Hydrographic Department, Japan Maritime Safety Agency conducted multichannel seismic reflection and refraction survey and Sea Beam bathymetric survey in the Toyama Trough extending northward in the southeastern margin of the Japan Sea. The purpose of the survey is to reveal the geological structure and tectonic landform of the Toyama Trough and to examine the hypothesis that the Toyama Trough is subduction zone. Seismic profiles across the Toyama Trough do not support the presence of distinct subduction zone along the trough because the basement of the trough is not traceable beneath the Sado Ridge. This suggests that the Toyama Trough is different in geological feature from typical subduction zone such as the Japan Trench or the Nankai Trough, where subducting oceanic basement is clearly traceable beneath the overriding plate in seismic profiles. Further, seismic profiles show that the sedimentary layers thicken to the east in some places and to the west in the other places. These features indicate that convergent plate boundary is not present, or only extremely incipient plate boundary is present along eastern margin of the Toyama Trough. However, the tectonic landform and geological structure characterized by the presence of active reverse faults indicate that Toyama Trough has been subjected to compressional stress.
The hypothesis proposed by Nakamura (1983) that the North-Eastern Japan is a part of the North-American Plate has been re-examined from geological and geomorphological veiw points. One of the difficulties is that active faults are not found along the entire length of the Itoigawa-Shizuoka Tectonic Line which is considered as the boundary between rigid plates. The beginning of the convergence at the Itoigawa-Shizuoka Tectonic Line is much older than the age speculated by SENO (1987). An alternative hypothesis that the Itoigawa-Shizuoka Tectonic Line is a moribund plate boundary is proposed.
Extensive studies on seismic activity and focal mechanism of major to moderate-size earthquakes that occurred along the eastern margin of the Japan Sea and the northern part of the Fossa Magna regions are reviewed and discussed in relation to regional tectonics. The validity of a prevailing hypothesis is also tested from various observations to see if the Northeast Honshu arc is part of the North American plate. Seismicity along the Japan Sea coastal regions extends southwestwards across the Toyama trough in one way, and also in other way to the northern Fossa Magna through a zone near the Shinano river. It has been noticed that the seismicity show temporarily successive properties similar to migration phenomena. Five major earthquakes along the Japan Sea regions have been interpreted as having thrust-type mechanisms, and the depth distribution of aftershocks of two larger events and the dip of submarine active faults suggest an eastward dipping fault plane. The maximum compressive stress derived from the focal mechanisms of more moderatesize earthquakes indicates E-W to ESE-WNW orientations, changing gradually from the northern to southwestern part of the coastal regions. In inland regions west of the northern Fossa Magna, the general trend appears oriented again in the ESE-WNW direction, which is found to be well consistent with the direction of principal compressive strains derived from geodetic triangulation surveys over the last 80 years. All the above evidence suggests that the regions under consideration may be a tectonically active, convergent zone, and might be regarded as a zone of a possible plate boundary. There is no direct evidence, however, suggesting an eastward incipient subduction of the Japan Sea lithospheric plate, from observations of submarine topography and upper crustal structure beneath the regions. Numerical calculations show that the suggested hypothesis could partly account for the observed directions of compressive stress in Northeast Honshu but is apparently inconsistent with those in Southwest Honshu. The results suggest that the Northeast Honshu arc may better be regarded as a “micro-plate” which receives strong compressive stress not only from the westward movement of the Pacific plate but also from the west side. One of possible sources of the driving stress from the west side might be an eastward movement of the “Amurian plate” which is one of micro-plates detached from the Eurasian plate. Another possible source could be bilateral extension of the central Japan Sea region or of the regions covering Northeast China and the Korean peninsula.
