A Case Study of an Earthquake Swarm in Uzbekistan

Abstract.

This paper presents a case study of an earthquake swarm consisting of low-magnitude seismic events recorded in the western sector of the Fergana intermontane basin, in the vicinity of the city of Kokand, Uzbekistan. The study area is situated between two seismically active regional fault systems, the North Fergana and South Fergana fault zones, yet does not coincide with either principal fault trace. Historically, seismicity in this specific locality has been sparse: only two earthquakes of magnitude 4.3 were documented here, both in 1990, prior to the swarm described in this work.

A catalog of 86 weak earthquakes occurring between 6 and 27 March 2025 was compiled from precise instrumental data, including origin time, latitude, longitude, focal depth, and magnitude. The first event occurred on 6 March at 19:21 UT (M 1.6, depth 15 km), and activity intensified sharply over the following two days: 27 earthquakes were recorded on 7 March and 38 on 8 March, together accounting for 65 of the 86 total events, before tapering off through late March. Magnitudes ranged from 1.0 to 2.8, and focal depths spanned 3 to 26 km, with no systematic progression in either parameter over time, indicating a temporally chaotic sequence rather than a mainshock–aftershock pattern.

Spatially, the epicenters cluster within the northeastern portion of the Kokand magnetic anomaly, a high-gradient geomagnetic feature reaching a maximum intensity of approximately 500 nT, previously interpreted as the surface expression of a buried granitoid intrusive body whose upper boundary lies at an estimated depth of 6–8 km beneath the sedimentary cover, though its lateral extent remains poorly constrained. The epicentral field shows a predominantly meridional and southwestern alignment relative to the first event of the sequence, extending more than 25 km to the north and about 12 km to the southwest. Given that hypocentral depths reach as deep as 26 km, some of the seismic sources may plausibly originate within or adjacent to the intrusive body itself, though this association remains speculative given the absence of precise constraints on the intrusion's geometry. Notably, the nearest major tectonic structure, the north–northwest-trending Kumbel–Kokand– Khaydarkan fault, lies to the east of the swarm zone and is inconsistently represented across existing tectonic maps, underscoring its uncertain regional significance.

To evaluate a possible external trigger, geomagnetic K-index data for the study period were compared against the earthquake time series; no geomagnetic storm occurred before or during the swarm, and no clear correlation between geomagnetic activity and the onset or evolution of seismicity was identified, in contrast to mechanisms proposed in some prior studies linking geomagnetic disturbances to seismic triggering. The occurrence of a spatially clustered, low-magnitude earthquake swarm away from the principal regional fault system, coincident instead with a buried intrusive body, represents a poorly understood geological phenomenon that warrants further investigation, particularly regarding the genetic mechanisms responsible for such intraplate seismic clustering.

Keywords: earthquake, magnitude, epicenter, focal depth, fault, magnetic field