2006) 173–185www.elsevier.com/locate/tecto

Tectonophysics 421 (

Attenuation study in the Straits of Messina area (southern Italy)

Tiziana Tuvè a,⁎, Francesca Bianco b, Jesus Ibáñez c, Domenico Patanè a,Edoardo Del Pezzo b, Antonio Bottari d

a Istituto Nazionale di Geofisica e Vulcanologia, sezione di Catania, P.zza Roma 2, Italyb Osservatorio Vesuviano, INGV, Italy

c Istituto Andaluz de Geofisica y Prevención de Desastres Sísmicos, Universidad de Granada, Spaind Osservatorio Sismologico-Di.C.T.A., Università di Messina, Italy

Received 13 June 2005; received in revised form 29 March 2006; accepted 16 April 2006Available online 21 June 2006

Abstract

The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquakeengineering, which is closely related to the seismicity and regional tectonic activity of a particular area. In this work, the seismicattenuation in the Straits of Messina, affected by several and strong historical earthquakes, was studied using waveforms recordedby a local seismic network composed of seven stations. We measured: the coda quality factor (Qc) in the Single Scattering modelhypothesis; the direct quality factor (Qd) applying the Coda Normalization method for S-waves; and the intrinsic and scatteringquality factor (Qi and Qs) by the Multiple Lapse Time Window Analysis (MLTWA) method. Coda Q values were obtained usingdifferent lapse times (40, 60 and 80 s) for the frequency bands centred at 1.5, 3, 6 and 12 Hz. Our findings indicate that Qc increaseswith increasing lapse time and that Qc is frequency dependent. This behaviour is usually correlated to the degree of tectoniccomplexity and to the presence of heterogeneities at several scales. In fact, by using the Coda Normalization method we obtainedlow Qd values, as expected for a heterogeneous and active zone.

Finally, by the MLTWA method we observe that the contribution of the scattering attenuation (Qs−1) prevails on the intrinsic

absorption (Qi−1) until 3 Hz. Conversely, Qi

−1 and Qs−1 seem to be of the same order in the higher frequency bands.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Coda waves; Intrinsic attenuation; S-waves; Scattering attenuation; Straits of Messina

1. Introduction

The southern Calabria and the northeastern Sicilyregions, including the Straits of Messina area, arelocated in the peri-Tyrrenian mountain belt of southernItaly and are characterized by intense post-collisiondynamic and seismic activity (Catalano et al., 2003).

⁎ Corresponding author. Tel.: +39 095 7165800; fax: +39 095435801.

E-mail address: tuve@ct.ingv.it (T. Tuvè).

0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2006.04.005

During the last 1000 years, and particularly in the lastcentury, the Straits of Messina has been affected byseveral strong earthquakes (i.e. an M=7.3 earthquake in1908, an M=4.8 earthquake in 1971, an M=4.7earthquake in 1975, and an M=4.0 earthquake in1985). In particular the December 28, 1908 Messinaquake was undeniably the most destructive to ever hitEurope and most of southern Italy's cities lost as manyas half of their residents.

Earthquake damage is primarily caused by seismicwaves and shaking is heavily influenced by the manner

174 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

in which seismic waves propagate through complexgeological structures. A good knowledge of attenuationand properties of the medium is a modern instrument todefine and to obtain information on earthquake sourceparameters in a given region. This is because the accuratedefinition of attenuation laws serves as a predictive toolfor ground motion parameters at a particular site in futureearthquakes. Fig. 1 (http://www.mi.ingv.it/) depicts thehazard map of the Italian region as Peak GroundAcceleration (PGA) commonly specify a 10% chanceof exceeding some ground motion parameters for anexposure time of 50 years, corresponding to a returnperiod of 475 years. The map chosen defines the actuallevel of hazard for the North Eastern Sicilian andSouthern Calabrian regions that shows high values ofPGA within the range 0.20–0.30 g. In Fig. 1, the Qvalues obtained, before this study, for different zones ofItaly are also reported.

Fig. 1. Hazard map of the Italian territory in terms of Peak Ground Acceleratmi.ingv.it/). Q values obtained in different zones of the territory are also repor2004), Umbria–Marche regions (Castro et al., 2002), Etna area (Del Pezzo et aof Messina area (Godano et al., 1992).

