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Finite-fault slip models for the 15 April 1979 (M
w
7.1) Montenegro
earthquake and its strongest aftershock of 24 May 1979 (M
w
6.2)
Christoforos Benetatos, Anastasia Kiratzi ⁎
Aristotle University of Thessaloniki, Department of Geophysics, 54124 Thessaloniki, Greece
Received 10 November 2005; received in revised form 9 March 2006; accepted 16 April 2006
Available online 27 June 2006
Abstract
We revisit the April 1979 Montenegro earthquake sequence to invert for finite-fault slip models for the mainshock of 15 April
1979 (M
w
7.1) and of the strongest aftershock of 24 May 1979 (M
w
6.2) using P, SH and SV waveforms, retrieved from IRIS data
center. We also used body waveform modelling inversion to confirm the focal mechanism of the mainshock as a pure thrust
mechanism and rule out the existence of considerable strike slip component in the motion. The mainshock occurred along a shallow
(depth 7 km), low angle (14°) thrust fault, parallel to the coastline and dipping to the NE. Our preferred slip distribution model for
the mainshock indicates that rupture initiated from SE and propagated towards NW, with a speed of 2.0 km/s. Moment was released
in a main slip patch, confined in an area of L∼50 km × W∼23 km. The maximum slip (∼2.7 m) occurred ∼30 km to the NW of
the hypocenter (location of rupture initiation). The average slip is 49 cm and the total moment release over the fault is 4.38e19 Nm.
The slip model adequately fits the distribution of the M
w
≥4.3 aftershocks, as most of them are located in the regions of the fault
plane that did not slip during the mainshock. The 24 May 1979 (M
w
6.2) strongest aftershock occurred ∼40 km NW of the
mainshock. Our preferred slip model for this event showed a characteristic two-lobe pattern, where each lobe is ∼7.5 × 7.5 km
2
.
Rupture initiated in the NW lobe, where the slip obtained its maximum value of 45 cm, very close to the hypocenter, and
propagated towards the south-eastern lobe where it reached another maximum value —for this lobe —of 30 cm, approximately
10 km away from the hypocenter. To indirectly validate our slip models we produced synthetic PGV maps (Shake maps) and we
compared our predictions with observations of ground shaking from strong motion records. All comparisons were made for rock
soil conditions and in general our slip models adequately fit the observations especially at the closest stations where the shaking
was considerably stronger. Through the search of the parameter space for our inversions we obtained an optimum location for the
mainshock at 42.04°N and 19.21° E and we also observed that better fit to the observations was obtained when the fault was
modeled as a blind thrust fault.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Earthquake mechanics; Finite-fault; Montenegro; Earthquake source; Slip distribution
1. Introduction
On 15 April 1979 (GMT 06:19:48) a strong
earthquake (M
w
7.1) struck the coastal regions of
Serbia –Montenegro and northern Albania. This event
caused extensive damage along 100 km of coastline
Tectonophysics 421 (2006) 129 –143
www.elsevier.com/locate/tecto
⁎Corresponding author. Tel.: +30 2310 998486; fax: +30 2310
998528.
E-mail address: kiratzi@geo.auth.gr (A. Kiratzi).
0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2006.04.009
(Fig. 1a, b), killed 129 people, injured 1554 in former
Yugoslavia and Albania and left homeless more than
80,000 (Papazachos et al., 2001; Papazachos and
Papazachou, 2003). The cities that suffered more were
Bar, Budva, Kotor and Ulcinj in Yugoslavia; massive
landslides, significant damage in harbours, and ancient
monuments along the Montenegro coastline have been
also reported. There is a note in Papazachos and
Papazachou (2003), without any reference to their
original source, that a sea wave was reported in the
Adriatic Sea, where a ship sunk and sea side houses
were taken off by the sea wave along a line of 15 km
The aftershocks of the sequence formed two distinct
clusters, which operated simultaneously and in the
northern cluster the strongest aftershock occurred on
24 May 1979 (M
w
6.2).
