Seismicity patterns of earthquake swarms due to fluid intrusion and stress triggering

Earthquake swarms are often assumed to result from an intrusion of fluids into the seismogenic zone, causing seismicity patterns which significantly differ from aftershock sequences. But neither the temporal evolution nor the energy release of earthquake swarms is generally well understood. Because of the lack of descriptive empirical laws, the comparison with model simulations is typically restricted to aspects of the overall behaviour such as the frequency-magnitude distribution. However, previous investigations into a large earthquake swarm which occurred in the year 2000 in Vogtland/northwest Bohemia, Central Europe, revealed some well-defined characteristics which allow a rigorous test of model assumptions. In this study, simulations are performed of a discretized fault plane embedded in a 3-D elastic half-space. Earthquakes are triggered by fluid intrusion as well as by co-seismic and post-seismic stress changes. The model is able to reproduce the main observations, such as the fractal temporal occurrence of earthquakes, embedded aftershock sequences, and a power-law increase of the average seismic moment release. All these characteristics are found to result from stress triggering, whereas fluid diffusion is manifested in the spatiotemporal spreading of the hypocentres.