Nowadays, the new economical development of thisregions, which includes a bridge connecting the ItalianPeninsula and Sicily, requires a better characterization ofthe seismic and attenuation features of the area.

It is well known that the evaluation of attenuationproperties of the crust at higher frequencies can beevaluated using the background seismicity of aparticular area, allowing to develop regionally calibratedattenuation relationships.

The attenuation of seismic waves, described by thequality factor Q, is a complex mechanism whichdepends on anelastic phenomena and scattering. Quan-tifying the relative contribution of scattering andintrinsic attenuation has been the subject of considerableinterest among seismologists in the last decades. Forglobal and regional crustal studies, the methods mostcommonly used to obtain and analyse attenuation in thefrequency domain are: i) Spectral Ratio (SR); ii)

ion (PGA) with a 10% exceedance probability in 50 years (http://www.ted: Southern Apennine (Bianco et al., 2002), Friuli zone (Bianco et al.,l., 1996), Southeastern Sicily (Giampiccolo et al., 2002), and the Straits

175T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

Spectral Decay (SD), (Aki, 1980); iii) Coda-Q (Aki andChouet, 1975); and (iv) Multiple Lapse Time WindowAnalysis (MLTWA), (Hoshiba et al., 1991). Thesetechniques provide information for calculating theattenuation coefficient, either as a function of time orinverse distance. The SR method provides the mostdirect approach to measure the energy loss in a seismicsignal. The SD method, similar to the SR method,calculates the attenuation by comparing the spectralamplitude of one signal to that of a reference signal. Inthe Coda-Q method, as originally proposed by Aki andChouet (1975), the problem is the ambiguity ininterpreting Qc (coda Q) in terms of Qt (totalattenuation), Qs (scattering) and Qi (intrinsic absorp-tion). Later, several methods were proposed to deter-mine the relative contribution of scattering attenuationand intrinsic absorption to the total attenuation (Aki,1980; Wennerberg, 1993). The Multiple Lapse TimeWindow Analysis (MLTWA) assumes multiple scatter-ing, and allows the possibility to separate intrinsicattenuation from the scattering effect considering theenergy in multiple consecutive time windows as afunction of the hypocentral distance.

The aim of the present paper is to obtain theattenuative properties of the crust beneath one of themost active Italian seismic regions, the MessinaStraits and the neighbouring zones (southern Calab-rian and north-eastern Sicily), by using local earth-quakes and the estimations of the quality factor of S-waves and coda waves. We will estimate the coda

Fig. 2. Map showing the epicentral location (circles) and seismic stations (trieastern Sicily is also reported: 1) volcanic products, 2) front of the Apennini

attenuation coefficient (Qc), the direct Q (Qd) and, inorder to obtain a separate estimate of the intrinsic andscattering attenuation coefficients, we will also applythe MLTWA method. Finally, since the MLTWAtechnique has been applied to several areas in theworld, differences and analogies observed among theareas investigated in this study and other tectonicsettings will be discussed.

2. Geological and tectonic setting of Messina Straitsarea

The Straits of Messina area is characterized by acomplex geodynamic process which affects both theCalabrian Arc compressive zone system and theTyrrhenian extensional tectonic system (Lentini et al.,1995). Geological field data and marine seismic profileshave allowed to delineate the main structural featuresand outline the presence of different fault systems in theinvestigated area.

The basal part of the sedimentary cover is formed of apelitic-sand–conglomeratic sequence of the early Serra-vallian Messinian age, deposited during an extensionaltectonic phase joined to a WNW–ESE and N–S normalfault system. The Messinian–Middle Pliocene depositsare characterized by evaporates, marls, sands, calcareniticand clays. The Late Pliocene–Early Pleistocene sandcalcarenitic and pelitic succession deposited during theactivity of the NE–SW fault system characterizes themain morphological features of the Tyrrhenian margin. A

angles) used in this study. In the insert the simplified structural map ofc–Maghrebian chain, 3) main normal faults, 4) main strike slip faults.

176 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

normal fault system with an ENE–WSW trend cuts theLate Pleistocene age Messina Formation, which lies in adownlap on an erosion surface (Del Ben et al., 1996).

