Investigation of Microearthquakes - On Earthquake Occurrence in the Crust -

Abstract

1. The author has tried to consider that seismicity is expressed only by earthquakes occurring independently each other. This is accomplished fairly well by eliminating earthquakes that occur within the same hypocentral domains in a certain period. A seismicity map made in this way resembles more to that of large earthquakes than the original one. 2. The coefficient m of Ishimoto-Iida's relation varies minutely by space or by time. 3. The focal mechanisms of the earthquakes that occur in the northern part of Kinki District and the eastern part of Chugoku District are nearly the same, regardless of magnitude, focal depth and pattern of occurrence. The direction of the principal compressional axis is latitudinal and that of the intermediate principal axis is vertical, with slight variations. Then the majority of these earthquakes are considered to be the strike slip type, but a few earthquakes are the dip slip type. This is consistent with the neotectonic evidences. 4. In explaining the flactuation of focal mechanisms by conjugate sets of fractures, even when the ratio of the conjugate fractures is assumed as small as 1/4, the fracture angle exceeds 40°. 5. To explain the low stress drop actually observed in natural earthquakes and the uniformness of their focal mechanisms at the same time, these earthquakes must be considered to occur where the stress is not large but the strength is weak. 6. If this weak strength is attributable to interstitial fluid material, various geophysical or geological phenomena can be explained.