HR: 16:45h
AN: V14B-04    [Abstracts]
TI: Unusual Volcanic Tremor Observations in Fogo Island, Cape Verde
AU: * Custodio, S I
EM: susana@crustal.ucsb.edu
AF: Instituto Superior Tecnico, Physics Department, IST, Av Rovisco Pais 1 , Lisbon, 1049-001 Portugal
AU: Heleno, S I
EM: sandra.heleno@ist.utl.pt
AF: Instituto Superior Tecnico, Physics Department, IST, Av Rovisco Pais 1 , Lisbon, 1049-001 Portugal
AB: Volcanic tremor is a ground motion characterized by well-defined frequencies, and has traditionally been explained by the movement of fluids, namely magma, in conduits or cracks (Chouet, 1996). Thus tremor has the potential to reveal key aspects of volcanic structure and dynamics. Two types of previously unreported seismic signals have been observed in Fogo volcano: a) tide-modulated seismic noise and volcanic tremor, and b) high-frequency low-attenuation harmonic tremor. Amplitude modulation of seismic noise can be detected by simple eye-inspection of raw data in some stations of the VIGIL Network, Fogo Volcano. A more detailed analysis shows that certain frequency bands which we interpret as volcanic tremor, mainly in the range 2.0-3.0Hz, are preferentially modulated. The main frequency of modulation is 1.93 c.p.d., which corresponds to M2, the semi-diurnal lunar harmonic. Air pressure and temperature, which are continuously monitored in Fogo Island, have been analyzed and cannot explain the observed periodicity. Thus we conclude that seismic noise and tremor amplitudes are controlled by tides (Custodio et al., 2003). A relation between the tidal modulation and hydrothermal systems activity is suspected and under investigation. High-frequency (HF) tremor (5-20 Hz) has been recorded simultaneously in several stations in Fogo Island and even in different islands of the Cape Verde archipelago (up to distances of 120 km). In volcanic environments high-frequency motions are normally recorded in a small area close to the source, due to the strong attenuation of seismic waves. Non-volcanic origins for HF tremor were examined: cultural noise, whale vocalizations, ship noise, electronic/processing artifacts and path and/or site effects were all considered and dismissed. Emergent arrivals and strong site effects render source location a difficult task, but the analysis of wave polarizations and amplitude distributions seems to point to an offshore source. Two alternative mechanisms are presently being considered: a) propagation in the ocean sound channel of T-waves generated by resonance in a shallow conduit/chamber, and b) existence of a deep strong source, such as a large fluid-filled crack, capable of producing tremor with a complex pattern that propagates to large distances.
DE: 8419 Eruption monitoring (7280)
DE: 8424 Hydrothermal systems (8135)
DE: 7280 Volcano seismology (8419)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting