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We deployed an on/offshore seismic network consisting of 36 three-component stand-alone digital stations in the Saronikos Gulf and surrounding area and observed the microseismic activity from 17 August to 9 December 2001. The geometry and location of the network were designed to observe the offshore activity to connect known active faults onshore with those offshore. An average of 15 events per day were recorded by a minimum of six stations. The focal parameters of 739 events could be defined. Within the network area we located 306 events with an rms travel-time residual of about 0.2 sec. The epicenters in the Saronikos Gulf are aligned mainly along an east–west trending fault with a length of nearly 30 km. This fault is located north of Aegina and could produce earthquakes as large as M s 6.5. It is the offshore continuation of a seismic zone located in east Corinthia. A second active fault was identified east of Aegina, trending northeast–southwest. It is probably linked to the northeast–southwest striking zone of the Salamis-Fili seismic region, activated by the September 1999 event of Athens. Outside the network we mapped an active area at the Parnis mountain, still active after the M s 5.9 event, and a second one further northeast at the western coast of Evia. This zone is close to Chalkis and the recently active Psachna area. The network also mapped intense activity at the northeastern coast of Evia, close to the city of Kymi and the area of Skyros, which was seriously damaged by an M s 5.8 earthquake in 2001. The onshore/offshore local network identified several regions of very high seismic activity not detectable by regional networks, and demonstrated once more the advantage of deploying local seismic networks for mapping active deformation in very short periods of time.
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Bulletin of the Seismological Society of America, Vol. 94, No. 3, pp. 920–929, June 2004
Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece
by J. Makris, J. Papoulia, and G. Drakatos
Abstract We deployed an on/offshore seismic network consisting of 36 three-
component stand-alone digital stations in the Saronikos Gulf and surrounding area
and observed the microseismic activity from 17 August to 9 December 2001. The
geometry and location of the network were designed to observe the offshore activity
to connect known active faults onshore with those offshore. An average of 15 events
per day were recorded by a minimum of six stations. The focal parameters of 739
events could be defined. Within the network area we located 306 events with an rms
travel-time residual of about 0.2 sec. The epicenters in the Saronikos Gulf are aligned
mainly along an east–west trending fault with a length of nearly 30 km. This fault
is located north of Aegina and could produce earthquakes as large as M
s
6.5. It is
the offshore continuation of a seismic zone located in east Corinthia. A second active
fault was identified east of Aegina, trending northeast–southwest. It is probably
linked to the northeast–southwest striking zone of the Salamis-Fili seismic region,
activated by the September 1999 event of Athens. Outside the network we mapped
an active area at the Parnis mountain, still active after the M
s
5.9 event, and a second
one further northeast at the western coast of Evia. This zone is close to Chalkis and
the recently active Psachna area. The network also mapped intense activity at the
northeastern coast of Evia, close to the city of Kymi and the area of Skyros, which
was seriously damaged by an M
s
5.8 earthquake in 2001. The onshore/offshore local
network identified several regions of very high seismic activity not detectable by
regional networks, and demonstrated once more the advantage of deploying local
seismic networks for mapping active deformation in very short periods of time.
Introduction
The region of Attica and, in particular, that of the city
of Athens have long been considered aseismic (Sieberg,
1932). This was mainly attributed to the fact that the his-
torical center of the City, built on schist of high rigidity, is
not vulnerable to earth tremors. However, since Athens has
been constantly expanding, densely urbanized areas haveen-
croached into seismogenic regions and Athens has become
vulnerable to earthquakes. Today, according to estimates of
Papadopoulos and Arvanitides (1996) the seismic risk of
Athens is among the highest in Greece. This was verified
dramatically by the M
s
5.9 earthquake of 7 September 1999,
which was the costliest natural disaster of Greece in recent
history.
Prior to this event and during the instrumental period of
Greek seismology, no significant earthquake activity had
been reported. The strongest shocks (M
s
6.0) to affect Athens
were located in the Thiva-Oropos zone at a distance of more
than 50 km from the historic center of the city (Papazachos
and Papazachou, 1997). Events of rather small magnitude
recorded from time to time in the vicinity of Athens, like
those of 3 April (M
s
4.2) and 4 December (M
s
3.5) 1965 and
a cluster of events ranging from 3.8 to 4.5 in November 1997
[see the National Observatory of Athens (NOA) bulletins],
did not attract public interest. It was therefore assumed that
a detailed examination of the microseismicity of the Athens
area was unnecessary.
The 1999 event, however, clearly demonstrated that the
low rate of seismicity alone is not a safe criterion to assess
the seismic potential of a region. Only a detailed tectonic
analysis and mapping of active faults and soil conditions
contribute toward a reliable seismic risk assessment. Had
such studies been undertaken before 1999, they certainly
would have revealed the potential risk of the active tectonic
structures that ruptured during the 7 September 1999 seismic
crisis.
Public awareness and the need for a thorough under-
standing of the regional seismic potential after the 7 Septem-
ber 1999 event motivated the Earthquake Planning and
Protection Organization of Greece to support an onshore-
offshore microseismicity survey in the region of the Saron-
ikos Gulf, whose purpose was to identify active faults on-
and offshore and to define their seismic potential. A local
Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 921
seismic network was deployed consisting of eight ocean bot-
tom seismographs (OBS) and 28 land stations (Fig. 1). This
network operated for 45 days.
Both marine and land stations were equipped with
SEDIS seismic recorders of GeoPro, Hamburg (Makris and
Moeller, 1990; GeoPro technical reports). These instruments
are capable of recording six seismic channels on hard disc
with 10-GB capacity. Onshore, an integrated Global Posi-
tioning System (GPS) receiver provides coordinates and tim-
ing at each location. Marine stations are housed in a 17-inch
glass sphere and record seismic signals directly on the sea
floor. A gimbal-mounted three-component geophone system
and a deep sea hydrophone are used for this purpose. The
instruments are autonomous and can be acoustically ad-
dressed by a surface-mounted system. Timing is provided
by an internal quartz clock. By measuring the quartz tem-
perature continuously and GPS time at the beginning and end
of the survey, time-drift corrections are obtained for each
instrument.
Regional Tectonics
The region of Attica as presented in the 1:50,000 scale
geological maps of the Institute of Geology and Mineral Ex-
ploration of Greece (IGME) (1989), belongs to an autoch-
thonous metamorphic basement of Paleozoic-Mesozoic age,
consisting of marbles and schists (see also Jacobshagen,
1986). Remnants of a “tectonic cover” have been identified
in several localities. A major tectonic boundary, dominating
the area, separates the mountains of Parnis to the northwest
from those of Pendeli and Immitos to the southeast. These
two are characterized by metamorphic rocks, mainly marbles
and schists belonging to the Cyclades Massif, which dip to
the northwest, below the nonmetamorphic limestones of
northwestern Attica. Normal faults along the Parnis moun-
tain are considered to be the eastward extension of the Cor-
inthiakos Gulf fault system and are of roughly east–west
orientation. During historic times and the more recent period
of instrumental earthquake observations in Greece, consid-
erable earthquake activity has been observed along this zone,
whereas the southeastern part of Attica appears to be nearly
aseismic (Galanopoulos, 1960, 1961; Makropoulos et al.,
1989; Papazachos and Papazachou, 1997).
The Saronikos Gulf is a neotectonic basin, divided by a
very shallow north–south-oriented platform into a western
and an eastern part. The islands of Methana, Angistri, Ae-
gina, and Salamis (see Fig. 1) are situated on this platform.
The western part of the Gulf consists of two basins. The
Epidaurus Basin is of west-northwest–east-southeast orien-
tation and more than 400 m deep, whereas the Megara Basin
is of east–west orientation with relatively shallow depth,less
than 250 m. The marginal extensional faults of the Epidaurus
Basin strike parallel to the bathymetry and have a 350-m
throw, forming a symmetric graben. The Megara Basin is
formed by east–west to east-northeast–west-southwest mar-
ginal faults. The Eastern Saronikos gulf is dominated by
northwest–southeast faults of relatively small throws and al-
ternating shallow basins and plateaus (Papanikolaou et al.,
1988).
