Historical Perspective and Critical Review of the Seismic Swarm Concept

Abstract

Earthquake swarms are clusters of seismic events occurring in a localized area over a short time span, without a dominant mainshock. Unlike typical mainshock‑aftershock sequences, seismic swarms feature earthquakes of similar magnitude and unpredictable temporal patterns. Their occurrence can be put in relation with crustal heterogeneity, fluid migration, and volcanic activity, including related geothermal processes. The concept of earthquake swarm emerged in the late19^th century, with the early key contribution from Josef Knett in Europe. In Japan, renewed interest in the mid‑20^th century – especially following the development of modern recording techniques and the Matsushiro swarm (1965‑1967) – led to major advancements made by researchers like Kiyoo Mogi and Takeo Utsu, who contributed two complementary approaches to swarm classification:
quantitative and statistical, respectively. Parallel developments occurred in the United States and Europe, gradually shaping earthquake swarms as a distinct seismological phenomenon. By providing a historical perspective on the concept of seismic swarms, this work establishes a context for our future research, which will focus on the application of statistical models, such as the Epidemic Type Aftershock Sequence (ETAS) model.