The region of occurrence of the sequence is located
at the border of the Adria plate with the Eurasian
platform, where the folded alpine Dinarides–Alba-
nides –Hellenides mountain ranges exist, and conti-
nental–continental collision takes place. This area is
dominated by the Ionian–Adriatic faults that form a
zone of 30 km to 40 km in width, located at the outer
side of the orogene (Aliaj and Muco, 1980). Based on
GPS data, Oldow et al. (2002) proposed that the Adria
plate is fragmented into blocks from which the south-
eastern, that includes south-eastern Italy, northwestern
Albania and the western coasts of former Yugoslavia,
is moving northwards (∼3 to 10 mm/yr) colliding
with stable Eurasia.
The Montenegro earthquake has been recorded by
stations at regional and teleseismic distances, of the
developing at that time GSN network, by a significant
number of short-period stations in the countries
surrounding the Adriatic Sea and by 29 SMA-1
accelerographs (Petrovski et al., 1979). The so far
published work concerns the characteristics of the
aftershock distribution (Kociaj and Sulstarova, 1980;
Hurtig et al., 1980; Console and Favali, 1981;
Karakaisis et al., 1985) the focal mechanism parameters
of the mainshock (Kanamori, 1979; Sulstarova, 1980;
Karakaisis et al., 1985; Baker et al., 1997) of the
foreshocks and aftershocks of the sequence (Kociaj and
Sulstarova, 1980; Karakaisis et al., 1985), as well as
stochastic strong ground motion simulations (Roume-
lioti and Kiratzi, 2002).
Here we study the rupture process of the two
strongest events of the sequence, of 15 April 1979,
Fig. 2. Focal mechanisms previously proposed for the mainshock and the solution obtained in this study using body wave waveform modelling.
Below each mechanism the strike/dip/rake is presented.
131C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
M
w
7.1 and of 24 May 1979, M
w
6.2 and we specifically
focus on the slip distribution onto the fault planes. We
use P and S waveforms from GSN stations, and the
finite-fault inversion technique of Antolik and Dreger
(2003). To better resolve our preferred slip models we
also invert for the focal mechanism of the mainshock as
there were diverse solutions available in the literature.
Furthermore we use our preferred finite-fault models to
calculate synthetic PGV (Peak Ground Velocity) maps
of the meizoseismal area, for both events, and compare
them with the maximum ground velocities that have
been recorded at the strong motion stations that are
available in the European Strong Motion Database
(Ambraseys et al., 2000).
2. Characteristics of the sequence
2.1. Focal mechanism of the mainshock (15 April 1979,
M
w
7.1)
Fig. 2 summarizes the previously published focal
mechanisms for the mainshock (see also Table 1).
The solutions vary from pure low angle thrust
faulting to reverse faulting combined with consider-
able strike slip motions. Most solutions indicate
faulting in a NW–SEdirectionparalleltothe
coastline with the east dipping fault identified as
the fault plane (Karakaisis et al., 1985).
To better resolve the mechanism of the mainshock we
obtained digital waveforms from stations of the Global
Digital Seismograph Network (GDSN) from the Incor-
porated Research Institutions for Seismology (IRIS)
data center. We then used MT5 (Zwick et al., 1994) and
the procedures as described previously (e.g. Kiratzi and
Louvari, 2001; Benetatos et al., 2004, 2005 and in the
references therein) to invert for the focal mechanism
parameters (strike/dip/rake), centroid depth and seismic
moment, assuming a source represented as a point in
space and described in time by a source time function
consisting of overlapping isosceles triangles. In total we
use 7 P-waves and 7 SH-waves from stations in the
distance range from 30° to 90°. Prior to the inversion
waveforms have been filtered between 0.01 and 0.1 Hz
and convolved with a typical WWSSN 15–100s long-
period instrument response. Green's functions have
been calculated using a half space of 6.5 km/s and
3.7 km/s for P- and S-waves, respectively and density
2.8 g/cm
3
. This model is adequate to explain the
simplicity of the Earth structure at the distance range we
use (Helmberger, 1974; Langston and Helmberger,
1975).
A single point source was adequate to obtain
satisfactory fit and our inversion yielded low angle
thrusting along a NW–SE line (Fig. 3 and Table 1). Our
solution is very close to those of Baker et al. (1997) and
Boore et al. (1981). The source time function has a total
duration of 27 s, maybe large for an M
w
7.1 event, but
also close to the duration obtained by Baker et al. (1997).
