We acquired high-resolution seismic reflection profiles and continuously cored boreholes to evaluate active flexures produced by major blind thrust fault systems within two densely populated Neogene-Quaternary sedimentary basins in Japan: the Fukaya Fault System near Tokyo in the Kanto Basin and the Uemachi Fault System in the Osaka Basin. The high-resolution seismic reflection survey made clear the length, geometry and growth history of fault-related folds, or flexures formed above the two blind thrusts. Continuously cored boreholes linked with high-resolution seismic profiles enabled us to estimate the uplift rate as defined by shallow stratigraphic horizons and constrain the age of the most recent growth of the flexures during earthquakes on the Fukaya and Uemachi fault systems. Even with the high quality of the data we collected, it is still not possible to exactly constrain the age of the most recent blind thrust earthquake recorded by flexure of these fault-related folds. Data presented in this paper form the basis for future efforts aimed at mechanical and kinematic models for fault growth to evaluate the activity of blind thrusts underlying urban areas.
The Beppu‐Shimabara graben, in Kyushu, Japan, has active seismic regions and active volcanoes. Studies using triangulation, gravity surveys and mechanism solutions of large earthquakes show that the graben is in a state of extensive stress associated with rifting. A temporary seismic study was carried out from February 20 to March 8 in 1992 in the region, during which time many seismic signals were observed. The microearthquake seismograms recorded inside the graben have a later phase dominant on the horizontal component following the direct S‐wave. Two‐dimensional ray‐tracing was applied to investigate the cause of the later phase. The phase was identified as an S‐reflected S phase (SxS‐phase) from a discontinuity with a depth of 20 km. The reflection points are located inside the Beppu‐Shimabara graben that is in a state of extensive stress associated with rifting.
A dense temporary seismograph network was set up in the aftershock area of the 1984 Western Nagano Prefecture Earthquake of M 6.8 by the group for the 1986 Joint Seismological Research in Western Nagano Prefecture. Seismograms of about 1, 500 events were obtained during the observation period of 52 days from September 1 to October 22, 1986, and 530 events in and around the aftershock area were located precisely. Extent of the aftershock area is about 20 km in the east-west and about 7 km in the north-south. The hypocenter distribution in the present study shows that the depths of the deepest aftershocks become deep toward the west. The maximum depth in the eastern part of the aftershock area is about 6 km and it becomes 10 km at the western end. Seismic activity is high at a belt near the maximum depths of the aftershock area and no events were detected below this seismic belt. It was also found that the size and location of the aftershock area found by the present observation are almost the same as that obtained by a temporary seismic observation in 1984, which was made just after the main shock. Since no events below the inclined lower oundary were found in either of the observations, it is suggested that the deeper region below the boundary cannot accumulate stresses sufficient to generate earthquakes, suggesting the existence of large locality in the lower boundary of seismogenic zone.
A viscoelastic bending moment theory is applied to the subducting slab of the Philippine Sea plate. The slab is steeply bent at the northern end of the Kyushu-Ryukyu subduction zone compared with at the southern part of the arc. The plate boundary on the cross section of the slab beneath Aki-Nada, Iyo-Nada and Bungo Channel, sea regions between Kyushu and Shikoku Islands, represents an arc of a circle, as demonstrated by the microearthquake distribution during over 10 years. The elastic bending moment is evaluated based on this geometry, where the deformation due to viscosity is reduced applying a Maxwell viscoelastic theory. The bending is too large in strength to ascribe it only to the force of negative buoyancy of the slab. We here assume the horizontal pressure generated by the spreading mantle flow accompanying upwelling magmas at the volcanic area in northern Kyushu, where extensive ground movements have been detected by geodetic measurements. The exerted uniform pressure to bend the slab is estimated to amount to (σ=) 72 MPa, when we take the following model constants as standards : the thickness of the slab h = 30 km, Young modulus of the slab (E=) 1.8 x 105 MPa, Poisson's ratio (ν=) 0.27, an average elapsed time since the subduction of the slab initiated (t=) 1.9 × 106 y, the time constant of the viscoelastic slab (τ=) 3.0 × 105y, and the excess density of the slab compared with the surrounding athenosphere (Δρ=) 0.06 × 103 kg/m3. The region of the highest seismic activity is located at the vicinity of the upper locked portion of the plate, where destructive earthquakes have frequently occurred. The 2001 Geiyo earthquake of M6.4 was one of them and took place at the eastern end of this active seismic zone. The focal mechanism of the event shows normal faulting as similarly as those of the majority of small earthquakes in the seismic active region. The main cause of this large earthquake is considered to be the bending force of the plate.
