Frequency‐magnitude distribution of earthquakes in Vrancea: Relevance for a discrete model

Abstract

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 (ML < 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 (ML = 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 ( Mw = 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.

Multifractal and Chaotic Analysis of Vrancea (Romania) Intermediate-depth Earthquakes: Investigation of the Temporal Distribution of Events

“…One of the remarkable characteristics of the Vrancea intermediate-depth seismicity is the nonlinearity of its frequencymagnitude distribution. Thus, TRIFU and RADULIAN (1991) showed that around M ¼ 3.5 there is a significant change in the slope of the frequency-magnitude distribution, which probably marks the transition between two different earthquakegenerating mechanism regimes. Previous studies also suggested that the Vrancea intermediate-depth seismogenic region consists of two seismic zones of distinct activity, centered around 70-110 km and 120-160 km.…”

Repeated Earthquakes in the Vrancea Subcrustal Source and Source Scaling

“…One of the significant results of our analysis is extension of the scaling laws over the entire magnitude range, from small-to-moderate earthquakes to the major earthquakes. This is not a trivial outcome if we take into account the class of models for the seismogenic process in the Vrancea source which assume a critical jump between the crack-like and asperity-like rupture processes at small-to-moderate scale to percolation-type processes at large scale [28]. Thus, seismic moment -magnitude, seismic moment -source radius and seismic moment -stress drop scaling appears to be self-similar over the entire magnitude range of the considered data set (3.0 ≤ M w ≤ 7.7).…”

Seismic Hazard of Romania: Deterministic Approach

“…The two focal depths selected in the computations are typical average depths for the Vrancea earthquakes, and correspond to the maxima observed in the depth distribution of the earthquake energy release (TRIFU and Table 1 The intensity scale MSK-76 and associated a6erage peak 6alues of ground motion (MEDVEDEV, 1977) Velocity ( RADULIAN, 1991;ISMAIL-ZADEH et al, this issue). The two focal depths selected in the computations are typical average depths for the Vrancea earthquakes, and correspond to the maxima observed in the depth distribution of the earthquake energy release (TRIFU and Table 1 The intensity scale MSK-76 and associated a6erage peak 6alues of ground motion (MEDVEDEV, 1977) Velocity ( RADULIAN, 1991;ISMAIL-ZADEH et al, this issue).…”