A brief summary is given of the symposium on neotectonics and its significance, which was held on August 26, 1983 in Shizuoka on the occasion of the Annual Meeting of the Japan Association for Quaternary Research. One of the purposes of the symposium is to discuss neotectonics of the area of the South Fossa Magna including the Izu Peninsula and the Suruga and Sagami Troughs, where the prediction of a future large earthquake has been made.
Landforms in and around the South Fossa Magna, which are shown in Figs. 2 and 3, and the history of their growth during the late Cenozoic are described. The pattern of the tectonic landforms and the history of their growth are explained by a tectonic model shown in Fig. 5. The South Fossa Magna consists mainly of strongly folded and thrust Neogene and Quaternary strata along the subduction and collision zone between the overriding Eurasian plate (or the Eurasian and North American plates) and the subducting Philippine Sea plate. The collision occurred on the north of the Izu Peninsula during the past 1-0.5 Ma. The characteristic landform in the folded zone is flights of elongated domes or ridges, called outer ridges. There are three areas with different topography and tectonic history in central Honshu to the north of the plate boundary; they are, from west to east, 1) the Akaishi Mts-Tokai lowland and offshore area, 2) the Kanto Mts-Misaka Mts-Tanzawa Mts area, and 3) the Kanto Plain-Miura-Boso Hills and their offshore area. To the south of the plate boundary, there are three different belts in the northeastern part of the Philippine Sea (PHS) plate; they are, from west to east, i) the Shikoku Basin, ii) the inner volcanic arc (Izu Inner Bar), and iii) the outer non-volcanic arc (Izu Outer Bar) of the Izu-Bonin arc. The PHS plate moved to the north after the spreading of the Shikoku Basin (30-15 Ma BP), and different physical property and somewhat different speed of movement of the three belts gave different tectonic features to the three areas in central Honshu as is shown in Fig. 5 (A). Then the PHS plate collided with Honshu on the northern side of the Izu Peninsula, and changed the direction of movement to the northwest in 1-0.5 Ma BP. Quaternary uplift of the Akaishi, Tenshu, Misaka, and Tanzawa Mts thus started, while the processes of making the outer ridges ceased under a rather extensional stress field in the Miura-Boso Hills and their offshore area of southern Kanto as shown in Fig. 5 (B). Collision between the northeast Japan arc (on the North American plate) and the Southwest Japan arc (on the Eurasian plate) may have occurred along the Fossa Magna in the Quaternary Period.
The Hydrographic Department has prepared nine profiles of multichannel reflection survey across the Sagami, Suruga and Nankai Troughs. An interpretation of these profiles is presented in this paper. The Philippine Sea Plate is subducting beneath the Eurasian Plate along the Nankai Trough, and the hemipelagic sediments and turbidite wedge sediments in the trough are accreted to the landward slope of the trough, building accretionary prisms with imbricated structure. The forearc basin inside the outer ridge which is the highest crest of the uplifted accretionary prisms, has been buried by terrigenous sediments. The forearc basin is differentiated and its extent is reduced by the folding of sediments. Three left lateral transcurrent faults parallel to the Suruga Trough suggest that they are relict but still active structures formed by the collision of the Philippine Sea Plate with the Eurasian Plate in the past.
There may be a subduction of the Philippine Sea Plate under the Eurasia Plate on the north side along the Sagami and the Suruga Troughs. This is demonstrated in multichannel and single channel seismic profiles in both troughs. The gentle and smooth basement on the Bonin Arc side declines toward the troughs overlaid with trough sediments, and the steep and ridged slope on the landward side of the troughs has been faulted and uplifted to make ridges and banks. It is presumed that there is an accretion associated with thrust faulting on the landward (west) side of the Suruga Trough. There is no remarkable accretion, however, on the landward (northeast) side of the Sagami Trough, where the banks and highs on the landward side may have been be formed by thrusting associated with the subduction of the Bonin Arc side under the highs.
