Precise, absolute earthquake location under Somma-Vesuvius volcano using a new three-dimensional velocity model

Abstract

Summary The Somma–Vesuvius volcanic complex and surroundings are characterized by topographic relief of over 1000 m and strong 3‐D structural variations. This complexity has to be taken into account when monitoring the background volcano seismicity in order to obtain reliable estimates of the absolute epicentres, depths and focal mechanisms for events beneath the volcano. We have developed a 3‐D P‐wave velocity model for Vesuvius by interpolation of 2‐D velocity sections obtained from non‐linear tomographic inversion of the Tomoves 1994 and 1996 active seismic experiment data. The comparison of predicted and observed 3‐D traveltime data from active and passive seismic data validate the 3‐D interpolated model. We have relocated about 400 natural seismic events from 1989 to 1998 under Vesuvius using the new interpolated 3‐D model with two different VP/VS ratios and a global search, 3‐D location method. The solution quality, station residuals and hypocentre distribution for these 3‐D locations have been compared with those for a representative layered model. A relatively high VP/VS ratio of 1.90 has been obtained. The highest‐quality set of locations using the new 3‐D model falls in a depth range of about 1–3.5 km below sea level, significantly shallower than the 2–6 km event depths determined in previous studies. The events are concentrated in the upper 2 km of the Mesozoic carbonate basement underlying the Somma–Vesuvius complex. The first‐motion mechanisms for a subset of these events, although highly variable, give a weak indication of predominantly N–S to near‐vertical directions for the tension axes, and ESE–WNW near‐vertical directions for the compression axes.

Three‐dimensional modeling of Mount Vesuvius with sequential integrated inversion

“…This is clearly visible if we compare the 3-D model reconstructed by Lomax et al [2001] with our 3-D SII interpolated model. Horizontal sections of the imaged volume by Lomax et al (Figure 15) well resolve a 8-km-wide depression to the north of Vesuvius, but little evidence is given for the high-velocity body below the volcano and for the low-velocity region SE of Mount Vesuvius.…”

“…Gauss Boaga projection is used for representation, and gravity data, expressed in mGal, are displayed with a contour interval of 5 mGal. ture characterized by velocity and density lows to the volcano feeding system and to the northeastern depression inferred by our 3-D seismic analysis and that conducted by Lomax et al [2001].…”

“…Gravity residuals are also computed for the density models recovered from the 2-D velocity models which have been used by Lomax et al [2001] to develop the 3-D velocity model used for earthquake locations. The residuals are quite large and density models need to be optimized, especially in those regions with poor seismic resolution (Figure 8).…”

“…[8] A 3-D P wave velocity model was obtained by the interpolation of shallow (up to 3 -4 km depth) 2-D velocity sections obtained from the nonlinear tomographic inversion of the seismic data we used in our analysis [Lomax et al, 2001]. It allowed the relocation of 400 earthquakes under Somma-Vesuvius volcano concentrated within $1 km of the Gran Cono crater axis, at depths of about 1 to 3.5 km below sea level (bsl).…”

Influence of topography on ground deformation at Mt. Vesuvius (Italy) by finite element modelling

“…Using a circumferential interpolation procedure a 3D P-velocity model and the topography of the top of the limestone basement were computed (Lomax et al, 2001). This model was used to refine earthquake location occurred in the Vesuvius area, using a non-linear global search location, using the probabilistic inversion presented by Tarantola and Valette (1982).…”