FREQUENCY-MAGNITUDE DISTRIBUTION OF EARTHQUAKES IN VRANCEA - RELEVANCE FOR A DISCRETE MODEL

A high-resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes (h > 60 km), extending over an interval of 9.3 years emphasizes a significant deviation at low magnitudes (M(L) < 4) in a linear frequency-magnitude distribution. It appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve. This is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity-like earthquakes and the transition zone (M(L) = 3.3 - 3.9) from crack-like earthquakes (background seismicity) to asperity-like events. These features are better pointed out on well individualized active zones. The background seismicity shows a tremendeous decrease in its b slope, from 1.03 to 0.57, during a 6-year interval before the occurrence of a major earthquake in 1986 (M(w) = 7.3), followed by a fast recovery to 1.03 in 2 years after this event. Such a behavior could be correlated to the continuous growth of the shear stress-free surface on the fault, as in a percolation process, followed by its sudden diminution due to the locking of the fault. The above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths.