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ANNALS OF GEOPHYSICS, VOL. 47, N. 6, December 2004
Key words seismicity of Greece – probabilisticseismic hazard –
peak ground acceleration – designearthquake procedure
1. Introduction
The basic lithospheric process in theAegean Sea is the
subduction of the Africanplate under the Eurasian plate south of
the Is-land of Crete, forming the Hellenic Arc andTrench
(Papazachos and Comninakis, 1971;Comninakis and Papazachos, 1972;
Makris,1973; Mercier, 1977; McKenzie, 1978; Deweyand Sengor, 1979;
Makropoulos and Burton,1984). Because of this process, Greece is
one of
A probabilistic seismic hazard assessmentfor Greece and the
surrounding regionincluding site-specific considerations
Theodoros M. Tsapanos (1), Päivi Mäntyniemi (2) and Andrzej
Kijko (3)(1) Aristotle University of Thessaloniki, School of
Geology, Geophysical Laboratory, Thessaloniki, Greece
(2) Institute of Seismology, University of Helsinki, Finland(3)
Council for Geoscience, Pretoria, South Africa
AbstractA probabilistic approach was applied to map the seismic
hazard in Greece and the surrounding region. The pro-cedure does
not require any specification of seismic sources or/and seismic
zones and allows for the use of thewhole seismological record,
comprising both historical and instrumental data, available for the
region of inter-est. The new seismic hazard map prepared for Greece
and its vicinity specifies a 10% probability of exceedanceof the
given Peak Ground Acceleration (PGA) values for shallow seismicity
and intermediate soil conditions foran exposure time of 50 years.
When preparing the map, the new PGA attenuation relation given by
Margaris etal. (2001) was employed. The new map shows a spatial
distribution of the seismic hazard that corresponds wellwith the
features of shallow seismicity within the examined region. It
depicts the level of seismic hazard in whichthe exceedance of the
PGA value of 0.25 g may be expected to occur within limited areas.
The highest estimat-ed levels of seismic hazard inside the
territory of Greece are found in the Northern Sporades Islands,
where PGAvalues in excess of 0.50 g are reached at individual
sites, and in the Zante Island in Western Greece, where PGAvalues
in the range of 0.35 g to 0.40 g are obtained at more numerous
localities. High values are also observedin the sea between the
Karpathos and Rhodes islands, near the Island of Amorgos (Cyclades
Archipelago) andin the Southwestern Peloponnesus. The levels of
seismic hazard at the sites of seven Greek cities (Athens,
Jan-nena, Kalamata, Kozani, Larisa, Rhodes and Thessaloniki) were
also estimated in terms of probabilities that agiven PGA value will
be exceeded at least once during a time interval of 1, 50 and 100
years at those sites. Theseprobabilities were based on the maximum
horizontal PGA values obtained by applying the design
earthquakeprocedure, and the respective median values obtained were
0.24 g for Athens, 0.28 g for Jannena, 0.30 g forKalamata, 0.21 g
for Kozani, 0.24 g for Larisa, 0.43 g for Rhodes and 0.35 g for
Thessaloniki. The probabilitiesof exceedance of the estimated
maximum possible PGA value were also calculated for the cities to
illustrate theuncertainty of maximum PGA assessment.
Mailing address: Dr. Päivi Mäntyniemi, Institute ofSeismology,
University of Helsinki, P.O. Box 68, FI-00014,Helsinki, Finland;
e-mail: [email protected]
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Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
the most seismically active regions in theworld. In one ranking
of 50 seismogenic coun-tries worldwide, Greece took the sixth
position(Tsapanos and Burton, 1991).
The objective of seismic hazard assessmentis to obtain long-term
probabilities of the oc-currence of seismic events of a specified
size ina given time interval. A large number of ap-proaches are
currently available for this pur-pose. Abundant literature on the
seismic hazardin Greece therefore exists for a number of dif-ferent
seismic quantities such as the maximumexpected macroseismic
intensity (Shebalin etal., 1976; Papaioannou, 1984; Papoulia
andSlejko, 1997), Peak Ground Acceleration(PGA) or velocity
(Algermissen et al., 1976;Makropoulos and Burton, 1985) and the
dura-tion of strong ground motion (Margaris et al.,1990; Papazachos
et al., 1992). One version ofthe geographic distribution of seismic
hazard inGreece, based on seismic sources, has been pro-vided by
Papazachos et al. (1993). Main andBurton (1989) estimated the
seismicity inGreece by considering two different processes,namely
subduction and stretching. Seismic haz-ard parameters have been
calculated for Greeceusing the maximum likelihood method
(Pa-padopoulos and Kijko, 1991), while Papaioan-nou and Papazachos
(2000) estimated the seis-mic hazard using new seismotectonic
data.
