HR: 17:00h
AN: S52I-05    [PDF]
TI: New Inferences on the Campi Flegrei Caldera Structure from microearthquakes P- and S- delay time tomography
AU: * Vanorio, T
EM: vanorio@geoazur.unice.it
AF: UMR-G\'{e}osciences Azur, Universit\'{e} de Nice, 250, Rue Albert Einstein, Valbonne, 06560 France
AU: Virieux, J
EM: viri@geoazur.unice.it
AF: UMR-G\'{e}osciences Azur, Universit\'{e} de Nice, 250, Rue Albert Einstein, Valbonne, 06560 France
AU: Zollo, A
EM: aldo.zollo@na.infn.it
AF: Dpt Scienze Fisiche, Universit\'{a} di Napoli, Campus Universitario di Mte Sant'Angelo, Viale Cintia, Napoli, 80126 Italy
AU: Capuano, P
EM: capuano@unimol.it
AF: Dpt di Scienze e Tecnologie per l'Ambiente e il Territorio, Universit\'{a} del Molise, Via Mazzini, 8, Isernia, 86170 Italy
AU: Russo, G
EM: gguido.russo@na.infn.it
AF: Dpt Scienze Fisiche, Universit\'{a} di Napoli, Campus Universitario di Mte Sant'Angelo, Viale Cintia, Napoli, 80126 Italy
AB: The recently upgraded dataset collected during the 1982-1984 bradyseismic crisis of the Campi Flegrei caldera has been used to image its shallow structure by a three-dimensional delay-time tomography. Data selection provided 2500 P- and 2027 S- weighted pick times from 403 well located earthquakes. Data have been modeled by using a tomographic method based on an accurate finite-difference travel-time computation which, simultaneously inverts P- and S- first-arrival times for both velocity and hypocenter locations. We explored the dependence of results on the grid parameterization as well as on the initial velocity model, through a statistical study of the final solution inferred from 400 randomly sough initial 1-D models. The interpretation of the retrieved P- and S- wave velocity models and of the deduced Vp/Vs and Vp*Vs images has been supported by rock physics modeling of a laboratory dataset available in this area. This allowed us for mapping zones with different fluid saturation and fracture accruement down to 6 km depth. Between the surface and 3 km depth, the slighter increase of Vs as a function of depth compared with Vp seems to account for the high Vp/Vs and low Vp*Vs anomalies. Anomalies are also spatially correlated to the seismicity occurring at this depth. These seismic signatures suggest that the shallow caldera structure is formed by fractured and brine bearing formations. A nearly circular shaped Vp*Vs anomaly characterizes the deeper structure between 3 km and 5 km. Its inner part shows low values in which seismicity also occurs and a low Vp/Vs superimposes. Both low Vp and Vs anomalies are responsible for the aforementioned low Vp*Vs and Vp/Vs features: we are more inclined to interpret the deeper part as a fractured medium having partial or predominant steam saturation. The low Vs values characterizing the inner part of the caldera between 1 km and 4 km suggest that fluids might stand overpressured.
DE: 7203 Body wave propagation
DE: 7280 Volcano seismology (8419)
DE: 8180 Tomography
DE: 8424 Hydrothermal systems (8135)
SC: Seismology [S]
MN: 2003 Fall Meeting