Showing papers in "Journal of the Seismological Society of Japan in 1980"
TL;DR: In this paper, a force-balanced pendulum tiltmeter was installed at the bottom of a borehole of which the depth is about 100m, and the measuring range is +2 x 10-4 radian, the resolution is 6 × 10-9 radian and the long term drift is smaller than a few micro-radians per year after half a year since the installation.
Abstract: Continuous monitoring of the crustal tilt is one of the most effective approaches for the purpose of earthquake prediction. Drilling a borehole is not so expensive as construction of an underground vault, which is necessary for installation of water-tube tiltmeters and horizontal pendulum tiltmeters, and a borehole station is possible to be made even in a plain area. The National Research Center for Disaster Prevention has not only developed the boreholetype tiltmeter itself but has improved borehole drilling techniques and installation methods for the tiltmeter. The force-balanced pendulum tiltmeter developed for down-hole observation is installed at the bottom of the borehole of which the depth is about 100m. The measuring range is +2 x 10-4 radian, the resolution is 6 x 10-9 radian, and the long term drift is smaller than a few micro-radian per year after half a year since the installation. Clear records of the earth tides demonstrate the high quality of the instrument. Tilt changes of the order of micro-radian caused by precipitation are remarkable in the data of some stations. But, the crustal tilt observation by using borehole-type tiltmeters is expected to be very effective for detecting anomalous tilt changes of the order of 5 x 10-8 radian per day with a time constant of several hours to several tens of days.
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TL;DR: In this article, repeated measurements of in situ P wave travel time changes were made near the Obaku fault, Kyoto, over a distance of 400m, and a stack of marine air gun shots in a water-filled hole were carried out every 2 hours spanning 32hours.
Abstract: Repeated measurements of in situ P wave travel time changes was made near the Obaku fault, Kyoto, over a distance of 400m. A stack of marine air gun shots in a water-filled hole were carried out every 2 hours spanning 32hours.By the use of a wave form correlation method, we found a half-percent variation of the travel time with an accuracy of 0.3msec. This variation of the travel time that was proved due not to errors caused by the instruments used correlates with variations in tidal stress and atmospheric pressure (10-20mb). This inferred high stress sensitivity of velocity changes (0.2bar-1) may be attributed to the presence of thin cracks. For the purpose of monitoring long term velocity changes, measurements similar to the present one are being planned over the same base line.
6 citations
TL;DR: In this article, the phase velocity of the vertical component of the microtremors is derived by means of frequency-wavenumber spectrum in the case of complex wave field, and the phase difference between two observation points in the simple wave field.
Abstract: Recently, microtremors are investigated for the application to earthquake engineering, and many of these studies pay much attention to power spectrum. But, power spectrum depends not only on the underground structure of the observation site but alse on the path of propagation and the source spectrum. It is therefore important to extract the quantities of microtremors which depend only on the under ground structure. One is the phase velocity of microtremors, and another is the linear relation between radial and vertical components, if microtremors are composed only of Rayleigh wave. In the present paper, the quantities of microtremors are derived in a statistical way. It is demonstrated that the phase velocity of vertical component of microtremors can be obtained by means of frequency-wavenumber spectrum in the case of complex wave field, and by means of phase difference between two observation points in the case of simple wave field. But, the linear relation could not be obtained, because of complex constitution of horizontal components or coexistance body waves and/or higher mode Rayleigh waves with fundamental mode Rayleigh wave.
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TL;DR: In this article, a simple method is developed to determine seismic moments of earthquakes by using tabulated data in usual seismological bulletins, qualified through the criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity to the instrumental response: At first, characteristic period T of each earthquake is defined as the average value of apparent periods of wavelets which give maximum amplitudes of ground displacement at epicentral distances between 200 and 700 km.
Abstract: A simple method is developed to determine seismic moments of earthquakes by using tabulated data in usual seismological bulletins. The method is qualified through the criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity to the instrumental response: At first, characteristic period T, of each earthquake is defined as the average value of apparent periods of wavelets which give maximum amplitudes of ground displacement at epicentral distances between 200 and 700 km. Secondly, amplitude information is taken into consideration, making a product of maximum amplitude, its period and epicentral distance. Seismic-moment factor Me for a given earthquake is defined at the characteristic period T, as the average value of those products evaluated from horizontal components at stations within epicentral distance range from 200 to 400 km. The narrow range of epicentral distance in evaluating Me is taken so as to reduce the uncertainty due to seismic-energy attenuation into a permissible range and to be able to obtain equal number of observations for small earthquakes to that for large ones. The relation between the seismic-moment factors and characteristic periods for 163 intraplate earthquakes in Japan from 1926 to 1977 clearly demonstrates that Me is proportional to the cube of T,. A scaling model of earthquakes that satisfies the empirical relations among surface-wave magnitude, JMA magnitude, and body-wave magnitude facilitates the estimate of static seismic-moments from calculated The seismic moments of 16 earthquakes determined by conventional analyses from nearand/or far-field observations are consistent with static seismic-moments thus estimated. This shows the potential in practice of the present method especially in routine processing of seismic data.
TL;DR: Using the Phinney's method which is based on the Thomson-Haskell matrix of the spectral responce of a layered crust, short-period P waves from distant earthquakes were analyzed from seismograms recorded at Mizumiiro, Toyone and Mikawa stations in the Tokai region as mentioned in this paper.
Abstract: Using the Phinney's method which is based on the Thomson-Haskell matrix of the spectral responce of a layered crust, short-period P waves from distant earthquakes were analyzed from seismograms recorded at Mizumiiro, Toyone and Mikawa stations in the Tokai region. By using the ratio of the vertical component to the horizontal one, the author obtained the structure of the crust and uppermost mantle beneath this region. The Mohorovicic discontinuities were obtained as depths of about 33km in the western Shizuoka and about 30km near Mikawa. In the Tokai region, an \"intermediate layer\" just beneath the Mohorovicic discontinuity was detected, for which the compressional wave velocity has been suggested as 7.5-7.8km/s by explosion seismic studies. Thickness of this layer was obtained to be 15, 15 and 10km beneath Mizumiiro, Toyone and Mikawa, respectively. The present model including an intermediate layer does not contradict with the result from the surface wave observation.