The northern boundary of the Philippine Sea plate is defined topographically by the Nankai, Suruga and Sagami Troughs and it is considered to form a subduction boundary along the troughs. Tectonic landforms in and around the Suruga and Sagami Troughs are described and their relations to the subduction are considered in this paper. There is a significant difference in geomorphology between the Suruga Trough and the Sagami Trough. The Suruga Trough is narrow and runs straight in the direction of N10°E, while the Sagami Trough is sinuous off the southern tip of the Boso Peninsula and has a broad basin in Sagami Bay. The tectonic landforms around the Suruga Trough indicate that a subduction is going on from the southeast with a rather steep inclination. Riedel shear faults running in the direction of N40°W occur along the southeastern part of the Sagami Trough. A steep fault scarp with the direction of N70°E develops off the southern tip of the Boso Peninsula and is considered to originate from reverse faulting. These tectonic landforms indicate that the northern end of the outer belt of Izu-Ogasawara arc is moving northwestwards along the Sagami Trough. Regional difference in the nature of crustal deformations deduced from the tectonic landforms can be explained in terms of the relationship between the direction of plate motion and the change in strikes of the troughs. The mode of crustal deformations of the“seaward”side of the troughs is distinctively different from that of the“landward”one. The seaward side of the troughs including the izu Peninsula bends and form a broad flexure before subducting beneath the troughs. The landward side of the troughs is characterized by severe active deformations such as faults and folds with short wavelengths.
Crustal strains are deduced from geodetic data of triangulations and repeated trilaterations in the southern Fossa Magna district, Central Japan. This area is located very close to the convergent boundary of the Philippine Sea plate, and has been attacked by great earthquakes several times during historical ages. For the investigation of the regional distribution of compressional strains, lines of contraction in the district are obtained from the deduced principal axis of crustal strains in the second order triangles during the interseismic periods. The detected compressional strain field can be interpreted as the drift of the Philippine Sea plate for the south Kanto with N31°W and 4.3cm/yr, and for the Tokai with N45°W and 5.3cm/yr. Representative values of maximum shear strain rate are also deduced from the distribution of maximum shear strain rates in the second order triangles. These values are 0.40μstrain/yr for the south Kanto, and 0.16μstrain/yr for the Tokai. The highest value is found to be 0.53μstrain/yr in and around the Izu peninsula, where contemporary seismic activities are very high. A high strain accumulation is detected along a zone in the Tokai district parallel to the Suruga trough, where microearthquake activities are also very high at present. Another high strain accumulation zone has been known in the Boso Peninsula, south Kanto, but no microearthquake activity has been found there until now. ISHIBASHI (1983) supposed the existence of a triple junction near Kofu, where the North American, Eurasian, and Philippine Sea plates meet together and discussed the possibility of a jump of the above mentioned triple junction to a new position at the north end of the Nishi-Sagami Bay fault. The present author points out that the present day triple junction can be located at the jumped point mainly on the hasis of the intense crustal activity, high strain rate, and the highest level of the Holocene marine terrace at the north end of the Nishi-Sagami Bay fault. The age of the jump is presumed to be approximately 6, 000 years ago.