Furthermore, geological and morphological datacombined with bio-stratigraphic analysis indicate thatin the Straits of Messina region, rifting processes haveoccurred at least since 600 ky. The distribution ofgeologic and morphologic fault-related features clearlyindicates the overall pattern of the major border faults onboth sides of the Straits, also suggesting the offshoreextent of the major fault segments. Finally, the differentvalues of uplift rates recorded on both sides of the riftzone provide useful information to constrain thegeometry of the major fault zone (Del Ben et al., 1996).

3. Data

We analysed data recorded in the period January 1st1994–December 31st 1995 by the seismic networks of theOsservatorio Sismologico Università di Messina (OSUM),and the Istituto Nazionale di Geofisica (ING), (Fig. 2).

The instrumentation managed by the OSUM consistsof four digital seismic stations (CPR, GBA, MES, MOS)equipped with 1 Hz 3C Willmore MKIIIA sensors, withdynamic range of 96 dB and a sampling rate of 75 samplesper second. The ING network operated with three shortperiod analogical stations (ATN, MO9, and SC9)equipped with vertical component (sampling frequencyof 50 samples per second). The coordinates and geologiccharacteristics of the stations are shown in Table 1.

We selected events exhibiting signal to noise ratiogreater than 6, eliminating clipped signals with

Table 1Seismic stations used for the site attenuation study

Code Stationname

Lat°N Long°E Elevation(m)

Lithology of thesites

CPR Capo Rafi 38.24 15.68 320 Biotiteparagneisses

GBA Gambarie 38.16 15.83 1225 Coarse aggregateand gravels sands

MES Messina 38.19 15.55 45 Pelitic-sand–conglomeraticsequence

MOS Mosorrofa 38.09 15.76 1060 Gravels to mostlycrystalline pebbles

ATN Antennamare 38.16 15.46 1124 Amphibolite rockMO9 Motta S.

Giovanni38.00 15.69 490 Bioclastic

calcarenites withalternation of clayand clay marl

SC9 Scilla 38.26 15.71 30 Biotiteparagneisses

Lithological characteristics at each station site are also reported.

electronic spikes. The selected data set consists of 602vertical recorded waveforms of 147 events with focaldepth between 0 and 35 km, and magnitude rangingbetween 2.4 and 4.4. Fig. 2 shows a map of theinvestigated area with the locations of the epicentres ofthe analysed earthquakes.

4. Method of analysis

We used both S and coda waves to characterize theattenuation pattern of the Straits of Messina, performinganalyses on the vertical components, and applying thefollowing techniques.

4.1. Single scattering model (Q-coda method)

The Single Backscattering model was proposed byAki and Chouet (1975) to explain the time dependenceof the scattered energy density at the source location inthe 3-D space. The coda envelope may be expressed as:

Aðf ; tÞ ¼ A0ðf Þt−1expð−kft=QcÞ ð1Þwhere f is the frequency, t−1 is the geometrical spreadingfunction (for body waves), A0(f) is the coda sourcefactor, t is the lapse time and Qc is the quality factor forcoda waves. This model assumes that the coda iscomposed of waves scattered at uniformly distributedheterogeneities in a constant velocity earth medium;moreover, it is assumed that the scattering is weakenough so that multiple scattering may be ignored. Therelationship (1) is valid at lapse times (measured fromthe earthquake origin time) approximately twice greaterthan the direct S-wave travel time ts (Rautian andKhalturin, 1978). In order to analyse the early part of thecoda, also the source–receiver distance must be takeninto account, according to Sato's single isotropicscattering model (Sato, 1977). Qc

−1( f ) can easily beestimated from the recorded seismograms, by fitting theenvelopes of the filtered seismic traces to the relation-ship (1) (Aki and Chouet, 1975).

4.2. Coda Normalization

The Coda Normalization method (Aki, 1980)determines the seismic quality factor, Qd, by comparingS and coda amplitudes of events at different distancesfrom the observer. This technique is based on theempirical observation that coda waves principallyconsist of S waves scattered at random heterogeneitiesin the earth; at a lapse time greater than the S-wavetravel time the seismic energy is uniformly distributed in

177T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

a volume surrounding the source. The direct S-waveamplitude is normalized to the coda amplitude measuredat a fixed time, tc, leading to the elimination of thesource power, site effect, instrument response from theobserved spectra of the direct S waves.

