Ryuichi Shichi

On the Cause of Long-Period Crustal Movement

Abstract It has been found by continuous observation in the vaults of crustal-movement observatories that the progress of crustal deformation is accompanied by a long-period fluctuation. The migration of this fluctuation between the adjacent stations and its simultaneity over a wide area has also been recognized. Some interpretations of the cause have been proposed from the viewpoint that fluctuations originate substantially from the tectonic source. In this paper, it is concluded that this long-period strain fluctuation is fairly well explained by precipitation changes at each site; the simultaneity may be explained by the identical precipitation conditions over a wide area, while the migration may be deceptive, attributed to the differences in the response to the precipitation at each station. In other words, prior to the discussion of the long-period crustal movement in relation to tectonics, the effect of precipitation on the crustal strain at that station should be carefully examined.

Inversion of gravity data to determine the terrain density distribution in southwest Japan

We propose a new method of inverting gravity data for the lateral density variation of the terrain above sea level. The density variation is modeled by a piecewise step function. We impose an a priori condition that the Bouguer anomaly distribution expressed by a third-order spline function is smooth relative to the topographic distribution. No a priori condition is imposed on the density distribution. The Akaikes Bayesian Information Criterion is used to obtain optimum trade-off parameters controlling the smoothness of the Bouguer anomaly surface against its fitness to the observed anomaly values. We apply the method to about 45,000 gravity observations in southwest Japan. The calculated density distribution, in general, correlates well with major geologic features: the density of sedimentary, volcanic, or granitic units increases as their age increases in a way consistent with laboratory measurements of rock density and age. In some regions, however, where the geologic features described in the literature are likely to be surficial and to be underlain by other geologic units, the estimated densities deviate from this general trend. Some active faults sharply offset topography and geology, resulting generally in a step in the Bouguer anomaly. In such cases the estimated densities tend to be unreasonably large or small. Synthetic tests indicate that a larger number of data points and spline knots are required near the offset to eliminate this problem.

Data Processing for the Japan Gravity Standardization Net 1996

The Japan Gravity Standardization Net 1996 (JGSN96) has been established by analyzing a data set of both absolute and relative gravity measurements performed by the Geographical Survey Institute (GSI) during the last 2 decades. The paper explains how to derive the JGSN96 from the viewpoint of the method of analyses and the data used.

The Establishment of the Japan Gravity Standardization Net 1996

A new gravity reference of the Japan Gravity Standardization Net 1996 (JGSN96) has been established from absolute values with FG5 gravimeters and accumulated data of relative gravity connections after the current reference of the JGSN75 was published. The paper describes a short history of this work as well as the results of JGSN96.

Hidden Fault Scarp Inferred from Gravity Analysis and Disaster Belt of the 1995 Hyogo-Ken Nanbu Earthquake

Dense gravity measurements were carried out together with the GPS positioning along five survey lines across the Rokko fault system. The subsurface structure relevant to the 1995 Hyogo-ken Nanbu earthquake was inferred from the Bouguer anomaly. A hidden fault was discovered on the southwestward extension of the Koyo fault underneath the sedimentary layer. The extension runs just on the edge of the earthquake disaster belt. The thickness of the sedimentary layer was estimated to decrease gradually toward the mountain side. The wedge-like structure of the soft layer and the hidden fault scarp under the Kobe plain may serve as a focusing lens of seismic rays during the earthquake.

Observation of Crustal Movements by Means of a Long Baseline Water-Tube Tiltmeter at Sagara, Shizuoka, Japan

It has been pointed out that a destructive earthquake will occur in the Tokai District, central Japan, by the interactive motions between the Philippine Sea Plate and the Eurasian Plate in the near future.