The Northern Sicily, September 6, 2002 earthquake (Ml = 5.6, MW = 5.9) is investigated under macroseismic aspect:
peculiar effects are collected besides standard effects normally used to define Mercalli-Cancani-Sieberg
(MCS) intensity. They include sound heard during the quake, fear felt and a simple qualitative description of
ground movement felt. Spatial coverage of such information is dense enough to be statistically processed, to give
an interpolated, smoothed field for each data type. Sound heard is compared with theoretical sound field produced
considering source geometry and transmission of waves to air, it also confirms the Southern Sicily amplification
disclosed by macroseismic intensity values. Fear felt is also in agreement with macroseismic intensity field while type of ground motion is a partly independent aspect

De Rubeis, V.

Gasparini, C.

Tosi, P.

Earth-prints Repository

1361
ANNALS  OF  GEOPHYSICS, VOL.  46, N.  6, December  2003
Key  words macroseismic intensity – Sicily – earth-
quake sound – macroseismic effects
1. Introduction
The principal aim of a macroseismic inves-
tigation is to define an intensity field follow-
ing a specific intensity scale. The definition of
a particular scale reflects the effort to mediate
heterogeneous damage information into a sys-
tematic and structured description. The func-
tionality of an intensity scale resides in its
proper effects and damage subdivision. Spe-
cific damage should fulfill certain require-
ments: it must be easily recognizable, it must
be specific of as narrow as possible a range of
energy released, it must be often associated
with other effects of the same degree but must
maintain a sufficient independence from them
Northern Sicily, September 6, 2002
earthquake: investigation on peculiar
macroseismic effects
Calvino Gasparini, Patrizia Tosi and Valerio De Rubeis
Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy
Abstract
The Northern Sicily, September 6, 2002 earthquake (Ml = 5.6, MW = 5.9) is investigated under macroseismic as-
pect: peculiar effects are collected besides standard effects normally used to define Mercalli-Cancani-Sieberg
(MCS) intensity. They include sound heard during the quake, fear felt and a simple qualitative description of
ground movement felt. Spatial coverage of such information is dense enough to be statistically processed, to give
an interpolated, smoothed field for each data type. Sound heard is compared with theoretical sound field pro-
duced considering source geometry and transmission of waves to air, it also confirms the Southern Sicily am-
plification disclosed by macroseismic intensity values. Fear felt is also in agreement with macroseismic intensi-
ty field while type of ground motion is a partly independent aspect.
Mailing address: Prof. Calvino Gasparini, Istituto Na-
zionale di Geofisica e Vulcanologia, Via di Vigna Murata
605, 00143 Roma, Italy; e-mail: gasparini@ingv.it
(this can be defined as the power of represen-
tation of a specific intensity degree). A partic-
ular macroseismic intensity scale is the set of
all damage and effects organized in a hierar-
chical order. Other effects are considered not
particularly useful, if not confusing, and they
are marginally treated.
Within this paper some not-standard effects
are presented because they received a sufficient
spatial completeness of data. These effects are:
– hearing of earthquake sound shortly be-
fore or during the shaking;
– kind of ground movement felt by people
during the event;
– percentage of people feeling fear.
The main shock occurred on September 6,
2002, 01:21 TU, localized by the National
Earthquake Center on the Southern Tyrrhen-
ian Sea (latitude 38.45 N, longitude 13.70 E)
at a distance of approximately 40 km from the
Sicilian town of Palermo, with a magnitude
Ml = 5.6. Aftershock events are distributed on
an ENE-WSW direction in agreement with
the focal mechanism (thrust type strike = 255,
dip = 49, slip = 121; Pondrelli et al., 2003). 
1362
Calvino Gasparini, Patrizia Tosi and Valerio De Rubeis
The analyzed data came from macroseismic
questionnaires that the Italian INGV (Istituto
Nazionale di Geofisica e Vulcanologia) send to
every village within a specific distance from the
earthquake epicenter. The questionnaire com-
prises 80 questions mainly based on the effects
described by MCS intensity scale.
Macroseismic effects (fig. 1) were noticed
across all over Sicily and in part of the Calabria
region and they were collected as 549 complet-
ed questionnaires from 360 different villages
(De Rubeis et al., 2003). Mercalli-Cancani-Sie-
berg intensity was defined on each location of
the territory following methods described in De
Rubeis et al. (1992). Macroseismic intensities
range from not felt to a few data of VII degrees,
with the majority of data occurring within IV