Interpreting S-coda as a random superposition ofscattered S-waves (Aki, 1980), the S-coda energy as afunction of the source, site and path effects, can bewritten as:

AS�coda f ; tð ÞlW Si ð f Þ

����NSj ð f Þ

���� e−Q−1c 2kft

tnð2Þ

where AS-coda is the S-coda velocity wavefield atfrequency f and lapse time t; Wi

S ( f ) is the energyradiation from source i in the same frequency band; ∣Nj

S

( f )∣ is the S-wave site amplification factor for site j; ntakes into account the geometrical spreading factor, andtakes different values depending on the predominance ofsurface, diffusive, or body waves; finally, e−Qc

−12pft is theattenuation function where Qc

− 1 is the S-coda waveattenuation. The S-wave spectrum for the ith source andjth station (at distance rij) can be written as:

AS�Directij ð f ; tÞ∝W S

i ð f Þr2ij

��NSj ð f Þjeð−Q

−1d 2kfrijÞ=ðm0Þ ð3Þ

where v0 is the velocity of the S-waves and Qd−1 is the

direct S-wave attenuation.Taking the logarithm of the Eq. (2), at fixed lapse

time tc we obtain:

lnrijjAS�Direct

ij ð f ; tÞjAS�codað f ; tcÞ ¼− Q−1

d ð f Þpf =m0� �

rij þ const ð4Þ

A least square regression analysis of the left handside of the relationship (4) vs. hypocentral distanceallows us to estimate Qd

−1 from the slope. Using thismethod, the site and source effects and S-codaattenuation are cancelled by the ratio (Eq. (4)). Formore details, see Aki (1982).

4.3. Multiple Lapse Time Window Analysis (MLTWA)

Several methods have been proposed to determine therelative contribution of scattering attenuation andintrinsic absorption to the total attenuation (e.g. Aki,1980; Wennerberg, 1993). Taking into account theRadiative Transfer theory, Wu (1985) introduced amethod to separate the contribution of scattering andintrinsic absorption, based on a theoretical model ofseismic energy propagation, where the scattering isassumed to be isotropic and multiple scattering is

included. Wu's formula predicts the spatial distributionof the integral of seismic energy over an infinite timelength. Hoshiba et al. (1991) have developed a method,called “multiple lapse time windows analysis”(MLTWA), in which they consider energy in threeconsecutive time windows as a function of hypocentraldistance. These data are then matched to a set of modelcurves (Zeng, 1991). Each model curve used in fitting iscalculated based upon an input value for the seismicalbedo (B0) and total mean path, also known as theextinction length (Le), the seismic albedo being B0¼ gs

gsþgi,

and the extinction length coefficient Le=1 / (ηs+ηi). TheLe value describes the distance over which the amplitudeof the seismic signal decreases by e−1, and ηs and ηirepresent scattering and intrinsic attenuation factors.

Seismic albedo ranging from 0 to 1 was proposed byWu (1985) to describe the proportions of energy lossdominated by intrinsic attenuation (B0<0.5) or scatter-ing attenuation (B0>0.5). In particular, media withstrong heterogeneity and no-intrinsic absorption havehigh albedo, while homogeneous media have zeroseismic albedo.

Under the assumption of multiple isotropic scatteringand uniform distribution of scatterers, the theoreticalcurves of the energy density at a given lapse time andhypocentral distance can be obtained by means of theequation described by Zeng et al. (1991):

E r; tð ÞiE0e−gmt d

t− rm

� �4kmr2

þ gsH t− r

m

� �4kmrt

ln1þ r

mt

1− rmt

� �

þ cH t−rm

� � 3gs4kmt

32

e−3gsr

2

4mt −gimt ð5Þ

with:

c ¼ E0t1−ð1þ gsmtÞe−gsmt b

4ffiffik

pZ ffiffiffiffiffiffi

3gsmtp

2

0e−a

2a2da

and E(r, t) is the scattered energy density; E0 is theenergy a t=0; r is the position of the receiver; η is thecoefficient of total attenuation; ηs is the coefficient ofscattering attenuation; ν is the S wave velocity; H is theHeaviside function and

a ¼ mtr:

Zeng (1991) called the solution of Eq. (5) hybrid-singlescattering-diffusion solution. We adopted thehybrid-single-scattering-diffusion approximation tomodel absorption and scattering in the crust of theanalysed area.