Another indication for long source duration is the fact
that in the published Harvard CMT solution for this
event, the centroid time shift (the difference between the
centroid time and the origin time) which is interpreted as
half of the total duration, is 14.8 s, which implies a total
duration of ∼29 s. In the lower part of Fig. 3 we
Table 1
Source parameters for the 15 April 1979 (M
w
7.1) mainshock and for the 24 May 1979 (M
w
6.2) strongest aftershock previously published
Epicenter Depth
(km)
M
w
Nodal plane 1 Nodal Plane 2 P-axis T-axis Method
used
Ref
φ°N λ° E Strike° Dip° Rake° Strike° Dip° Rake° Az° Dip° Az° Dip°
15 April 1979
MS 06:19
––40 7.0 324 50 115 108 46 63 37 2 300 71 INV 1
332 55 1 241 89 145 292 23 191 25 INV 2
42.03 19.04 –7.2 287 11 63 134 80 95 220 35 50 55 FMP 3
42.13 19.06 22 6.9/7.1 308 15 88 130 75 91 220 30 41 60 INV/FMP 4
––– 6.7 ML 319 85 42 225 48 174 84 24 191 32 FMP 5
41.93 18.99 1 7.1 312 44 45 186 61 124 252 10 146 59 FMP 6
42.09 19.21 12 6.9 316 14 90 136 76 90 226 31 46 59 INV 7
42.04 19.21 7
(±5)
7.1 300
(+10/−8)
14
(+7/−6)
88
(+12/−5)
120 76 90 210 31 31 59 INV 7
24 May 1979
AF 17:23
––– 6.2 ML 37 41 154 147 73 52 265 19 17 48 FMP 4
42.15 18.71 7 6.3 299 50 31 188 67 136 247 10 146 47 FMP 5
42.26 18.75 6 6.2 320 35 90 140 55 90 232 10 52 80 INV 6
References: 1) Kanamori (1979),2)Sulstarova (1980),3)Boore et al. (1981),4)Console and Favalli (1981),5)Karakaisis et al. (1985),6)Baker et al.
(1997), 7) this study.
INV: Inversion, FMP: First motion polarities.
132 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
Fig. 3. a) Minimum misfit solution for the 15 April 1979 mainshock showing P- and SH-observed (solid) and synthetic (dashed) waveforms, which
are arranged azimuthally around the focal spheres. Station locations are plotted as capital letters on the focal spheres. Pand Taxes are also marked.
The second line on the header shows the strike, dip, rake, centroid depth (in km) and the scalar moment. This solution is similar to Baker et al. (1997).
b) Test of a solution when a strike-slip mechanism is assumed (arrows point out polarity mismatch).
133C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
investigate the fit to the waveforms if we assume
considerable strike-slip motion in the focal mechanism,
keeping all other parameters constrained. We clearly see
that the P-waves are not very sensitive, but in the SH
waveforms the fit deteriorates in most of the stations,
with polarity mismatch. Thus, for our further modelling
we accepted a thrust mechanism for the mainshock. For
the 24 May 1979, M
w
6.2 strongest aftershock we adopt
the focal mechanism of Baker et al. (1997) as listed in
Table 1.
2.2. Spatial distribution of the aftershock sequence
Fig. 4a summarises the temporal and spatial
distribution of the entire (April 1979–January 1980)
aftershock sequence, with M
w
≥4.3 (data as relocated
Fig. 4. a) Distribution of aftershocks with M ≥4.3 of the period 15 April 1979–23 January 1980. Note that they form two distinct clusters clearly
connected with the two strongest events. b) Distribution of aftershocks with M≥4.3 until 23 May 1979, prior to occurrence of the strongest
aftershock, which shows that the northern cluster was already in operation. Epicenters as relocated in Karakaisis et al., 1985.
134 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
by Karakaisis et al., 1985). It is seen that the aftershocks
form two distinct clusters, one at the southern part of the
activated area, clearly connected with the mainshock
and the other at the northern part, clearly connected with
the strongest aftershock of 24 May 1979 (M
w
6.2). The
northern cluster was already in operation prior to the
occurrence of the strongest aftershock (Fig. 4b).
Actually the first M
w
4.3 event was recorded on the
northern cluster 12 min after the occurrence of the
mainshock (Karakaisis et al., 1985). The region between
the two clusters is free of aftershocks, at least with
M
w
> 4.3 that the data covers. This is an indication that
this part has already slipped during the mainshock and
was not capable to produce aftershocks at later stages, a
remark assumed in Karakaisis et al. (1985) and shown
later in this paper. The depth distribution of aftershocks
is constrained to the upper 15–20 km (Console and
Favali, 1981).
