Table of Contents Riassunto i Abstract iii Table of Contents a

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Riassunto

Questa tesi di dottorato va a descrivere un lavoro che comprende due diverse parti:

scenari di pericolosità sismica e stima degli effetti di sito e magnitudo di coda. La prima parte tratta di scenari di pericolosità sismica in un’area che copre la regione Friuli Venezia Giulia, la parte occidentale della Slovenia e la parte orientale del Veneto. Poichè parte della tesi è stata svolta nell’ambito del progetto Interreg Italia- Austria, Hareia, "Historical and recent earthquakes in Italy and Austria", è stato trattato anche un evento localizzato nella regione Trentino Alto Adige.

La pericolosità sismica, espressa in termini di accelerazione o velocità massima attesa sul territorio, è stata dedotta in base al calcolo di molteplici scenari realistici di scuotimento del suolo con eventi connessi a faglie segnalate in letteratura come le più significative dell’area in esame. Questo approccio permette di ottenere una buona stima sull’eventuale pericolosità dell’area, importante per mitigare gli effetti di potenziali terremoti futuri.

Le sorgenti sismogenetiche prese in esame sono state tredici. Tutti i calcoli sono stati eseguiti usando un modello di faglia estesa, applicando un modello di velocita’ di propagazione della rottura costante e variando la posizione dell’epicentro lungo la superficie di faglia. La distribuzione del momento sismico è stata considerata sia omogenea che, applicando il modello k², non uniforme. In questo modo, per ogni modello di faglia, possono essere calcolati diversi scenari. I sismogrammi sintetici sono stati calcolati con una frequenza massima di 1 Hz e per ogni punto di una densa griglia di ricevitori posizionati in modo equidistante dal centro della sorgente.

Quindi, da ognuno di questi ricevitori, e’ stato possibile ottenere il valore massimo dell’accelerazione e delle velocità del sismogramma, poi plottato sulla mappa finale.

Nella seconda parte della tesi, proprio perchè nella metodologia adottata gli scenari di scuotimento del suolo non tengono conto degli effetti di sito, si è applicata la metodologia di Mayeda et al. (2003), per poter ottenere, a diverse strette bande di

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Come input a questa metodologia, è stato scelto un database costituito da 200 eventi avvenuti fra il 2006 ed il 2009, con magnitudo locale da 2.5 a 4, registrati da 25 stazioni slovene, austriache ed italiane. Di questi terremoti è stata infine calcolata anche la magnitudo da momento di coda. I valori di magnitudo così ottenuti sono risultati coerenti con le stime ottenute da altri Autori usando metodologie diverse.

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Abstract

This PhD thesis describes a work that includes two different parts: the scenarios of seismic hazard and the estimate of site effects and coda moment magnitudes.

The first part deals with seismic hazard scenarios in the Friuli Venezia Giulia region, the western part of Slovenia and the eastern part of the Veneto region. We treat also one event located in Trentino Alto Adige region because it was part of one Interreg Italia-Austria project called Hareia, "Historical and recent earthquakes in Italy and Austria".

The seismic hazard, expressed in terms of the maximum accelerations or velocity for a particular zone, is derived from the calculations of realistic ground motion shaking scenarios. From the literature, we have chosen 13 active faults as source models.

Active faulting data are used to define input fault models for computing ground shaking scenarios, in the extended source approximation and for frequencies up to 1 Hz. For each fault model, we consider different nucleation points along the fault, a constant rupture propagation model and both a uniform and a non-uniform seismic moment distribution. In this last case, the seismic moment is computed according to the k² model. We calculate synthetic seismograms (acceleration and velocity) for a set of equally spaced receivers and we obtain contour maps of their maximum amplitudes.

We note that the scenarios are useful scientific tools for assessing seismic hazard, and consequently for mitigating the effects of future large earthquakes.

In the calculation of these scenarios, we did not consider site effects. They are treated in the second part of the thesis, where Mayeda et al. (2003) methodology is used to obtain all the site responses of the seismic stations for a series of frequency bands.

The results will be able to “correct” the seismic hazard estimates computed on rocks.

As input for this methodology, 200 events recorded between 2006 and 2009, for a

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magnitude. The results values were compared with the values, for the same events, calculated with other methodologies, and found to be coherent.

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CHAPTER 1 1.1 The study area

The study area includes the central Friuli plain, the Julian Prealps and part of Alps, the Carnic Alps and the Slovenian external Dinarides. We can say that this area comprises the whole eastern Southalpine chain (ESC) starting from its frontal zone, located in the Friulian Plain south of Udine, to the Gailtal line, a segment of the Periadriatic Lineament (Poli et al., 2002).

This area is also known as Alps-Dinarides Junction (NE Italy-Western Slovenia) and it is characterized by a very complex active deformation pattern. This complexity is due to the presence of both compressive and transcurrent seismogenic faults (Fig. 1).

Fig. 1. The area of study extends from northeastern Italy to western Slovenia, at the Junction between the Southeastern Alps (Carnic and Julian Alps) and the External Dinarides.

Let us have a look at the subdivision of the Alps-Dinarides Junction area in geotectonic units (Fig. 2): Eastern Alps, north of Gail line, in the Austrian-alpine domain; Southern Alps, between the Gail line and the South-alpine Front; External

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Dinarides, between the Dinaric Front and the South-alpine Front and the Adriatic Foreland (Adriatic Micro-plate) (Ponton, 2010).

Fig. 2. Geotectonical areas: AO: Eastern Alps; AM: Southern Alps; DE: External Dinarides; DI:

Internal Dinarides; BP: Pannonic Basin; AA: Adriatic Foreland (Ponton, 2010).

The area represents the northeastern portion of the deformed margin of the Adria microplate, where complex interaction between two chains occurs. The area comprises the zone between the eastern sector of the Southern Alps and the NW part of the External Dinarides. Both systems reflect in shape and in geodynamic evolution the effects of the collision between the Adria microplate and Europe. Collision and fragmentation occurred at different times creating compressive structures and shear movements. The high seismicity which is characterized in the Friuli area by descructive earthquakes is connetted, in the Southern Alps, to the interaction of the Adria microplate with the European Plate. Towards the east this interaction zone changes in character because of the presence of the Dinarides chain and of the Pannonic fragment.

A geodynamic cross-section of the studied zone is reported in Fig. 3:

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Fig. 3. Geodynamic situation of the studied zone (Doglioni & Carminati, 2008).

The main geological elements are (Carulli et al., 1990):

1. The metamorphic basement, exposed south of the Periadriatic lineament in the Southern Alps and more widely north of the lineament in the Austro-Alpine domain;

2. The Paleozoic units of the Austrian and Slovenian Alps and of the Hercynian non metamorphic and anchimetamorphic sedimentary cover, which are widely present in the Paleocarnic chain and have different levels of metamorphism;

3. The Mesozoic units of the Southern Alps, the Dinarides, the Gailtal Alps and the Northern Karawanken, consisting of massive Triassic or Cretaceous carbonate platform units, detached and thrust over plastic, mainly evaporitic layers;

4. The Tertiary flysch and molasses, often involved in the thrust fold system and Neogene deposits of the Styrian and Pannonian basins;

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6. The Tertiary Periadriatic intrusive masses and the Tertiary lava effusion.

The external margins of the Southern Alps chain and the Dinarides chain show strong recent tectonic deformation that reaches its maximum in the Friulian piedmont arc.

The motion of the Adria microplate has been controlled by the African plate since the early Mesozoic. A platform/basin morphology dominated the area and was generated during the Triassic continental rifting by transcurrent faulting, differential crustal extension and subsidence (D’Argenio and Horvath, 1984). It was severely deformed during the late Cretaceous to Recent especially at the Adria margin.

In the Eastern Alps, the present structure is the result of the suturing of Europe into several small continental fragments starting in the late Jurassic (Horvath and Channel, 1977). A continental collision commenced in the Eocene with subsequent shortening and imbrication of continental crust. In the Western Alps, this phenomenon began in the upper Cretaceous and successively extended towards the Eastern Alps and Eastern Carpathians. The Adria-Europe motion was partially accommodated by intracratonic shortening within the Dinarides where SW-verging thrusts began after the Jurassic in the internal units and progressed outwards until the external units were emplaced in Eocene-Oligocene times. By late Oligocene a dextral shear had developed along the Periadriatic-Vardar fault system (Laubscher, 1988).

South of the Periadriatic lineament, the Apulia-Europe convergence was accommodated by the E-W trending, south-vergent backthrusting (Doglioni, 1987).

This activity reached its maximum in the Friuli area during the Pliocene (Zanferrari et al., 1982). The tectonic activity continued during the Quaternary with notable areas of deformation in the Veneto-Friuli piedmont arc, partially in western Slovenia and along the Croatian coast (Arsovski, 1976). At present, the Friulian pre-Alpine sector is uplifting while the front plain is subsiding (Zanferrari et al., 1982). Anderson and Jackson (1987) explained this movement as a continuing anticlockwise rotation of Apulia which is compressing the edge of the Southern Alps (Periadriatic overthrust belt). The presence of active dextral Dinaric lines on the eastern margin of Apulia indicates a NW movement of Apulia (Cavallin et al., 1984) with associated thrusting under the Southern Alps margin (Castellarin, 1984).

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In the Southern Alps, the main tectonic structures are E-W oriented thrusts. North- verging structures are present in the northern sector between the Gail line (eastern part of the Periadriatic lineament) and the Fella-Sava line (Castellarin and Vai, 1982) and south-verging structures are evident in the southern sector. Minor sub-vertical faults, oriented N-S to NNW-SSE, can be observed in the hinge area where they act as a mechanical disengagement (Carulli et al., 1982). This hinge area corresponds to the zone of maximum crustal shortening in the Southern Alps. Moving towards the east, the major tectonic orientation changes from Alpine to Dinaric near Tolmin (Carobene and Carulli, 1981).

The Dinaric area is characterized be lateral, sub-vertical faults with directions ranging between NW-SE and NNW-SSE, the most important of which is the Idrija line with its auxiliary faults (Placer, 1981). Seismic reflection data indicate buried overthrusts of Dinaric direction also in the northern Friulian plan (Finetti et al., 1979). In the Friuli-Istria area, the Dinaric crustal shortening consists of a few tens of kilometers (Doglioni and Bosellini, 1987). In northern Slovenia, the main structures remain E-W oriented. They are the eastern continuation of the Gail line, the Fella-Sava line and the Southern Alps thrusts. These last continue eastwards, gradually attaining a NW- SE direction in the Sava trough area (Arsovski, 1976). In the region between Ljubljana and Zagreb, the Dinaric style is complicated by NE-SW oriented faults, a result of the tensional tectonics dominating the western parts of the Pannonian domain (Carulli et al., 1990).

1.2 Tectonic setting

The area of study (Fig. 3) is located at the Junction between the Southeastern Alps and the External Dinarides, a region covering northeastern Italy and western Slovenia, respectively.

The area is characterized by a rather complex active deformation pattern (Fig. 4): the Alpine compressive system, in the Friuli region, is active on structures trending

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by segmented high-angle right-lateral strike-slip faults (Fitzko, 2003). The Gail valley delimits this region at its northern part, striking ESE-WNW.

Fig. 4 Active deformation and geometry of the structures at the Alps-Dinarides Junction (Fitzko, 2003).

The region of Friuli consists of east-west trending mountain ranges and valleys and thrusts of similar trend.

The western part, covering the Veneto and Friuli plains until the Lessini mountains, is a Quaternary front of the Southern Alps chain, a fan of low-angle thrusts trending from WSW-ENE to WNW-ESE and verging SSE. The base of Venetian and Carnic Prealps are characterized by the WSW-ENE striking Polcenigo-Maniago and Arba- Ragogna thrusts. The Venetian Prealps is a sector denominated by the SE verging Bassano-Valdobbiadene thrusts and the NW-SE trending Schio-Vicenza strike system; other important faults are the Montello thrust and the Bassano-Cornuda and Thiene-Bassano thrusts, two minor system trending WSW-ENE and dipping NNW (Galadini, 2005) (Fig. 5).

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Fig. 5. Structural model of the Southern Alps area; the black circles represent the towns (TH = Thiene;

GE = Gemona; GO = Gorizia) and the black lines define the faults (TC = Trento-Cles fault; SV = Schio-Vicenza fault; TB = Thiene-Bassano fault; BC = Bassano-Cornuda fault; BV = Bassano- Valdobbiadene fault; BL = Belluno fault; VS = Valsugana fault; FU = Funes fault; AN = Antelao fault;

MT = Montello fault; CA = Cansiglio fault; PM = Polcenigo-Maniago fault; AR = Arba-Ragogna fault; PE = Periadriatic thrust; PU = Pinedo-Uccea fault; DA = Dof-Auda fault; SA = Sauris fault; BC

= But-Chiarsò fault; FS = Fella-Sava fault; VR = Val Resia fault; VV = Val Venzonassa fault; BE = Bernadia fault; BU = Buia fault; ST = Susans-Tricesimo fault; UD = Udine-Buttrio fault; PZ = Pozzuolo fault; MD = Medea fault; PA = Palmanova fault; ID = Idrija fault; PR = Predjama fault) (offer Galadini et al., 2005).

The western Dinaric system is characterized by the Kobarid structure representing the easternmost segment of a system of south-verging compressions with a E-W trend;

furthermore, there are evidences of an active reverse fault called the Gran Monte fault (Aoudia, 1998) and the Kobarid structure is the easternmost continuation of the E-W Gran Monte ridge (Fitzko et al., 2004). The Idrija fault is a right-lateral strike-slip fault extending in the NW-SE direction with a strike of 320° (Fitzko et al., 2005). A parallel fault to the N is the Knežje Ravne or Tolminka fault, a strike slip system extending in the NW-SE direction with a strike angle of 300° (Fitzko, 2003). This fault is located south of the 1998 Bovec earthquake epicenter and Bajc et al. (2001)

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suggest that it could be a locked segment of the Bovec-Krn (Ravne) fault with a difference in the strike value of 5°.

Galadini et al. (2005) proposed several seismogenic sources in the Southern Alps area using geological and available seismological data: ten possible faults responsible for strong earthquakes are selected (Fig. 6). To the east of the Tagliamento river there is a large amount of geological and seismological data connected with the seismic sequence occurred in 1976. In particular, the Susans-Tricesimo thrust may be responsible of the Friuli mainshock, while the deeper Trasaghis fault was activated during the earthquake on September 15, 1976. The Medea fault, located SE of Udine with a length of about 23 km and a depth ranging from 5 to 8 km might also be capable to generate earthquakes with magnitude higher than 6. The Gemona-Kobarid thrust is placed at the border between Italy and Slovenia and Galadini et al. (2005) suggest its activation during the seismic sequence of 1976. At the west of Tagliamento river the Venetian and Friulian plains are crossed by a thrust about 100 km long which is separated into different segments: in the Veneto region there are responsible of the historical earthquakes occurred in the same zone. In the Friuli Venezia Giulia region, the Cansiglio fault is located to the south of the Cansiglio massif (western margin of the Friuli plain) and it is considered responsible of the 1936 earthquake (Sirovich et al., 2000; Valensise and Pantosti, 2001) with a strike angle ranging between 212° and 232°. The Polcenigo-Maniago thrust and Arba- Ragogna thrust are placed to the east of the Cansiglio fault and numerous geological data evidence the presence of these faults in the area. All described sources are located at the border between the plain and the mountains and no active faults are located within the Alps zone itself (Galadini et al., 2005). Although, these seismogenic sources may be responsible of seismic events with magnitude larger than 6, the recurrence interval for each singular source is very long (many centuries).

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Fig. 6. Map of the seismogenic sources in the Thiene-Udine sector of the eastern Southern Alps: 1- Thiene-Bassano; 2- Bassano-Cornuda; 3-Montello-Conegliano; 4-Cansiglio; 5-Polcenigo-Maniago; 6- Arba-Ragogna; 7- Gemona-Kobarid; 8- Susans-Tricesimo; 9- Trasaghis; 10- Medea (Galadini, et al., 2005).

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CHAPTER 2 THE STRONG GROUND MOTION SCENARIOS 2.1 Introduction

The first step for the strong motion estimation for seismic hazard assessment is the identification and the characterization of the seismic sources. To understand the physical process of the source and to obtain an assessment of potential earthquakes effects, realistic ground shaking scenarios are estimated in the area of the southern-eastern Alps. The available parameters related to possible potential seismogenic sources in the region are taken from Burrato et al. (2008) and from Wells and Coppersmith (1994) and used in a deterministic approach. The retrieved ground motion parameters are the maximum spectral horizontal displacements, velocities and accelerations and they are computed for a set of localities in the area. The computations are performed using an extended source model, varying the hypocenter position along the fault surface and applying a constant rupture propagation velocity model. The seismic moment distribution is considered, at first to be uniform, subsequently, a K² model (Herrero and Bernard, 1994) is used for a non-uniform seismic moment density distribution. In this way, for each fault model, several scenarios are calculated in order to analyze both the effect of the directivity of the source and of the seismic moment distribution on the estimated ground motion fields. The synthetic seismograms are computed for an upper frequency cutoff of 1 Hz, even if they should be estimated for a frequency band as large as possible. But the deterministic evaluation of displacement, velocity and acceleration at frequencies larger than our 1 Hz limit, is influenced by the rupture propagation process complexities and by the seismic waves scattering along the source-receiver path. To extend the considered frequency range up to 10 Hz, we would need a lot of structural details, at the order of some tens of meters. Therefore, the usual approaches for the ground motion evaluations at such high frequencies are stochastic and not deterministic. We use an average structural model for the region proposed by Costa et al. (1992).

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The synthetic seismograms are computed over a dense grid of equally spaced receivers and the results are presented with contour maps.

2.2 The extended source

We use a kinematic approach to calculate the synthetic seismograms, in the 2D extended-source approximation. In this type of approach, we assume an “a-priori”

seismic moment distribution, which is a function of time and space. In this way, we do not relate it to the stress that caused it. The fracture process is described purely by the slip vector, as a function of the coordinates on the fault plane and of the time. For strong motion estimation purposes for seismic hazard studies this kinematic approach is sufficient. There are some parameters required as input to the fault model in the extended source model that describe the source geometry and the details of the source rupturing processes. In fact, we usually consider the rupture surface to be planar with the following characteristics:

• L, the source length;

• W, the fault width.

Both, L and W, are related to the scalar seismic moment M0.

The rupture propagation model requires of fixing the position of the nucleation point on the fault surface that confide the time dependency of the seismic moment release. We also have to fix the rupture propagation velocity, vr, and a characteristic rise time for all the points composing the source (Heaton, 1990).

The rupture velocity vr is usually defined as a percentage of the S-wave velocity β and it can vary when β changes in successive earth’s layers in which the fault plane is lying. The consequence of a non-constant rupture velocity on the fault plane results in non-spherical rupture propagation fronts.

The rupture propagation velocity, vr, is assumed to be constant in our computations and not depending on the seismic moment. It is ranging from a maximum value equal to the P-wave velocity, to a minimum value somehow less than the Rayleigh-wave velocity. Geller (1976) suggests an average value of vr

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in the range between 2.4 km/s and 3.0 km/s. In this thesis, we will consider a constant rupture velocity equal to 0.7β.

2.3 Seismograms computation

Synthetic seismograms are computed from each point source located on a grid into which the seismogenetic source is discretized. The resulting seismogram is the sum at a given receiver site of the contributions from all such source points.

In Fig. 1, we show the grid of receiver sites and, at the center, the red rectangular is the seismogenetic source taken in consideration.

Fig. 1 The grid of receiver points with, at the center, the red rectangular representing the fault.

The fault is embedded in a half space composed by homogeneous horizontal anelastic layers. This is one-dimensional approximation for the description of the

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earth properties, since the elastic parameters of the earth in general vary both in the horizontal and in the vertical directions.

The seismogram is computed resolving the following equation:

The function ^sn

( )

^t describes the time dependency of the seismic moment release from the n^th source element and they are related to the kinematic model used for the rupture propagation description. The * symbol denotes time convolution.

The source function ^sn

( )

^t can be divided:

( ) (

n

)

n

n a t t

s = ∗δ −τ 1.2

Where ^an

( )

^t is the time function for the n^th source element (at t₀=0) and τ_n^is the time delay when the rupture propagation front reaches this element. The functions τ_n are computed adopting a rupture propagation model, a rupture velocity υ_r and the hypocenter position along the fault plane. The vector ^gn

( )

^t is the impulse response of the considered site to a unitary seismic moment release by the n^th source element with a fixed focal mechanism. These vectors are named Green’s Functions. The Green’s Functions ^gn

( )

^t depend on the approximation used for the earth structural model. In the case described here, the earth structural model, as we said before, is one-dimensional. The functions ^gn

( )

^t are calculated using the method of the surface waves modal summation (Haskell, 1953; Schwab and Knopoff, 1972; Panza, 1985).

The contributions of each cell to the complete seismogram are added on the frequency domain following the equation:

( ) ( ) ( ) ( ) ( )





=^FT⁻ ^f

∑

^N ^a ^g ⁱ

t

u ¹ ω ω ω exp ωτ ^1.3

( ) ∑ ( ) ( )

=

∗

= ^N

n

n t g t

s t

u

1

1.1

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Where f(ω) is a filter used for selecting a certain frequency band contribution to the ground motion.

Fig. 2. Discretized fault surface divided into a grid of regular elements. We can choose one of these elements as position of our nucleation point. The three parameters, strike, dip and rake give the orientation of the fault and relative motion on it, permit to have the geographical position of the seismogenetic source. In this case we have 10 elements along strike, and three along width

The discretizated source surface is composed into a dense grid of points regularly spaced along the length (L) and the down-dip (W) directions.

The three parameters, strike, dip and rake permit us to have the geographical position of the fault plane in the space, the latitude φ, longitude λ and dept h of a given reference cell. In the fault reference system, a starting depth is fixed along the z-axis and the grid is numbered along the directions of strike and down dip. In particular, the rows are counted from the upper most one to the deepest one while the columns are counted along the strike direction.

The two parameters L and W are defined by the number of cells in each row and column and by a grid spacing ∆x, ∆h selected for the extended source discretization (Fig. 2). The choice of the grid spacing is very important because in this way we should simulate the continuity of the rupture propagation process along the fault surface. If the grid spacing is not well defined, aliasing problem can occur.

Now we take in consideration, as an example, two neighboring point sources separated by a distance ∆x. They will produce an impulse when the rupture

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propagation front traveling at a velocity vr will reach them. In the case of a sampling in the time domain, the contribution of the two point sources will be indistinguishable if the time difference ∆t between their arrival times is less than the sampling time step δt. This will result in a doubled amplitude. The sampling time step δt defines the maximum possible resolvable frequency considered or the Nyquist Frequency

( )

^t

f_Ny δ 2

= 1 . The problem of the grid step definition is related

to the discrete representation of a continuous process, which is the rupture propagation. There are some empirical relations that can be used in the grid step definition and we give in the following some examples.

If cmin is the minimum wave velocity and vr the rupture propagation velocity, given a maximum frequency, f_Ny, the corresponding grid spacing should be:

(

^c ^v

) (

^c^c ^v^v

)

^t

f v x c

r r r

Ny

r δ

= +

≤ +

∆

min min min

min

2

1.4

If for example the Nyquist Frequency is 1 Hz and

kms v

c_min≈ _r≈2 , the maximum grid spacing should be about0.5 km. If the Nyquist Frequency is 2 Hz and

kms s v

c_min≈0.6 km , _r≈2.2 them ∆x decreases to about 120 m (Fitzko, 2002).

We will consider now another case regarding the position of the receiver with respect to the direction of the rupture propagation.

There are two opposite cases:

1. First case: the site is in a forward direction, i.e. the rupture propagates towards it;

2. Second case: the receiver is placed in a backwards direction i.e. the rupture propagates away from it.

If two point sources are separated by a distance ∆l, the minimum frequency fc that allows the two sources to be distinguishable for a receiver at an angle θ from the direction of rupture propagation depends on the shear-wave velocity and the

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

 



 −

= ∆

β θ

β ^cos

2

1

r c

v

f l ^1.5

If we have a too coarse grid spacing, the result will be an excess in high frequencies and a consequent overestimation of the amplitudes.

