A method for multidimensional analysis of earthquake frequency-magnitude distribution with an application to the Vrancea region of Romania

• Published in: Tectonophysics Vol. 261; no. 1; pp. 9 - 22
• Main Authors: Trifu, Cezar-Ioan, Shumila, Vladimir I.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.08.1996
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/0040-1951(96)00053-4

A novel method is proposed for the time-space analysis of the frequency-magnitude distribution of earthquakes (FMD). The FMD is calculated using a non-parametric technique, based on the nearest neighbourhood statistics, and the FMD parameters, namely the occurrence rate at a reference magnitude and the b-value, are subsequently estimated by a parametric (log-linear) regression. The method's potential is examined by using a microearthquake catalogue of the intermediate depth ( h > 60 km) seismicity in the Vrancea region (Romania) spanning 18 yr. For the magnitude intervals of M L 2.8–4.0 and 4.0–5.2, the calculated occurrence rates exhibited different linear decays with increasing magnitude on a log-linear scale. Quasi-linear correlations were found between the FMD parameters over selected time-space intervals. In several cases, the results showed that changes in the b-values were inversely related to changes in the occurrence rates of magnitudes at the limits of the first magnitude interval. The three large events ( M W ∼ 7) reported in the catalogue were preceded by time-space regions of relatively low b-values lasting for a few years at the depth of these events. Three episodes of distinct behaviour were identified: (i) b-values are inversely related to changes in the occurrence rates of magnitudes at the limits of the first study range ( M L 2.8 and 4.0); (ii) a decrease in the b-value occurs when an increase in the occurrence rate of relatively larger magnitude events ( M L 4.0) is observed; (iii) the occurrence rate of larger events tends to saturate, whereas the rate of small events tends to fluctuate. The results suggest that multi-dimensional b-values and occurrence rates over selected magnitudes have the potential to outline characteristic trends in the recorded seismicity, and provide relative constraints for the modelling of the earthquake cycle.