Seismicity
Present day activity
Present day activity
In the current period of quiescence at Somma-Vesuvius, low
to moderate seismic activity is typical.
There are approximately 20 earthquakes per month, with spikes in
activity, or “crises”, every few years (Figure 7, Vilardo et al., 1996).
Figure 7. Plot showing
number of earthquakes per month in the Somma-Vesuvius area (Saccorotti et al., 2002).
Figure 8. Plot showing number of earthquakes
per month in the Somma-Vesuvius area (Saccorotti et al., 2002).
These earthquakes all have magnitudes less than 3.6, and cluster
beneath the cone of Vesuvius between 2 and 3 km below sea level (Figure 8,
Saccorotti et al., 2002). The maximum
depth is approximately 6 km.
Almost all of the focal mechanisms show strike-slip motion,
with P and T axes consistent with local fault trends (Vilardo et al., 1996).
Figure 9. Focal mechanisms for
earthquakes beneath Vesuvius (Vilardo et al., 1996).
Real time
seismograms can be viewed at the Vesuvius
Volcano Observatory.
Historical documents indicate that seismicity may increase
(in occurrence and magnitude) prior to and during an eruption. The oldest discussion of seismicity at
Vesuvius is from Pliny the Younger, who documented the 79 AD eruption:
"for
several days before (the eruption) the earth had been shaken, but this fact did
not cause fear because this was a feature commonly observed in
Nostros et al. (1998) calculated
Coulomb failure stress changes due to both eruptions of Vesuvius and normal
faulting Apennine earthquakes. They suggest
that earthquakes may promote eruptions at Vesuvius by compressing the magma
chamber and dilating conduits to the surface, and eruptions may promote
earthquakes since voiding a magma chamber beneath Vesuvius may bring suitably
located Apennine faults closer to failure.
Figure 10. Temporal patterns of southern
Apennine earthquakes (M ≥ 5.5) and of final eruptions of Vesuvius that
occurred since 1600 (Nostro et al., 1998).
Tomography
P-wave delay-time tomography beneath Somma-Vesuvius shows
high seismic velocities (Figure 11, blue color) below 3 km below sea level
immediately beneath the volcano. These
are interpreted to indicate the top of the carbonate basement. The high velocities between 1 km and 3 km
below sea level are consistent with either slowly cooled magmatic dikes or
intense hydrothermal alteration (Zollo et al., 1996).
Figure 11. P-wave delay-time
tomography beneath Somma-Vesuvius (Zollo et al., 1996).
Reflection seismology experiments (Auger et al., 2001)
indicate a strong increase in energy at ~8 km depth (Figure 12). Amplitude variations with incidence angle at the
reflector were used to estimate P- and S-wave velocities below the
interface. P-wave velocity was estimated
to be ~2 km/s, and S-wave velocity less than 1 km/s. Auger et al. (2001) conclude that the high
energy at ~8 km and low velocities are the result of an extended sill of magma interspersed
in a highly fractured rock layer. This flat layer is at least 400 km2
in area, but is of unknown thickness.
Figure 12. PP and PS migrations along
a NE-SW profile through Vesuvius. The
stack function on the right is the summation of energy along horizontal lines (Auger
et al., 2001).