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Tectonophysics 371 (2003) 175–186
Perceptible earthquakes in the broad Aegean area
G.Ch. Koravosa, I.G. Mainb, T.M. Tsapanosa,*, R.M.W. Mussonc
aGeophysical Laboratory, School of Geology, Aristotle University of Thessaloniki, 54006 Thessaloniki, GreecebDepartment of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
cBritish Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
Received 22 October 2002; accepted 16 May 2003
Abstract
A probabilistic estimate of seismic hazard can be obtained from the spatial distribution, of earthquake sources, their
frequency–magnitude distribution and the rate of attenuation of strong ground motion with distance. We calculate the
earthquake perceptibility, i.e. the annual probability that a particular level of ground shaking will be generated by earthquakes of
particular magnitude, by weighting frequency–magnitude data with the predicted felt area for a given level of ground shaking at
a particular magnitude. This provides an earthquake selection criterion that can be used in the anti-seismic design of non-critical
structures. We calculate the perceptibility, at a particular value of isoseismal intensity, peak ground acceleration and velocity, as
a function of source magnitude and frequency for the broad Aegean area using local attenuation laws. We use frequency–
magnitude distributions that were previously obtained by combining short-term catalogue data with tectonic moment rate data
for 14 tectonic zones in Greece with sufficient earthquake data, and where contemporary strain rates are available from satellite
data. Many of the zones show a ‘characteristic earthquake’ distribution with the most perceptible earthquake equal to the
maximum magnitude earthquake, but a relatively flat perceptibility between magnitudes 6 and 7. The maximum perceptible
magnitude is in the fastest-deforming region in the middle of the Aegean sea, and tends to be systematically low on the west in
comparison to the east of the Aegean sea. The tectonic data strongly constrain the long-term recurrence rates and lead to low
error estimates (F 0.2) in the most perceptible magnitudes.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Perceptibility; Ground motion; Seismic hazard; Aegean area
1. Introduction
Different physical, numerical, and statistical mod-
els have been applied over the last decade in order to
define the seismotectonic environment and the future
behaviour of a region (e.g. Ito and Matsuzaki, 1990;
Cowie et al., 1993; Lomnitz-Adler, 1993; Rundle and
0040-1951/03/$ - see front matter D 2003 Elsevier B.V. All rights reserve
doi:10.1016/S0040-1951(03)00216-6
* Corresponding author.
E-mail address: [email protected] (T.M. Tsapanos).
Klein, 1993; Kagan and Jackson, 1994; Main, 1995).
These models have various proportions of determin-
istic and probabilistic elements, and are capable, in
principle of validation in particular regions once
sufficient data become available. All of these models
predict that the frequency–magnitude distribution at
low magnitudes takes the form of the Gutenberg–
Richter law
logN ¼ a� bm; ð1Þ
d.
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186176
where a and b are model parameters and N is the
cumulative number of earthquakes with magnitude
greater than or equal to m. This relation defines the
annual probability of occurrence of a given earth-
quake magnitude in a given region, and is a strong
primary constraint on seismic hazard. However, the
distribution at high magnitudes is less certain due to
the small number of data points for the rare, large
events.
A variety of different approaches have been adop-
ted in an attempt to overcome this problem of uncer-
tainty in recurrence rates for large events in the
calculation of seismic hazard (Burton et al., 1984;
Makropoulos and Burton, 1985; Goes and Ward,
1994; Lutz and Kiremidjian, 1995; Shimazaki et al.,
1999; Papazachos, 1999; Papaioannou and Papaza-
chos, 2000). In particular, geological and geodetic
data on deformation rates can be used to place strong
constraints on the long-term recurrence. For example,
Goes and Ward (1994) applied a seismicity model
based on the concept of fault segmentation and the
physics of static dislocation, that allows for stress
transfer between fault segments. The application of
their model allows to obtain recurrence statistics and
long-term probability estimates for shocks with
Mz 6.0 on the San Andreas Fault. In this approach,
constraints are provided by geological and seismolog-
ical observations of segment lengths, characteristic
magnitudes and long term recurrence rates. Interaction
between faults is also a theme in the stochastic
earthquake occurrence model (Lutz and Kiremidjian,
1995). This process can be used to estimate the hazard
due to large, spatially and temporally, dependent
earthquakes. A similar stochastic method known as
‘‘renewal process’’ is described by Shimazaki et al.
(1999) and applied to the recurrence of large earth-
quakes in Japan.
