
34 hior
Fig. 2. A. Pre- and postseismic vertical deformations along the routes from Mikura to Kakegawa, and from Mikura trr Omaezaki, respectively (Imamura, 1945; Sato, 1970). B. Index map of the epicentral regions along the eastern Nankai trough and the levelling routes.
after the completion of the 1944 (December) survey, the Tonankai earthquake occurred. The next survey was repeated promptly along the same levelling route. This postseismic survey discovered a very surprising movement, namely that Kakegawa rose up as much as 11 cm relatively to Mikura and Omaezaki. It is hard to tell the precise time of this local deformation, by levelling data only. However, it is sure that the movement was not associated with a rapid fracturing, because no local earthquakes were reported from any of the three seismological stations (J.M.A.) which are only 40 km or less distant from this area. One possible explanation for this anomalous movement may be provided by hypothesizing a creep-like slow slip at depth. This seems very likely, because the fault plane of the Nankai trough thrusting is considered to extend eastward as far as this area. Then, the anticlinal movement (Fig. 2A) can be well understood by a partial slip along this buried fault, which occupies the bottom part of the D-plane (see Fig. 3). Referring to Ando (1974a), its fault parameters are supposed to be as follows: dip direction: N20”W; dip angle:
73 I38’E I
I
1
Fig. 3. Synthetic vertical displacement to explain the observed vertical movement in Fig. 2. Solid and dashed contours are for uplift and subsidence, respectively. The hatched small rectangle illustrates the part of the fault plane of the
25”; fault dimensions: 20 km X 15 km; depth to the top side: 20 km; dislocation: 1 m (reverse dip-slip/right-lateral slip: 3/l). Though the location and geometry of this slip plane is not precisely determined owing to scanty data, the writer presumes that this slip occurred in the D-plane, not extending further to the east. This assumption is supported by the levelling data at Mikura, Kakegawa and Numazu (Fig. 4). The step-like movement (uplift) is noticed at Kakegawa, but it is not seen at Numazu. From these considerations, it is concluded that the 1944 Tonankai earthquake occurred by a faulting in the C-plane including a small part of the D-plane. The slow slip at depth, as previously discussed, may be evidence for triggering of the shallower faulting. Thus, we must say that the risk of a large earthquake is very high there. Present crustal deformations Preseismic strain accumulation associated with the underthrusting lithosphere have been discussed by many authors (Plafker, 1965; Mogi, 1970; Fitch and Scholz, 1971; Shimazaki, 1974). Although their work does not necessarily refer to plate tectonics, this hypothesis seems to work very well to explain their results, basically. Fig. 5 shows a schematic model for vertical and horizontal movements. In the preseismic stage, the continental lithosphere is dragged downward by the descending oceanic lithosphere, resulting in horizontal compression. The typical premonitory effect had been observed in the eastern part of Hokkaido (G.S.I., 1972;
Fig. 4. Vertical Mikura relative
movement at Kakegawa to Numazu (after G.S.I.,
relative 1970).
to Mikura
and Numazu,
and that at
Shimazaki, 1974), where we had a strong earthquake (M, = 7.7) recently (June 17,1973), as mentioned before. This type of premonitory deformation is also seen in the Tokai district. Mogi (1970) stressed the tectonic significance of the landward compression here, referring to the first-order triangulation data by Harada and Isawa
Force
Oceanic Plate
A Fig. 5. A. Cross-sectional view of the island arc being strained by the sinking oceanic plate: upper, preseismic stage; lower, postseismic stage. B. Typical pattern of the horizontal displacement in the island arc: upper, pattern of strain accumulation; lower, pattern of strain release by a large earthquake (modified from Mogi’s (1970) figure). Direction of the force vectors is considered here to represent the slip direction of the oceanic plate.
75
I
-*
Im
l ”
Fig. 6. Horizontal displacements of triangulation points for the period from 1883-1904 to 1948-1964 (after Harada and Isawa, 1969). Note the seaward vectors in the source areas of the 1944 Tokai and 1946 Nankaido earthquakes, and the landward ones in the assumed source area.
