The Off-East Izu Peninsula has been a realm of active swarm earthquakes since the end of 1978. Earthquake swarms with a magnitude of around 5 have occurred once or twice a year in a belt extending in a NW-SE direction. The trend of the seismic belt coincides with the regional pressure axis inferred from many active strike-slip faults. The seismic belt is situated in the Fuji Volcanic Belt that is characterized by active volcanoes and high geotherm. The purpose of this study is to detect a mode of horizontal crustal deformation that is expected to occur before and after an earthquake swarm. A new automatic electronic distance measuring device was manufactured for this investigation. The device uses an electronic distancemeter as a sensor and is controlled by a personal computer. A measuring line was laid between Ito on the east coast of the Izu Peninsula and the Hatsushima Island in Sagami Bay. The distance between the two points is about 9.7 km. Laser beam is shot from Ito and reflected from Hatsushima. Measurements are carried out every 10 minutes. The observation was started on April 14 of 1989. Although seismicity was low and no change in distance was observed toward the end of June, a new earthquake swarm commenced to occur on June 30 in the region covered by the measuring line. Day by day earthquakes increased in number and magnitude and eventually a large earthquake with a magnitude of 5.5 struck the region on July 9. The earthquake resulted from right-lateral strike-slip faulting with a trend of N 86 °W. In accordance with seismicity the distance on the measuring line gradually increased and attained to an elongation of 22.4 cm just after the occurrence of the large earthquake. Right-lateral slip of 30 cm along the fault and opening of 10 cm across the fault were estimated on the basis of the amount of elongation and another data of 16 cm elongation that was observed on the different measuring line by the Geographical Survey Institute. A submarine volcanic eruption occurred in the central part of the fault four days after the appearance of the fault. But no considerable elongation was observed on the measuring line. Aftershocks clustered in the tension region of the right-lateral strike-slip fault. This fact as well as the opening across the fault and the volcanic eruption suggests the increase of pressure of pore-filling liquid within the crust that results from a convection current of thermal water heated by magma. The author concludes that the present earthquake swarm is a phenomenon of shear failure due to decreasing effective stress in concurrence with increasing pore pressure in the rock.
The 1872 Hamada earthquake (M=7.1±0.2) is one of the major historical earthquakes which accompanied a coseismic uplift and subsidence in the coastal area. This paper intends to examine an implication of the Hamada earthquake for geomorphological development of the Hamada area, based on the investigation of marine terraces, emerged sea level indicators such as sea caves, benches, fossil beds and beach deposits, and fault topography. Two steps of Pleistocene marine terraces are found in the coastal area. Terrace I, c. 40-60 m high, is underlain by weathered beach gravel covered by dune sand which is interbedded by at least four horizons of paleosol. It is considered, that a major interglacial period resulting in strong weathering of Terrace I deposits occurred after the formation of this terrace. Thus, Terrace I can be correlated to the penultimate interglacial, and Terrace II, c. 15-25 m high, underlain by rather fresh gravel bed, to the last interglacial. Accepting this correlation, the uplift rate of the study area must be small, c. 0.1-0.2 m/ka, and is no significant difference in the uplift rate over the study area. Height of emerged sea level indicators associated with the Hamada earthquake is 0.9-1.7 m above the present mean sea level. At least one sea level indicator higher than the emerged sea level at the time of the 1872 Hamada earthquake was found at several locations through the study area including the coast where coseismic subsidence occurred in 1872. Northeast-southwest trending lineaments predominate in the study area and a fault exposure is observed on one of the lineaments suggesting that they are fault origin. Areas of coseismic uplift and subsidence can be seens by turns along these faults. Characteristics of the 1872 Hamada earthquake are summarised as follows, based on the above mentioned geomorphological observation : 1) Coseismic deformation as associated with the Hamada earthquake has not accumulated through the late Quaternary, and this earthquake must be a very rare event with coastal deformation during the Holocene. 2) Distribution pattern of coseismic deformation, that is, the occurrence of small blocks with coseismic uplift and subsidence by turns can be interpreted as an effect of strike-slip movement on the faults.
Akagi volcano situated in the North Kanto district of central Japan is a Quaternary volcano. The eruptive history of this volcano during the last 200, 000 years is clarified by the tephrochronological study. The plinian pumice fall deposits derived from Akagi volcano are as follows in ascending order; the Moka Pumice (MoP), Akagi-Mizunuma Pumice-16. …-12, -10…-1 (MzP-16…-12, -10…-1), Namekawa Pumice-2, -1 (Nm-2, -1), Yunokuchi Pumice (UP) and Kanuma Pumice (KP) (Fig. 1). Stratigraphy, distributions and petrographic characteristics of these tephras are described (Figs. 5, 6, 8, 9 and Tables 1, 3). The MoP pumice fall deposit covering the most part of the eastern part of the North Kanto district, erupted in the penultimate glacial stage preceding the Last Interglacial Stage. Moreover, the stratigraphic relations of the MzP-10…-1, Nm-2, -1 and UP with the well dated widespread tephras, which are the K1P-7 (ca. 130 ka), DPm, On-Pm I (ca. 80 ka), K-Tz (ca. 75-80 ka), Aso-4 (ca.. 70 ka) and DKP (ca. 45-48 ka), are clarified (Fig. 7). These data can give the chronological framework for the eruptive history. The MzP series, Nm-2, Nm-1 and UP erupted during the stage called the younger stratovolcano (YS) of the Akagi volcano in previous work (Fig. 10). The total volume of the plinian pumice fall deposits from the MzP-10 to the UP amounts to 28 km3. This corresponds to the discharge rate of the pumice equivalent to 0.33 km3/1, 000 years and the frequency of the plinian eruption 0.15/1, 000 years. Before the formation of the central cone, it occurred the most eruptive episode of Akagi volcano. This is represented by the members : the KP pumice fall deposit (ca. 31-32 ka) and the Mizunuma lithic (chert lapilli) fall deposit (CLP). The volume of the KP deposit which amounts to 25 km3, is the largest volume of the plinian pumice fall deposits derived from Akagi volcano.