Elisabetta Giampiccolo

Attenuation and source parameters of shallow microearthquakes at Mt. Etna volcano (Italy)

We estimated the attenuation laws of high-frequency seismic waves in the shallow crust (depth 5 km) and earthquake source parameters by using a se- lected data set of 320 shallow events (2.6 MD 4.2), recorded at Mt. Etna volcano during the last two flank eruptions occurring in 2001 and 2002-2003. The quality factor (Q) was estimated from spectra of P and S waves for 24 stations of the local permanent network by applying a spectral ratio technique. The results show varia- tions in both QP and QS as a function of frequency, according to the power law Q Q0f n , with n ranging between 0.3 and 1.3 for P waves and between 0.2 and 0.9 for S waves. As typical of volcanic environments, strong azimuthal variations of QP were also found, suggesting the presence of local strong lateral heterogeneities and/or of fluid-filled cracked volumes. After correction for attenuation, we estimated the source parameters (seismic moment, source radius, and stress drop) of a subset of 66 shallow events, under the assumption of a circular dislocation. The estimated seismic moments M0 range from 10 13 to 10 15 N m. The source radii (r) are confined between 100 and 1000 m and stress drop (Dr) ranges between 0.2 MPa and about 4 MPa. Combining the source parameters obtained in this study with those calculated by Pataneet al. (1997) for an old data set of smaller microearthquakes (10 9 M0 10 14 N m) recorded in the same area, we re-evaluated the scaling relationship between seismic moment (M 0 ) and corner frequency ( f c ) for the earthquakes with M 0 ranging between 10 13 and 10 15 N m. We confirm that microearthquakes at Mt. Etna seem not to obey a

Attenuation in Southeastern Sicily (Italy) by applying different coda methods

The attenuation in Southeastern Sicily has been investigated using 40aftershocks of the December 13 1990, earthquake. The quality factor ofcoda waves (Qc) was estimated in the frequency range 1.5–24 Hz,applying three different methods in time and frequency domains. On thewhole, a clear dependence of Qc on frequency was observed,according to the general law Q = Q0(f/f0)n . Thefrequency dependence relationships obtained from the analysis of codawaves at three lapse time windows (10, 20 and 30 seconds) show that, forall methods, Q0 (Qc at 1 Hz) significantly increases with lapsetime. In particular, Q0 is approximately 20 at short lapse time (10s) and increases to about 70 at longer lapse time (30 s). This is attributedto the fact that larger lapse times involve deeper parts of the crust andupper lithosphere which may be characterized by larger quality factors.Moreover, the value of the exponent n decreases with increasing codalengths from about 1.3 to 0.9, suggesting a decrease in heterogeneity ofthe medium with depth.Finally, Qc-values here found are of the same order as thosereported from other tectonic regions like the Anatolian Highlands orSouthern Spain, while significantly higher than in the neighboring volcanicarea of Mt. Etna.

Two-dimensional seismic attenuation images of Stromboli Island using active data

In this work we present intrinsic and scattering seismic attenuation 2-D images of Stromboli Volcano. We used 21,953 waveforms from air gun shots fired by an oceanographic vessel and recorded at 33 inland and 10 ocean bottom seismometer seismic stations. Coda wave envelopes of the filtered seismic traces were fitted to the energy transport equation in the diffusion approximation, obtaining a couple of separate Qi and Qs in six frequency bands. Using numerically estimated sensitivity kernels for coda waves, separate images of each quality factor were produced. Results appear stable and robust. They show that scattering attenuation prevails over intrinsic attenuation. The scattering pattern shows a strong concordance with the tectonic lineaments in the area, while an area of high total attenuation coincides with the zone where most of the volcanic activity occurs. Our results provide evidence that the most important attenuation effects in volcanic areas are associated with the presence of geological heterogeneities.

Attenuation of Short-Period P Waves at Mount St. Helens

An increase of seismicity occurred at Mount St. Helens between May and July 1998, with more than 900 events (Md 2.2) recorded by the Pacific North- west Seismograph Network. This article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before (January 1995-April 1998), during, and after (August-December 1998) the period of higher seismicity and recorded on three different one-component seismic stations. Epicentral distances of the studied events ranged between 0.5 and 15 km. We performed the analysis distinguishing the deep (depth 5.5 km) from shallow (depth 5.5 km) events. We used the frequency decay method to estimate the quality factor Q and station site correction S for P waves in the frequency bands 2-7 and 18-30 Hz. The results show that the attenuation varies from site to site systematically and decreases with depth. Lower Qp values are obtained for focal depths less than 5.5 km. Moreover, the spatial variations of Qp show minimum values (30) in the crater area that are interpreted as due to a low-density mass distribution under the crater. We find that Qp was 30% higher before the period of high seismicity than after. This change may be attributed to an increase of pressure in the magma chamber producing new cracks and/or reopening of pre-existing cracks, which are the most viable mechanisms for increasing attenuation.

