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Introduction

In so-called 'earthquake swarms', numerous earthquakes occur locally over an extended period without a clear sequence of foreshocks, main quakes and aftershocks. The Swiss Seismological Service (SED) registers several of earthquakes swarms every year. They are therefore nothing extraordinary. Swarms usually end after a few days or months. Only seldom does the strength and number of earthquakes increase over time or do occur single, damaging events. How an earthquake swarm develops over time is just as difficult to predict as earthquakes are in general.

Many earthquake swarms occur in regions with complex contiguous fracture systems. The theory is that they are related to the movement of fluid gases and liquids in the Earth’s crust.

Intro
Graphical representation of an earthquake swarm and a typical earthquake sequence. It should be noted that, in a typical sequence, foreshocks do not occur in every case (see frequently asked question "Can earthquakes be predicted?").