The coseismic vertical crustal movements in the Suruga Bay region, the Philippine Sea coast of central Japan, during three historical large earthquakes are briefly reviewed with special reference to the late Quaternary seismic crustal movement in the region. According to abundant historical documents, at the time of the 1854 Ansei-Tokai earthquake of magnitude around 8.4, many places on the west coast of Suruga Bay were remarkably uplifted, whereas the east coast was scarcely displaced or slightly subsided. The general pattern of that crustal deformation strongly suggests that the earthquake was basically due to a large-scale thrust faulting along a plane dipping westerly from the Suruga trough running north-south in the middle of the bay so far as the eastern part of its rupture zone is concerned. The pattern of the 1854 coseismic crustal deformation is generally in good harmony with the distribution of late Quaternary tectonic landforms, submarine topography and active faults in the Suruga Bay region, which implies that the 1854-type faulting has recurred many times during the late Quaternary due to the northwestward underthrusting of the Philippine Sea plate at the Suruga trough. There is no positive record of uplift nor subsidence concerning the coseismic crustal deformation in the Suruga Bay region at the time of the 1707 Hoei earthquake of magnitude around 8.4, which is considered on the basis of macroseismic and tsunami data, to have been basically an 1854-type faulting so far as the Suruga Bay region is concerned. At the time of the 1498 Meio earthquake of magnitude around 8.6, which is also considered on the basis of macroseismic and tsunami data, to have been basically an 1854-type faulting so far as the Suruga Bay region is concerned. a point on the west coast of Suruga Bay considerably subsided. This subsidence is interpreted as deformation of the hanging-wall side of the 1854-type reverse faulting. The difference among the coseismic vertical crustal movements on the west coast of Suruga Bay in 1854, 1707, and 1498 suggests that secondary reverse or normal faults on the hanging-wall side of the main thrust contribute much to the surface deformations and that these subsidiary faultings do not always accompany the plate boundary main ruptures. These features of coseismic crustal deformations in the Suruga Bay region should be taken into account in the morphotectonic investigation of the region.
Recent vertical crustal movements in the Tokai area are briefly reviewed from leveling data. The main features in the area are as follows; (1) large and steady uplift in the Akaishi mountain range, (2) steady and rapid subsidence on the western coast of Suruga Bay, and (3) anomalous uplift in the Izu Peninsula. The plate tectonic hypothesis may give a general framework to interpret these movements. However, the uplift of the Akaishi mountains is still open to further researches. A map of vertical crustal movements of the area is compiled, which excludes the effect of the Tonankai earthquake of 1944. The obtained result shows that the uplift of central Honshu extends further south into the offshore area in the Sea of Enshu. This result is consistent with the uplift rate estimated from sea level data. Thus, obtained data on the recent crustal movements can be compared with some other geological/geophysical data such as Quaternary crustal movements, topography, gravity, and so on. These various data should be interpreted synthetically to understand the neotectonics of the area in detail, which would be also important for the prediction of a forthcoming large earthquake of the area.
In southern Fossa-Magna, direct stress measurement such as over-coring, hydraulic fracturing, X-ray, and acoustic emmision has been carried out at 12 localities by 1982. Field study of active faults has been conducted energetically in this area, too. The results of both stress measurement and active faults are almost agreeable. In conclusion, tectonic stress in this area comprises two stress systems, (1) northwest-southeast (NW-SE), and (2) north-south (N-S). The NW-SE stress is found nearly all over the area. However, it is most dominant around the west coast of the Izu Peninsula and the Izu Islands. In this stress system, the maximum principal stress is nearly horizontal and trends northwest to southeast, and the medium and minimum principal stresses are respectively horizontal northeast-southwest and vertical. The medium and minimum principal stresses are almost equal. Therefore, in some areas, alternatively the vertical stress is the medium and the horizontal NE-SW is the minimum principal stress. On the other hand, the N-S stress is more restricted in area. In this stress system, maximum principal stress is nearly horizontal, trending NNE to SSW. Medium and minimum principal stresses are respectively horizontal WNW-ESE and vertical. Both NNE-SSW and WNW-ESE horizontal stresses are much larger in shallow depth than the vertical stress. However, the NW-SE and N-S stress system presumably do not divide area of distribution between each other, but are in state of co-existence in some places. Acting in the intermediate area between the NW-SE or WNW-ESE stress of southwestern Japan and the E-W stress of northeastern Japan, the NW-SE stress is the most common stress system in southern Fossa-Magna. Its tectonic source is a lateral compressional force applied from southeastwards. The N-S stress system develops mainly in the narrow zone bounded by the Ashigara-Kanogawa line in the east, which is called the Nishi-Izu zone in this report. The Nishi-Izu zone is probably under structural control of both the NW-SE and N-S stress systems. Tectonic origin of the N-S stress is not clear yet. However, the stress system is possibly closely related with the N-S stress acting in several places in southwestern Japan along the Pacific Ocean.