3. Slip distribution obtained from body waveform
inversion
3.1. Data and method of analysis
We examined the slip models of the mainshock and
of the strongest aftershock using data from 10 GDSN
stations that were in operation in 1979 (Fig. 5). The data
set consists of 7 P and 10 S waveforms in the distance
range from 1218 to 9830 km. Initial records have been
deconvolved from the instrument response, integrated to
obtain ground displacement and bandpass filtered
Fig. 5. GSN locations of the stations (white triangles) whose waveforms were used in the inversions. The waveforms form GRFO and TATO (black
triangles) were not used in the inversion due to low signal to noise ratios.
Fig. 6. Variation of the variance reduction (%) obtained from the finite-
fault inversion for the mainshock slip model, with different rupture
speeds tested. The highest variance reduction achieved is for rupture
speed of 2.0 km/s.
135C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
between 0.01 and 0.15 Hz. We model 40 s of the P-
waves and 80 s of the S-waves (SV and SH
components). Green's functions were computed using
a frequency-wavenumber integration method (Saikia,
1994) and the Iasp91 velocity model (Kennett and
Engdahl, 1991) which has been proved adequate to
explain the timing and the frequency content of the
waveforms at the distances used here. Actually,
FKRPROG is a plane-layered frequency integration
code, which can be used also at teleseismic distances
when an earth flattening algorithm is used to flatten the
velocity structure, as applied here.
The inversion method is a linearized least-squares
scheme presented previously (e.g., Hartzell and Hea-
ton, 1983; Wald and Heaton, 1994; Dreger and
Kaverina, 2000; Kaverina et al., 2002; Antolik and
Dreger, 2003), that places several constrains on the
solution which include slip positivity and spatial-
derivative smoothing. The fault plane is divided into
nodes in both coordinate directions and the rupture is
constrained to propagate at a constant velocity from the
hypocenter. The slip velocity history of each subfault
can be a delta function or an isosceles triangle with a
constant duration (adopted here), and each node is
allowed to slip only once as the (circular) rupture front
passes. The rake angle is also forced to remain constant
over the fault plane. The total system of equations to be
solved is:
WdG
kD
gI
0
@1
AðmÞ¼
Wd
0
0
0
@1
A:ð1Þ
Fig. 7. a) Fault plane view of our preferred slip distribution model for the 15 April 1979 Montenegro (M
w
7.1) earthquake obtained using least-squares
inversion of digitally recorded waveforms. The white star denotes the initiation of rupture onto the fault plane. Rupture propagated from SE towards
NW. Moment is released in a main slip patch (L∼50 km × W∼23 km); the maximum slip (∼2.7 m) occurred ∼7 km to the NW of the locus of rupture
initiation. b) Surface projection of the slip model along with the aftershocks (M≥4.3) that followed its occurrence. There is a good anti-correlation
between the area that slipped during the mainshock and the regions that slipped during the aftershocks.
136 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
The matrix Gconsists of the subfault Green's
functions, Wweights and normalizes the data
according to the inverse amplitude, Dis a gradient
smoothing matrix for both coordinate directions, mis
the vector of scalar moment values for each subfault,
λand ηare empirically determined constants that
control the weighting of the constraints relative to the
data fit, and dis the data vector. The slip positivity
constraint requires m≥0, indicating slip along the
rake direction. The values of scalar moment for each
subfault are converted to equivalent slip values using
a rigidity of 3.5 × 10
11
dyn cm
−2
.
Fig. 8. P, SH and SV observed (solid lines) and synthetic (dashed lines) waveforms produced using our preferred slip model for the 15 April 1979
event.
137C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
4. Slip models of the events
4.1. Slip model for the 15 April 1979 Montenegro
mainshock (M
w
7.1)
To perform the inversions we used a fault plane of
dimensions 90 km × 50 km, larger than what is expected
for an earthquake of magnitude M
w
7.1, in order to
allow the slip to move to its preferable position without
constraining it due to limited fault dimensions (Das and
Suhadolc, 1996). The orientation of the fault and the
hypocenter depth adopted are those derived from our
teleseismic inversion (see Table 1). The fault plane was
divided into subfaults of dimensions 2 km × 2 km along
strike and dip, resulting in 45 nodes along strike and
25 nodes along dip direction.