Table 2Values of coda attenuation, Qc, estimates at different frequency bandsand lapse times

Lapse time(40 s)

Lapse time(60 s)

Lapse time(80 s)

f (Hz) Qc ΔQc Qc ΔQc Qc ΔQc

1.5 91 54 137 66 167 543 129 73 208 72 263 606 215 102 309 104 434 8012 250 150 537 154 338 116

The attenuation standard deviation is also reported.

178 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

After modelling the theoretical curves in terms of Le−1

and B0, we follow the approach described in Bianco etal., 2002, to compare the experimental curves to thetheoretical ones in order to obtain a separate estimate ofintrinsic and scattering attenuation.

5. Data analysis and results

5.1. Qc and Qd analysis

For the Qc analysis we selected the seismograms onthe basis of signal to noise ratio, considering threedifferent coda lengths, corresponding to lapse times of40, 60 and 80 s. We first filtered the seismograms infrequency bands centred at fc equal to 1.5, 3, 6 and12 Hz.

Then we estimated the envelope of the coda at eachfrequency band as Â(fc, t)= rms(s(fc, t)) where s(fc, t) isthe seismic trace filtered at frequency fc. Qc is estimatedby least squares fitting formula (1) to the data Â(fc, t).

The resulting values of Qc and their associated errorsare plotted in Fig. 3 and listed in Table 2. The plots showhow the Qc values for local earthquakes in the Straits ofMessina and its nearby regions are clearly frequency andlapse time dependent.

The present results concord well with those previ-ously obtained for the same area by Godano et al.

Fig. 3. Estimates of coda Q obtained with a lapse time (t)

(1992), which were interpreted as being due to anincrease in Qc with depth. Godano et al. (1992) analyseda different data set consisting of about 120 earthquakeswith local magnitudes greater than 1.8. Since weconsider our data set more “complete”, we are confidentenough that our estimate of Qc may be more robust dueto the greater number of analysed earthquakes.

We computed Qd by using the same data setselected for the estimate of Qc. We calculated thequantity As-Direct( f ), defined in formula (3), fordifferent frequency bands, centred at 1.5, 3, 6 and12 Hz, taking the rms value of the filtered traces in a 5-second-long time window, starting from the onset ofdirect S-waves. We call this estimate Ad. Then, wenormalized Ad for the average coda amplitude computed

between 40 and 80 s for different frequency bands.

Table 3Qd values and relative standard deviation and correlation coefficientobtained using the Coda Normalization method

f (Hz) Qd ΔQd ρQd

1.5 88 6 0.63 114 12 0.76 186 21 0.712 341 30 0.8

179T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

at lapse time tc=40 s, that we denote as Ac. Taking thelogarithm of the ratio between Ad and Ac for all thestation-event couples (i, j) we can linearly invert for Qd

using formula (4).The quantity ln (Ad/Ac) estimated for each frequency

band is plotted as a function of hypocentral distancetogether with the best-fit linear regression (Fig. 4). Table3 reports the estimated Qd values with the correspondingcorrelation coefficients.

Frequency dependence of Qd can be fitted by thewell-known empirical relationship Q=Q0 ( f/f0)

n, whereQ0 is the quality factor at a reference frequency f0 (1 Hz)and n is the frequency parameter, which is close to 1 andvaries from region to region on the basis of heteroge-neity of the medium (Aki, 1981).

Fig. 4. Plots of the Coda Normalization method fo

For the Straits of Messina, we obtained (correlationcoefficient ρ=0.97)

Qdðf Þ ¼ 61f 0:7

similar to that estimated for coda waves at 40 s of lapsetime. The obtained Qd values are generally comparable

r different frequency bands versus distance.

180 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

with those measured in several tectonically active andheterogeneous zones (Akinci and Eyidogan, 1996).

