Diapositiva 1

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Cities On Volcanoes 4 23-27 January 2006
Mount Etna eruptions and damaging earthquakes
new insights from recent broad band data by
Giuliano Milana, Giuliana Mele, and Antonio
Rovelli
INGV
Between 26 and 29 October 2002, the Mt. Etna area
was struck by a seismic swarm that counted some
hundred events some of them well felt by the
population. A first group of epicenters is
located in the E-NE part of the volcano, reaching
a maximum local magnitude of 4.6 on October 27 at
0250 UT. A second group, occurring since October
28, is located in the SE flank close to the town
of Zafferana Etnea and culminated with the ML
4.4 earthquake of October 29 at 1002 UT. The
seismic swarm was concomitant with the opening of
a system of fractures in the eastern part of the
volcano and the reactivation of the eruptive
fracture system originated by the 2001
eruption. The most damaging event was the ML 4.4
earthquake of October 29, which occurred in a
densely urbanized territory and attained
intensity VIII on the European Macroseismic Scale
(EMS). In spite of its small local magnitude, the
shock caused significant damage to many buildings
including reinforced concrete ones in an area
that extends for about 4 km in a NNW-SSE
direction and is centered around the villages of
S. Venerina and Guardia. A long system of
surface fractures originated along the damaged
area. Other similar and even stronger events of
the 2002 swarm in the northern zone did not cause
diffuse damage since they occurred in a not
densely urbanized area however, they were highly
destructive for the few sparse buildings close to
the epicenters. The sequence was well recorded by
the Broad Band Station of Antillo (Mednet
Network), the most energetic events also
triggered the Strong Motion Stations of Bronte
(BRNT) and Catania (CATA). The latest part of
the sequence (Dec. 2003 Jan. 2004) was recorded
by the local micronetwork installed in Santa
Venerina (SVN). Two of these stations were also
operating in Santa Venerina from Nov. 2003 to
Aug. 2004. Moreover, data from a broad band
station installed in Catania are available from
Mar. 2001 to Dec. 2001.
Red circles Events recorded by strong motion
Instruments Blue diamonds Aftershock recorded by
local seismic stations.
EMS-98 Macroseismic Intensities for the 2002 Oct.
29 1002 4.6 event.
Map of the study area.
Damage on reinforced concrete buildings produced
by 2002 Oct. 27 0252 (left) and 2002 Oct. 29
1002 (right).
Strong motion data enhance an anomalous
low-frequency content

• The activation pattern of strong motion stations
  reflects the temporal and spatial evolution of
  the swarm.
• The accelerometer of BRNT was triggered by three
  of the events of October 27 located in the N-NE
  area of Mount Etna.
• The ML 4.6 event of October 27 at 0250 UT is the
  only one that triggered both BRNT and CATA.
• The accelerometer of CATA was also triggered by
  two of the events of October 29 occurred in the
  SE flank of Mount Etna, very close to Santa
  Venerina area.
• Waveforms indicate a 20-s long strong motion
  phase characterized by predominance of low
  frequencies.
• Horizontal PGA and PGV values deviate from the
  statistical expectations of the Sabetta and
  Pugliese regression (SP87). Even if the SP87 did
  not include earthquakes of volcanic origin we
  believe that this comparison still is useful to
  enhance the inadequacy, for volcanic events, of
  the commonly adopted ground motion scaling laws.
  This is especially important for the hazard
  assessment in the Mt. Etna area.
• Recorded PGAs are always smaller than
  predictions, in contrast recorded PGVs are always
  larger, and the discrepancy tends to increase for
  the strongest events.
• The low-frequency amplitudes of the strongest
  volcanic events are larger than those of tectonic
  events at the same magnitude and distance whereas
  the opposite occurs for the high-frequency band,
  implying a source scaling completely different
  between tectonic and volcanic events.
• Acceleration spectra of CATA show a significant
  depletion in the high-frequency amplitude and a
  large spectral bump in the frequency band 0.1 1
  Hz.
• BRNT station has the same tendency, although the
  exaggeration of low-frequency amplitudes has a
  smaller extent.
• The difference in the spectral bump between BRNT
  and CATA for the same event is not due to a site
  effect at CATA since the station does not show
  amplification at low frequency in the spectra of
  the local tectonic events.
• The evident deviation from the conventional Brune
  spectrum is therefore the origin of the
  anomalously small accelerations and large
  velocities of volcanic events.