In the present work, the seismic hazard inGreece was assessed in
terms of PGA using theapproach described in Kijko and Graham(1998,
1999) and using the recent Greek atten-uation relationship derived
by Margaris et al.(2001). Seismic hazard analysis was done onthe
basis of the whole seismological recordavailable for Greece,
including historical obser-vations as well as the instrumental data
record-ed during the past decades, covering a period ofabout two
and a half millennia and comprisingsome 600 destructive earthquakes
(Papazachosand Papazachou, 1997). Only the shallow seis-micity was
taken into consideration, becausethe attenuation relationship given
by Margariset al. (2001) was derived for shallow earth-quakes, and
they account for the vast majorityof the observed events. A seismic
hazard mapwas created for the Greek territory. In this map,the
estimated seismic hazard is specified in
terms of the PGA with a 10% probability of ex-ceedance in 50
years, corresponding to a returnperiod of 475 years. In addition, a
detailedanalysis of seismic hazard was carried out forthe Greek
cities of Athens, Jannena, Kalamata,Kozani, Larisa, Rhodes and
Thessaloniki. Thisincluded an assessment of the maximum possi-ble
PGA at the sites of the cities and the calcu-lation of the
probabilities that a given PGA val-ue will be exceeded at least
once during timeintervals of 1, 50 and 100 years at each site.
Themaximum PGA values for the seven sites wereobtained by applying
the design earthquakeprocedure, assuming the occurrence of
thestrongest possible earthquake at the distance of15 km from the
site. The probabilities of ex-ceedance of the maximum possible PGA
valueswere also calculated to illustrate the uncertain-ty of
maximum PGA estimation.
2. The data
The examined region covers the territory ofGreece and its
vicinity, including the Greekmainland, parts of its northern
contiguouscountries, the Aegean Sea and Western Turkey,between
latitudes 33.0°-43.0°N and longitudes19.0°-29.0°E. Figure 1 shows
the epicentres ofshallow main shocks of magnitude M ≥ 5 forthis
region.
Information on the seismicity of Greece ex-ists from the 6th
century B.C. onwards, as de-scriptions of earthquake effects were
made bythe ancient Greeks, Romans and Byzantines.The data used in
the present study were re-trieved from the databank of the
GeophysicalLaboratory of the University of Thessaloniki.This
databank comprises information on a largenumber of Greek
earthquakes since 550 B.C.(Papazachos et al., 2000). As knowledge
of thecompleteness, homogeneity and accuracy ofearthquake data is
necessary for a reliable esti-mation of various seismicity
parameters, theuncertainties and thresholds for complete re-porting
were assessed as follows: the catalogueof shallow seismicity is
complete for the areaunder investigation for magnitudes M ≥
8.0since 550 B.C., for M ≥ 7.3 since 1501 A.D.,M ≥ 6.0 since 1845,
M ≥ 5.0 since 1911, M ≥ 4.5
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
since 1950, M ≥ 4.3 since 1964 and for magni-tudes M ≥ 4.0 since
1981 (Papazachos et al.,2000). All magnitudes are given on a
scaleequivalent to the moment magnitude (for moredetails see:
Papazachos et al., 1997). During theinstrumental measurement era
after 1911, the er-ror for the epicentres is less than 20 km,
whilethe uncertainty of magnitudes is 0.25 magnitudeunits
(Papazachos and Papazachou, 1997). Theerrors affecting the
historical data between 550B.C. and 1910 are of the same order,
mainly be-cause these were strong shocks with substantial
macroseismic information available. Thus, forthe historical
earthquakes the uncertainties forthe epicentre and magnitude are
less than 30 kmand 0.4 magnitude units, respectively (Papaza-chos
and Papazachou, 1997). If the number ofmacroseismic observations
available for an eventis less than ten, the uncertainty of
magnitudemay reach 0.5 magnitude units.
In the present study, foreshocks, aftershocksand earthquake
swarms were omitted from theinitial earthquake catalogue. This
removal wasbased on a relationship derived by Papazachos
Fig. 1. An epicentre map of shallow main shocks of magnitude Mw
≥ 5 for Greece and the surrounding regionbetween 550 B.C. and 1999.
The letters mark the sites of the cities of Athens, Jannena,
Kalamata, Kozani, Lar-isa, Rhodes and Thessaloniki for which
site-specific analyses of seismic hazard were performed.
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Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
and Papazachou (1997), in which the durationof the aftershock
sequence depends on the mag-nitude of the main shock. The remaining
shal-low main events served as the input data for theseismic hazard
analyses.
3. Outline of the methodology
The method used to estimate the level ofseismic hazard in terms
of PGA has been de-scribed in detail in Kijko and Graham
(1998,1999). The first part of their work focuses onthe development
and presentation of statisticaltechniques that can be used for the
evaluation ofthe maximum regional magnitude, mmax. Thesecond part
delineates a methodology for prob-abilistic seismic hazard
assessment at a givensite. In the present study, emphasis is placed
onthe second aspect.
Site-specific analyses of seismic hazard require a knowledge of
the attenuation of theselected ground-motion parameter a,
usuallyPGA, as a function of earthquake magnitudeand distance.
According to the adopted method-ology, the attenuation relationship
of PGA isassumed to be of the type
( ) ( )ln a c c m r1 2 $= + + +z f (3.1)
where c1 and c2 denote empirical coefficients, mis the
earthquake magnitude, φ (r) is a functionof earthquake distance and
ε is a normally dis-tributed random error.
To express seismic hazard in terms of PGA,the aim would be to
calculate the conditionalprobability that an earthquake of random
mag-nitude, occurring at a random distance from thesite, will cause
a PGA value equal to, or greaterthan, the chosen threshold value,
amin, at thesite. We accept the standard assumption (e.g.,Page,
1968) that the random earthquake magni-tude m, in the range of
mmin≤ m ≤ mmax, is dis-tributed according to the doubly
truncatedGutenberg-Richter relation with a CumulativeDistribution
Function (CDF) of
( ) .exp exp
exp expF m
m m
m m
min max
min
M =- - -
- - -
b b
b b
_ _
_ _
i i
i i
(3.2)
In eq. (3.2), mmin is the minimum earthquakemagnitude
corresponding to amin, which is theminimum value of PGA of
engineering interestat the site, mmax is the maximum credible
earth-quake magnitude and β = bln10, where b is theparameter of the
Gutenberg-Richter magnitude-frequency relation. It can be shown
(Kijko andGraham, 1999) that choosing eq. (3.1) as amodel for
attenuation of PGA and eq. (3.2) as adistribution of earthquake
magnitude, is equiv-alent to the assumption that the CDF of the
log-arithm of PGA, x, is of the form
( ) .exp exp
exp expF x
x x
x x
min max
min
X =- - -
- - -
c c
c c
_ _
_ _
i i
i i(3.3)
Above, xmin = ln(amin), xmax = ln(amax), amax is themaximum
possible PGA at the site, γ = β/c2where c2 is the coefficient
related to the attenu-ation formula (3.1) and β is the parameter
relat-ed to the Gutenberg-Richter distribution ofearthquake
magnitude (3.2). One should notethat CDF (3.3) was derived under
the conditionthat the spatial and magnitude distributions
aremutually independent. In practice, the assump-tions of
independence mean that within the areasurrounding the specified
site, the parametersof earthquake magnitude distribution mmin,
mmaxand β are the same.
It can be seen from formula (3.3) that thelogarithm of the PGA
at a given site follows thesame type of distribution as the
earthquake mag-nitude, i.e. doubly truncated negative exponen-tial
– the form of the Gutenberg-Richter distribu-tion in eq. (3.2). The
two distributions differ on-ly in the value of their parameters. If
the param-eter of the magnitude distribution is equal to β,the
parameter of the distribution of x = ln(PGA)is equal to β/c2.
From an engineering point of view, the largestPGA expected at
least once at a given site duringa specified time interval, t, is
of special interest.The CDF of the logarithm of the largest PGA
val-ue, x, to be observed at least once at the site dur-ing a
specified time interval t, can be written as
x( )( )
exp
exp expF
t
t F x t
1
1max
X
X
=- -
- - - -
m
m mt
]
]
g
g6 @# -
(3.4)
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
where λ is the site-specific activity rate ofearthquakes that
cause a PGA value, a, at thesite, exceeding the threshold value
amin. Clearly,this CDF of the largest PGA values is
doublytruncated: from below, by xmin = ln(amin), andfrom above, by
xmax = ln(amax). The distributionin eq. (3.4) was derived under the
assumptionthat the earthquakes that cause a PGA value a,a ≥ amin,
at the site are distributed according tothe Gutenberg-Richter
relation (3.2) and, intime, follow the Poisson process with mean
ac-tivity rate λ(x) = λ [1−FX(x)], where x = ln(a).
The maximum likelihood method is used toestimate the
site-characteristic seismic hazardparameters λ and γ. If a1,…, an
are the largestPGA values recorded at the site during n suc-cessive
time intervals t1,…, tn, the likelihoodfunction of the sample x1,…,
xn, where xi = ln(ai)and i = 1,…, n, for a specified value amax can
bewritten as
1
x, (L f maxX i in
=m c=
)ti
%_ i (3.5)
where fXmax(xiti) is the probability density func-
tion of the logarithm of the largest PGA valueobserved at a
given site during a given time in-terval t. By definition, the
probability densityfunction x x( (f xdmax maxX i i X i i=t t) )Fd .
For a giv-en value of xmax (or equivalently, the maximumpossible
PGA at the site), maximisation of thelikelihood function (3.5)
leads to the determi-nation of the parameters λ and γ. However,
thisprocedure for the estimation of the hazard pa-rameters is used
only when the b-value (orequivalently β) of the Gutenberg-Richter
fre-quency-magnitude relationship is not known.When the b-value is
known, parameter γ is cal-culated as β/c2 and the maximum
likelihoodsearch (3.5) reduces to the estimation of
thesite-specific, mean seismic activity rate λ. In or-der to create
seismic hazard maps, the proce-dure is repeatedly applied to grid
points cover-ing the area of interest.
4. Attenuation relationship
A new attenuation relationship has recentlybeen proposed for
Greece and the surrounding
region by Margaris et al. (2001). They used adata set consisting
of records of 142 mainlynormal faulting earthquakes with magnitudes
inthe range of 4.5< Mw < 7.0 and epicentral dis-tances
between 1 and 150 km to derive the at-tenuation relationship. A
total of 744 records ofhorizontal components and 338 records of
ver-tical components of the peak ground accelera-tion, velocity and
displacement were available.They took into consideration only the
shallowearthquakes within the examined region. Theyconsidered soil
classification according to theNEHRP (1994) recommendations and
suggest-ed empirical predictive relations for the hori-zontal
components of strong ground motion.
The new attenuation relation for PGA takesthe following
form:
h
( )
.
ln
ln
c c M c
D c c
PGA
S/
w1 2 3
2
0
2 1 2
4 5
$ $
$ $ !
= + +
+ +^ h(4.1)
Above, D is epicentral distance (in km) and h0== 7 km. The
parameters of the model are c1 == −3.36, c2 = 0.70, c3 = −1.14 and
c4 = 0.12 andthe standard deviation of ln(PGA) is c5 = 0.70.With
these values, the attenuation relationshipof eq. (4.1) provides
acceleration values in unitsof g. The constant c3 in front of the
term ln(D2++h0
2)1/2 includes both anelastic and geometricattenuation. The
letter S describes soil classifi-cation by taking values 0, 1 or 2,
correspondingto rock (hard), intermediate and alluvium
(soft)conditions, respectively.
Earlier attenuation relationships for theGreek territory were
proposed by Makropoulosand Burton (1985), following
Makropoulos(1978), and also by Papaioannou (1984). Therelationships
derived by Theodoulidis (1991)were regarded as the official
attenuation rela-tionship for Greece during the 1990s. In orderto
obtain more insight into the most recent at-tenuation relation
given by Margaris et al.(2001), it was compared with the previous
rela-tionships. Figure 2a-c shows the accelerationvalues as a
function of distance computed ac-cording to the four different
relationships forthree different magnitudes assuming intermedi-ate
soil conditions. It can be seen that the newattenuation
relationship gives lower values than
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Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
the other relationships especially at near dis-tances. This is
most pronounced in the case ofMw = 7.0, when the acceleration
values based onthe most recent relation are the lowest up to
adistance of 130 km. For magnitudes Mw = 5.0and Mw = 6.0, the
corresponding distances are80 km and 100 km, respectively. For
magni-tudes Mw = 6.0 and Mw = 7.0, only the relation-ship proposed
by Papaioannou (1984) giveslower acceleration values for large
distancesthan the most recent relationship, whereas formagnitude Mw
= 5.0 also the acceleration valuesfrom the relationship of
Theodoulidis (1991)are lower for distances in excess of 80 km.
Acomparison between the attenuation relation-ships of Theodoulidis
(1991) and of Margaris etal. (2001) shows that the difference
betweencomputed acceleration values increases with in-creasing
magnitude. For example, for a distanceof 20 km, assuming
intermediate soil condi-tions and a magnitude Mw = 5.0, the
differencebetween median PGA values is only 0.0042 g;for Mw = 6.0
it is 0.042 g and for Mw = 7.0 it is0.18 g, the higher values
resulting from the ear-lier attenuation relationship.