5.2. MLTWA analysis

We measured the time integral of the signal energy asa function of the hypocentral distance in threeconsecutive time windows, each 12 s long, the firstone starting from the S-wave onset. We fixed theaverage S wave velocity at 3.5 km s−1 following Bottariet al. (1979), and then normalized the energy in eachtime window by the coda energy estimated at a referencetime of 40±10 s from the origin time.

Since the energy integrals are a function of theseismic albedo and the total attenuation, B0 and Le

−1,respectively, (see formula (5)) we can find the pair ofvalues of Le

−1 and B0 which best fit the theoreticalexpression after correcting for geometrical spreading(multiplying by 4πr2). We calculate the best fit betweentheoretical and experimental values using a grid-searchapproach, in which Le

−1 and B0 are let to vary in a rangeof possible values (0.005<Le

−1 <0.1 and 0.05<B0<1).

Fig. 5. Observed integrated seismic energies obtained for the 40–100 km hyenergy. The rhombus, the crosses and the squares represent the measuremen

We found that the seismic albedo increases withdecreasing frequency, being higher than 0.5 below3 Hz; this indicates that scattering attenuation dominatesover intrinsic absorption. For each couple Le

−1 and B0 wecalculate the sum of the squared residuals betweenobservation and theory; the minimum corresponds to thecouple of Le

−1 and B0 which best fit the data distribution.In Fig. 5 the theoretical curves that give the best fit of theobserved data are plotted for all frequency bands (1.5–12 Hz) in the investigated hypocentral distance range(40–100 km).

In Fig. 6, we plot the ratios between residuals andtheir minimum, for all the frequency bands and thehypocentral range defined. We considered those solu-tions as entirely different with a normalized residualgreater than 1.2, which corresponds to the confidenceareas calculated by the application of the F distributiontest at 70% confidence. The pair Le

−1 and B0 ofminimum residual is indicated by a cross for eachfrequency band. The minimum near to the right boundof the B0 values for the 1.5 Hz may be an artefact due tothe limited grid extension.

pocentral distance range. The continuous line represents the syntheticts for all three windows (0–12 s; 12–24 s; 24–36 s), respectively.

181T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

We also calculated the expected codaQ−1 value usingthe relationship proposed by Mayeda et al. (1992):

Q−1cexpected ¼ Q−1

s 1−C2 þ 2C3gmt þ 3C4ðgmtÞ2 þ N

1þ C2gmt þ C3ðgmtÞ2" #

þ Q−1i

ð6Þwhere Cn is the coefficient for the nth order scatteringreported by Hoshiba (1991), that we cut off at the 10thorder for our estimate.

Results of Le−1 and B0, with their estimated errors,

intrinsic inverse Q (Qi−1), inverse scattering Q (Qs

−1),Qcmeasured

−1 and Qcexpected−1 are reported in Table 4.

In Fig. 7, scattering, intrinsic, total and codaattenuation parameters (both measured and expected)versus frequency are reported.

Fig. 6. Plot of residuals (black stars) calculated by means of the F test for

For the 40–100 km distance range, it can be seen thatat 1.5, 3 and 6 Hz Qcmeasured

−1 measured is close to Qi−1, but

for high frequencies (12 Hz), Qcmeasured

−1 is close to Qt−1.

Moreover, Qs−1 is close to Qi

−1 in the frequency range6–12 Hz. This means that both heterogeneities whichgenerate the scattering phenomena (explained with thepresence of different direction of faults, medium withstrong layering, etc.) and absorption play a similar rolein determining the high total seismic attenuation at theStraits of Messina. It is possible to relate both low Qvalues using the explanation of a heterogeneousmedium highly fractured. Scattering is related to thesize and distribution of fractures of the area under study.At the frequencies observed these phenomena, from 6 to12 Hz, we expect fractures of small size. The intrinsicabsorption can be explained by thermal dissipation ofheat in spatial domains at the microphysical scale Leary

all frequency bands. The grey cross indicates the minimum residual.