2.4 Non-Uniform seismic moment distribution: the K² Model

In this work we take in to consideration the seismic moment distribution both as uniform and non-uniform. The simplest approach is to consider a constant seismic moment density on the fault plane and to smootly bring it to zero along the bordes in order to have a physically ground slip distribution and thus to avoid possible border effects (Fig. 3).

Fig. 3. Uniform momentdistribution (Fitzko, 2002).

Since uniform distributions are rather an exception we considered also cases in which the largest amount of the seismic moment is concentrated in small parts of the source known as asperities, in which the slip vector, proportional to M0, is statistically 1.5 times larger than its average value (Sommerville et al, 1999). It seems that the number of asperities per event is 2.6 and it is not related to the magnitude of the event. Moreover, there is no evident correlation between the hypocenter position and the maximum seismic moment release areas or the asperity locations (Fitzko, 2002).

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Fig. 4. Seismic moment distribution defined with the k² model for a single asperity (Fitzko, 2002).

Fig. 5. Seismic moment distribution defined with the k² model for two asperities (Fitzko, 2002).

The seismic moment distribution on the rupture surface can be described using the bi-dimensional Fourier Transform, where k is the wave number (

λ 2π

=

k ) and λ is the wavelength:

(

1 2

) (

1 2

)

⁽ ⁾ 1 2

2 2 1

, 1

,^k ^m ζ ζ ^s ^ζ ^ζ ^dζ ^dζ

k

m ⁻ⁱ^k ⁺^k

∞ +

∞

−

∫∫

= 1.6

For | m(k) |, with ^k⁼

(

^k¹²⁺^k²²

)

²¹ and denoting L as the fault length, we can search for a relation like (Herrero and Bernard, 1994):

( ) ( )

k L L if

k k m k m

n 1

0 1  >



 



= 

= 1.7

( ) ( )

k L if k

m k

m 1

0 <

=

= 1.8

If m is the average seismic moment density on the fault surface and it is proportional to the fault length and the stress drop; we can write (Geller, 1976):

L C

m= ∆σ 1.9

Then:

( )

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Substituting 1.10 into 1.7, we obtain:

( )

if k L

L L k C k m

n 1

3 1  >







∆ 

= σ ^1.11

It is verified (Herrero and Bernard, 1994) that the spatial variations of the seismic moment density, at a small scale, are not correlated to the final dimensions of the rupture area and therefore the space dependency of the amplitude spectrum, taking n=2, is:

( )

if k L

k C L k

m 1

2 >

∆

= σ ^1.12

These relations are known as the K² Model for the seismic moment density distribution on the fault surface.

Sommerville (1999) proposes an empirical scale relation, considering different wave numbers for the x and y directions:

W

x M

k 1.72 0.5

log ₀= − 1.13

W

x M

k 1.93 0.5

log ₀= − 1.14

So, the K² Model can complicate the seismic moment distribution on the fault surface with a fixed amplitude of the spatial spectrum. The phase of the spatial spectrum is defined by means of a casual number generator that selects the large wavenumbers; the small wavenumbers instead are fixed in order to control the number of the asperities and their position. The use of the Fourier anti-transform leads to negative values of seismic moment that are put equal to 0 in this work.

Moreover, a cosine tapering function is applied at the fault edges in order to avoid the asperities to be sharply cut and generate border effects.

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CHAPTER 3 THE SEISMIC SOURCES

Fig. 1. This map shows the location of the 13 faults that we have analyzed in this work.

3.1 Introduction

The border between northeastern Italy and western Slovenia is located at the northernmost tip of the convergent margin between the Adriatic and European plates.

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(Bajc et al., 2001; Zupančič et al., 2001),. This seismic activity testifies that the geodynamic processes, which led to crustal thickening and to the building of the Eastern Southern Alps (ESC) and the northern portion of the External Dinarides are still active (Burrato et al., 2008). This region has experiences earthquakes with magnitudes up to 7 (Idrija 1511 event). Fault plane solutions of major instrumental earthquakes and active tectonic studies show that thrust faulting is the dominant mechanism on the Italian side (Slejko et al., 1999; Poli et al., 2002), whereas dextral strike-slip faulting prevails on the Slovenian side (Herak et al., 1995; Poljak et al., 2000).

GPS studies estimate N-S shortening rates of the order from 2 mm/a (Serpelloni et al., 2005) to 4 or 5 mm/a (Anderson and Jackson, 1987) across the epicentral region of the 1976 Friuli earthquake sequence.

According to paleoseismological observations, the mean return time of major earthquakes on a given fault in Italy is in the order of 10 years (Valensise and ³ Pantosti, 2001a).

All the geometrical and kinematical parameters of the sources proposed in this PhD work are taken from the Burrato et al. (2008) work, that presents an overview of the seismogenic sources of northeastern Italy and western Slovenia certained in the Database of Individual Seismogenic Sources (DISS). All the seismogenic sources taken in consideration are responsible for Mw > 5.5 earthquakes.

DISS is a georeferenced repository of active tectonic, fault and paleoseismological information for the Italian territory and surrounding regions. The main objects of the database are the Individual Seismogenic Sources and the Seismogenic Areas. We assume that each individual seismogenic source ruptures the entire fault plane assigned to it thereby generating its maximum allowed earthquake. In a seismogenic area instead, the seismogenic potential and the state of segmentation are more loosely defined and each one of them may span an unspecified number of Individual Sources (Burrato et al., 2008).

The last version of DISS includes, for the Veneto-Friuli Region, an improvement of the scheme of seismogenic sources potentially responsible of M ≥ 6 earthquakes compiled by Galadini (2005).

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The individual seismogenic sources mapped in Fig. 2 are these used in this work to compute realistic ground shaking scenarios.

Fig. 2. Following the Burrato et al. (2008) work, here we find the individual seismogenic sources.

They are each represented with a rectangle that is the projection of the fault plane onto the ground surface. The red triangles are the seismic stations used in this work.

All geometrical and kinematical parameters related to each source model are reported in a single table within the chapter describing each source. The Mw value is that reported obtained from the Burrato et al. (2008). Afterwards, the length and the width of the fault are calculated using the empirical relationships proposed by Wells and Coppersmith (1994). The M0 value is estimated using the analytical relationships of Hanks and Kanamori (1979).

In the Fig. 3 there is the investigated area, that comprehends the Veneto-Friuli pre- Alpine region, included the Italian-Slovenian border and the oriental part of Slovenia.

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Fig. 3. Structural model of northeastern Italy and of western Slovenia. Legend (cities): TH, Thiene;

GE, Gemona; GO, Gorizia. Legend (structures): TC, Trento-Cles fault; SV, Schio-Vicenza fault; TB, Tiene-Bassano fault; BC, Bassano-Cornuda fault; BV, Bassano-Valdobbiadene fault; BL, Belluno fault; VS, Valsugana fault; FU, Funes fault; AN, Antelio fault; MT, Montello fault; CA, Cansiglio fault; PM, Polcenigo-Maniago fault; AR, Arbe-Ragogna fault; PE, Periadriatica fault; PU, Pinedo- Uccea fault; DA, Dof-Auda fault; SA, Sauris fault; BC, But-Chiarsò fault; FS, Fella-Sava fault; VR, Val Resia fault; VV, Val Venzonassa fault; BE, Bernadia fault; BU, Buia fault; ST, Susans-Tricesimo fault; UD, Udine-Buttrio fault; PZ, Pozzuolo fault; MD, Medea fault; PA, Palmanova fault; ID, Idrija fault; PR, Predjama fault (from Galadini et al., 2005)

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3.2 The Belluno fault

Fig. 4. Location of the Belluno fault.

The 29 June 1873 event associated to this fault has mainly affected the Alpago valley, in the northern part of the Cansiglio plateau. This area is characterized by strike-slip movements with a NNE-SSW direction.

There are several hypotheses about this source, but it is possible to validate some models with the available seismo-tectonic informations. First of all, we mention the Valensise and Pantosti (2001) work in which they have supposed that the source called Fadalto is the responsible of the 1873 event. This structure represents the easternmost part of the Bassano-Valdobbiadene line of Castaldini and Panizza (1991) (Fig. 5).

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Fig. 5. Active fault map (Castaldini & Panizza, 1991).

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Sirovich and Pettenati (2004) retrieved geometric and kinematic information about this source by inverting its regional macroseismic intensity pattern and their results are in agreement with morphotectonic evidences of activity of the Cansiglio Thrust (Galadini et al., 2005). They propose a SSW fault orientation with a strike = 217° and this is consistent with the branch of the Bassano-Valdobbiadene (BV) line.

Instead, Galadini et al. (2005) propose the Polcenigo – Maniago (PM) fault as the source of the 1873 event (Fig. 3).

Burrato et al. (2008) propose that these are almost the same zones that suffered the strongest effects during the 1873 Bellunese Mw=6.3 earthquake, that we suggest to associate to the nearby Polcenigo-Maniago source on the basis of the damage pattern.

On this side of the Eastern Southern Alps, the Cansiglio and Polcenigo-Maniago sources are the most external thrusts and are located along the Carnian Prealps mountain front. The same structural position at the mountain front is occupied to the east by the nearby Maniago and Sequals sources, that show a change of strike from NE-SW to ENE-WSW. To their north, the Periadriatic Thrust is active and generates intermediate-size earthquakes.

Input file model

The Belluno source model:

Source/Fault Lat Long Z L W Strike Dip Rake Earthquake Mw Belluno 46.05 12.52 7.0 15.0 18.0 220 40 80 29.6.1873 6.4 The geographic coordinates refer to the westernmost upper corner of the fault plane.

The magnitude of the associated earthquake is taken from Burrato et al. (2008). The two parameters L, the length of the fault, and W, the width of the fault, were calculated using the empirical relationships of Wells and Coppersmith (1994). Z represents the top of the fault.

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COMPUTED SCENARIOS

3.2.1 Uniform seismic moment distribution

In the following we show all the realistic strong ground motion scenarios calculated with the selected geometric and kinematic features of the Belluno source (we will used the same procedure for all the following sources considered in this work).

At first, we calculate the simplest scenario, with a uniform seismic moment distribution on the fault plane. In this case, the seismic moment density is tapered at the fault edges by means of a cosine tapering function. In the second case of the damaging scenario computation, we choose a non-uniform seismic moment distribution. We used the K²Model to build a single-asperity and a double-asperity moment density. The total seismic moment on the fault plane is 5.1∗10²⁵ dyne*cm and corresponds to the given Mw=6.4 event (Hanks & Kanamori, 1979).

For each seismic moment distribution, the nucleation position has been placed at the center of the fault and at one and other fault end: at its northwestern and southeastern corner. The rupture propagates with a constant velocity equal to 70% of the S-waves velocity.

The synthetic seismograms are computed assuming the sites are lying on bedrock and therefore no site effects are taken in to consideration.

The Fig. 5 shows the results related to the first strong ground motion scenario presented with a contour map. The nucleation point is located at the center of the fault and at a depth of 7 km. As an effect of the bilateral rupture propagation, two high- velocity lobes are located off the northwestern and off the southeastern fault edges. In Fig. 6 it is possible to see the maximum horizontal accelerations, in Fig. 7 we report the maximum horizontal velocities.

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Fig. 6. Contour map of the maximum horizontal accelerations computed for a scenario using the Belluno fault model. The rupture propagates bilaterally from the nucleation point located at the center of the fault at 7 km depth. The seismic moment distribution is uniform.

In Fig. 8 and Fig. 9 we present the results of the second damaging scenario computed applying a uniform seismic moment distribution on the fault plan, but with a different position of the nucleation point respect to the bilateral position seen before. The nucleation is positioned at the northeastern fault edge. In Fig. 10 and in Fig. 11, the rupture propagates southeastwards. The source directivity produces a single high- acceleration or high–velocity zone that is elongated in the direction to which the rupture propagates.

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Fig. 7. Same as Fig. 6, but for the maximum horizontal velocities.

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Fig. 8. Same as Fig. 6, but for a nucleation point at the northeastern fault edge.

Fig. 9. Same as Fig. 6, but for maximum horizontal velocity and a nucleation point at the northeastern fault edge.