The recent Greek attenuation relationship,eq. (4.1), was also
compared with that derivedfor the European territory by Ambraseys
et al.(1996). Since this relation only provides accel-eration for
magnitudes expressed on the Msscale, the version given by Wahlström
andGrünthal (2001), with Ms converted to Mw, wasused. The PGA
values were computed as afunction of distance for the magnitude Mw
= 6.0and a focal depth of 7 km assuming hard rockconditions (fig.
3). It can be seen that the medi-an values of acceleration
(corresponding top = 0) given by the Greek relation are
consider-ably lower than the values computed accordingto the
Ambraseys et al. (1996) relationship for
a Fig. 2a-c. A comparison of Peak Ground Accelera-tion (PGA)
values as a function of distance for mag-nitudes (a) Mw = 5.0, b)
Mw = 6.0 and (c) Mw = 7.0computed for intermediate soil conditions
accordingto the new Greek attenuation relationship given byMargaris
et al. (2001) and the relationships proposedearlier for Greece by
Makropoulos (1978), Papaioan-nou (1984) and Theodoulidis
(1991).
b
c
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
distances in excess of 5 km. The Greek valuescorresponding to
the 84th percentile level(p = 1), computed by taking into account
thestandard deviation of the normal distribution ofln(PGA), are
lower than those computed withAmbraseys et al. (1996) relationship
for dis-tances larger than about 20 km. This behaviourwas also
observed for magnitudes Mw = 5.0 andMw = 7.0.
5. Results and discussion
5.1. Seismic hazard map
Figure 4 shows the new seismic hazard mapfor Greece, computed
according to the above
procedure and the recently developed attenua-tion relation
(4.1). The seismic hazard is ex-pressed in terms of PGA values with
a 10%probability of exceedance at least once in 50years,
corresponding to a return period of 475years. The map was prepared
for shallow seis-micity, assuming intermediate soil conditionsand
using a grid point mesh of 0.20°. The b pa-rameter was assumed to
take the constant valueb = 1 over the whole region of interest
(see, e.g.,Papazachos and Papazachou, 1997).
The overall spatial distribution of seismichazard as depicted in
the map corresponds wellwith the main features of tectonics and
shallowseismicity in the examined region. A belt of in-creased
seismic hazard runs along the HellenicTrench, the Island of Crete
and in WesternGreece, continuing north into Albania, whereasits
eastern part joins the west coast of Turkey.This zone is related to
the active convergentboundary between the Aegean and the
Africanplates in the south and to the continental colli-sion
between Apulia and the Aegean in the west.The highest estimated
levels of seismic hazard inthis zone are located in the Zante
Island in West-ern Greece. High values are also observed in thesea
between the Karpathos and Rhodes islands,near the Island of Amorgos
(Cyclades Archipel-ago) and in the Southwestern Peloponnesus.
Thehighest estimated levels of seismic hazard insidethe territory
of Greece are found in the NorthernSporades Islands, which
corresponds to thewestern termination of the North AnatolianFault.
Intermediate values dominate in the Gulfof Corinth, in the Athos
Peninsula (Chalkidiki-Northern Greece), Thessaloniki and in the
Ion-ian Islands. Northeastern Greece, the Sea ofCrete and
Northwestern Greece appear as areasof the lowest seismic hazard.
Most of the zone ofincreased seismic hazard corresponding to
theHellenic Arc and Trench, as well as the NorthAegean and Western
Turkey, shows PGA valuesin the range of 0.20 g to 0.25 g, while
values inexcess of 0.25 g are reached within limited ar-eas. The
maximum computed PGA values insidethe territory of Greece are in
excess of 0.50 gand can be found at individual sites in the
North-ern Sporades Islands, whereas PGA values in therange of 0.35
g to 0.40 g are obtained at morenumerous localities in the Ionian
Islands.
Fig. 3. A comparison of Peak Ground Acceleration(PGA) values as
a function of distance for the mag-nitude Mw = 6.0 and a focal
depth of 7 km computedaccording to the new Greek attenuation
relationshipgiven by Margaris et al. (2001) and that derived forthe
European territory by Ambraseys et al. (1996).The displayed PGA
values were computed for hardrock conditions. Notation p= 0 stands
for medianPGA values, whereas in the case of p= 1 the PGAvalues
were computed by taking into account thestandard deviation of the
normal distribution ofln(PGA), corresponding to the 84th percentile
levelof non-exceedance.