Estimates of seismic hazard depend not only on
the recurrence rates of events of different magnitudes,
but also on the attenuation of seismic energy with
distance (due to a combination of elastic geometric
spreading and anelastic absorption). Burton et al.
(1984) combined earthquake magnitudes obtained
through Gumbel’s third asymptotic distribution of
extreme values with regional intensity attenuation
laws to determine the relative ‘perceptibility’ of
earthquakes of different magnitude at a given level
of ground shaking. The earthquake perceptibility
P(x |m) is defined as the conditional probability that
a site perceives ground shaking at least of level x
(where x may be intensity, peak ground acceleration,
velocity, etc.) given the annual probability of occur-
rence of an earthquake of magnitude m (Burton,
1978). Hence the most perceptible earthquake is the
event that is most likely to occur and be perceived or
felt at a given level of ground motion at a site or in
an area.
The concept of the ‘‘most perceptible earthquake’’
essentially involves a trade-off between small earth-
quakes, which are common but not widely felt, and
large earthquakes, which are uncommon but felt
widely. For any site, there will be a magnitude that
represents a maximum, determined by the variables
governing frequency and impact.
The concept is analogous to that of ‘‘design
earthquake’’ in hazard analysis. In a very recent
study Tsapanos (2003) applied the ‘‘design earth-
quake’’ concept and estimated the ‘‘design PGA’’
values for a seismic hazard scenario in the main
cities of the island of Crete. A given level of ground
motion hazard is associated with a specific probabil-
ity (the typical output of a hazard study for design
purposes), so it is reasonable to ask which earth-
quake parameters are most likely to produce this
ground motion. Is the hazard mostly from small local
earthquakes or large distant ones? The answer will be
different for different sites. This problem of disag-
gregation of hazard can be addressed analytically
(McGuire, 1995) or through simulation (Musson,
1999).
Makropoulos and Burton (1985) used the notion of
perceptibility to assess seismic hazard in Greece using
attenuation laws for ground acceleration. Most recent-
ly Papaioannou and Papazachos (2000) estimated
time-dependent hazard based on the whole earthquake
catalogue (not just the extreme values). They calcu-
lated the conditional probability of occurrence for
macroseismic intensity for 144 Greek cities, based
on a method that assumes fluctuations in b to be much
lower than those in a in Eq. (1) above (Papazachos,
1999). This is based on the hypothesis that b values
depend on material properties and the seismotectonic
setting and therefore should vary relatively smoothly
in space.
There have been many studies of seismic hazard in
Greece, due both to the availability of different types
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186 177
of data and practical concerns for the population. On a
global scale, Greece is ranked sixth in terms of
seismic hazard (Tsapanos and Burton, 1991). The
country has often experienced strong and catastrophic
earthquakes, with significant casualties and damage to
buildings infrastructure, e.g. the recent Athens earth-
quake (m = 5.9, 7 September 1999). The geographical
distribution of seismic hazard, based on such seismic
sources, has been determined by Papazachos et al.
(1993), and more recently quantified as a function of
various design parameters. These include: the maxi-
mum expected macroseismic intensity (Shebalin et al.,
1976; Papaioannou, 1984), the peak ground acceler-
ation or velocity (Algermissen et al., 1976; Makro-
poulos and Burton, 1985), and the duration of strong
ground motion (Margaris et al., 1990; Papazachos et
al., 1992).
In this paper we calculate the perceptibility of
ground motion (intensity, peak ground acceleration
and velocity) for the Aegean area. We use frequency–
magnitude relations from a recently-published data set
of historical and instrumentally recorded earthquakes,
constrained by recent satellite data on deformation
rates in Greece and the surrounding areas (Holt et al.,
2000; Koravos et al., 2003). We allow the distribution
to deviate from Eq. (1) at high magnitudes, and
calculate the annual probability of ground shaking
at a particular level as a function of earthquake
magnitude, based on appropriate ground attenuation
laws for intensity acceleration and velocity. The
results follow from a previous paper (Koravos et al.,
2003) where we used a new catalogue for earthquake
recurrence in Greece and the surrounding area to
determine the best-fitting frequency–magnitude rela-
tions some of the 16 tectonic zones defined by a
geodetic study of crustal deformation rates (Holt et
al., 2000). These authors considered two zones to
have insufficient data for the comparison of seismic
and tectonic information rate (zones 15 and 16), and
Koravos et al. (2003) found a further two (zones 1
and 2) insufficient for analysis of maximum magni-
tude. The deformation rates were used to constrain
extrapolations from the frequency–magnitude rela-
tion, and to estimate the maximum credible magni-
tude, a crucial parameter of interest to earthquake
design engineers. Here we combine these frequency–
magnitude distributions with local attenuation laws to
predict the distribution of perceptibility of ground
motion as a function of magnitude for the same
tectonic zones.