The geologic feature of the northern Fossa Magna region is characterized by a well developed folded belt system. Recent microseismicity over 10 years and historic earthquakes reveal that there are seismic zones pararell to the axes of this structure: Itoigawa-Shizuoka Tectonic Line, the western side of Shinano River, the western margin of the Central Belt of Uplift, and the eastern margin of the Central Belt of Uplift. The last zone was recently recognized by the earthquake sequence with the main shock of M 4.9 in 1986. The fcal mechanism solutions and other geologic and geodetic evidence indicate the region is compressed in the direction perpendicular to the folded belts. We can also find seismic activity lines perpendicular to or oblique to these folding axes: the Chikumagawa Tectonic Line separates the Central Belt of Uplift with a left lateral offset of 10km; the seismic line along the eastern side of the volcanic line of Myoko-san, Kurohime-yama and Iizuna-yama makes the eastern edge of the seismic gap around the northern end of the Itoigawa-Shizuoka Line. A 20-25 years recurrence time is found in the occurrence of the major earthquake events in the central part of the northern Fossa Magna in recent 140 years. Seismic active regions are grouped into some rectangular blocks with their bases being either pararell or perpendicular to the folded belts. The activity in a block became active nearly simultaneously, and it sometimes migrated to other blocks with some delay time. The high activity migrated from the block region around the Chikumagawa Tectonic Line to both the northern activity block around the Japan Sea coast and the active region in the southern Fossa Magna during the period from 1986 to 1987. The migration of the seismic activity along the Fossa Magna region may be due to delayed transmission of the stress field in the viscoelastic or elastoviscous medium. The exponential time dependence of the decay rate of the Matsushiro swarm activity for a long period more than 20 years since 1965 is found to be reasonably interpreted by the relaxation process of stresses within the elastoviscous medium. The viscosity changed from 1.1×1019 poise to 2.7×1020 poise as time elapsed. The first value is comparable with that of a ground surface rock body estimated by secular ground tilts and strains. The latter is the same order of the values obtained in the long period second creep experiments using granite and gabbro. The Matsushiro region is one of the region where the viscosity might have been very low due to numerous micro-cracks.
Recently, an idea was proposed that the eastern Japan Sea deformation zone represents a nascent convergent zone between the North American and Eurasian plates. This supposed convergent plate boundary extends southwards through the Fossa Magna, a colliding sector, to the Suruga Bay forming a triple junction near Mt. Fuji or the Kofu Basin where the three convergent boundaries among Eurasian, North American and Philippine Sea plates meet. In order to evaluate this idea, the seismic activity including focal mechanisms of large and small earthquakes in the South Fossa Magna and the northern margin of Philippine Sea plate are examined. The subducting Philippine Sea plate, descending northwestwards along the Suruga and Sagami troughs was well defined by using the hypocenters and focal mechanisms of high resolution and accuracy. However, the plate boundary passing through the inland region, the South Fossa Magna, between the Sagami and the Suruga troughs was not defined due to poor seismic activity. On the basis of available seismic data, the newly proposed convergent plate boundary passing through the South Fossa Magna could not be found.
Newly developed idea of the nascent plate boundary along the eastern Japan Sea has become to be discussed in the last five years. The boundary extends southwards through central Japan, where the Itoigawa-Shizuoka Tectonic Line (ISTL), one of the great tectonic lines in Japan, traverses to Suruga Bay. In the south of central Japan, the Philippine Sea plate is subducting under Japanese Islands, which lie on the North American plate and the Eurasia plate, and, therefore, a triple junction is formed there, if we acceptthe new idea of the plate boundary. Central Japan is, then, considered to be situated under the complex tectonic stress fields due to the plate interactions, and has the structures resulting from ongoing or past geodynamical process. Many seismic probings have been undertaken in central Japan and the derived structure will open informations on the geodynamic process and verifications of the nascent boundary. New resolution methods in seismic probing of the crust and the upper mantle can derive a three-dimensional velocity structure. The derived structure in central Japan, including ISTL, shows a low velocity body beneath the Hida mountains at the east of ISTL. However, the difference between the North American plate and the Eurasia plate has not beenresolved over the whole area along ISTL from the P-wave velocity of viewpoint. Many explosion profiles have been conducted in central Japan, some of which cross ISTL. The result at the north of ISTL shows a distinctive reverse fault dipping east near the Matsumoto Basin, though the fault location is slightly different eastwards from the geologically estimated tectonic line. The dip of the fault is concordant to that derived from the newidea of subducting the Eurasia plate under the North American plate. We cannot find other seismic profiles showing the similar structure in this area. In the southern part of ISTL, the northsouth fault is derived, which may be deformed by the plate interaction between thePhilippine Sea plate and the Eurasia plate. Unraveling a highly and widely resolved structure will be required to step forward to verify the new plate boundary hypothesis, and to put it practice, it is necessary to develop observation systems as well as the processing techniques. We address a request ofdeep seismic sounding which has controlled sources on a long-range profile with seismic waves penetrating deep into the crust and the upper mantle on a scale to discusss the new plate boundary hypothesis. Almost simultaneously, seismic tomography using natural sources, having a resolution of a three-dimensional image, will provide important information for better understanding of the geodynamical process in central Japan.
