Giuseppina Tusa

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.

Source Parameters of Microearthquakes in Southeastern Sicily, Italy

We computed the source parameters of 135 local earthquakes that occurred in southeastern Sicily (Italy) between 1994 and 2001. Magnitudes ranged from 0.6 to 2.5. The source parameters were determined from the spectral analysis of P waves, after corrections for attenuation and site effects. P -wave quality ( Qp ) and site ( S ) factors are known in the two frequency ranges 3–9 Hz and 16–27 Hz (Giampiccolo et al. , 2003). Because the correction for Qp and S over the whole spectrum could not be performed, a new approach to estimate spectral parameters was applied. We defined the new notion of “holed spectrum” to specify a spectrum lacking corrected spectral amplitude values within a certain frequency range, whereas we referred to a “whole spectrum” to specify when the entire analyzed frequency band was used. A detailed statistical analysis showed that dealing with the holed spectrum did not lead to significant differences in the estimate of spectral parameters. Displacement holed spectra, corrected by the attenuation and site response, were then used to estimate source parameters. Seismic moments ranged from 1017 to 1020 dyne cm, and source dimensions from 150 to 500 m. In general, the stress-drop values obtained analyzing P -wave spectra were very low, because they mainly span the range of 0.05–5 bars.

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.

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.

High‐Frequency Spectral Decay in P‐Wave Acceleration Spectra and Source Parameters of Microearthquakes in Southeastern Sicily, Italy

In widely used ω −2 source models the characteristics of high-frequency radiation are described as being flat for frequencies between the source corner fre- quency and an upper limiting frequency fmax. Deviations from this behavior are de- scribed in a parameter κ, which is understood as a general measure of the changes the signal undergoes on its way from the source to the receiver. In this study, we calculated κ in southeastern Sicily by using microearthquakes belonging to three different seis- mic sequences occurring in the area in 1990, from 1999 to 2000, and in 2002. The selected events form four different clusters whose seismic sources are located within a 2-km radius. Although the source-to-station paths are approximately the same inside a given cluster, the values of κ change considerably at the same recording site from one event to another, also in the case of events having the same magnitude. We parame- terized κ in terms of event (κ E ) and path (κ P and κ Diff ) contributions. The term κ P represents the contribution on total κ of both the whole source-to-station path and the near-surface geology, while κ Diff models the possible spatial variation in the parameter measured with respect to a reference source-station direction. Results show that the source contribution is not negligible and that a positive correlation with source size exists. Moreover, the hypothesis of a laterally homogeneous crustal structure within the area in question is not appropriate, and significant variation in attenuating proper- ties of the medium may occur in a very small distance range (also in the order of a few tens of meters). Our analysis suggests that the origin of the previously mentioned variability is located near the recording site. Synthetic spectra are also computed in order to verify the actual significance of the parameterization employed and its capacity to separate the source and the path contribution to κ. We describe our spectra as a product of a Brune-type source spectrum and an exponential shaping term accounting for propagation effects. The seismic moments range between 3:8 × 10 11 and 5:2 × 10 13 N · m, the source radii range between 176 and 669 m, while the stress drop varies from 0.01 to 0.67 MPa. Online Material: Tables of stations in the deployed portable seismic networks, coordinates, and average values of κ.

From Seismic Input to Damage Scenario: An Example for the Pilot Area of Mt. Etna Volcano (Italy) in the KnowRISK Project

In this paper we present a multidisciplinary approach aimed at assessing seismic risk regarding non-structural damage. The study has been carried out in the framework of the European KnowRISK Project and focuses on the pilot area of Mt. Etna volcano (Italy). Both instrumental data and as well as macroseismic observations provide unique opportunities for testing innovative and classical approaches for assessing seismic risk. Starting from the seismic hazard analysis, we first identify a test site (Zafferana) affected by non-structural damage. We produce seismic scenarios based on macroseismic and ground-motion data and finally obtain the relevant risk map using the Italian census data to classify buildings into vulnerability classes and a model to predict damage distribution.