Several volcanoes and volcanic outcrops of Plio-
Quaternary age are also situated in the Gulf and are part of
the western active Aegean volcanic arc. The westernmost
group of volcanic outcrops are those of the Loutraki-Sousaki
area. They are locally more than 70 m thick (Freyberg,
1973). The peninsula of Methana consists mainly of a large
volcanic dome and volcanic rocks of Pliocene and Quater-
nary age which are widely exposed (Gaitanakis and Dietrich,
1995). The most recent eruption is dated around 250 B.C.
and was desrcibed by Strabo (Stothers and Rampino, 1983).
To the northwest they appear to truncate all but the upper-
most 100 m of Quaternary sediments (Papanikolaou et al.,
1988). Volcanic rocks are also found to the south of the
island of Poros (Fytikas et al., 1986). Clearly, volcanic ac-
tivity in the Gulf is extensive, recent, and very close to Ath-
ens. It is poorly understood and its hazard has never been
assessed.
From displacements at faults and the distribution of sed-
iments it is shown that the Saronikos Gulf is more active in
the west than in the east. This has also been verified by the
microseismic activity, as will be presented subsequently.
The most seismically active area in the vicinity of Ath-
ens is the Corinthiakos Gulf, which exhibits deformation
typical of the extension that dominates central Greece and
the Peloponnese (Jackson et al., 1982; Collier et al., 1992;
Armijo et al., 1996), and an annual extension rate on the
order of 1 to 7 mm. About 20% of this extension is linked
to east–west-striking normal faults of eastern Corinthia (Pa-
pazachos and Kiratzi, 1996). The continuation of this fault
system to the east was also mapped by high-resolution,
single-channel reflection seismic profiles (Papanikolaou
et al., 1988) offshore in the Saronikos Gulf. A significant
part of the observed seismicity coincides with this system of
faults, as will be discussed subsequently.
Microseismicity
Within 45 days of field observations we recorded 739
microearthquakes above a threshold magnitude of M
L
0.3
(Fig. 2), which were observed by at least six stations.
Magnitudes were defined by the coda length of the seis-
mograms, calibrated by using earthquakes recorded also by
the Seismograph Network of the NOA. For the magnitude
estimates, we derived a mean function from four stations
having similar site response and applied this relationship to
the full temporary network (Fig. 3).
The locations of the hypocenters were obtained using
HYPOINVERSE software (Klein, 1989). HYPOINVERSE
allows the application of spatially varying local velocity
models for the hypocenter location. The velocity model used
922 J. Makris, J. Papoulia, and G. Drakatos
Figure 1. Location of the on and offshore seismic network used to record mi-
croearthquake activity in the area of Saronikos gulf, Greece. Stations are represented
by triangles.
was determined by a controlled source seismic experiment
between Kythnos and Eratini, crossing the Saronikos Gulf
from southeast to northwest (Makris et al., 2002). However,
to avoid local geological complexities, we adopted a mean
velocity model, as presented in Table 1.
Within the network most travel-time residuals after lo-
cation have an rms 0.2 sec. This corresponds to an epicentral
accuracy of the order of 2 km, depending on the local
velocity model used, whereas the hypocentral depths are af-
fected by an error twice this value. Outside the network,
errors are significantly higher (Fig. 4).
The frequency-magnitude distribution of the recorded
events is shown in Figure 5. Within the network the thresh-
old magnitude of completeness is M
L
1.7. Outside the net-
work this increases to M
L
2.3.
To demonstrate the advantage of using local networks
over regional ones in assessing the seismic activity in this
area, we present in Figure 6 a comparison between historic
events (Fig. 6a), instrumentally recorded events published
by the NOA regional network (Fig. 6b), and events recorded
by this temporary local network (Fig. 6c). The advantage of
using a local network of densely spaced stations is obvious.
Even a short period of observations is enough to resolve
active structures that a regional network would require tens
of years to identify due to the different threshold of com-
pleteness.
Within the network and its immediate vicinity we lo-
cated 306 events (Fig. 7). Most of them are within the upper
15 km of the crust, whereas only a few were found at depths
exceeding 30 km. The distribution of the foci is illustrated
in Figure 8. The southwest–northeast oriented profile (Fig.
