Tokihiko Matsuda

Fault mechanism and recurrence time of major earthquakes in southern Kanto district, Japan, as deduced from coastal terrace data

The southern Kanto region has had two shocks of magnitude 8 or greater during the past 1,000 years. They were the 1703 and 1923 earthquakes, which occurred along the Sagami trough, a northeastern boundary of the Philippine Sea crustal plate in contact with the Asian plate. Although they occurred in nearly the same region, the 1703 earthquake was significantly different from the 1923 earthquake in the distribution of coastal uplift and tsunami height. The 1703 earthquake deformation is described on the basis of the height of the marine terraces along the coast of the southern Kanto region. The 1703 earthquake is interpreted, as is the 1923 earthquake, as the result of low-angle right-lateral faulting with a thrust component at the plate boundary. However, the fault surface in 1703 was longer (about 200 km) and was located farther east than that of the 1923 earthquake. On the basis of the pattern of coastal uplift and the trend of the Sagami trough, the fault surface of the 1703 earthquake can be divided into three planes, which involve the eastern part of the source region of the 1923 earthquake to the west (plane A), the Kamogawa submarine cliff in the middle (plane B), and a segment near the source region of the 1953 Boso-Oki earthquake (M equals 8.0) to the east (plane C). The Boso and Miura Peninsulas in the sourthern Kanto region have been uplifted during at least the last 6,000 years, and major uplifts have been accompained by earthquakes like those of 1703 and 1923 many times. The recurrence time of similar uplifts is estimated at 800 to 1,500 years on the basis of the numbers of the uplifted Holocene terraces in the Boso Peninsula, the rate of upheaval during the last 6,000 years, and the present geodetic data. Thus, it is unlikely that major earthquakes such as the 1703 and 1923 earthquakes will occur in the same segments in the near future. The Oiso area, however, which is located west of the western end of the 1703 faulting, seems higher in seismic risk than the other parts of the Sagami trough fault, because the sum of the recent uplift in the 1703 and 1923 earthquakes in that area is significantly less than the average rate of uplift there during the past 6,000 years. /Author/

Quantitative distribution of late Cenozoic volcanic materials in Japan

The late Cenozoic orogeny in Japan is briefly reviewed. Amounts of volcanic materials in the three periods of the orogeny are estimated at: early Neogene 150 × 103 km3 (mafic 40 %, salic 60 %), middle and late Neogene 20 × 103 km3 (mafic 70 %, salic 30 %), Quaternary 5 × 103 km3 (mafic 80 %, salic 20 %). The largest volume per unit time is in the early Neogene, and the smallest in the middle and late Neogene. Volume per unit area becomes larger towards the southeastern margin or «front» of the volcanic belt. Thermal energy transported by volcanic materials is compared with the terrestrial heat flow in the belt.

Atera Fault and Its Displacement Vectors

The northwest-southeast-trending Atera fault, which is expressed geomorphologically and geologically, cuts many terraces of the Kiso River in central Japan. The authors measured the vertical and horizontal displacement of the fault from the offset of the terrace surfaces and faces (scarps). Seven displacement vectors were calculated in the fault plane. They show that (1) the horizontal displacement is about five times larger than the vertical; (2) the faulting, to date, has been consistently left lateral; (3) the rate of faulting seems to have been almost consistent (about 2–4 m/1000 yrs.). It is noted that the Neo Valley fault is subparallel to the Atera fault, and that its displacement is also left lateral. Data on recent strike-slip fault displacements in central Japan, in part from this specific study, show a regular pattern of geographic distribution of strains.

CRUSTAL STRUCTURE OF THE SOUTH FOSSA MAGNA, JAPAN, INFERRED FROM THE GEOLOGICAL DATA

Crustal structure in the South Fossa Magna, Japan, a Tertiary geosynclinal area,. is inferred from study of stratigraphic profiles prepared for each stage of development and the modification of those profiles which must have resulted from subsequent crustal deformation. The general sequence of events was: 1) deposition of early Miocene Misaka formation; 2) crustal warping; 3) deposition of the late Miocene Fujikawa series with contemporaneous erosion of Misaka rocks in upwarped areas; 4) quartz diorite intrusion; and 5) formation of crystalline schist along the southern limb of the Tanzawa upwarp. It is concluded that the Mohorovicic discontinuity lies at a depth of 15 km to 20 km in the area and increases in depth northward. The Mohorovicic discontinuity must have relief of 10 km or more corresponding to the amplitude of Miocene folds. The quartz diorite magma was generated in the lower part of the crust during orogeny. M. Russell