The Geiyo earthquake occurred on June 2 in 1905 in the western Seto Inland Sea between the Honshu and Shikoku Islands, Japan. The seismograms of the earthquake obtained at the stations of Central Meteorological Observatory were newly found at Earthquake Research Institute of the University of Tokyo. They are recorded by the Omori seismometers and tromometers, which are superior to former seismometers with respect to continuous recording. For the estimation of the magnitude and source mechanism from the seismograms, we digitize 3 records, which are Hongo EW component, Hitotsubashi EW component and Tokyo tromometer. We have to know the response of the Omori seismometer to estimate the ground motion during the earthquake. In order to know the frequency characteristics of the seismometers, we calculate their Fourier amplitude spectra. The spectra of the Hongo EW component and Hitotsubashi EW component show clear peaks which may be considered as the natural periods of the seismometers. The natural periods of Hongo EW component and Hitotsubashi EW component are about 60s and 25s, respectively. The damping constant estimated from the free oscillation record of Omori seismometer at Ishinomaki observatory is less than 0.01, and the friction is 1.7mm.
The detailed three-dimensional P and S wave velocity structure in the focal region of the 1984 Western Nagano Prefecture Earthquake occurring close to an active volcano, Mt. Ontake, central Japan, is derived from a tomographic inversion of travel time data obtained by the 1986 Joint Seismological Research in Western Nagano Prefecture. The data set includes 7, 693 P-wave and 6, 070 S-wave arrival times observed at 49 stations from 212 local earthquakes and 2 explosions. The velocities in the shallow portion of upper crust are determined at each grid point with its spacing of 1-2 km and good resolutions are obtained from the Earth's surface to a depth of 4 km. There exist strong lateral heterogeneities especially from the surface to a depth of 1 km. The seismic velocity map obtained in the height of 1 km above sea level well corresponds to the maps of surface geology and Bouguer gravity anomaly. This correspondence, however, is not so clear in the deeper layers. And, the greater the depth, the less heterogeneous the velocity structure. The velocity distribution on the fault seems to correspond to the distribution of dislocation and rupture front estimated from an analysis of strong motion and geodetic data: the low velocity region has the large amount of dislocation and the retarded rupture front.
An analysis of seismic waves from regional microearthquake networks shows that the seismic leading edge of the Philippine Sea plate may have reached the uppermost mantle beneath the southern to central part of the western Chugoku region, Japan. The epicenters are located close to the volcanic front in the region. The most possible cause of the very low seismicity may be the initiation of melting of the Philippine Sea plate.
Recently the telemetering seismic network of Shiraki Micro-Earthquake Observatory, Earthquake Research Institute, University of Tokyo, has been constructed in Chugoku District and its vicinity, Western Japan by the fourth stage of National Earthquake Prediction Project. There are seven seismic observation sites in the network, three of which are linked by the telephone lines and four of which, newly installed, are linked by radio to Shiraki Micro-Earthquake Observatory. The latter four are located in the southern part of the network. Thus, the accuracy of hypocenter determination has been improved, especially in the southern part. According to hypocenter determination based on the revised Ichikawa-Mochizuki travel-time table, the following features of the seismic activity in the period from July, 1983 to October, 1984 were obtained:1. In the land area of Chugoku District, earthquakes occurred in the upper part of the crust.2. In the southern region of 34°N latitude line, most of earthquakes took place in the upper mantle. An east-west cross section of hypocenters clearly shows a simple Wadati-Benioff seismic zone with a thickness of about 20km, but a north-south cross section of hypocenters shows complicated seismic zones, possibly more than two. The dip of the seismic zone west of 132°E is a little larger than that east of 132°E in the east-west cross section.Furthermore, detailed description is given to main events during the period from July, 1983 to October, 1984, such as earthquakes M6.8 and M5.2 on August 26, 1983 and June 25, 1984, respectively.
Focal mechanisms around Ontake Volcano, central Japan, were investigated by using the data set obtained by a seismic observation project of the 1986 Joint Seismological Research in Western Nagano Prefecture (JSR'86). The data set included P-wave first motions of aftershocks of the 1984 Western Nagano Prefecture Earthquake (M=6.8) mainly. A total of 556 events was selected for the focal mechanism analysis. The P-wave data from the surrounding university routine networks were also utilized additionally. Four hundred two events were successfully analyzed. Based on the types of focal mechanisms and on the spatial distribution of the events, we classify these events into the following 4 groups: (1) events of strike-slip fault type with a common nodal plane parallel to the ENE-WSW striking nodal plane in the fracture zone of the 1984 main shock; (2) events of reverse fault type in the area of the preexisting earthquake swarm to the northeast of the main shock, in a possible relation with an earthquake swarm activated by the main shock; (3) events of reverse fault type that occurred near the largest aftershock (M=6.2), but are not the continued aftershock sequence of the largest aftershock; (4) events of strike-slip fault type at depths to the easternmost part where no activity had been observed before the operation of JSR'86. The plural types of focal mechanisms obtained and the spatial distribution of foci suggest a complex seismo-tectonic structure that consists of at least 2 kinds of fracturings around Ontake Volcano.
