• volcanic swarm;
  • Guagua Pinchincha;
  • b value;
  • multifractal

[1] Temporal variations of spatial multifractal dimensions of Quito swarm seismicity (Ecuador) are related to the August 1998 to December 1999 Guagua Pichincha volcanic activity. Multifractal dimensions decrease a few days before the five main seismic energy peaks and increase again before or just after these peaks. This behavior reveals a self-organization of earthquakes that precedes main seismic energy peaks. The Quito swarm is also characterized by a bimodal Gutenberg-Richter law with two b values: b1 = 0.95 ± 0.15 and b2 = 1.48 ± 0.15. This bimodality may reveal the superposition of two different processes: one related to a classical elastic rupture and the other related to hydraulic fracture (leading to high b values) resulting from magma and overpressurized groundwater movements. Groundwater expulsion may be driven by heat released from deeper upgoing magma and manifested on the surface by the occurrence of phreatic explosions (beginning 7 August 1998) and by the formation of eight dacitic lava domes (beginning 25 September 1999).