(1969) (Fig. 6). This movement can be understood reasonably as a preseismic effect by the previous model (Fig. 5B). That is, if no great shallow earthquake occurs during the resurvey period (recent 50-60 years, in the present example), the island is compressed into the inner direction relative to the coast of the Japan Sea (Fig. 5, upper part), and with the occurrence of a great shallow earthquake along the coast, a reverse trend appears in the corresponding region of the land (Fig. 5, lower part). From these considerations, we may say that the Tokai district has a potentiality for a large earthquake in the future. This prediction is indirectly supported by the postseismic horizontal movements in the epicentral regions of the 1944 Tonankai and 1946 Nankaido earthquakes, all rebounding oceanward (Fig. 6). It must be remarked, however, that the horizontal displacements thus deduced are not free from some uncertainty due to the arbitrariness in the net adjustment (e.g. Fujita, 1973). But these horizontal displacements seem too large to be explained by the factor mentioned above. Contrary to Fujita (1973), it should not be denied after all that the Tokai district is now being increasingly compressed. Patterns of the vertical movements are not so simple as to harmonize with the model (Fig. 7). Subsidences in the Suruga bay area and uplifts in the Akaishi range are notable features in the figure. Profiles A-B and A-C in Fig. 7 show that the sea side is sinking relatively to the land side. They are typical preseismic vertical movements as inferred from the above-mentioned model. However, a question may arise as to why Omaezaki and its environs have not subsided so much. That is, if the oceanic plate is under-thrusting
76
3P
Fig. 7. Vertical land displacements during 1889-1967 (after dashed contours are for uplift and subsidence, respectively.
G.S.I.,
1974).
Solid and
against the continental plate to drag it down, Omaezaki, which presumably represents the movement of the hypothetical epicentral region, would have subsided much more. One possible answer to the question is that where the thrusting of the oceanic plate against the continental plate is not deep enough to drag the continental plate down, the preseismic sinking does not necessarily occur on the land. Actually, there is evidence for the fact that horizontal compression predominates here in comparison with other places along the Nankai trough (Ando, 1974b). Thus the inactive vertical movement at Omaezaki is not a serious contradiction to the above-stated idea. Furthermore, we may positively attribute the uplifting of the Akaishi range and its environs to the crustal buckling caused by this predominant horizontal stress. SEISMOTECTONIC EARTHQUAKE
EFFECTS
ASSOCIATED
WITH THE HYPOTHETICAL
Fault model
The hypothesis can be advanced that the future earthquake, if it occurs in the D-plane, will be similar to the past events in the same area with respect to their fault location and parameters (Ando, 1974a) (Fig. 2B). The case of the 1854 Ansei I earthquake may be notable from this point of view. Thus, the fault strike was taken parallel to the local strike of the Nankai trough, N60” W. The assumed fault parameters are as follows: dip direction: N20” W;
dip angle: 25” ; fault dimension 100 km X 70 km; dislocation: 4 m (reverse dip-slip: 3.8 m; right-lateral strike-slip: 1.3 m). The eastern extremity of this fault is not located definitely, but it is sure that the fault plane does not extend deep under Suruga bay, judging from the distribution of crustal deformations in the 1854 previous event. Crustal deformation
In computing the crustal deformations we employ Voltera’s dislocation model in a semi-infinite homogeneous medium (Maruyama, 1964). Practically, we take the simple exact expressions by Manshinha and Smylie (1971) for the computation. For simplicity’s sake, it is assumed here that the fault plane is a rectangle and that the dislocation is uniform both in amplitude and direction over the whole fault plane. Under these conditions, the static horizontal and vertical displacements are computed as shown in Fig. 8. Uplift in and near Omaezaki and subsidence over the Ise bay, Mikawa bay and Atsumi peninsula characterize the deformation pattern. A similar aspect was also observed at the Ansei I earthquake of 1854 (Ando, 1974a). This mode of the displacement pattern has been observed not only in this district but also in many other earthquakes of a low-angle thrusting type: the 1923 Kanto earthquake (Ando, 1971); the 1944 Tonankai earthquake (Ando, 1974a), the 1946 Nankaido earthquake (Fitch and Scholz, 1971), the 1960 Chile earthquake (Plafker and Savage, 1970), the 1964 Alaska earthquake
35’N
50 \
‘\\
1
.
‘:.
i
km
I
Fig. 8. Vertical and horizontal displacements synthesized for the hypothetical earthquake. Solid and dashed contours are for uplift and subsidence, respectively. Arrows indicate the horizontal displacements, vectorially. The rectangle illustrates the surface projection of the hypothetical fault plane, dipping to the northwest. The upper edge of the fault is set on the earth’s surface.