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Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
Makropoulos and Burton (1985) presentedseismic hazard maps for
Greece in terms ofPGA. They applied extreme value analyses
andemployed an attenuation relationship based onthe averaging of
eight formulae reported inworldwide studies. The spatial
distribution ofseismic hazard depicted in their maps is
rathersimilar to that shown in fig. 4. Makropoulos andBurton (1985)
obtained a high PGA hazardaround Cephalonia and the Levkas islands
aswell as around Lesvos and the Northeastern Spo-rades Islands.
They reported a 30% probabilityof the PGA exceeding 0.2 g in 50
years withinthese areas.
5.2. Sites of the cities
Figure 5a-g shows the probabilities of thegiven PGA values being
exceeded within timeintervals of 1, 50 and 100 years at the
sitesAthens, Jannena, Kalamata, Kozani, Larisa,
Fig. 4. A new seismic hazard map for Greece based on the
attenuation relationship of Margaris et al. (2001).The hazard is
expressed in terms of Peak Ground Accelaration (PGA) values for
intermediate soil conditionswith a 10% probability of exceedance at
least once in 50 years.
Fig. 5a. Probabilities that the given Peak Ground Ac-celeration
(PGA) values will be exceeded in 1, 50 and100 years at the Athens
site. The maximum PGA val-ues are the median values obtained using
the designearthquake procedure.
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
b
c
d
e
Fig. 5b-g. Probabilities that the given Peak Ground Acceleration
(PGA) values will be exceeded in 1, 50 and100 years at the sites:
b) Jannena, c) Kalamata, d) Kozani, e) Larisa, f) Rhodes and g)
Thessaloniki. The maxi-mum PGA values are the median values
obtained for each site using the design earthquake procedure.
f
g
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Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
Rhodes and Thessaloniki. These probabilitycurves are based on
the estimated values for γ,λ and amax in formulae (3.3) and (3.4).
The val-ues for the maximum possible PGA, amax, wereobtained using
the attenuation relation (4.1) andassuming the occurrence of the
strongest possi-ble earthquake, m̂max, at a distance of 15 kmfrom
the site. The maximum credible magni-tudes, m̂max, were computed
using data for a ra-dius of 180 km from each city centre and the
K-S-B procedure (see Kijko and Graham, 1998:eq. 54). The maximum
(median) PGA values,amax, were 0.24 g for Athens, 0.28 g for
Jan-nena, 0.30 g for Kalamata, 0.21 g for Kozani,0.24 g for Larisa,
0.43 g for Rhodes and 0.35 gfor Thessaloniki. The probability plots
forAthens and Thessaloniki can also be found inMäntyniemi et al.
(2004).
Figure 6 illustrates the uncertainty of maxi-mum PGA assessment
for the cities of Athens,Jannena, Kalamata, Kozani, Larisa, Rhodes
andThessaloniki. It gives the probabilities that an
earthquake of maximum credible magnitude, asestimated for each
area, will occur at a hypo-central distance of 15 (± 5) km from the
respec-tive site and produce a PGA value exceedingthe estimated
maximum value. In the calcula-tions it was assumed that the
standard deviationof estimated maximum values of the PGA is
af-fected by three factors: 1) the uncertainty in de-termination of
the earthquake epicentral dis-tance; 2) the maximum earthquake
magnitudemmax and 3) the uncertainty of attenuation rela-tionship.
This kind of approach can be regardedas a probabilistic extension
of the deterministic,design-earthquake scenario. The
probabilitiesobtained for the exceedance of a high PGA val-ue, for
example 0.70 g, range between 36% forRhodes and 27% for Kozani. The
probabilitiescomputed for Athens and Larisa overlap, andalso the
assessed maximum PGA values forthese sites are the same.
The computed exceedance probabilities ofthe given acceleration
values at the site of eachcity for an exposure time of 100 years
are com-pared in fig. 7. Rhodes stands out from the oth-er cities
with the highest expected PGA ex-
Fig. 6. The probabilities that an earthquake of max-imum
magnitude, as estimated for each area, will oc-cur at a hypocentral
distance of 15 (±5) km from therespective site and produce a Peak
Ground Accelera-tion (PGA) value exceeding the estimated
maximumvalue at the sites Athens, Jannena, Kalamata, Kozani,Larisa,
Rhodes and Thessaloniki. The probabilitiescomputed for Athens and
Larisa overlap.