Body wave attenuation heralds incoming eruptions at Mount Etna

How fast, and how foreseeable, is magma ascent is one of the most compelling and unanswered issues of volcanology. The velocity of the magma upwelling depends on the local conditions of the volcanic conduit and rheology of the magma. During magma emplacement in the shallow crust, transient variations of physical properties underneath active volcanoes are expected and in a few cases observed. The predictability of such changes strongly depends on how fast this process is, compared to our ability to handle geophysical data and consistently resolve transient anomalies in the physical properties of the medium. Mount Etna (Italy) is a perfect natural laboratory to investigate such issues, due to the almost continuous magmatic activity and the high quality of seismologic and geodetic data. Here we show, for the first time, that seismic attenuation of local earthquakes strongly increases due to the emplacement of magma within the crust, forecasting an incipient eruption at Mount Etna.

A reappraisal of seismic Q evaluated at Mt. Etna volcano. Receipt for the application to risk analysis

A new approach in dealing with seismic risk in the volcanic areas of Italy, by taking into account the possible occurrence of damaging pre- or syn-eruptive seismic events, is exciting the scientific interest and is actually the topic developed in several research projects funded by the European Community (e.g., UPStrat-MAFA, www.upstrat-mafa.ov.ingv.it/UPstrat/) and the Civil Defense Department of Italy. To achieve this goal, it is necessary to have a detailed knowledge of the local attenuation-distance relations. In the present paper, we make a survey of the estimates of the seismic quality factor of the medium reported in literature for the Etna area. In the framework of a similar paper published for the Campi Flegrei zone in Southern Italy, we first review the results on seismic attenuation already obtained for Etna and then apply a standard technique to separately measure intrinsic and scattering attenuation coefficients from passive seismic data recorded by the Etna seismological network. Indications are then given for the correct utilization of the attenuation parameters to obtain the best candidate quality factor Q to be used in this area for seismic risk purposes.

Faulting processes and earthquake source parameters at Mount Etna: State of the art and perspectives

Seismicity at Mt. Etna, as in other volcanic areas, is usually defined as seismo-volcanic activity. However, a large class of earthquakes, named volcano-tectonic (VT), shares similar characteristics to those of seismic events acquired in tectonic environments. In the past, most of the ambiguities engendering a general mistrust in using simple source models originated from poor constraints on foci or unsatisfactory coverage of the focal sphere. Recently, after the Mt. Etna seismic network was boosted, more detailed analyses have been performed, allowing to verify that most of the earthquakes can be interpreted in terms of shear failure and their mechanisms can be considered to derive from the classical double-couple system of forces. This is supported by structural and geological evidences, as well as by the space-time features of seismicity and focal mechanism results, which confirm the important role played by regional tectonics at Mt. Etna. In this work we will present the state of the art of the source mechanism studies performed on Etnean VT earthquakes and discuss the role of attenuation effects and site response which strongly contribute to determine the appearance of seismograms. The results of the evaluation of the seismic moments and fault dimensions will be also discussed in order to compare Mt. Etna source scaling with those of other volcanic and non-volcanic areas in the world. Finally, some considerations on faulting processes, on the directivity effects and on the presence in the first P-pulse of a nucleation phase preceding a phase of high-speed rupture propagation will be provided.

Regionalization and dependence of coda Q on frequency and lapse time in the seismically active Peloritani region (northeastern Sicily, Italy)

The Peloritani region is one of the most seismically active regions in Italy and, consequently, the quantification of attenuation of the medium plays an important role for seismic risk evaluation. Moreover, it is necessary for the prediction of earth ground motion and future seismic source studies. An in depth analysis has been made here to understand the frequency and lapse time dependence of attenuation characteristics of the region by using the coda of local earthquakes. A regionalization is likewise performed in order to investigate the spatial variation of coda Q across the whole region. Finally, our results are jointly interpreted with those obtained from recently published 3D velocity tomographies for further insights.