RESEARCH ARTICLE Long-period seismicity during magma movement at Volcán de Colima Nick R. Varley & Raúl Arámbula-Mendoza & Gabriel Reyes-Dávila & John Stevenson & Rob Harwood Received: 29 September 2009 / Accepted: 22 June 2010 / Published online: 16 July 2010 # Springer-Verlag 2010 Abstract During the period from February to September 2005, Volcán de Colima produced 30 Vulcanian explosions of sufficient magnitude to produce pyroclastic flows of variable size, with a total volume of at least 2.5×10 6 m 3 . Swarms of long-period events were associated with each event, their duration ranging from about 6 h to 3 days and each swarm containing up to 886 events. The characteristics of the swarms have been studied to understand the source mechanism and their relationship with the Vulcanian explosions. In total, 12,548 long-period events were analysed using various comparative and statistical methods. Patterns were not apparent in the data with no correlation between different properties of the swarms (duration, magnitude or frequency of occurrence of LP events) and the magnitude of the associated Vulcanian explosion, whether recorded by seismicity, volume of pyroclastics or altitude of the eruption column. This, along with other characteristics of the swarms, such as the continuation of the swarm after the explosion, with an increase in long- period event amplitude in some cases, suggests that the mechanism is not merely associated with the pressurization under an impermeable cap and resulting pressure differ- entials between adjacent volumes within the system. It is more likely that the production of long-period events is dominated by brittle fracturing on the margins of an ascending magma body. A model is proposed whereby the unloading above the ascending magma column produced by a Vulcanian explosion resulted in an increase in ascent rate, reflected in the increasing amplitude of long-period events. The results reflect the complexity of non-linear processes involved during magma ascent, degassing, crystallization and rupture of the impermeable plug during the Vulcanian process. At Volcán de Colima, as at many volcanoes, long-period events represent a useful precursor for eruptive activity. For monitoring, this paper highlights some useful analyses that can be carried out, which could illustrate certain characteristics of an eruptive episode. A preliminary model is presented of the conduit processes at work during the cyclic extrusive and explosive activity during 2005. Keywords Long-period seismicity . Volcano monitoring . Vulcanian activity . Conduit processes Introduction Geological setting Volcán de Colima is located in the western part of the Trans-Mexican Volcanic Belt (TMVB, Fig. 1). It is a typical subduction-originated calc-alkaline volcano and the subject of a concerted multiparametric monitoring effort. Since 1998, its activity has been at its most intense since the last major eruption (Plinian) in 1913. This culminated in a period lasting about 8 months in 2005, when the volcano Editorial responsibility: M. Ripepe N. R. Varley (*) : J. Stevenson : R. Harwood Facultad de Ciencias, Universidad de Colima, Colima, Mexico e-mail: nick@ucol.mx R. Arámbula-Mendoza Instituto de Geofísica, Universidad Nacional Autónoma de México, México City, Mexico G. Reyes-Dávila Centro Universitario de Estudios Vulcanológicos, Universidad de Colima, Colima, Mexico Bull Volcanol (2010) 72:1093–1107 DOI 10.1007/s00445-010-0390-7