Fig. 7. The probabilities of exceedance of the givenPeak Ground
Acceleration (PGA) values at the sitesAthens, Jannena, Kalamata,
Kozani, Larisa, Rhodesand Thessaloniki during 100 years. The
probabilitycurves for Athens and Kalamata overlap.
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
ceedance probabilities. The city of Jannena dis-plays the second
highest long-term probabili-ties, but they are much lower than
those forRhodes. Despite its proximity to the HellenicArc and
Trench, Kalamata does not rank veryhigh in the comparison. The
computed proba-bilities for this site are practically identical
tothose for Athens, whereas higher values wereobtained for the
northern cities of Kozani andThessaloniki. This is in contrast with
expecta-tions, as Kozani is situated in northwesternGreece, a
region of negligible historical seis-micity (Papazachos and
Papazachou, 1997).The instrumentally recorded seismicity thereseems
to be mainly related to the filling of theartificial Lake Polyfytos
(Papazachos et al.,1995). The area has been assumed to be of
verylow seismic hazard, and therefore the destruc-tive Kozani
earthquake (Ms= 6.6) of 13 May1995 was quite unexpected (Papazachos
et al.,1995). Northwestern Greece appears as an areaof low seismic
hazard also in fig. 4.
The seismicity features in areas adjacent toThessaloniki seem to
be characterised by earth-quake sequences of long duration,
extending tothe northern contiguous countries (Papazachoset al.,
1979). The city of Larisa has the lowestprobabilities for the
exceedance of the givenPGA values. The probabilities computed
forthis site for exposure times of 1 and 100 yearsdo not deviate
very much from each other (fig.5d), which can be attributed to the
small activi-ty parameter λ S for this site. The
computedprobabilities decrease rather quickly with in-creasing PGA
values for all sites. For example,for the sites of Rhodes and
Jannena, the PGAvalues with a 50% chance of exceedance with-in 100
years are 0.058 g and 0.040 g, respec-tively (fig. 7).
Previous studies have also provided someestimates of seismic
hazard in terms of themaximum expected PGA at the sites of
thecities investigated in the present work.Makropoulos and Burton
(1985) estimatedground-motion for the cities of Athens,
Thessa-loniki and Rhodes, among others. They appliedextreme value
analysis to compute ground ac-celeration values and to produce
seismic hazardparameters. The acceleration values they re-ported
that had a 70% probability of non-ex-
ceedance for 50 and 100 years were, respec-tively, 0.09 g and
0.11 g for Athens, 0.15 g and0.17 g for Thessaloniki and 0.075 g
and 0.08 gfor Rhodes. Thus, for the city of Rhodes theirestimated
PGA values are the lowest, althoughin terms of earthquake size the
area surroundingthis city has a high seismic potential. In the
de-terministic approach used to compute the amaxvalues of the PGA
shown in fig. 5a-g, the max-imum credible magnitude plays an
importantrole, thus the highest amax value is obtained forthe site
of Rhodes. Also the highest long-termprobabilities for the
exceedance of the givenPGA values were obtained for this site (fig.
7).
Lyubushin et al. (2002) evaluated the levelof seismic hazard in
terms of maximum valuesof PGA for six sites in Greece, including
Kala-mata and Kozani, using a Bayesian procedure.They estimated a
maximum PGA value of 0.36g for Kalamata and 0.38 g for Kozani. The
at-tenuation relationship of Theodoulidis (1991)was employed to
derive these estimates. Themaximum PGA values computed by
Lyubushinet al. (2002) are higher than those estimated inthe
present study, which probably largely fol-lows from the use of
different attenuation rela-tionships. As discussed in Section 4,
the mostrecent attenuation relationship elucidated byMargaris et
al. (2001) gives faster attenuationthan do the previous Greek
relations up to dis-tances between 80-130 km depending on
themagnitude.
Directly measured ground motion valuesbecame available following
the magnitude Mw== 6.0 event of 1986 near Kalamata
(Anagnos-topoulos et al., 1987) and the magnitude Mw== 6.6 event of
1995 near Kozani (Theodoulidisand Lekidis, 1996). The observed
value was0.27 g for an epicentral distance of 12 km fromKalamata.