The South Fossa Magna in central Japan is thought to be a collisional zone between the Philippine Sea plate and the Asia plate. In this region, intense tectonic movements such as folding, faulting and other crustal movements have been proceeding since the early Quaternary. Several active faults which have an extremely high vertical slip rate are distributed along the north end of the Izu bar. Three tectonic sub-regions can be distinguished on the basis of the sense of fault movement and other tectonic features. That is, the area of the southwestern foot of Mt. Fuji, the area of the south end of the Tanzawa Mts. and the Ashigara-Oiso area. The first area is located in the northern extension of the Suruga trough. This area is divided into three blocks by N-S trending active faults. The easternmost block shows distinct subsidence and the others upheave with a high rate. In the second area, an E-W trending high angle reverse fault bounds the south end of the Tanzawa Mts. The topographic feature of this fault is indistinct. The third area is located in the northwestern extension of the Sagami trough. The NW trending Kozu-Matsuda fault which has a sence of reverse and right lateral slip divides the area into the Oiso hills and the Ashigara plain. The former shows a distinct upheaval which occurred in recent geological ages and the latter suggests the decrease of its subsidence rate. Studies of the crustal movements in each sub-region reveal the following common tectonic features. That is, each crustal block distant from the Izu bar has changed its movement style from the phase of intense subsidence to that of upheaval through the transitional phase. A newly created fault or a subsidence zone has always occurred in a block closer to the Izu bar. As a consequence, the latest tectonic zone in the South Fossa Magna seems to have migrated from a distant position closer to the Izu bar. Fig. 3 shows a concept of the tectonic process across the tectonic zone in the South Fossa Magna. This tectonic process is the process of accretion of the sediment to Honshu in the landward region of the Suruga and Sagami troughs. The active faults in this region are considered to be neither interplate faults nor branches from the main boundary fault but the imbricated thrust faults in the accretional sediments.
Active faults and neotectonics in and around the southern part of the Tanzawa Mountains and the Oiso Hills in the southern Fossa Magna region are summarized on the basis of new evidences obtained from fault analyses and tephrochronological studies. The active faults in this area can be classified into the following four systems; the E-W Kannawa thrust, the E-W high-angled reverse faults, the NE-SW left-slip faults and the NW-SE right-slip faults. Most of the strike-slip faults have dip-slip components. The Kannawa thrust probably moved during the Middle Pleistocene, and the other faults have been active since the late Middle Pleistocene, under the regional compressional stress field of N-S direction. The accumulation and deformation of the strata in this area have been strongly affected by displacements along the above mentioned fault systems.
A new view on the mechanism of the occurrence of a future Tokai Earthquake is presented in this paper. Conventional hypothesis concerning the problem regards the Suruga Trough in the Tokai district of Central Japan as a subduction zone that juxtaposes the Phillippine Sea and Eurasian Plates. But the hypothesis does not appear to be sustained by full substantial evidence. Careful examination of submarine topography, seismic reflection profiles and magnetic data suggests that the Suruga-Bay fault along the Suruga Trough is not a plate boundary fault, but a great left-lateral strike-slip fault. Recently, an important fault named the Fujikawa fault was discovered in the land area. The fault strikes north and corresponds to the northern extension of the Suruga-Bay fault. The Fujikawa fault is the most active fault in Japan and has an average left-lateral slip of 3.3cm/y. From historical documents we can conceive that a great earthquake (M=8.4) occurred in 1854 as a result of a displacement of both the Suruga-Bay and Fujikawa faults. Recent crustal activity shows that strain accumulation is steadily going on around the two faults. In the near future, the crustal strain will provoke a left-lateral slip of these faults and result in a Tokai Earthquake.
