Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW Bohemia

[1] The characteristics of earthquake swarms can neither be described by simple laws nor are the underlying mechanisms presently understood. Swarm activity is often assumed to be caused by an intrusion of fluids into the seismogenic zone. We have studied the earthquake catalog of the large earthquake swarm that occurred in the year 2000 in Vogtland, SE-Germany and NW-Bohemia, an area well known for its episodic swarm generation. We observe a significant decrease of the Gutenberg-Richter b value during the swarm evolution as well as a fractal temporal clustering of the earthquakes. The spatial spreading of the swarm's activity, which is approximately confined to one plane, cannot simply be explained by a process of fluid diffusion. Instead, we observe a simple relationship between the spatial spreading and the seismic moment release, which is in good agreement with empirical relationships derived for tectonically driven earthquakes and theoretical crack growth models. This observation points to a progressively growing main fracture underlying the swarm activity. In addition, we find that the swarm earthquakes themselves trigger aftershocks near the border of their rupture area. The stickslip behavior of the rupture propagation can be explained by stress transfers and induced fluid flows due to earthquakes in a fluid-permeated critically loaded fault zone. However, during the first phase, the temporal behavior is found to be different, pointing to intrusion of fluids initiating the swarm activity.