and V degrees. III, II and not felt degree are
shown within an almost horizontal central belt,
which introduces a peculiar character to the
field: an anomalous increment in intensity in the
south of Sicily. Intensity data were filtered and
interpolated using a kriging method based on a
modeled experimental semivariogram connect-
ed to a fractal dimension of intensity data. This
choice permitted a separation of regional field
from noise (De Rubeis et al., 2003).
2. Data analysis
Many people reported having heard a rumble
shortly before or during the earthquake. Infor-
mation on this effect was collected in two ways:
one was a questionnaire aimed at describing the
sound heard that people voluntarily compiled on
line; the other was the standard macroseismic
questionnaire in which a question  was pertinent
to the hearing of earthquake sounds during the
quake. We received 38 compiled questionnaires
Fig. 1. Macroseismic data of September 6, 2002 event with smoothed and interpolated field reconstruction
(black isoseismals). Original intensity values are represented by color dots, while epicenters of the sequence are
identified by red stars.
1363
Northern Sicily, September 6, 2002 earthquake: investigation on peculiar macroseismic effects
Fig. 2. Earthquake sound heard. Original data with its interpolated and smoothed spatial reconstruction (color
shading), compared with the theoretical sound pressure level in relative values (violet contour of equidistance 10
dB) obtained using focal mechanism data. 
of the sound oriented kind. The analysis of these
shows that many people in Palermo city or near-
by were able to identify the source direction,
stating that the rumble was coming from the
north. In most cases, the sound loudness was
compared to that of heavy traffic, often capable
to waking people up. 
A larger data set came from the macroseis-
mic questionnaire. Such data are treated statisti-
cally in a simple way, to assure the mitigation of
anomalies and the spatial continuity of answers.
Errors may be due to misinterpretation of
sounds (buildings or other structures produce
sound during the shaking) or, by simply under-
estimating the process due to attention driven by
other more involving effects (shaking, damage,
etc.). This binary information is converted by
simply counting the percentage of affirmative
perception on total answers, weighted by a pow-
er 2 of distance, inside a circular area of 30 km.
Such filtered and interpolated data are com-
pared with a theoretical model of the acoustic
pressure transmitted from the P-waves to the air
in each point, calculated using the focal mecha-
nism parameters of the event and, consequently,
the radiation pattern of P-waves (Tosi et al.,
2000). Results are depicted in fig. 2 where point
data (heard, not heard) are plotted with their
smoothed interpolation and theoretical sound
pressure level. The theoretical field is represent-
ed in the figure with a contouring of decibel rel-
ative value, as the absolute sound pressure level
can vary widely in relation to the soil properties
in each site. Sound was heard by many people
until distances of around 150 km in an ESE di-
rection from the epicenter, and until distances of
110 km to the south. Then after a belt of practi-
cally total absence, sound was heard again on
the Iblei Mountains, within a range of 165 to
220 km from the epicenter. 
1364
Calvino Gasparini, Patrizia Tosi and Valerio De Rubeis
The second effect investigated concerns the
kind of movement felt: the choice is between
subsultory and undulatory. The resulting field is
represented with data in fig. 3. There is a preva-
lence of undulatory response over most of the
island of Sicily and the overall pattern is differ-
ent from that of the other investigated effects. It
is interesting to note that in the southern part of
the island (Mt. Iblei area), corresponding to a
relatively high MCS intensity, the kind of
movement in prevalence is subsultory. Also for
earthquake sound this zone is particular, in fact,
several locations here reported hearing a quake
rumble at more than 160 km of distance from
the epicenter. Such a coincidence supports the
hypothesis of site amplification behavior in the
area. 
Similar attention was given to another effect
not considered for standard intensity evaluation.
Complete questionnaires contained information
on the degree of fear felt by people: nobody, few,
many or all. These categories were ranked and
used to build a continuous smoothed and interpo-
lated field. The method followed is similar to that
used to build a sound field. Fear can be due to
several factors such as the MCS intensity level,
earthquake sound, people’s excitability and their
group influence, etc. Results shown in fig. 4 re-
veal an overall pattern close to both the filtered