The largest aftershock of the Kala-mata earthquake was of magnitude
Mw = 5.4 andthe respective recorded PGA value about 0.05 gat an
epicentral distance of approximately 1 km(Anagnostopoulos et al.,
1987). In the presentstudy, the design earthquake procedure
yieldeda value of maximum PGA equal to 0.30 g,based on the maximum
credible magnitude of7.50 (± 0.25). This maximum magnitude ex-ceeds
the range of observed magnitudes avail-able in the derivation of
the PGA attenuation re-
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1686
Theodoros M. Tsapanos, Päivi Mäntyniemi and Andrzej Kijko
lationship of Margaris et al. (2001), and there-fore the
computed attenuation can be consid-ered more uncertain than that
for lower magni-tudes. As displayed in fig. 6, the estimated
max-imum PGA values can be exceeded with thegiven probabilities.
Also, the observed PGAvalue may have been affected by site effects,
asKalamata is built on the natural deposits of adried up river
(Anagnostopoulos et al., 1987).The measured ground motion value
followingthe Kozani earthquake was 0.21 g for a distanceof 19 km
from the city (Theodoulidis andLekidis, 1996), which equals the
present valuerelying on a maximum credible magnitude of7.0 (±
0.25). Both the observed values for theKalamata and Kozani main
shocks are lowerthan the maximum PGA values estimated byLyubushin
et al. (2002).
6. Conclusions
In the present study, seismic hazard mapsand site-specific
assessments of seismic hazardwere prepared for the territory of
Greece in-cluding its vicinity and the sites of seven Greekcities
(Athens, Jannena, Kalamata, Kozani, Lar-isa, Rhodes and
Thessaloniki) using themethodology developed by Kijko and
Graham(1998, 1999) and the recent PGA attenuationrelationship
derived for Greece by Margaris etal. (2001). The applied technique
has been es-pecially developed for probabilistic seismichazard
assessment at a specified site, and itdoes not rely on the
definition of seismicsources and/or seismic zones. Only the
shallowearthquakes were taken into consideration, be-cause the
attenuation relationship given byMargaris et al. (2001) was derived
using them,and they account for the vast majority of the ob-served
events. A seismic hazard map was creat-ed by applying the procedure
repeatedly to gridpoints covering the area of interest. In the
newmap, the estimated seismic hazard is specifiedin terms of the
horizontal PGA with intermedi-ate soil conditions with a 10%
probability ofexceedance in 50 years. It depicts a spatial
dis-tribution of the seismic hazard that correspondswell with the
features of shallow seismicitywithin the examined region. It shows
a level of
seismic hazard in which the exceedance of aPGA value of 0.25 g
may be expected to occurwithin limited areas. The highest estimated
lev-els of seismic hazard inside the territory ofGreece are found
in the Northern Sporades Is-lands, where PGA values in excess of
0.50 g arereached at individual sites, and around theZante Island
in Western Greece, where PGAvalues in the range of 0.35 g to 0.40 g
are ob-tained at more numerous localities. High valuesare also
observed in the sea between theKarpathos and Rhodes islands, near
the Islandof Amorgos (Cyclades Archipelago) and in theSouthwestern
Peloponnesus. Intermediate val-ues dominate in the Gulf of Corinth,
in theAthos Peninsula (Chalkidiki-Northern Greece),Thessaloniki and
in the Ionian Islands. North-eastern Greece, the Sea of Crete and
North-western Greece appear as areas of the lowestseismic hazard.
The levels of seismic hazard atthe sites of the seven Greek cities
were assessedin terms of probabilities that a given PGA valuewill
be exceeded at least once in 1, 50 and 100years at the sites of the
cities. The maximum(median) PGA values obtained by applying
thedesign earthquake approach were 0.24 g forAthens, 0.28 g for
Jannena, 0.30 g for Kalama-ta, 0.21 g for Kozani, 0.24 g for
Larisa, 0.43 gfor Rhodes and 0.35 g for Thessaloniki.
Theprobabilities of exceedance of the estimatedmaximum possible PGA
values were also cal-culated for the cities to illustrate the
uncertain-ty involved in this kind of assessment. Some ofthe
directly measured PGA values found in theliterature exceed those
obtained through the de-sign earthquake procedure, although the
respec-tive observed earthquakes were only of moder-ate magnitude.
This discrepancy can probablyin part be attributed to site effects.
Also, the re-cent Greek attenuation relationship employedin this
study predicts rather rapid decrease inmedian PGA values as a
function of distance incomparison with many previously published
at-tenuation formulae. This applies especially tolarge earthquake
magnitudes. The maximumcredible magnitudes used in the design
earth-quake procedure also exceed the range of ob-served magnitudes
available for its derivation,which adds to the uncertainty of the
computedacceleration values.
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A probabilistic seismic hazard assessment for Greece and the
surrounding region including site-specific considerations
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(received July 16, 2002;accepted July 20, 2004)