The Itoigawa-Shizuoka Tectonic Line (ISTL) forms the western margin of the Fossa Magna separating the Northeast Japan from the Southwest Japan. The conventional idea insists that both the Northeast Japan and the Southwest Japan belong to the Eurasian plate (EUR). A hypothesis has recently proposed, however, that the Northeast Japan belongs to the North-American plate (NAM), while the Southwest Japan belongs to EUR. If we approve the hypothesis, it follows that ISTL is a plate boundary between these two plates. The purpose of this paper is to define ISTL as a kind of plate collision boundaries on the basis of gravity data. ISTL runs across a positive Bouguer anomaly belt with the width of about 30km located on the Japan-Sea coast. The Bouguer anomaly there is about 30 mgal higher in the west side of ISTL than in the east side. This implies that, if we assume ISTL to be a plate subduction boundary, the eastern plate sinks down the western one along ISTL. On the other hand, the inland part of ISTL between Omachi and Matsumoto cities is characterized by a large NS-trending thrust called “East Matsumoto Basin Fault”, the west side of which subsides down the east side. The existence of this thrust is confirmed by both gravity and explosion seismic data. The movement of this thrust is evidently opposite to the plate sinking on the Japan-Sea coast. ISTL turns its way to the NW-SE direction at the south end of the East Matsumoto Basin Fault. The left-lateral slip is predominant from Suwa to Kofu cities, and its horizontal displacement amounts to about 12 km long from geological considerations. The leftlateral movement there can be interpreted from a crustal compression in the EW direction which is caused by a plate collision. The graben lies along this part of ISTL. We find a gravity difference amounting to 20-30 mgal at its maximum between the inside and the outside of the graben. Assuming that a density contrast of basement with sediment buried in the graben is 0.5g/cm3, we estimate the vertical displacement of the graben wall as 1-1.4 km. Our estimation is consistent with the fact that a 800m deep drilling does not reach the basement in the Suwa basin. If we approve the previously mentioned hypothesis, the Kofu basin is considered to be a tripple junction of three plates: EUR, NAM and the Philippine-Sea plate (PHS). Accordingly, the southernmost part of ISTL is a subduction boundary of PHS sinking down EUR, extending southward to the Suruga trough in the Pacific Ocean. Meanwhile, the NAM-PHS boundary runs southwestward from the Kofu basin to the Sagami trough. The highpass-filtered Bouguer anomaly on the inland part of PHS intensifies NE-SW trending short-wavelength undulations of gravity field, which may reflect the northeastward movement of PHS. Geoscientific investigations can not define the Northeast Japan as a part of NAM, but find sufficient evidence for the fact that ISTL is a collision boundary between EUR and the other continental plate except for the PHS area. The gravity data also support this conclusion.