The origin of the northward-convex structure in and around the South Fossa Magna, central Honshu is discussed in relation to the collision of the Izu Block on the Izu-Mariana arc with Honshu. The Izu Block has been pushing Honshu since the middle Quaternary when the collision occurred. The 15-30km contraction between the Izu Block and collided Honshu is presumed to have been consumed in the manner as follows: 1) internal deformation of the Izu Block, 2) thrusting, folding and uplift of the Neogene and Quaternary deposits in the South Fossa Magna, and 3) compressive uplift and strikeslip faulting dominant in central Japan outside the Fossa Magna. Geological evidences suggest that the convex structure of the Neogene of this region was formed, inheriting the pre-existing bend of the pre-Miocene terrains surrounding the South Fossa Magna and that the Quaternary collision of the Izu Block contributed to the compression of the region, though it did not strengthen the northward-convex structure significantly.
Neogene and Quaternary structures of the area around the Suruga Trough in the South Fossa Magna and their neotectonics are examined from the viewpoint of microplanktonic biostratigraphy and biochronology. Folds and N-S trending thrusts of the South Fossa Magna are considered to have been affected by the stress field with the σ Hmax axes of NW-SE to WNW-ESE directions since the latest early Miocene time at 16Ma. The undulatory deformations of the terrace surfaces of the area have also been affected by the same stress field. A series of N-S trending and high-angle thrust-faults of the area has successively appeared from west to east, and an inferred active thrust-fault along the lower Fuji River is presumably a northern extention of an inferred active thrust-fault along the Suruga Trough which bears major dip- and probable strike-slip components. The above-mentioned aspects seem to be concordant with the northwest subduction of the Philippine Sea plate along the Suruga Trough. On the basis of fossil evidences obtained from the Izu Peninsula, it is most likely that the peninsula, an ancient volcanic island in the southern tropical ocean in middle Miocene time at 11Ma, drifted with the Philippine Sea plate north- and/or northwestward, and came into collision with the Honshu arc. All of the N-S trend of the Suruga Trough, intensive folds and thrusts of the South Fossa Magna, and active Quaternary undulations of the area can be well explained by the still continuing collision of the Izu block. The above-mentioned neotectonics of the area may closely be related to the mechanism of large earthquakes off the area.
Neotectonics of the coastal areas of Sagami and Suruga Bays in the South Fossa Magna region are diseussed with special reference to Holocene marine terraces of the area, where the Eurasian Plate is considered to have collided with the Izu Peninsula on the Philippene Plate along the Sagami and Suruga troughs. In the coastal areas of the Oiso Hills, the Miura Peninsula and the south tip of the Boso Peninsula, the adjacent areas northeast of the Sagami trough, there are several sets of Holocene marine terraces, suggesting remarkable upheavals. Active faults are developed throughout these areas, a notable one of which is manifested by the Kohzu-Matsuda fault known as the northern extension of the Sagami trough. Along the Kohzu-Matsuda fault, the upheaval of the Oiso Hills is recognized at the rate of about 3.4m in about 1, 000 years. The area of the Izu Peninsula is characterized by a downward movement of the west coast adjacent to the Suruga trough, and an upheaval of the east coast. A terrace of the maximum phase of the Jomon transgression is found only on the northeastern coast. On the southern coast, Holocene terraces of the highest level are dated from 3, 000 years B.P. On the west side of the Suruga trough, crustal movements are prominent on the coast of the delta of the Fuji River, which is regarded as the northern extension of the Suruga trough. That is manifested by an upheaval of the lwabuchi area in the west and a subsidence of the area of Ukishimagahara in the east. The amount of the dislocation in the area seems to be much larger than that of the Kohzu-Matsuda fault, and to decrease towards the Omaesaki point.