2. Data used and method applied
Papazachos et al. (2000) compiled a comprehen-
sive catalogue of instrumental and historical earth-
quakes in Greece and the surrounding area for the
time span 550 BC–1999 AD. The magnitudes are
quantified in terms of the seismic moment magnitude
scale which we should denote m. The completeness of
the catalogue and the corresponding errors in magni-
tude, depth and epicenter estimates are given by
Papazachos et al. (2000). The published catalogue is
available at (http://www.jjlahr.com/iaspei/europe/
greece/the). In a later study (Koravos et al., 2003), it
was shown that some of the large events included in
Papazachos et al. (2000) catalogue have systematical-
ly higher values. Because they were calibrated using
the events at the beginning of the 20th century (1900–
1912 or so) which were not corrected for the effect of
using undamped narrow-band seismic instruments
(Abe and Noguchi, 1983). For example, we corrected
the event of 1904 (41.80N 23.00E) that occurred in
zone 14 from ms = 7.7 to ms = 7.1, as well as the event
of 1905 (040.26N 24.33E) generated in zone 10 from
ms = 7.5 to ms = 6.8, based on the instrumental deter-
mination of Abe and Noguchi (1983) and Ambraseys
(2001). Since these events are amongst the largest
recorded events in the catalogue, they also have a
disproportionate effect on the calibration of the his-
torical catalogue for the largest large events. This
systematic effect is on average of the order of � 0.4
(Koravos et al., 2003) and this was then applied to the
historical data. We also added one additional year of
data (for the year 2000) from the preliminary annual
bulletins of the seismological station of the Aristotle
University of Thessaloniki. The data were divided
geographically into the 14 tectonic zones considered
to have adequate data by Holt et al. (2000), and used
to estimate the range of complete reporting for differ-
ent magnitude thresholds, the frequency–magnitude
relation and maximum magnitudes, constrained by the
tectonic deformation rates (Koravos et al., 2003). Fig.
1 depicts the examined zones and shows the spatial
distribution of the epicenters in the study area for the
time period 550 BC–2000 AD. For consistency the
Fig. 1. Map of the area of study, divided into 16 numbered regions after Holt et al. (2000), showing the spatial distribution of the epicentres of
shallow seismicity (hV 50 km) in the Aegean and the surroundings for 550 BC–2000 AD. The data are from Papazachos et al. (2000).
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186178
numbers of the tectonic zones correspond to those in
Holt et al. (2000).
Here we calculate the perceptibility based on the
felt area at a given level of seismic intensity I
(modified Mercalli scale), ground motion acceleration
a in cm/s2 and velocity v in cm/s. The attenuation
formula for I as a function of hypocentral distance R
for the broad Aegean area is:
I ¼ 1:063þ 1:5222Ms � 1:1021lnR� 0:0043R ð2Þ
where Ms is the surface magnitude (Musson, 2000).
The corresponding attenuation law of Eq. (2) in terms
of epicentral distance D in km (for D>h) is:
I ¼ 1:063þ 1:5222Ms � 1:1021ln� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
D2 þ h2p �
� 0:0043� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
D2 þ h2p �
ð3Þ
where h is the depth in km.
The attenuation relationship for horizontal peak
ground acceleration, ag (in cm/s2) and velocity, vg(in cm/s), in the broad Aegean is:
lnag ¼ 4:37þ 1:02Ms � 1:65lnðD þ 15Þ þ 0:31S
þ 0:66P* ð4Þ
lnvg ¼ �0:18þ 1:29Ms � 1:62lnðD þ 10Þ þ 0:22S
þ 0:73P* ð5Þ
(Theodulidis and Papazachos, 1992), where Ms is the
surface magnitude, D is the epicentral distance in km,
the site effect S takes the value of zero at ‘‘alluvium’’
sites and one at ‘‘rock’’ sites. The parameter P*
accounts for the scatter in the data about the best
fitting line. P* = 0 for 50 percentile values and = 1 for
84 percentile values. The attenuation law of Eqs. (3),
(4) and (5), is shown in Fig. 2A, B and C, respectively
for Ms = 6.0, h = 10 km, S= 0.5 and P* = 0.