Fig. 10. Same as Fig. 6, but for a nucleation point at the southwestern fault edge.

Fig. 11. Same as Fig. 6, but for maximum horizontal velocity and a nucleation point at the

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Fig. 12 shows the contour map in which we have plotted the maximum values of all the three types of scenarios seen before. In this way, we have obtained the map with the maximum values that we could have in the uniform moment seismic distribution case.

Fig. 12. Contour map with the maximum values of all three scenarios for the uniform moment distribution cases.

3.2.2 1 asperity cases

Hereafter, we present the results related to scenarios computed applying a non- uniform seismic moment distribution characterized by a single asperity. This test is related to a bilateral rupture propagation model (Fig. 13), with the nucleation point fixed at the center of the fault surface. Applying a non-uniform seismic moment distribution, we observed higher values of the maximum horizontal acceleration.

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Then, in Fig. 13 and Fig. 14, we show the results for a non-uniform seismic moment distribution,

Applying a non-uniform seismic moment distribution, we observe higher values of the maximum horizontal acceleration than for the related uniform distribution. In Fig.

14 and Fig. 15 we show the results for a non-uniform seismic moment distribution, with the nucleation point first located at the northeastern edge of the fault and then located at the southwestern edge of the fault. In these two cases, the high-acceleration zones are elongated in the direction of the rupture propagation and they are characterized by accelerations higher then 160 cm/s².

Fig. 13. Contour map of the maximum horizontal acceleration computed for a scenario using the Belluno fault model. The rupture propagates bilaterally from the center of the fault located at 7 km depth. The seismic moment distribution is non-uniform, with 1 asperity.

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Fig. 14. Same as Fig. 13, but for the nucleation point at the northeastern fault edge.

Fig. 15. Same as Fig. 13, but for the nucleation point at the southwestern fault edge.

Fig. 16 shows the contour map in which we have plotted the maximum values of all the three scenarios seen before.

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Fig. 16. Contour map with the maximum values of all the three scenarios for the non-uniform one- asperity moment distribution case.

3.2.3 2 asperities cases

In Fig. 17, the contour map related to the scenario computed applying a double- asperity seismic moment distribution. The rupture propagates almost bilaterally from the center of the fault, but we note higher values at the western side. In particular, there are only two high-acceleration lobes, with accelerations higher then 160 cm/s².

This is a consequence of the presence of two asperities characterized by different values of associated seismic moment of the fault. The asperity located at the southern portion of the fault releases a larger amount of seismic moment.

In Fig. 18 and Fig. 19, we report the results obtained with different positions of the

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propagates from the northeastern edge of the fault, while in Fig. 19, the rupture propagates from the opposite side.

Fig. 17. Contour map of the maximum horizontal acceleration computed, using the Belluno fault model, for a scenario in which the rupture propagates bilaterally from the center of the fault located at 7 km depth. The seismic moment distribution is non-uniform, with two asperities.

In Fig. 20, we show the map with the maximum values that we could have in the non- uniform moment seismic distribution case, with two asperities, with the three considered nucleation point positions.

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Fig. 18. Same as Fig. 17, but for the nucleation point is at the northeastern fault edge.

Fig. 19. Same as Fig. 17, but for the nucleation point is at the southwestern fault edge.

Fig. 20. The contour map with the maximum acceleration values of the three scenarios for the non- uniform moment distribution cases, with two asperities.

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The last scenario computed for the Belluno fault (Fig. 21) is obtained plotting all the maximum values found in all the different considered scenarios.

Fig. 21. The maximized scenario computed for the Belluno fault is obtained plotting all the maximum values of accelerations found in all the considered scenarios.

This last scenario (Fig. 21) is the recapitulation of the 9 different scenario types, computed using the Belluno fault, as input model. They have been calculated using three rupture propagation models for each of the three seismic moment distributions applied.

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3.3 The Bovec 1998 fault

Fig. 22. Location of the Bovec fault.

The Bovec fault mechanism is fixed according to the Ms = 5.8, April 12 1998 Bovec- Krn Mountain (northwestern Slovenia) event. This event is the best constrained one, in Slovenia, within the active zone collision zone between Eurasia and Adria at the junction between the southeastern Alps and the Dinarides (Bajc et al., 2001). This deforming area undergoes 4 to 5 mm/years of crustal shortening (De Mets et al., 1990). The epicenter is about 40 km east of the destructive Friuli 1976 thrust faulting earthquake (Aoudia et al., 2000) and is located near the junction between Alpine structures trending E-W and external Dinaric structures trending NW-SE (Aoudia, 1998). Its focal mechanism corresponds to an almost pure strike-slip faulting in agreement with the NW-SE trend of Dinaric structures. The aftershock zone (over 4000 aftershocks have been recorded till the end of June 1998) does not match any already known active fault (Bajc et al., 2001) (Fig. 23), but runs along the

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Fig. 23. The 1998 Bovec-Krn earthquake sequence. Yellow star is the main shock: red dots are the 548 relocated earthquakes; triangles are the closest temporary stations (Bajc et al., 2001).

The Bovec earthquake fault represents the northernmost segment of a larger fault zone, the Idrija right-lateral fault system that exhibits witnesses of recent activity (Aoudia, 1998) and should be of concern in terms of hazard.

Input file model

The Bovec source model:

Source/Fault Lat Long Z L W Strike Dip Rake Earthquake Mw Bovec-Krn 46.32 13.61 5 13 7 313 82 171 12.4.1998 5.8 The geographic coordinates refer to the westernmost upper corner of the fault plane.

The magnitude of the associated earthquake is taken from Burrato et al. (2008). The two parameters L, the length of the fault, and W, the width of the fault, were calculated using the empirical relationships of Wells and Coppersmith (1994). Z represents the top of the fault.

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COMPUTED SCENARIOS

3.3.1 Uniform seismic moment distribution

As in the previous Belluno model, the input fault model used in the scenario computation follows the Burrato et al. (2008) work. The strike is 313°, consistent with the active deformation setting of the area, the dip is fixed to 80° and the rake 176°. The given magnitude is 5.8, corresponding to a total seismic moment of

1024

237 .