Table 4Inverse extinction length, seismic albedo, scattering attenuation, intrinsic attenuation, total attenuation, expected and measured coda attenuationvalues obtained from uniform model

f (Hz) Le−1 B0 Qs

−1 Qi−1 Qt

−1 Qcexpected−1 Qcmeasured

−1

1.5 0.087 0.61 0.01961 0.01266 0.03226 0.01492 0.010993 0.082 0.55 0.00840 0.00680 0.01515 0.00722 0.007756 0.081 0.52 0.00388 0.00364 0.00752 0.00372 0.0046512 0.077 0.50 0.00179 0.00178 0.00357 0.00178 0.00400

182 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

(1995). Local stress concentrations at microfractures,flaws, cracks and grain will induce thermal gradients ofsufficient magnitudes to allow for the thermal absorptionneeded for the seismic wave absorption observation.

The physical meaning of coda attenuation within thecontext of the single scattering theory was intended torepresent the total attenuation. Nevertheless, some labora-tory experiments (Matsunami, 1991) and theoretical studies(Frankel and Wennerberg, 1987; Shang and Gao, 1988;Hoshiba, 1991) demonstrated that Qc

−1 is not a measure oftotal attenuation but mainly reflects intrinsic attenuation.

From Table 4 and Fig. 7, we clearly see that theexpected coda Q−1 is nearly identical to the observedintrinsic Q−1.

6. Conclusive remarks

We have been working in the Straits of Messina areaand neighbouring region, analysing its attenuationproperties. This region represents a complex tectonicregion where an intense post-collision dynamic is stillactive. The evidence of this dynamic is reflected by theintense seismic activity.

We have made a systematic estimate of shear waveseismic attenuation in this zone, separating intrinsic

Fig. 7. Scattering, intrinsic, total, expected and measured coda Q versus

absorption from scattering attenuation. By this separa-tion it is possible to recover a better comprehension ofthe physical mechanisms governing attenuation proper-ties in the crust of this region.

We applied the Single Scattering model (Aki andChouet, 1975) to a data set composed of 147 events toinvestigate Q-coda frequency and lapse-time depen-dence. Q-coda appears to be almost linearly frequencyand lapse-time dependent, with the sole exception of80 s lapse time at 12 Hz. This is due to a small number ofdata and to the presence of a large effect of noise,probably induced by sea waves (e.g. Chung and Sato,2001). The present results show that for the Messinaarea Q0 results comparable with what was found insoutheastern Sicily (Giampiccolo et al., 2002) eventhough n is lower. The lapse time dependence isgenerally interpreted (see e.g. Ibáñez et al. (1990)) asdue to a depth dependence of the seismic attenuation,which generally decreases with depth.

Qd, the quality factor for direct S-waves, has beenestimated as a function of frequency for a distanceinterval between 40–100 km. The frequency depen-dence fits well the widely used empirical relationshipQ=Q0 ( f/f0) with Q0 and n values reported in Table 5.As reported by many authors Qd and Qc have similar

frequency for the analysed hypocentral distance range: 40–100 km.

Table 5Average values of the Q laws as a function of the frequency

Q Q=Q0 (f− n/f0) ρQ

Qd 61 f 0.7 0.97Qc 76 f 0.5 0.99Qi 53 f 0.9 0.99Qs 33 f 1.1 0.99

The correlation coefficients are also reported.

Fig. 8. Comparison of (a) Qi−1 for different regions: Southern

Apennines (Bianco et al., 2002), Central California, Hawaii andLong Valley (Mayeda et al., 1992), Canary Islands (Canas et al., 1998),Northern Chile (Hoshiba et al., 2001), North Anatolian fault andAndalusia-Spain (Akinci et al., 1995), southeastern Sicily (Giampic-colo et al., 2006) and Messina Straits (present study); (b) Qs

−1 for thesame regions shown in (a).

183T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

values, indicating that probably mechanisms thatinfluence the attenuation of coda-waves are more relatedto intrinsic than scattering attenuation. The lowQd valueobtained in this work is the evidence that an intensetectonic regime affect the investigated area.

MLTWA approach gives a separate estimate of Qi−1

and Qs−1. At frequencies lower than 3 Hz scattering

attenuation predominates over intrinsic absorption.Thereafter, Qi

−1 and Qs−1 are approximately equal one

to each other, within experimental errors. In any case ourestimation of Qi

−1 and Qs−1 are sufficiently large to

ensure that the region strongly affect to the propagationof the seismic waves producing high attenuation.

