Earthquake

Abstract The article presents the results of a study of a swarm of weak earthquakes that occurred in one of the regions of Uzbekistan. The earthquake swarm occurred in the western part of the Fergana intermontane basin. This area is located between the seismically active North Fergana and South Fergana zones of regional faults. In 1990, only two earthquakes with a magnitude of 4.3 occurred in this area. Of particular interest is the fact that the area containing the epicenters of the weak earthquakes occupies the northeastern part of the Kokand Magnetic Anomaly, which has a maximum intensity of 500 nT. According to geophysical investigations, this anomaly is caused by a granitoid intrusion. The top of the intrusion is located at a depth of approximately 6–8 km; however, no reliable data are available on its horizontal dimensions. The hypocenters of the earthquake swarm are located at depths of 5–26 km. The only regional fault in the vicinity, the Kumbel–Kokand–Haydarkhan Fault, which has a north–northwestern strike, is located to the east of the area where the swarm of weak earthquakes occurred. This combination of geological, geophysical, and seismic characteristics represents a rare and poorly studied phenomenon. The few available publications on this subject are reviewed, in which various mechanisms have been proposed to explain the occurrence of this type of earthquake swarm. Considering the geological and tectonic conditions, as well as the specific geophysical characteristics of the area where this seismic event occurred, it is difficult for us to attribute the observed phenomenon to any of the proposed mechanisms with confidence. A geomagnetic storm was observed before and during the occurrence of these weak earthquakes. Therefore, other mechanisms may also be involved. Further cases of weak-earthquake swarms need to be documented and accumulated to allow a more reliable assessment of the mechanisms responsible for their occurrence.