Article,
Precise, absolute earthquake location under Somma-Vesuvius volcano using new three-dimensional velocity model
Geophysical Journal International, 146 (2): 313--331 (2001)
DOI: 10.1046/j.0956-540x.2001.01444.x
Abstract
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.
