Analysis of seismicity and micro-seismicity associated with the October–November 2010 Sampeyre swarm, Southwestern Alps

• Seismogram cross-correlation is performed to identify families of earthquakes.
• Events joined into families are located using the double-difference method.
• Micro seismicity is detected through an STA/LTA-based algorithm and analyzed.
• We analyze the b-value of the Gutenberg and Richter relationship.
• Results suggest the activation of different sources.

The study presents a detailed analysis of the swarm that occurred in the area surrounding Sampeyre (Piedmont, Italy) during the period October–November 2010. The crisis caught the attention of seismologists because of its intense seismic productivity that has never been observed in the past. During the swarm, 550 earthquakes were recorded and located by the Regional Seismic network of Northwestern Italy — RSNI (http://www.distav.unige.it/geofisica/) in the study area. However, including the micro-seismicity (mainly, earthquakes of magnitude lower than 0) detected through an STA/LTA- (short-term average/long-term average) based algorithm, the number of events goes up to approximately 3250. Among all these events, approximately two hundred exceeded a local magnitude (Ml) of 1.0 but only two earthquakes exceeded Ml = 3.0, reaching up to Ml = 3.2.Following a general description of the swarm evolution, the study presents the results of a waveform similarity analysis and double-difference relocation, aimed at identifying groups of earthquakes belonging to common genetic sources. A statistical analysis of the resulting earthquake families has revealed a variation of the scaling exponent (b-value) of the Gutenberg and Richter size-frequency model with depth. In particular, our results have evidenced a slightly higher b-value at shallower depths where a distinct family of earthquakes was identified through seismogram cross-correlation. This may reflect variability in the distribution of fractures in the rock volume involved in the swarm.