Fig. 2. Ground motion attenuation with distance for (A) modified
Mercalli intensity, (B) peak horizontal ground acceleration and (C)
peak horizontal ground velocity, for m= 6.0 and h= 10 km.
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186 179
The surface-wave magnitude Ms is related to the
moment magnitude m by
m ¼ 0:56Ms þ 2:66 : 4:2VMsV6:0
m ¼ Ms : 6:0VMsV8:0ð6Þ
(Papazachos et al., 1997). The felt area of an event of
magnitude m producing an intensity of at least I at
distance r from the epicentre is:
AðIÞ ¼ p½rðIÞ�2 ð7Þ
The annual perceptibility of ground motion at a level I
(a or v) is then
PðI=mÞ ¼ FðmÞAðIÞ=Amax ð8Þ
where F is the incremental annual frequency of
occurrence of a magnitude in the range mF dm/2,and Amax is the total area of study.
3. Results
The perceptibility for the 14 zones is shown in Fig.
3 for I=VIII and a typical focal depth h = 10 km. The
solid lines represent the functional form for Eq. (8),
given the frequency distributions of Koravos et al.
(2003) and the attenuation laws described above,
plotted against the data. The finite focal depth implies
that there is a minimum threshold magnitude at the
surface that can produce ground shaking at a given
intensity (Main, 1995). In Fig. 3, this threshold
magnitude for perceptibility of I=VIII is above mag-
nitude 6.4. For the first two zones (zones 1 and 2) of
the examined area, the absence of data above the
threshold magnitude of 6.4 for Greece (Koravos et al.,
2003) made it impossible to estimate the maximum
credible magnitude. These regions are not considered
further. Above the threshold magnitude, most of the
remaining areas show a steady increase in percepti-
bility to a relatively flat local maximum or inflexion
point, and then a peak at the maximum magnitude.
For these plots, the most perceptible magnitude is
equal to the maximum magnitude. This is due to the
fact that the best fitting frequency–magnitude distri-
bution for these zones has a characteristic peak at the
largest event sizes (Koravos et al., 2003), consistent
Fig. 3. The perceptibility P=F(m)A(I)/Amax for the different tectonic zones identified by Holt et al. (2000), predicted from the frequency–
magnitude data and curve fits of Koravos et al. (2003), using I =VIII and h= 10 km.
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186180
Fig. 4. Perceptibility of ground motion at 0.2 g peak horizontal ground acceleration, assuming S= 0.5 for the site conditions and P*= 0 in Eq. (4).
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186 181
Fig. 5. Perceptibility of ground motion at 10 cm/s peak horizontal velocity, assuming S= 0.5 for the site conditions and P*= 0 in Eq. (5).
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186182
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186 183
with a ‘supercritical’ distribution (Main, 1995). Zone
9 in contrast shows a distinct maximum at m = 6.9
and zone 10 at m = 7.4, although zone 10 has a wide
range of magnitudes (m = 7.0–7.7) where the percep-
tibility remains constant. Main (1995) showed that
the perceptibility remains relatively constant for all
magnitudes above the threshold for the ‘critical’
(Gutenberg–Richter) distribution. This is consistent
with the results for zone 10.
The perceptibility for ground-motion acceleration
at a level of 0.2 g, assuming an average site condition
S = 0.5 and P* = 0 is shown in Fig. 4. In this case the
threshold magnitude of perceptibility is m = 5.6. Most
of the curves show features that are qualitatively
similar to the corresponding curves for intensity VIII
in Fig. 3. However, the peak probability occurs
systematically at lower magnitudes for acceleration
0.2 g in Fig. 4. For example, in zone 9 the most
Fig. 6. Spatial distribution of the examined zones and the perceptible magn
is also given in parenthesis for both magnitudes.
perceptible magnitude mp is in the range 6.1(F 0.1)
for acceleration cf. 6.9(F 0.1) for intensity. These low
error values in mp reflect the strong constraint of
deformation rates on the results. In zone 10 the broad
maximum in perceptibility has a lower and narrow
range for acceleration (m = 6.1–6.4), when compared
to the range of intensity (m = 7.0–7.7).
The perceptibility for ground-motion velocity at a
level of 10 cm/s for S = 0.5 and P* = 0 is shown in Fig.
5. The threshold magnitude in this case is m = 5.5 and
the general pattern in this figure is similar to the
corresponding curves for acceleration in Fig. 4. In
three zones 4, 6 and 7 the perceptible magnitude
shows a difference of the order of F 0.1 in correlation
with perceptible magnitudes estimated from intensity
and acceleration.
