Three dimensional velocity structure of the Alban hills volcano with the seismic tomography

A detailed P-wave tomographic image of the Alban Hills Volcano (Central Italy) has been determined by inverting local earthquake arrival times. 1314 P wave arrival times from 163 selected events have been inverted by using Local Earthquake Tomography Technique with a normal approach. Starting models used in the inversion consist of four constant velocity layers with different grid spacings (2 km, 1.5 km, 1.0 km). For each inversion a suitable damping parameter is chosen by investigating data and model variance txade-off curves. Detailed resolution analysis has been performed to determine which model is more reliable. This study reveals the complex velocity anomalies beneath the volcano at l to 4 km depths. Lower velocity anomalies are dominant in the center and surround the caldera. Two high velocity anomalies are located in the western and the easthern parts of the caldera rim in the first layers (at l km and 2 km depths). They are connected to each other in the layer at 4 km depth. No velocity changes have been found in last layer located at 6 km depth. Based on the geology of the surrounding regions and subsurface data, the horn shaped high-velocity feature is most likely due to presence of uplifted limestone and dikes originating from the deep magma body. An important improvement of hypocentral determination have been obtained in the 3D solution and the errors are less then 0.6 km. Smaller RMS residuals for the earthquakes are obtained by the computed model with grid spacing 1.5 km according to two other computed models. Key Words: Seismic Tomography, Local Earthquake Tomography, inversion, Volcanological Seismology, Quatemary Volcano, Alban Hills.