Abstract
In order to determine information relating to large earthquakes from crustal deformation at a distance of 100 km, for example, we need to observe strains as small as 10-7. The surface and the crust are formed of many blocks which have individual elastic and plastic coefficients. Therefore, a complex pattern of strain is usually observed near the surface even under a uniformstress pattern. Besides, changes of creep velocity, viscosity and strength of the crust forming materials depending on temperature, watersaturation and pressure are much larger than those of elasticity. Therefore, the gradient of distribution of the plastic deformation is usually complex and large. The short period strains caused by seismic waves, the earth tidal force and so on chiefly consist of elastic deformation, but the long period crustal movement consists both of elastic and plastic deformations. In addition, the stress caused by atmospheric phenomena is very complex at shallow depths. So, the pattern of the long period strains is more complex than that of the short period strain near the surface. In order to avoid the trouble produced by the complexity of the ground structure and the ground noise on observation of crustal deformation, we require placing the instrument in a deep place. However, increase of efficiency of the station decreases rapidly with depths. There are two methods to offset the ground noise and to smooth the irregularity. The first method is group setting, in other word; array, which takes the average of observations of the same component at several observatories distributed in a specified area whose radius is smallerthan few kilometers. The second method is the using of a long base line with a length longer than few hundred meters. An experiment to determine the relativemerit between these two methods was conducted at old Osakayama tunnel of the former Tokaido Line.