A considerable number of initial inversions were
devoted to test the parameter space in terms of different
locations for the initiation of the rupture on the fault
plane and of the position of the fault plane relative to
the hypocenter. We additionally tested different rupture
velocities ranging from 1.4 to 3.0 km/s with a step of
0.2 km/s and the highest total variance reduction and
hence the better fit to the waveforms was for a rupture
velocity of 2.0 km/s (Fig. 6), which indicates a rather
slow rupture process. Moreover, we tested the
Fig. 9. a) Fault plane view of our preferred slip distribution model for the 24 May 1979 strongest aftershock obtained as before. The white star denotes
the initiation of rupture onto the fault plane. Note the characteristic two-lobe pattern, (each lobe ∼7.5 × 7.5 km
2
). Rupture initiated in the NW lobe,
(maximum slip value 45 cm), and propagated towards the south-eastern lobe (maximum slip value 30 cm). b) Surface projection of the slip alongwith
the aftershocks (M ≥4.3) that followed its occurrence, for reason of comparison as in Fig. 7b.
138 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
sensitivity of our inversions in terms of the focal
mechanism parameters and their uncertainties, varying
the strike, dip and rake within their error limits. During
these tests some stations (e.g. ANTO) were more
sensitive to these changes and improved the fit, while
for other stations the fit was worse, so we decided it
was a good compromise to adopt the values from our
teleseismic inversion in the finite-fault inversion. From
these initial runs of the inversion we saw that our
inversions were stable and the position and magnitude
of slip remained also very stable. We observed that we
were getting a better fit to the waveforms and we were
predicting more accurately the level of strong ground
shaking, as it will be presented in the following
sections, when the epicenter of the mainshock was
taken at 42.04° N and 19.21° E (slightly modified from
Baker et al., 1997) and when we modelled the fault as a
blind thrust fault that did not reach the surface. When
we allowed the fault to break the surface we were
getting very high values of ground shaking on land,
which contradicted the observed intensities and the
observed accelerations.
Our preferred slip distribution solution for the 15
April 1979 mainshock is presented in Fig. 7a.
Rupture initiated from SE and propagated towards
NW. The main slip patch is confined in an area of
L∼50 km × W∼23 km that is in agreement with
empirical relations for reverse faults (Wells and
Coppersmith, 1994; Papazachos et al., 2004). The
main moment release occurred ∼30 km to the NW
of the hypocenter (location of rupture initiation) with
a maximum slip value of 2.7 m. The average slip is
49 cm and the total moment release over the fault is
4.38e19 Nm, in agreement with the results of our
moment tensor inversion. In Fig. 7b we present a
map view of the slip distribution of the mainshock
along with the aftershock epicenters (Karakaisis et
al., 1985) that occurred until 24 May 1979, day of
the occurrence of the largest aftershock (M
w
6.2).
The data set consists of 74 events with M
w
≥4.3.
There is a good anti-correlation between the areas
that have slipped during the mainshock and the
locations of the aftershocks. The slip model (Fig. 7a)
provides a good overall fit between the observed
Fig. 10. P and SH observed (solid lines) and synthetic (dashed lines) waveforms produced using our preferred slip model for the 24 May 1979 event.
139C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
Fig. 11. Synthetic Shake map, contours of Peak Ground Velocities (in cm/s) using forward modelling and our preferred finite-fault slip model (Fig. 7a)
for the mainshock. Black triangles denote strong motion station locations and to the right of each station code (see Table 2) the maximum observed
PGV (in cm/s ) is shown, for comparison. Note that our model adequately predicts the observed values especially those of the closer to the epicenter
(see text for further details).
Fig. 12. As in Fig. 11 but for the 24 May 1979 aftershock using the slip model of Fig. 8a (see Table 2 and text for more details).
140 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
and the synthetic P-, SH- and SV-waveforms (Fig.
8).
4.2. Slip model for the 24 May 1979 aftershock
(M
w
6.2)
We used a fault plane with strike 320° dip 35°
rake 90°(Baker et al., 1997) having dimensions
30 km × 20 km, with the same reasoning as before.