Sample acceleration and velocity waveforms for
BRNT and CATA stations.
Date Time ML Distance PGA SP PGA Obs. PGV SP PGV Obs.
2002/10/27 0128 4.3 19 28.01 4 0.91 0.7
2002/10/27 0158 4.6 12.5 49.89 14 1.86 2.3
2002/10/27 0250 4.6 17 38.27 9 1.39 2.4
2002/10/27 0250 4.6 27 24.89 4 0.89 1.6
2002/10/29 1002 4.4 18 31.58 15 1.07 4.5
2002/10/29 1639 4.1 16 28.4 6 0.87 0.9
Comparison between observed and predicted strong
motion parameters.
Comparison between observed source and Brune
model horizontal spectra for Etna events (a-e)
and for a southern Sicily tectonic event (f).
Weak motion data confirm the predominant
low-frequency radiation

• Between 2002 Dec. 3 and 2003 Jan. 25 five local
  seismic stations were operating in S. Venerina
  (SVN)
• Eleven good S/N ratio events were recorded with
  magnitude between 2.2 and 3.7.
• Events were located in the N-NE and S-SE sectors
  of Mount Etna.
• The majority of the records shows a low-frequency
  content regardless of epicentral distance.
• Some high-frequency events were also recorded
  mainly at the end of the recording period.

Comparison between records and Fourier spectra
for two weak motion data.
The Brune model spectrum for the high
frequency event is also showed.
Fourier spectra for weak motion data. Epicentral
distance are a) 3-5 km, b) 13-16 km.
Does the high-to-low frequency amplitude ratio
relate to volcanic activity?
Seismic records are low- and high-pass filtered
with a 1 Hz Butterworth filter, PGV are then
evaluated on filtered signals (PGVHP, PGVLP)
along with their ratio. It is possible to find
some temporal variations of the high-to-low
frequency amplitude ratio. This procedure is
applied to i) 2001 data recorded at Catania, ii)
2002-2003 data recorded at Antillo and Santa
Venerina, iii) 2003-2004 data recorded at Santa
Venerina.
enlarged in the next plot
2002-2003
2003-2004
2001
PGVHP / PGVLP
PGVHP / PGVLP
PGVHP / PGVLP
The large low-frequency content is responsible
for high damage

• 5 damped pseudovelocity response spectra for the
  strongest of the October 2002 earthquakes
  compared to statistical expectations of the
  Sabetta and Pugliese regression (SP96) show
  again a significant (up to a factor of 3)
  depletion in the observed high-frequency range.
• In contrast, at low frequency, response spectra
  computed from the local recordings are fit by
  spectral ordinates statistically corresponding to
  ML 5.5 to 6 of tectonic earthquakes in Italy. In
  this frequency range the effects of volcanic
  earthquakes on buildings could be as large as
  those experienced in tectonic regions of Italy
  during much stronger (up to ML 6) earthquakes.
• The low-frequency content brings to high values
  for ground displacement, also the damage of the
  2002 events were consistent with high values of
  displacement.
• The frequency band 0.3 to 1.5 Hz gives the
  largest contribution to the pseudovelocity
  response spectra of volcanic events of Mt.Etna.
• For a given earthquake, it is possible to define
  for each recording site a measure of ground
  shaking evaluating the Housner intensity in
  0.3-1.5 Hz frequency band and to determine the
  magnitude that, in the SP96 regression, would
  attain the same spectral amplitude.

• CONCLUSIONS
• The Housner spectral intensity can be evaluated
  almost in real time as an indicator of potential
  damage produced by volcanic events even when
  their magnitude is as low as 4.
• The volcanic event can cause local damage
  comparable to the tectonic events associated to
  the main seismogenetic structures of Eastern
  Sicily and must be taken into account in hazard
  evaluation.

Istituto Nazionale di Geofisica e
Vulcanologia, Via di Vigna Murata 605, 00143,
Roma, ITALIA milana_at_ingv.it