78
Height
20'
IO' I
137% I
of the unit
Fukue
Terrace
(0
I my)
L
: m
-34’4dN
1
I
Fig. 9. Tectonic tilting of the Fukue terrace (1.2-1.3 peninsula (from Ishikawa and Ota, 1967). Contours shorelines above the present sea-level.
lo4 years of age) in the Atsumi represent the height of the former
(Plafker, 1965; Savage and Hastie, 1966) and the 1968 Tokachi-Oki and 1969 Hokkaido-Toho-Oki eartquakes (Abe, 1973). This hypothetical mode of the crustal deformation is also consistent with the long-term tectonic data, i.e. the distribution of the coastal terraces. That is, if the coseismic land uplifts have not recovered fully during the subsequent intra-seismic period, the residual part would have been accumulated over a long period to form a series of marine terraces. Ishikawa and Ota (1967) discovered that the Pleistocene marine terraces (0.1 million years of age) in the Atsumi peninsula tilt NNW as shown in Fig. 9. It is clear from Figs. 8 and 9 that the pattern of the hypothetical coseismic vertical movement in the future earthquake is in positive correlation with these terraces. On the other hand, no positive correlation between the coseismic and the tectonic aspects is seen with respect to the horizontal movements. This is not strange, because a thrusting fault at the plate boundary will accumulate little horizontal displacement. Tsunamis
Tsunamis are most efficiently excited if the submarine vertical movement takes place abruptly. Let us suppose that the submarine topographic change takes place in a manner as computed from the model in Fig. 8. Programs for the numerical calculation of tsunamis generation due to sea-bottom movement are referred to by Aida (1969). Fig. 10 shows the region and the sea depths used for computation, where the region is bounded by, two kinds of lines, namely, the coast line (thick line in the figure) and the outer border of
79
Fig. 10. Region for the numerical experiments and water depths (in meters) there. This region is specified by the coast lines (thick solid line) and the outer c;eaward borders (thick dashed line). Scale on the borders represents the grid lines (120 x 66) with separation of 2 km. Circles with numbers affixed are the points of tsunami observations. The rectangle shows the surface projection of the hypothetical fault plane.
the test area (thick dashed line). The water transportation normal to the coast is fixed zero at the coast line. At the outer boundary, on the other hand, the water level and the water transportation are related by a progressive long wave. The numerical region is divided into 120 X 66 meshes with unit side length As, 2 km. Then, the disturbances are traced stepwise with time interval At, 6 sec. The rise time of the crustal deformation is assumed as three minutes. Under these conditions, the theoretical tsunami records are computed at the twenty-seven points as given in Fig. 10. Fig. 11 shows the tsunamis for 3m-I hours after the origin time. The accuracy of this technique has been tested successfully for the 1964 Niigata and 1968 Tokachi-Oki e~thquakes by Aida (1969) by comparing the computed tsunamis with the observed tide records. However, it should be remarked that the numerical data tend to underestimate the tsunami effect by two factors. Firstly, the theoretical sites of stations cannot be set at the actual shore precisely, tending to be shifted offshore, by two kilometers (the grid separation) in the worst case. And, secondly, the run-up effect at the coast is ignored. Nevertheless, the reduction factor for these sorts of errors may not be larger than 2, unless the local amplification effect is not exceptionally large. Therefore, we approximately reduce the theoretical values by 1.5 for the shoaling beach of depth 10 m or less at more than 2 km offshore and by 2.0 for the other places. Based on this assumption, Fig. 12 shows the maximum height of tsunamis thus estimated for the twenty-seven observation sites in Fig. 10.
80 Ise
Bay
Mlkowa
Bay
22 Suruga
Boy.
Enshyu-No
Kumano-Nada
I
-
Lp&+M/q::
(hour)
Fig. 11. Synthetic records of tsunamis the vertical land movements.
at the 13 points
in Fig. 10. Heights
corrected
for
Seismic intensity Seismic intensity depends seriously on the high-frequency components in the ground motion. Such high-frequency waves are probably excited by the irregular developments of the fault slippage, reflecting the irregularities in the stress conditions at the growing fault edge. In addition they are also generated or modified by the soft alluvial layers and the lateral heterogeneity in the structures. Therefore, it is extremely difficult to forecast the seismic intensity deterministically by the present simple faulting model. In the following, therefore, we shall examine the seismic intensity only empirically, referring to the previous two earthquakes, the 1944 Tonankai and 1854 Ansei I earthquakes. Seismic intensity and damages of the 1944 event are presented by J.M.A. (1945), Minakami and Utibori (1946), Omote (1946b) and Miyamura (1946). Those of the 1854 event by Omori (1913) and Hagiwara (1971). With these data, we estimate the hypothetical intensity of the future earthquake as shown in Fig. 13. The intensity is represented in the J.M.A.
81
Fig. 12. Heights of tsunami waves along the coast. Solid and dashed lines represent, respectively, the maximum height corrected for the vertical land movements and the inundation height corrected for both vertical land movements and coastal effects. Fine solid line denotes the arrival time of the first waves, when we observe the water surface moving above the post-seismic sea-level (corrected for land movements) by 10 cm.