Fig. 6 shows the spatial distribution of the studied
zones and the perceptible magnitudes for intensity,
itudes for intensity, acceleration and velocity. The standard deviation
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186184
acceleration and velocity with their uncertainties.
Inspection of this figure shows that perceptible mag-
nitudes are over 7.0 in almost all the regions with
sufficient data, indicating that the seismic hazard is
really dominated by the rare, large events in the
Aegean area.
4. Discussion
Papazachos and Kiratzi (1996) examined the seis-
mic deformation velocities for the area of Greece and
its surroundings. They found high seismic deforma-
tion velocities for the middle part of the Aegean sea
having in general north–south direction, high to
intermediate values for the eastern part and low
values for the western part of the Aegean area where
the direction changes to east–west. Crustal deforma-
tion strain rates were determined by Kahle et al.
(1998, 2000) based on the use of GPS measurements
in the eastern Mediterranean region. They found high
values for the area of the north Anatolian fault (170
nstrain year� 1) while lower (90–120 nstrain year� 1)
and lowest (less than 40 nstrain year� 1) values were
determined for the southeastern Aegean and central
and southwestern Aegean, respectively. Our results
are consistent with the more recent data of Holt et al.
(2000) deduced by the combination of SLR and earth-
quake mechanisms. The obtained results (P(x |m), mp)
confirm that local deformation rate exercises a first-
order control on the seismic hazard. The highest value
for mp is in the fastest-deforming part in the middle
of the Aegean sea and tends to be systematically low
on the west in contrast to the east of the Aegean sea
(Fig. 6). For example in zone 11 where the defor-
mation velocity is 24 mm/year according to Papaza-
chos and Kiratzi (1996) and 28 mm/year according
to Holt et al. (2000), mp magnitudes (deduced from
intensity, acceleration and velocity) are equal to 7.6.
The other one is zone 12 where the seismic defor-
mation velocity is 16 mm/year (Papazachos and
Kiratzi, 1996) and 17 mm/year (Holt et al., 2000),
mp magnitudes are equal to 7.4. The Aegean Sea
coincides from north to south with zones 14, 10, 11,
6, 7 and 3 of Holt et al. (2000). We observed that
generally the easternmost zones (12, 8 and 4,
corresponding to the western part of Turkey), have
larger mp values than the westernmost ones (13, 9, 5
and 1, i.e. mainland Greece). These results confirm a
very good correlation that exists between the seismic
deformation velocities, deformation strain rates and
mp.
5. Conclusion
The perceptibility of ground motion has been
estimated for seismic zones corresponding to a grid
used to determine tectonic deformation rates, for
intensity, acceleration and velocity. Such results
can be used to constrain seismic hazard based on
combining spatially variable frequency–magnitude
and satellite strain rate data with attenuation laws.
Most of the deformation zones examined here ex-
hibit a characteristic or supercritical frequency–mag-
nitude distribution, where the moment release and
the seismic hazard is dominated by largest magni-
tudes, and the most perceptible earthquake is equal
to the maximum magnitude. The tectonic data
strongly constrain the long-term recurrence, and lead
to rather low estimates of the error in perceptible
magnitudes, on the order of F 0.1 or F 0.2 magni-
tude units. Only two zones, zones 9 and 10 showed
critical behaviour with a most perceptible magnitude
in the middle of the perceptible range. The location
of this peak is lower for acceleration than intensity
in both of these zones. Three zones had insufficient
data for the analysis presented here, while in the rest
of the zones we observed a good correlation be-
tween the seismic deformation velocities and the
perceptible magnitudes. Zones of high seismic de-
formation rates are correlated with high values of
perceptible magnitudes (mp). The middle of the
Aegean has the highest deformation rate and the
highest value for the mp. Intermediate to high
deformation rates and mp values are found on the
eastern side of the Aegean area, while low defor-
mation rates are associated with relatively mp param-
eters on mainland Greece, in the western part of the
examined area.
Acknowledgements
We thank James Jackson and John Haines for
providing the coordinates of the tectonic zones, and
G.Ch. Koravos et al. / Tectonophysics 371 (2003) 175–186 185
for access to the original principal average strain
rate data used to calculate the tectonic moment
release rate. The contribution of RMW Musson to
this paper is made with the permission of the
Executive Director of the British Geological Survey
(NERC).
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