Seismic amplification effects and soil-to-structure interaction study nearby a fault zone: the Tremestieri fault and Madre Teresa di Calcutta School (Catania)

Results of passive seismic surveys, in terms of both amplification and polarization effects in a section of the Tremestieri Etneo Fault (Sicily Eastern center - Catania) are discussed. For the purpose, velocimetric and accelerometric records of seismic ambient noise were analyzed. The polarization analysis of particle motion was performed and azimuthally dependent resonant frequencies were estimated. Ambient noise data were also used to assess the dynamic properties of a reinforced concrete building, located on the fault zone. The fundamental modes have been estimated through ambient noise recordings acquired by three-directional accelerometers, installed at the highest accessible floor and outside the building. The study revealed a clear oriented seismic amplification in the fault zone. This effect was observed in intensely jointed rock masses, located inside the fault area, as the result of specific geometries and significant directional impedance contrasts characterizing the area under study. The analyses show that the direction of the largest resonance motions has transversal relationship with the dominant fracture orientation. The directional amplification is inferred to be produced by stiffness anisotropy of the fault damage zone, with larger seismic motions high angle to the fractures. The results obtained are in complete agreement with those obtained by a previous study which analyzed the fault section located to the north-west. Finally, comparing the dynamic properties of the school building and the vibrational characteristics of the soil in the direction of maximum amplification, no clear resonant effect in the soil-structure interaction has been observed.

Structural health monitoring and Earthquake Early Warning: preliminary studies for application in eastern Sicily

In this work, the reduction of seismic hazard in eastern Sicily is addressed by both studying the fundamental resonant frequency of strategic buildings through low cost geophysical investigation techniques, and exploring a practical approach to earthquake early warning (EEW) system. The fundamental period and the corresponding amplification of some selected strategic buildings has been estimated using ambient vibration and earthquake data. We analyzed the basic dynamic parameters of buildings chosen as target, by using continuous vibration measurements at different floors. The dynamic behavior of structures was evaluated considering both small strains induced by ambient vibrations and larger excitation levels due to the earthquakes occurrence. A practical approach to earthquake early warning in the investigated area was dealt with by using empirical relationships between parameters measured on the initial portion of seismic recordings and related to the earthquake magnitude and peak ground motion. In particular, we performed the first preliminary tests by using empirical relationships calibrated for the considered area and taking into account the geometry of the existing permanent seismic network deployed in the eastern Sicily. The estimated relationships have been used to provide onsite warning around a given seismic station and evaluate the potential damaging effects. The joint of EEW system and geophysical investigation shown in this work may be deemed a useful guide for the future implementation of the in real time seismic monitoring in the region.

Estimation of Earthquake Early Warning Parameters for Eastern Sicily

Abstract Earthquake early warning systems (EEWSs) are becoming a suitable instrument for seismic risk management in real time. In fact, they are implemented or are undergoing testing in many countries around the world because EEWSs represent an effective approach to mitigating seismic risk on a short timescale. EEWSs are based on the use of relationships between some parameters measured on the initial portion of seismic signal after the onsets. Here, we address the first approach to the implementation of EEWS in eastern Sicily, a region that has been hit by several destructive earthquakes. We estimated the peak displacement amplitude of the first portion of P and S waves P d , the ground‐motion period parameter τ c , and the peak ground velocity (PGV) from earthquakes with M L ≥2.8 recorded by the broadband stations operated by the Istituto Nazionale di Geofisica e Vulcanologia. We found that the P d is correlated with the size of the earthquake and may be used to compute the magnitude for an EEWS in this area. We also derived the relationships between τ c and M L , and between P d and PGV, which can be used to provide on‐site warning in the area around a given station and to evaluate the potential damaging effects. These relationships may be deemed a useful guide for future implementation of the EEWS in the region.