The fracturing in volcanic edifice before an eruption: the June–July 1998 high-frequency earthquake swarm at Volcán de Colima, México

The recent eruption of Volcán de Colima, México, began in November 1998 and was preceded by a 12-month period of seismic activity, which consisted from a few swarms of numerous microeathquakes. We study the source characteristics of earthquakes that occurred during June–July, 1998, swarm within the edifice of Volcán de Colima, which consists of the dormant Nevado de Colima and active Volcán de Fuego. A three-component digital seismic station, added to the one-component seismic network of five stations during this period, allowed us to analyze S-wave spectra for our study. Thirty-two microearthquakes with hypocentral distances up to 7 km and magnitudes Mw between 0.5 and 1.8 were analyzed in the frame of Bruneʹs model. These events were characterized by high-frequency radiation with corner frequencies from 3.9 to 9.0 Hz. Their seismic moments varied from 0.9×1010 to 7.2×1011 N-m and stress drops were obtained in the range between 0.05 and 13 bars. We found three groups of events with different corner frequencies that may reflect the presence of three fracture zones with distinct characteristic lengths of fissures formed by the subsurface dike intrusion before the eruption. The first stage (from June 16 to July 3) of swarm development was characterized by simultaneous fracturing within two fracture zones. The first zone was represented by a system of fractures with length of about 80–90 m and was situated below Nevada de Colima, the second zone was represented by a system of fractures with length of about 130–150 m and was situated below the crater zone of Volcán de Fuego. The second stage (from July 4 to 6) of swarm development was characterized by simultaneous fracturing within three fracture zones. A third zone was added to the first two zones with the fracture length of intermediate size about 110 m. The majority of foci that occurred from June 16 to July 3 were situated along the SE slope of Nevado de Colima; the events of July 4 to 6 occurred mainly below the crater zone of Volcán de Fuego. The fracturing in the volcanic edifice occurred within a system of fractures of ENE direction tracing by epicenters and coinciding with the general trend of Tamazula regional fault that intersects the volcanic structure. We suggested that this development of the earthquake swarm reflects either (1) the migration of dike to the active crater of Volcán de Colima or (2) the growth of branches of the dike-swarm tree within the volcanic edifice.