We divided the fault plane into nodes of 1 km× 1 km
along strike and dip, respectively. The location of the
aftershock was taken at 42.15° N, 18.708° E (Karakaisis
et al., 1985), placing it in the north western part of the
activated area. Initial runs concerned the sensitivity of
our inversion to the location of the fault relative to the
hypocenter, different values for rupture velocity (span-
ning the range from 1.2 km/s to 3.0 km/s; increment of
0.2 km/s ). These tests showed that the slip was very
stable and not sensitive to the position of the fault
relative to the hypocenter, while the optimum rupture
velocity was 2.6 km/s.
Our preferred slip model for the 24 May 1979
aftershock (Fig. 9a) shows a characteristic two-lobe
pattern which is also reflected in its surface projection
(Fig. 9b). Each lobe is ∼7.5 × 7.5 km
2
. Rupture initiated
in the NW lobe, where the slip obtained its maximum
value of 45 cm, very close to the hypocenter. Then
rupture propagated to the south-eastern lobe where it
reached another maximum value —for this lobe —of
30 cm, approximately 10 km away from the hypocenter.
In Fig. 10 the fit between the observed and synthetic
waveforms is presented. We were able to achieve a
satisfactory overall fit for P- and for the single S-
waveform used.
5. Predicted ground motions (Shake maps)
To calculate the predicted ground motions by our
finite-fault models we constructed a grid spacing
0.02° × 0.02° around the region of study. At each point
we calculated synthetic velocity records for rock site
conditions, following the method of Dreger and
Kaverina (2000), using our preferred finite-fault model
(moment weights distributed on the fault plane). Green's
functions were calculated, as previously, using the
discrete wavenumber code (FKRPROG; Saikia, 1994).
For this application we adopted a more refine velocity
model applicable to the area (Ambraseys et al., 2004).
Table 2
Information on the sites of the strong-motion stations that recorded the ground motions of the Montenegro mainshock and the largest aftershock
Station Lat
(°N)
Lon
(°E)
Description Site class 15 April 1979 mainshock 24 May 1979 aftershock
Distance
(km)
Observed
PGV cm/s
PGV (for Rock
class, B cm/s)
Distance
(km)
Observed
PGV cm/s
PGV (for Rock
class, B) cm/s
BAR 42.095 19.101 Bar,
Skupstina Opstine
Stiff soil 16 47 40.7 32 15.9 14.4
ULA 41.919 19.221 Ulcinj,
Hotel Albatros
Rock 21 21 21
ULO 41.911 19.249 Ulcinj,
Hotel Olimpic
Stiff soil 24 42.8 38.6 54 2.9 1.8
PETO 42.204 18.948 Petrovac,
Hotel Oliva
Stiff soil 25 31.3 27.4
TIS 42.43 19.261 Titograd,
Seismoloska Stanica
Rock 55 3.4 3.4
TIG 42.442 19.264 Titograd,
Geoloski Zavod
Rock 56 4.0 4.0
HRZ 42.457 18.531 Herceg Novi,
O.S.D. Pavicic
Rock 65 13.3 13.3 32 5.4 5.4
DUB 42.656 18.091 Dubrovnic–
Pomorska Skola
Rock 105 3.3 3.3
BUD 42.284 18.831 Budva–PTT Alluvium 8 22.8 11.9
PET 42.204 18.949 Petrovac–
Hotel Rivijera
Alluvium 16 13.8 7.3
KOTN 42.418 18.773 Kotor Nas Raki Rock 21 3.9 3.9
TIVA 42.425 17.708 Tivat–Aerodrom Alluvium 22 7.8 3.1
KOTZ 42.436 18.769 Kotor–Zovod za
Biologgiju Mora
Alluvium 23 7.9 3.4
(Distances refer to the epicentral distances that were used in the empirical attenuation relations).
141C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
Green's functions were band pass filtered between
0.01 Hz and 5 Hz in order to incorporate the higher
frequencies in the estimation of the near source ground
motions. In each grid point we calculated an average
velocity value (geometric mean), using the maximum
amplitudes of the two horizontal components.
The Shake maps for the mainshock and for the
aftershock are presented in Figs. 11 and 12, respectively.