We note that all attenuation parameters arefrequency-dependent and the coefficient n rangesbetween 0.5 and 1.1. The highest value corresponds toQs. As suggested by several other authors (e.g. Mayedaet al., 1992; Akinci et al., 1995; Canas et al., 1998;Akinci and Eyidogan, 2000; Giampiccolo et al., 2006)this strong frequency dependence could be related to thesize of heterogeneities. In such a hypothesis, a strongfrequency dependency of Qs

−1 occurs when the hetero-geneities responsible for the scattering are, at least,comparable with the wavelength for the lowest frequen-cies analysed. In other words, the seismic scatteringwhich is related to fracture and lithological heterogeneityin the crust can be considered an indirect indicator ofmacroscale Earth heterogeneity (Leary, 1995).

Using the integral equation of Zeng et al. (1991) wehave determined the expected coda Q−1 at hypocentraldistance 40–100 km. Expected coda Q−1 is close toobserved Qi

−1, rather than Qs−1. This conclusion is in

agreement with theoretical results of Frankel andWennerberg (1987), analytical studies (Shang and Gao,1988) and with laboratory experiments of Matsunami(1991). Shang and Gao (1988) showed that when thescatters are randomly distributed and intrinsic attenua-tion is uniform, the coda Qc

−1 is identical to the intrinsicattenuation. Then the MLTWA has improved ourknowledge of the attenuation parameters in this area,allowing a better characterization of propagationmechanisms. It is noteworthy that, as recently suggestedby several authors (e.g. Margerin et al., 1999; Hoshiba et

al., 2001) the use of a more realistic earth models, whereintrinsic absorption and scattering vary with depth andcrustal velocity structure, could be more appropriate thana homogeneous earth model for modelling attenuation inthe crust. Notwithstanding the approximation of ahomogeneous model used in this study, our results canbe considered useful to compare with those of otherItalian and worldwide tectonically active zones.

Previous authors have tried to estimate Q attenuationin different regions, using the same methods (e.g. Japan,Yoshimoto et al., 1993; northern Greece, Hatzidimitriou,1995; Turkey, Akinci and Eyidogan, 1996; Horasan andBoztepe-Guney, 2004 and southeastern Sicily, Giampic-colo et al., 2006), in which processes such as faulting arelikely to introduce strong heterogeneities.

184 T. Tuvè et al. / Tectonophysics 421 (2006) 173–185

A useful comparison is reported in Fig. 8 (a, b), wherethe pattern of intrinsic and scattering attenuation para-meters estimated through the world is reported. TheStraits of Messina area and neighbouring regions showthe highest intrinsic and scattering attenuation. Inparticular, the exhibited values for intrinsic attenuationprove very close to those observed for central California,and close to the North Anatolian fault and Andalusia.Scattering attenuation exhibits almost the same values asthose observed at low frequencies at Long Valley caldera.

An interesting interpretation can be carried at localscale comparing scattering and intrinsic Q−1 obtainedat Mt. Etna volcano (Del Pezzo et al., 1996) and insoutheastern Sicily (Giampiccolo et al., 2006) with ourresults. The volcanic area of Mt. Etna, located nearbythe southern margin of our investigated area, revealsboth scattering and intrinsic absorption slightly higherthan our study, with same behaviour in Qs

−1 and Qi−1.

The complex structure of this volcanic area easilyexplain the low Q values observed. In southeasternSicily Giampiccolo et al. (2006) observed slight lowerQ values than those calculated in the present work.These results support the different geological andtectonic setting characterizing both sectors. In fact,seismicity in southeastern Sicily is related to adifferent stress pattern with respect to the northeasternarea (more extensive to the north and compressive tothe south). Therefore we are confident that our resultsreflect a strong relationship between high attenuationand the tectonic regime.

Acknowledgements

This work was partially carried out during a researchstage by T. Tuvè at the Instituto Andaluz de Geofisica,Observatorio de Cartuja and at the Osservatorio Vesu-viano. The work was supported by grants from theIstituto Nazionale di Geofisica e Vulcanologia, Sezione diCatania. We thank Dr. Antonio Marino for permitting usto use data recorded by the Osservatorio Sismologico-Università di Messina. Hans Thybo, Elisabetta Giampic-colo and the anonymous reviewer are kindly acknowl-edged for their stimulating comments and suggestions.

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