ALBERT

Description: A single mobile observation was made in Hokkaido to complement its crustal structure which had been poorly known. A swarm of Usu volcanic earthquakes, whose source region was concentrated within a diameter of 1 km, was used as a “source”. Because of the small size of the source region and the good time keeping, errors in travel times are as little as 0.2 sec, which is enough for travel time studies. Thirty-nine stations were temporarily operated on two profiles, whose length was 320 km and 100 km, respectively. The seismometer was basically an ocean bottom seismometer which was slightly modified for the land use. The small size, the wide dynamic range, the low power drain and the sturdiness were helpful for the efficient observations. The obtained crustal structure illustrates: (1) The granitic layer, with the velocity of 5.8 km/sec, is shallow at the top and the thickness is about 20 km. (2) The basaltic layer, with the velocity of 6.6 km/sec, is immediately beneath the granitic layer. Its thickness is also about 20 km. Hence the crust is thicker than usual compared to other regions in Japan. (3) The upper mantle velocity is 7.8 km/sec. The profiles were inherently not reversed, however one of them is so planned as to be reversed by possible future earthquake swarms of Teshikaga region, which have occurred recurrently in the past. © 1981, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Various approaches have been explored to elucidate the nature of the core-mantle boundary, such as its position, physical shape, and the distribution of physical constants. In this study these investigations are reviewed and the possibility is examined to study the unknown factors of the core-mantle boundary characteristics when more abundant seismic data become available in the future. In order to determine the physical constants of the lower mantle close to the core-mantle boundary, it is quite important to investigate the effect of diffraction due to the core. An attempt to obtain Q distributions in the mantle by taking spectral ratio of direct waves at different stations has been made. If the effect of diffraction is unknown, however, it is impossible to make correction for observed spectral amplitudes to find correct Q values in the lower mantle. To elucidate the nature of the core-mantle boundary, other method for obtaining the attenuation function along the boundary by taking spectral ratio of the core-diffracted waves has been developed. For this case, it is necessary to know the attenuation function for a wide variety of the core-mantle boundary structures which are suggested from other evidences. To eliminate the effect of diffraction, it is promising to analyze the spectra of waves reflected at the core boundary. But even for this case, it would be required to solve the problems of determining the reflection coefficients when spherical waves are incident onto spherical boundary having the gradual change of physical constants. In the present paper, the model studies of these bottlenecks are planned. First, the precise diffraction pattern by an impenetrable sphere is obtained from sonic wave experiments. Second, the difference between the diffraction patterns due to the various structures of the core-mantle boundary is studied by use of two-dimensional models which have core-mantle boundary structures with continuous velocity change, as well as irregular boundary structures. The difference between the diffraction patterns is found to be too large to make the diffraction compensation to the spectral ratio of the direct waves possible with an aim of getting Q values in the lower mantle. The attenuation functions of the diffracted waves in the shadow zone are also obtained by ultrasonic model experiments and compared with seismic data. The results show that a kind of irregular core-mantle boundary structure having a thickness of 30 to 100 km accounts for the seismic data. PcP spectra are generally insensitive to the structure with continuously changing velocities and other physical constants, while they are sensitive to the layered structure having discontinuous boundaries. This result suggests the possibility to discriminate whether or not there is a layered structure at the core-mantle boundary by use of the spectra of PcP. © 1969, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: This study presents a 500-km-long crustal transect across the Lofoten volcanic passive continental margin, N. Norway, by compiling the results of two successive Ocean Bottom Seismographic (OBS) experiments performed in 1988. The OBS profiles were acquired from the Norwegian mainland, across the continental shelf, over an area covered with landward flood basalts, to the Lofoten basin. The land side end of the crustal model represents a thinned continental structure. The crust in this part has strong structural complexity, mainly due to faulting during pre-Tertiary continental thinning phases. Between the continental shelf and the seaward dipping reflectors (SDR), the model represents an extremely thinned continental crust and ocean/continent transition zone. This region is interpreted to be dominated by an early Tertiary continental rifting phase that progressed until early Eocene. The observed lower crustal reflectors, which are interpreted as intrusions in the lower crust, as well as the landward flood basalts indicates an extensive magmatic activity during the continental rifting phase. Between the SDR and magnetic anomaly 21, an oceanic crust with thick lower crust and a high velocity layer at the bottom of the crust (7.3km/s) are obtained. This high velocity layer is believed to be created by anomalously hot asthenospheric material rising around a hot spot. The comparison of the crustal structure across the Lofoten margin with the structure of the Voring-More margin shows significant differences in the volume of the lower crustal high velocity layer, which can be interpreted in terms of a NE-ward decrease of the influence of the hot spot. © 1995, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Studies on the crustal structure in the profile across central Japan close to the longitude 137°E were started in 1966 with 20 explosions made off the Atumi Peninsula. In 1967, twenty-one reverse shots off the Noto Peninsula and a supplementary three-ton explosion at the Ebisu Mine were observed at stations on land specially arranged for the derivation of crustal structure. Most of the stations were equipped with magnetic-tape-recording systems which provide us a filter technique to detect signals of low S/N ratio. The quality of record was extensively increased through filtering, and the following characteristic features were revealed: A. Pn waves were extensively observed in the explosions off the Atumi Peninsula but not in those in the Sea of Japan. B. Waves belonging to a granitic layer were clearly observed in the explosions off the Noto Peninsula but were absent or almost absent in the explosions off the Atumi Peninsula except for shots near the peninsula. These marked differences