I 134-E
I 136’
_
I 13.9’
I 139.
36%
Fig.
13. Seismic
intensity
(J.M.A.
scale)
of the hypothetical
earthquake
(empirical).
82
intensity scale. The pattern is very similar to that of the 1854 event. In the Tokai district, Miyamura’s (1946) detailed survey shows that the seismic intensity is seriously affected by the local geological conditions. In fact, the coastal region of the Tokai district, far from the epicentral region, was severely damaged by the 1944 shock, in contrast with relatively slight damages in the eastern part of the Kii peninsula, adjacent to the fault plane. Such an anom~ous pattern was also observed at the 1854 event. Seismic intensities as high as 6 and ‘7 (equivalent to the M.M. scale of IX, and X-XII, respectively) were recorded in the eastern district including Suruga bay and its environments. These anomalous features of seismic intensity distribution may be attributed to the thick alluvium covering the coast of the Tokai district and the Tenryu river. It is reasonable to conclude that this area will receive relatively high intensities from the future earthquake. DISCUSSIONS
AND CONCLUSIONS
The writer has pointed out a possibility of a large earthquake in the Tokai district in the near future and has discussed its seismotectonic effects from various viewpoints. The seismicity gap and the local strain accumulation of crustal deformations are the primary data of concern. A creep-like slow slip in the fault plane at depth was hypothesized in the present study as a basic precursory phase of a large earthquake. This idea is in good harmony with laboratory data for the frictional characteristics by Scholz et al. (1972). They show that a small amount of stable slip always precedes every stick-slip, amounting to 2-5% of the final slip amplitude. Their experimental results for the frictional characteristics is in good harmony with the present precursor model. This model is of course speculative, but it is given in order to underscore the possible significance of this effect. In these circumstances, it seems worthwhile to monitor crustal deformations for the purpose of prediction. High-sensitivity strain meters and tilt meters will be useful for this purpose. Fortunately crustal-movement observatories with these instruments are available at three sites, Mikawa, Inuyama and Fujigawa, around the hypothetical source region, which are all expected to detect preseismic and precursory movements if they appear. If a creep-like slip occurs at depth (dip direction: N20”W; dip angle: 25”; fault dimensions: 20 km X 15 km; depth to the top side: 25 km; dislocation: 1 m, see Fig. 14), the cooperative work of these three stations will be particularly useful for detecting its location. In the present study, the hypothetical fault plane is assumed not to extend into Suruga bay. In other words, it is assumed that no major earthquakes related to the plate boundaries will occur there in the future. This assumption is made by considering the fact that the occurrence of crustal deformations from the historical earthquakes were not reported here, and that this area has no geomorphological evidence for the unde~hrusting. This scrt of information suggests that the principal faulting does not reach the
x3
Fig. 14. Maximum strain change on the earth’s surface synthesized for the hypothetical slow slip. Small hatched and large rectangles illustrate the part of the hypothetical fault plane of the slow slip and that of the major earthquake, respectively (surface projection).
Suruga bay area. Rather, Suruga bay is considered as a marginal region where the two sea-plate units are now colliding (Ando, 197413). The tsunami height inshore is subject to the complex topographic effect. Thus some uncertainty is unavoidable, particularly in predicting the tsunamis at the rias-type valleys in the Shima peninsula. Referring to t,he past tsunamis in this region, it seems possible that they are amplified inshore to a height 2 -4 times as large as the present prediction. Thus, in the worst case, there is fear of large tsunamis reaching 4-6 m in the Shima peninsula. The southwestern part of the Izu peninsula experienced large tsunamis in the 1854 Ansei I earthquake (Hatori, 1974). But this tsunami pattern is not deduced from the present model. The coastal topographic effect is too small to explain this discrepancy. Consequently the fault plane, as hypothesized above, might better extend for about 20 km to the east or accompany a small secondary fault in Suruga bay. The hypothetical earthquake will complete the present migration cycle since 1944, so that, after its occurrence, the whole Nankai trough system will become quiescent for a long time until the next cycle begins. ACKNOWLEDGEMENT
I am grateful to Prof. Keichi Kasahara for critically reading the manuscript and giving many suggestions. I benefited from discussions with Prof. Takashi Mikumo and Drs. Isamu Aida, Katsuyuki Abe, Yoichiro Fujii and Tokutaro Hatori. Dr. Isamu Aida generously allowed me to use his computer program for synthetic tsunami wave forms.
84
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