Anomalies in the magnitude-frequency relation of earthquakes in the Garm region, Soviet Central Asia

Abstract

We examine magnitude-frequency relations for earthquakes in the Garm region of Soviet Central Asia using 17 years of data gathered by the Complex Seismological Expedition (CSE) network. Earthquakes in most of the region obey the linear magnitude-frequency (Gutenberg-Richter) relation. However, when we focus on the highly active area in the western portion of the Peter the First Range, we observe a significant departure from the G-R relation in the form of an excess in the number of earthquakes with magnitudes in the range of 3.5 to 5.5. We show there is less than a 5 per cent probability that this anomaly is due to random chance. We speculate that this anomalous behavior may be related to the fact that the Peter the First Range is an active fold and thrust belt in which the deformation is dominated by moderate-sized events on imbricate thrust planes. The linear portion of the magnitude-frequency relation at smaller magnitudes represents the self-similar response of the fault system on smaller scale structures.