suggest an asymmetric crustal structure in the profile. A time-term method is satisfactory for the derivation of sediments under the Sea of Japan, but not for the derivation of the Moho discontinuity. A new method utilizing long wave components, was developed and used in the present study resulting in many possible structures. The crustal structures thus obtained are, in general, continental under the mountain area, oceanic under the Pacific, and intermediate under the Sea of Japan. The most probable one out of many is shown Fig. 17. Gravity data were also used in the selection of the best model. The thickness of the basaltic layer remains unknown, but it should be very thin in the southern half of the profile. It forms a striking contrast with the structure in the profile along the longitude 139°E which is located within a short distance of only 220km. A velocity gradient for the upper mantle was also obtained from the explosions off the Atumi Peninsula. © 1972, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Model experiments were carried out for relatively short body waves compared with the dimension of the model to obtain their travel times, amplitudes and wave forms. A plexiglass disk of radius of 1m with thickness of 2mm is used as a two-dimensional model of the earth. The cylinder-shaped source transducer without directionality is devised so that the amplitudes observed at any point on the surface can be analyzed and compared. The time duration at the source is about 0.03 with the unit of a/Vs, Where a is theradius of the disk and Vs the shear velocity. The disk was also preferred as themodel by OLIVER et al. (1954) and by KATO and TAKAGI (1956), where time durationswere about 0.3 and 0.04 with the same units, respectively. Theoretical seismograms were synthesized by SATO and USAMI (1964) and ALTERMAN and ABRAMOVICI (1965)with the time durations of 0.19 and 0.03, respectively. The seismograms are obtained at the surface with 5° intervals of epicentral distances for four models where distances between the source and the center of the diskare 0.775m, 0.850m, 0.925m and 0.997m. The rules of phase change for reflected waves such as pP, PP, pPP, PPP, etc. derived theoretically are verified by the model experiments. Diffracted pulses which appear at shorter epicentral distances than those of cusp points of travel time curves for pP, sS and pS are identified. When attenuation due to the lossy medium is taken into account, observed amplitudes of pP and PP pulses show good agreement with theoretical values when these pulses are not interfered by other phases. The present experiments suggest that the possibility of further study when themedium has some velocity gradient with depth and a core is feasible. Seismograms from real earthquakes are compared with those obtained in the experiments. When both focal mechanism and the phase shift rule are taken into account, it is found that wave forms of reflected waves such as pP and PP can be interpreted. This implies that not only P and S waves but also the reflected waves are useful for determining the focal mechanism. © 1965, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The Urakawa-Oki earthquake (M=7.1) occurred offshore of Urakawa, Hokkaido, Japan, on March 21, 1982. In order to investigate the aftershock activity of this event, we deployed four ocean bottom seismographs (OBS's) off Urakawa. During an observation period from March 29 to April 6, the OBS network detected more than 4,500 earthquakes. Magnitude range of these events is from -2.0 to 5.0. Aftershock distribution of the Urakawa-Oki earthquake was determined on the basis of the OBS and land observations. In the analysis, we selected about 250 aftershocks with M〉2.0, because the seismic signals from these events were recorded clearly both by the OBS's and the land stations around the aftershock area. Hypocenter determination was carried out using a method of inversion analysis. Most of the aftershocks were located in a region of 42°00’-42°20’ N and 142°25’-142°40'E. These events were distributed in a depth range of 10-30 km. The characteristic dimensions of the aftershock area were estimated as 35 km x25 km. The events in the southern part (south of 42°15'N) were distributed on a plane with an area of 20-25 km x 10-15 km dipping 20-30° northward. In the northern part of the aftershock area, we found another trend of aftershocks which inclined southward with a high angle of 60-70°. The events in this group were located on a plane with a relatively narrow area of 10 km x 10-15 km. The aftershock distribution obtained in the present analysis suggests that the fracture mechanism of the Urakawa-Oki earthquake was very complicated. © 1983, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: A very accurate system to observe the temperature change of a deep groundwater has been developed. A specially made quartz crystal is adopted as a sensing unit which enables the temperature measurement to the accuracy of 1/1,000°C. Other advantages of the system are the simplicity of the instrument, ease of the installation, and low cost, which make possible a number of installations in an area. The concentration of the observations is especially important since the natures of wells might be different from place to place. More than 25 sets of the instruments were made and 15 sets have been already installed in Hokkaido and Honshu. According to the preliminary results, a coseismic signal was observed at a well in eastern Hokkaido when an earthquake of magnitude 7.5 took place at the distance of 200 km. Other than the coseismic signal, a number of unexpected temperature variations have been observed. © 1980, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Five ocean bottom seismographs (OBSs) were dispatched to the source region of the 1983 Japan Sea earthquake (occurred on May 26; M=7.7) by a helicopter only three days after the mainshock. The source region was 40 to 100 km off the western coast of northern Honshu. The major part of the aftershock area was covered by the OBS network, which recorded ground motion on magnetic tape continuously for 12 days. Among the vast number of aftershocks recorded, 500 events were selected for reproduction. A precise aftershock distribution was obtained from these data. The aftershock area, which ran along the eastern margin of the Japan Basin, was 140 km long in north-south with a width of 40 km. The focal depths of the aftershocks were concentrated in a range of 8 to 21 km. Since the lithosphere of this region is estimated to be no less than 30 km thick, the fracture did not span the entire lithosphere. The geometry of the main fault was suggested by a plane arrangement of the aftershock distribution, which is 30 km long in north-south and dips eastward by 15 to 25°. This observation is consistent with the mechanism solution of the mainshock obtained from land data. Not a few hypocenters which do not belong to the eastward-dipping plane may suggest the existence of another major fault plane. © 1985, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.