Crustal structure of the Alban Hills volcano inferred by RF studies.

The Alban Hills is a Quaternary quiescent volcano, located a few km southeast of Rome (Italy). During the past decade, seismic swarms, ground deformation and gas emissions occurred in the southwestern part of the volcano, where the last phreatomagmatic eruptions (27 ky) developed, building up several coalescent craters. In the frame of a DPC-INGV project aimed at the definition and mitigation of volcanic hazard, a temporary array consisting of 20 seismic stations has been deployed on the volcano and surrounding areas. The main goal is the reconstruction of the crustal and uppermost mantle structure by using seismological data. Each recording site is equipped with a Reftek130 digitizer, continuously recording, coupled with a 5s or 40s broadband sensor. The array geometry is designed to focus on the volcano uppermost structure with a station spacing of 2-3 km while broadening and deepening the illumination from seismic rays in the surrounding regions (10 km of spacing). During the first year, around XX teleseismic events are recorded. We present the first results obtained using Receiver Functions (RF) analysis. We applied both classical RF computation and inversion methods (Langston, 1979; Sambridge,1999) and an harmonic angular analysis of the RF data-set (Girardin &Farra,1998; Farra &Vinnik, 2000) focusing on the first 5 seconds of the RF. RFs from some representative stations, located upon the volcanic edifice, reveal the main discontinuities related to the volcanic plumbing system. Each station shows different features and conversions, indicating a local complexity of the crustal structure with several shallow discontinuities. In particular, a striking Vs reduction (negative pulse at 2.0 s) suggests the presence of magma in the middle crust.