6 ∗ dyne*cm, using the Hanks & Kanamori (1979) relation.

Also in this case, the ground motion parameter chosen to assess the seismic hazard is the maximum horizontal acceleration and we report some cases in which the maximum horizontal velocity is calculated. The results are presented as contour maps of the acceleration (or velocity) distribution.

In analogy with the previously studied Belluno model, we consider both uniform and non-uniform seismic moment distribution on the fault plane. For each seismic moment distribution, three nucleation positions along the fault plane are tested: at its center and at its northwestern and southeastern corner. The rupture propagates with a constant velocity equal to the 70% of the S-waves velocity.

The synthetic seismograms are computed supposing the site lying on the bedrock and therefore no site effect is taken into account. The synthetic signals are computed using the method of the Modal summation for an extended source (Panza, 1985;

Panza et Suhadolc, 1987; Florsch et al., 1991; Panza et al., 2001).

Fig. 24 shows the results related to the first scenario in a uniform seismic moment distribution hypothesis. The nucleation point is located at the center of the fault and at a depth of 5 km. As an effect of the bilateral rupture propagation, two high-velocity lobes are located off the northwestern and off the southeastern fault edges, with the maximum horizontal acceleration value of 19 cm/s². Bovec is the town more affected of the area.

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Fig. 24. Contour map of the maximum horizontal accelerations computed for a scenario using the Bovec fault model. The rupture propagates bilaterally from the nucleation point located at the center of the fault at 5 km depth. The seismic moment distribution is uniform.

The second scenario shows the maximum horizontal velocity values computed applying a uniform seismic moment distribution on the fault plane. The nucleation point is located at the center of the fault (Fig. 25). Here, the maximum velocity value is of 5.5 cm/s².

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Fig. 25. Same as Fig. 24, but for the maximum horizontal velocities.

Now we report the maximum horizontal acceleration values (Fig. 26 and Fig. 28) and the maximum horizontal velocity values (Fig. 27 - 29) plotted in the following scenarios in which the nucleation positions are changed. In particular, the first two cases (Fig 26 and Fig. 27), the rupture propagates northwestwards. The contrary case is in Fig 28 and Fig. 29.

In both cases, the source directivity implies the presence of a single high velocity zone that is elongated in the direction to which the rupture propagates.

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Fig. 26. Same as Fig. 24, but for a nucleation point at the southern fault edge.

Fig. 27. Same as Fig. 24, but for maximum horizontal velocity and a nucleation point at the southern fault edge.

Fig. 28. Same as Fig. 24, but for a nucleation point at the northern fault edge.

Fig. 29. Same as Fig. 24, but for maximum horizontal velocity and a nucleation point at the northern fault edge.

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Fig. 30 shows the contour map in which we have plotted the maximum values of all the three types of scenarios seen before. In this way, we have obtained the map with the maximum values that we could have in the uniform moment seismic distribution case.

Fig. 30. Contour map in which the maximum values of all three scenarios for the uniform moment distribution cases.

3.3.2 1 asperity cases

In Figures 31, 32 and 33 are shown the results of scenarios calculated applying a non- uniform seismic moment distribution, characterized by a single asperity moment density.

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When the nucleation position is located at the center of the fault plane, we computed accelerations of 34 cm/s². Similar values are found for the other two cases, with different nucleation positions.

Fig. 31. Contour map of the maximum horizontal accelerations computed for a scenario using the Bovec fault model. The rupture propagates bilaterally from the center of the fault located at 5 km depth. The seismic moment distribution is non-uniform, with one asperity.

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Fig. 32. Same as Fig. 31, but for a nucleation point at the southeastern fault edge.

Fig. 33. Same as Fig. 31, but for a nucleation point at the northwestern fault edge.

In the Fig. 34, we have obtained the map with the maximum values that we could have in the uniform moment seismic distribution cases seen before.

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Fig. 34. Contour map with the maximum values of all the three scenarios for the non-uniform one- asperity moment distribution case.

3.3.3 2 asperities cases

In Fig. 35 are reported the results of a scenario in which a non-uniform seismic moment distribution, characterized by two asperities, is considered. In this case, we have a bilateral rupture model. One big high acceleration lobe is present, in which we obtain values larger than 35 cm/s².

In Fig. 36, the nucleation is located at the southeastern fault edge and the rupture propagates northwestwards. The opposite is Fig. 37.

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Fig. 35. Contour map of the maximum horizontal acceleration computed, using the Bovec fault model, for a scenario in which the rupture propagates bilaterally from the center of the fault located at 5 km depth. The seismic moment distribution is non-uniform, with two asperities.

In Fig. 38, we obtained the map with the maximum values that we could have in the uniform moment seismic distribution case, with the three considered nucleation point positions.

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Fig. 36. Same as Fig. 35, but for the nucleation point at the southeastern fault edge.

Fig. 37. Same as Fig. 35, but for the nucleation point at the northwestern fault edge.

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Fig. 38. Contour with the maximum values of the three scenarios for the non-uniform moment distribution cases, with two asperities.

This last scenario (Fig. 39) is the maximezed one of the 9 different scenario types, computed using the Bovec fault, as input model. They have been calculated using three rupture propagation models for each of the three seismic moment distributions applied.

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Fig. 39. The maximized scenario computed for the Bovec fault is obtained plotting all the maximum values of accelerations found in all the considered scenarios.

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3.4 The Bovec 2004 fault

Fig. 40. Location of the Bovec fault.

On 2004 July 12, an MD=5.1 earthquake occurred close to the 1998 main shock, generating a sequence that lasted until the end of November 2004; there were about 300 aftershocks with MD varying from 1.1 to 3.6 (Bressan et al., 2009). The 2004 main shock was characterized by a strike-slip focal mechanism (Bressan et al., 2007) very similar to that one the 1998 main shock and also related to the right lateral strike-slip Bovec-Krn fault.

The seismogenic tectonic structure is a near-vertical dextral strike-slip fault, about NW-SE oriented and is quite similar to the one of the 1998 main shock. The focal mechanisms of the 1998 and 2004 sequences reveal common features. The main shocks are similar and consistent with the large-scale orientation and sense of motion of the Bovec-Krn strike-slip fault.