微小地震の活動様式

近畿地方中北部の微小地震活動について

抄録

In the middle and northern parts of Kinki District, microearthquakes are occurring almost stationarily. In order to investigate their activity in relation to the tectonic structure and also to simulate the occurrence of larger events, a telemetry network of observation stations was spread densely over these areas. The data, thus acquired, were analyzed and filed with a data processing system.
By using these data of about seven thousand events (1976 to 1981), we got a most detailed spatial distribution of microearthquakes and compared it with the fine tectonic structure in this region.
As the result of analyses, it is clarified that microearthquakes are not distributed uniformly over the tectonic lines or the active faults, but they form some arrays or clusters systematically along a north-south direction. It may be possible to conceive of the shape and direction of arrays or clusters being provided apriori under a tectonic field of northsouth direction. An earthquake family, then, seems to be composed of microearthquakes belonging to an arrays, because the cutoff value (M=4-5) in the frequency distribution of magnitude corresponds well to the size of the array.
Next, the time series of microearthquake activity is examined by applying the Weibull distribution. The mode of activity is divided into two portions, having respectivedly the successive and random properties, at a certain value of time interval of earthquake occurrence. This characteristic value varies among the earthquake families, and also has a negative correlation to the Gutenberg-Richter's b value. The value, therefore, seems to be a parameter that indicates the state of medium and/or the temporal concentration of tectonic stress.
Thus, we intend to obtain more data to deduce a local concentration of tectonic stress or a change of pattern of stress field through detailed studies on the spatial and temporal distributions of microearthquakes in stationary states.