Geoscience Reference
In-Depth Information
At Etna, a ground-based Doppler radar operated jointly by the INGV in Catania and the
OPGC in Clermont-Ferrand has been continuously monitoring the tephra emissions of the
summit craters since 2009 (Donnadieu et al., 2009a, 2012) from a shelter at La Montagnola,
about 3 km to the south. Echoes are recorded in 11 range bins, 150 m deep and 9° wide in
azimuth and elevation, defining a 1650 m-long truncated conical volume covering the
summit craters. These were very active in 2010-2012, showing different eruptive styles,
including short-lived ash plumes (e.g. new SE Crater April 8 2010; Bocca Nuova, August 25
2010) and large ash columns several kilometers high sustained for several hours from lava
fountains, such as the eruptive episodes of the new SE Crater in 2011 (Donnadieu, 2012;
http://wwwobs.univ-bpclermont.fr/SO/televolc/voldorad/).
Figure 16 presents records from VOLDORAD 2B of simultaneous activity of Bocca Nuova
and new SE Crater during cloudy weather. Radar monitoring is not hampered by clouds
that sometimes make visual observations impossible. As shown by the echo power curves
from the range gates at 3135 m and 3735 m, the short-lived explosion forming a weak plume
at Bocca Nuova can be discriminated from the strong and dense column fed from the lava
fountain originating in the new SE Crater and sustained for about 1.5 h. While the former
cause a power increase of only a few dB, the latter result in much more powerful echoes (>
20 dB), with a progressive onset and more abrupt waning phase. Radial velocities in the
convective ash and lapilli plume above the lava fountain commonly reach 30-40 m/s.
7.2 Tracking of rock falls
Not only the explosive activity can be monitored using transportable radars, but also lava
flow or dome instabilities (Wadge et al., 2005; Hort el al., 2006). Viscous basaltic andesite
lava flows continuously outpour from the summit of Arenal volcano and slowly flow on top
of loose pyroclastic material down the steep and unstable upper slopes. Due to the joint
actions of cooling and pushing by new lava, instabilities occur and generate repeated rock
falls, sometimes evolving into small pyroclastic flows. While monitoring the ash emissions
with VOLDORAD 2 from the west between January 26 and March 4 2009, signals from very
frequent rock falls could be recorded in several range gates because their lowest part hit the
volcano's upper slopes, where destabilizations occurred toward the SW.
The radar signature of rock falls is characterized by echoes with only radial velocity
components toward the radar in contiguous bins at slant distances consistent with the
location of the volcano's upper slopes (4013 and 3878 m on Fig.17). Radial velocities are
typically low (<20 m/s). The amplitude of the backscattered power is less in closer bins, as
expected if not all the destabilized material goes all the way down the slope. As seen from
the power curves, signal onsets are delayed from the most remote range gates to the closest,
as the destabilized material tumbles down. It is interesting that many small rock fall events
detected by the radar are not always well recorded on seismograms, and thus both
techniques appear complementary as noticed by Vöge et al. (2008).
An estimate of maximum block velocities during rock falls (
V
rf
) can be retrieved from the
radial velocities (
V
r
) measured by the radar on the upper slopes when the geometry is known:
1
VV
(13)
rf
r
cos(
)cos(
)
rf
ant