To validate our results we used the observed PGV values
(also shown in Figs. 11 and 12) at the locations of the
available strong motion records (Table 2) for these two
events (Ambraseys et al., 2000). To have a common
reference all observed PGV's were converted to rock
site conditions (B class, NEHRP classification), using
empirical attenuation relations (Tromans and Bommer,
2002), and these are plotted on the maps. More
specifically, we used the empirical relations to calculate
the PGV values for rock, stiff soil and alluvium site
conditions and the difference in PGV's was subtracted
from the observed values of either stiff soil or alluvium
sites. In this way we unified the PGV values at all sites
as they were all situated on reported rock conditions.
In general, the predicted PGV's for most of the
stations are in satisfactory agreement with the observa-
tions. For the mainshock the maximum ground shaking
(∼140 cm/s ) is predicted offshore, to the NW of the
epicenter and we do model adequately the high values
observed at the closest stations e.g. BAR, ULA and
PETO. We do not model the high value at ULO, which
is close to ULA where the recorded PGV is a factor of 2
smaller; besides ULO is not at the direction of the
rupture, so this high value could not be expected. In any
case, the mismatch can be attributed to model
insufficiency, misclassification of the site conditions of
the station, among other reasons. For the strongest
aftershock the maximum ground shaking (∼34 cm/s ) is
predicted in the sea and our slip model satisfactorily
predicts the PGV values at the closest stations (e.g BUD,
PET).
6. Conclusions
We revisited the April 1979 Montenegro earthquake
sequence, aiming to provide finite-fault slip models for
the mainshock of 15 April 1979 (M
w
7.1) and of the
strongest aftershock of 24 May 1979 (M
w
6.2) together
with other details of the rupture, using the available
waveforms from the IRIS network. We used body
waveform modeling inversion to recalculate the focal
mechanism of the mainshock and confirm its pure
thrust mechanism, (Boore et al., 1981; Baker et al.,
1997) and rule out the existence of considerable strike
slip component in the motion (Console and Favali,
1981; Karakaisis et al., 1985). Thus, the mainshock
occurred along a shallow (depth 7 km), low angle
(14°) thrust fault, parallel to the coastline and dipping
to the SE.
Our preferred slip distribution model for the main-
shock indicates that rupture initiated from SE and
propagated towards NW, with a speed of 2.0 km/s.
Moment was released in a main slip patch, confined in
an area of L∼50 km × W ∼23 km. The maximum slip
(∼2.7 m) occurred ∼7 km to the NW of the hypocenter
(location of rupture initiation). The average slip is 49 cm
and the total moment release over the fault is
4.38e19 Nm, in agreement with the results of our
moment tensor inversion. The slip model adequately fits
the distribution of the M
w
> 4.3 aftershocks, as most of
them are located in the regions of the fault plane that did
not slip during the mainshock.
The strongest aftershock of 24 May 1979 (M
w
6.2)
occurred ∼40 NW of the mainshock. Our preferred slip
model showed a characteristic two-lobe pattern. Each
lobe is ∼7.5 × 7.5 km
2
. Rupture initiated in the NW
lobe, where the slip obtained its maximum value of
45 cm, very close to the hypocenter. Then rupture
propagated to the south-eastern lobe where it reached
another maximum value —for this lobe —of 30 cm,
approximately 10 km away from the hypocenter.
We used the available strong motion records for the
two events to validate our slip models in an indirect
manner, by simply producing synthetic PGV (Shake
maps) and comparing our predictions with the observa-
tions. All comparisons were made for rock soil
conditions and in general our slip models adequately
fit the observations especially at the closest stations
where the shaking was considerably stronger.
At last, from the inversion parameter search, we
constrained the location of the mainshock epicenter. The
epicenter location which produced the best results, in
terms of matching the regions of ground shaking, was at
42.04° N and 19.21° E. In addition, in the same
parameter search we observed that we obtained the best
matching both in the synthetic waveforms and also to
the values of strong ground shaking when we model the
fault as a blind thrust fault that did not break the surface.
Acknowledgments
We acknowledge with thanks financial support from
the General Secretariat of Research and Technology
(GSRT), Ministry of Development of Greece. We are
grateful to Douglas Dreger, from the Seismological
Institute of Berkeley University, for his continuous
142 C. Benetatos, A. Kiratzi / Tectonophysics 421 (2006) 129–143
support with the inversions. We thank the two
anonymous reviewers and the Editor for the constructive
comments and suggestions. Thanks are also due to the
IRIS consortium and all the affiliated networks for
providing the digital waveforms. Most of the figures
were produced using the GMT software (Wessel and
Smith, 1998).
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