intensity field and sound. Thus, all fields collabo-
rate to confirm a general pattern of effects pro-
vided by this seismic event.
3. Discussion
From the analysis of the earthquake sounds
it can be seen that the area where the rumble was
heard by most people is in good accordance
with the modeled acoustic pressure given by the
P-waves. On the contrary, the comparison with
the Sv radiation pattern showed a minimum in
Fig. 3. Prevailing type of ground movement perceived by people during the event (subsultory or undulatory).
Original data are represented with their interpolated and smoothed spatial reconstruction (color shading).
1365
Northern Sicily, September 6, 2002 earthquake: investigation on peculiar macroseismic effects
the same zone. It is interesting to note that in the
central belt of the island, where the sound was
not heard, there is a prevalence of undulatory
movement. The explanation can be that here the
distance from the epicenter is 100 km. Accord-
ing to Suhadolc and Chiaruttini (1987) at these
distances the largest component of motion is the
horizontal one, due to the Lg-wave group. In the
Mt. Iblei area, beyond 150 km, people reported
having heard the earthquake sound and the
ground movement is again, as near the source,
subsultory. Using the event focal mechanism it
was possible to calculate that in the SE direction
there was a minimum in the radiation pattern of
the S-wave, thus Lg phase (resulting from the in-
terference of multiple S-wave reflections at the
Moho discontinuity) was no longer dominant.
The sound here could have been due to the crit-
ically refracted P-wave traveling in a low atten-
uation basement, probably in association with
site effect.
4. Conclusions
Peculiar macroseismic effects, not strictly re-
lated to a particular intensity degree or to a spe-
cific energy release, can be of interest if suffi-
ciently collected and interpreted. Answers on
simple perception of sound heard during the
quake, fear felt and description of felt ground
movement were systematically collected for the
event that struck Sicily on September 6, 2002
(magnitude Ml = 5.6, MW = 5.9). Original data
were composed basically of qualitative informa-
tion: heard sound and ground movement are di-
vided into two categories (heard, not heard and
subsultory, undulatory respectively), felt fear was
ranked into four levels (nobody, few, many or all).
Spatial distribution of all this information fol-
lowed village locations and a simple averaging
statistics was implemented to give continuity to
data. Smoothed and interpolated sound fields
showed a correlation with both fear felt and
Fig. 4. Fear felt by people during the event with its interpolated and smoothed spatial reconstruction: compare
these results with the macroseismic intensity field (fig. 1). 
1366
Calvino Gasparini, Patrizia Tosi and Valerio De Rubeis
macroseismic intensity field: the southern ampli-
fication of effects was confirmed. The general
theoretical behavior of sound sufficiently agreed
with experimental data. Ground movement was
generally undulatory except for the western side
of Sicily and southern side where it had a remark-
able amplification character for most macroseis-
mic effects. Finally, the spatial distribution of the
fear effect showed that the overall pattern of the
MCS intensity field can be reconstructed even us-
ing very simple transient effects.
REFERENCES
DE RUBEIS, V., C. GASPARINI and P. TOSI (1992): Determi-
nation of the macroseismic field by means of trend and
multivariate analysis of questionnaire data, Bull. Seis-
mol. Soc. Am., 82, 1206-1222.
DE RUBEIS, V., P. TOSI, C. GASPARINI and A. SOLIPACA
(2003): Macroseismic analysis of Northern Sicily,
September 6, 2002 earthquake: results of application
of kriging technique, Bull. Seismol. Soc. Am. (sub-
mitted).
PONDRELLI, S., A. MORELLI, G. EKSTRÖM, S. MAZZA, E.
BOSCHI and A.M. DZIEWONSKI (2003): On Line Cata-
log of Quick European-Mediterranean Regional Cen-
troid-Moment Tensors, http://www.ingv.it/seismog-
lo/RCMT/.
SUHADOLC, P. and C. CHIARUTTINI (1987): A theoretical
study of the dependence of the peak ground acceler-
ation on source and structure parameters, in Strong
Ground Motion Seismology, edited by M.O. ERDIK
and M.O. TOKSOZ (Reidel, Dordrecht), 143-183.
TOSI, P., V. DE RUBEIS, A. TERTULLIANI and C. GASPARINI
(2000): Spatial patterns of earthquake sounds and
seismic source geometry, Geophys. Res. Lett., 27,
2749-2752.
(received October 10, 2003;
accepted December 15, 2003)


Northern Sicily, September 6, 2002earthquake: investigation on peculiarmacroseismic effects

http://www.earth-prints.org/bitstream/2122/734/1/1361_1366Gasparini.pdf

Northern Sicily, September 6, 2002earthquake: investigation on peculiarmacroseismic effects

Abstract

Similar works

Full text

Available Versions

Earth-prints Repository