Extensive magnetic surveys, which have been undertaken in the Islands of Japan and surrounding regions, have revealed marked magnetic anomalies in various regions including the Fossa Magna region. Some of the anomalies are likely to be closely related to tectonics. Magnetic data also provide valuable information on the temperature distribution in the crust through the determination of an isotherm corresponding to the so-called Curie point ; crustal rocks lose their magnetization at temperatures higher than the Curie point. In this paper, magnetic anomalies are presented with special reference to tectonics in the Fossa Magna region as well as the Curie point depth distribution there. In the Southern Fossa Magna region, a regional magnetic anomaly extends as shown in Fig. 1; a positive anomaly is located near the northern-most edge of the Philippine Sea plate and a negative anomaly extends behind the Sagami and the Suruga troughs. This anomaly can be interpreted as indicating the subduction of the oceanic Philippine Sea plate, the top of which is likely to be highly magnetized. Then the configuration of subducting plate is inferred from block models which can account for the observed magnetic anomalies, as clearly shown in Fig. 2 for some profiles. The depth to the subducting plate thus inferred seems to be somewhat shallower than that derived from the distribution of hypocenters of microearthquakes. Recently, a complicated configuration including fracture and flexure has been proposed and hence a threedimensional interpretation seems to be required to cope in a realistic way with such a complicated structure, whereas in block models a rather two-dimensional structure is implicitly assumed. In view of this defect, the discrepancy is not meaningful and we may conclude that the block models largely represent the depth to the subducting Philippine Sea plate. The Itoigawa-Shizuoka tectonic line turned out to be located at a steep gradient of Curie point depth, which is deeper in the west (see Fig. 4). This fact implies that the Itoigawa-Shizuoka line is in fact a tectonic boundary. Some tectonophysicists claim that the eastern margin of the Japan Sea is a plate boundary. As seen in Fig. 3, the Curie point depth is generally shallow there, whereas it is fairly deep at the Japan trench where the Pacific plate sinks. However, this fact would not contradict with the new hypothesis if we take into account the age of seafloor of the Japan Sea (about 20 Ma). The Curie point depth data also provide useful information on the magnitude of a possible largest earthquake. On the one hand, earthquakes are not generated at high temperature because of ductile behavior of rocks. On the other hand, the earthquake magnitude is determined by the area of rupture zone. Hence the magnitude should not be large in areas where the Curie point depth is shallow. This relation can be quantified and then the zoning of the magnitude for the largest earthquake was derived as shown in Fig. 5. The magnitude of 7.7 for the earthquake which occurred at the eastern margin of the Japan Sea exceeds the upper bound if the region is not supposed to be a plate boundary. This difficulty is removed if the relation for interplate earthquakes is applied in this region, supporting the new hypothesis that the eastern margin of the Japan Sea is a plate boundary. The electrical conductivity is also expected to provide important information on tectonic processes. Although high conductivity zones have been found in the crust beneath the Fossa Magna region, their relation to tectonics is not clear at present. In particular, the high conductivity zone beneath the Northern Fossa Magna (see Fig. 6) extends in a direction rather perpendicular to the Itoigawa-Shizuoka line. No clear conclusion seems to be derived until a systematic study is undertaken in the entire Fossa Magna region.
Horizontal crustal deformation in the northern and central Fossa Magna regions, central Japan, has been revealed by the geodetic distance survey. Regional variations of the horizontal deformation are recognized along the Itoigawa-Shizuoka Tectonic Line. In the northern region, there is no discontinuity of the horizontal deformation between Itoigawa and Otari along the Itoigawa-Shizuoka Tectonic Line, and it seems that the boundary of the horizontal deformation jumps to the Naoetsu-Otari Tectonic Line. It seems that the collision, slightly oblique, occurs, and the gravity anomaly and the earthquake fault suggest that the southwestern Japan has subducted under the northeastern Japan along the Itoigawa-Shizuoka Tectonic Line. In the central region, it seems that the left-lateral strike slip fault motion occurs along the Itoigawa-Shizuoka Tectonic Line. Therefore, it is considered that the northern part of the Itoigawa Shizuoka Tectonic Line may be a convergence boundary and the central part may be a transcurrent boundary.