Abstract

VARGAS, Carlos A.; PUJADES, Lluis  and  CANEVA, Alexander. ATTENUATION STRUCTURE OF THE GALERAS VOLCANO, COLOMBIA. bol.geol. [online]. 2012, vol.34, n.2, pp.149-161. ISSN 0120-0283.

Spatial variation of coda-Q (Q_c) was analyzed in the Galeras volcano, located in the south western Andes of Colombia. The Q_c value was calculated using the Single Backscattering model and using seismograms filtered in the band centered on 3.0 Hz. We used the estimated values as the mean attenuation that represents the total effect of the simple scattering process inside an ellipsoidal shell volume. An inversion procedure was implemented to estimate the spatial variation of Q_c as function of volume fractions. Synthetic tests in 2D and 3D allow us to extend the procedure with available data from the seismological network located at the Galeras volcano. The inversion of 2855 Q_c values in a grid of 8 x 8 x 8 nodes associated to 435 earthquakes and 19 stations permitted resolve the presence of two levels of attenuation anomalies which are assumed to be related to bodies of partially melted rocks, the first located around 6.0 km depth and the other one at >13.3 km depth from the summit. These bodies could be responsible for the vulcanian-type explosions as a consequence of mixture of shallow magma with depth basic magma. The shallow body seems extend along the summit and may represent the presence of plugs and fluids that are constantly fed from the deeper body and produce the actual hydrothermal activity with eruptive events. From 2002 to nowadays several eruptions has happened, e.g. at November, 2005, the volcano generated evacuation of aprox. 9000 persons from four towns around the Galeras volcano (Mapachico, Genoy, La Florida and Nariño). Along these episodes, including the one on December 6^th, 2010, it looks like a large and deep magma volume is involved in this process and that more eruptive events may be expected.

Keywords : Coda Q attenuation; 3D tomography Galeras volcano; Eruptive activity.

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