HR: 0800h
AN: S51A-0983    [Abstracts]
TI: Seismic Tomography of Central Sao Miguel, Azores Islands (Portugal)
AU: Almendros, J
EM: alm@iag.ugr.es
AF: Instituto Andaluz de Geofísica,University of Granada, Campus de Cartuja s/n, Granada, Gr 18071 Spain
AU: * Zandomeneghi, D
EM: daria@iag.ugr.es
AF: Instituto Andaluz de Geofísica,University of Granada, Campus de Cartuja s/n, Granada, Gr 18071 Spain
AU: Saccorotti, G
EM: gilberto@ov.ingv.it
AF: Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano 328, Naples, Na 80124 Italy
AU: Barclay, A
EM: andrew@thompson.ocean.washington.edu
AF: School of Oceanography,University of Washington, BOX 357940, Seattle, WA 98195 United States
AU: Ibáñez, J M
EM: jibanez@ugr.es
AF: Instituto Andaluz de Geofísica,University of Granada, Campus de Cartuja s/n, Granada, Gr 18071 Spain
AB: The Azores Archipelago consists of nine volcanic islands, located at about 38° N and 28°W, in the triple junction of the American, Eurasian and African plates. The largest island is Sao Miguel. It has rift zones mainly trending NW-SE and E-W; calderas, as Fogo and Furnas, at the intersection of these tectonic lineaments; and thermal springs and fumaroles distributed along these fault systems. Furnas, Sete Cidades and Fogo are the most active volcanic complexes: central volcanoes with a dominantly trachytic production. Furnas is the youngest and consists of a steep-sided caldera structure formed during several collapses. The most important thermal features lie on an E-W lineament which cuts the Furnas caldera complex. The Fogo volcanic edifice is built over an older submarine lava basement and composed by lava flows, domes and pyroclastic flows deposits, with the summit truncated by a caldera.Thermal manifestations are associated with a NW-SE fault system and consist mainly of fumarolic activity.
São Miguel was selected as a site for a seismic experiment in a European Union-sponsored project with the aim of quantifying the seismicity of various quiescent volcanoes in inhabited areas.
The 3D distribution of P- and S-wave velocities is derived for central São Miguel, by traveltime tomography. We use P- and S-wave arrival times of 289 local earthquakes by a network of 23 seismometers. The model has good resolution in the shallowest 5 km. There are several Vp anomalies, referred to a composite picture of geologic deposits, volcanic structures and tectonic features. Furnas caldera has a shallow, low Vp value probably marking volcaniclastic sediments. A negative Vp anomaly is associated with the geothermal field of Ribeira Grande. Another low Vp area is related to the highly fractured NW-SE tectonic lineament connecting two geothermal areas in central São Miguel. Conversely, high velocity zones mark a central seismogenetic zone at 4-5 km of depth and the Altiprado region. The Sao Bras high Vp is instead interpreted in terms of high-density deposits. These interpretations are supported by distribution of Vp/Vs in the area.
DE: 6982 Tomography and imaging (7270, 8180)
DE: 7200 SEISMOLOGY
DE: 8419 Volcano monitoring (7280)
SC: Seismology [S]
MN: Fall Meeting 2005