8) extends from eastern Peloponnese (point A) crossing the
Saronikos Gulf to central Evia (point B). Hypocenters con-
tained in a 100-km-wide zone, parallel to line AB, were pro-
jected in the surface defined by AB and the zaxis extending
to 40 km depth. Their distribution shows some distinctzones
of foci concentration that can be associated with known
faults. At 90 km (see Fig. 8) the hypocenters are located at
the “Aegina fault” as presented in Figures 2 and 7. The linear
distribution of the foci along the fault extends to a depth of
more than 15 km and marks a deep fracture of the thinned
Saronikos crust. The next concentration, 20 km further
Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 923
Figure 2. Microseismic activity recorded by the Saronikos seismic network in a 45-
day period. The map shows the epicentral positions of 739 events, each having been
identified by a minimum of six stations. Depth of the hypocenters is indicated by the
color scale.
northeast coincides with the faults east of the island of
Salamis, which were reactivated by the “Athens Event” of
7 September 1999. They do not seem to have a deep exten-
sion and probably originate at the thrust front between the
Cyclades Massif and west Attica. In the Parnis area, 30 km
further northeast, we encounter the Fili fault zone. The seis-
micity shows that all faults around the Parnis epicenter of
7 September 1999 are still active and that the activityextends
through the complete crust.
At 45-km distance to the northeast we cross an active
seismic zone that is associated with the Chalkis area (see
Fig. 2). The Psachna region nearby is presently very active
924 J. Makris, J. Papoulia, and G. Drakatos
Figure 3. Calibration functions used to define lo-
cal magnitudes by the coda-duration method. The av-
eraged values were derived from functions defined at
four different location of comparable recording con-
ditions and therefore influenced by similar site effects.
Table 1
Local Velocity Model Used in the Location of Events
(Makris et al., 2002)
Velocity Vp (km/sec) Depth (km)
4.7 0.0
5.7 3.0
6.8 7.0
8.1 17.0
and has hosted several M
s
4.5 events. Another 35 km to the
northeast we encounter the Kymi fault zone with intense
crustal seismicity. This system of faults separates the north-
ern part of the island of Evia from the Skyros basin and
truncates the entire crust. Large-magnitude events have been
reported from this zone (Makropoulos and Burton, 1984;
Papazachos and Papazachou, 1997). Finally, around the
Skyros area, 65 km to the northeast off the coast, we en-
counter a seismically active region with a diffuse distribution
of foci. This area is notorious for its intense seismicity and
has hosted several large earthquakes (see, e.g., Makropoulos,
1978; Makropoulos et al., 1989). The large distance of the
Saronikos network from the island of Skyros and the limited
period of monitoring does not permit us to associate the
recorded microseismicity with the existing faults. A special
study with a local network is needed because the position
and size of the active faults around Skyros are not yet clearly
mapped.
In general, below Peloponnese and particularly the Sa-
ronikos gulf, the foci are located much deeper than below
Attica, Evia, and the Aegean domain (see Fig. 8). This phe-
nomenon points to the existence of two regional stress fields
that dominate the deformation of the Hellenides and the Ae-
gean region. Deeper seismicity, at subcrustal levels, as ex-
pressed below the Saronikos Gulf, is driven by the subduc-
tion of the lonian oceanic plate below Greece. Shallow
seismicity, on the other hand, concentrated at crustal levels,
is associated with extensional stresses due to deformation of
the upper crust. The oceanic lithosphere is subducted to the
northeast, whereas the continental crust and thinned litho-
sphere are uplifted and stretched westward. The crustal de-
formation is very complex involving rotation of blocks,
normal faulting, and crustal fragmentation. Significant west-
ward motion of the Hellenic napes creates a complex topog-
raphy that complicates even more the tectonic phenomena,
defining a third level of deformation at shallow depth, as
recognized by GPS observations (Kahle et al., 1995). This
complex deformation has been extensively discussed in the
geological and geophysical literature (see, e.g., Le Pichon
and Angelier, 1979; McKenzie, 1978; Rotstein, 1985) and
is also suggested on Figure 8.
Correlation of Seismicity with Tectonic Elements
Within the Saronikos Network
To correlate the seismicity with the tectonic elements
we used two sources of tectonic information: one is the Seis-
motectonic Map of Greece, published by IGME (1989) for
the onshore areas, and the other is the submarine neotectonic
map of Saronikos (Papanikolaou et al., 1988). Although ex-
tensive literature exists on the regional deformation of the
Hellenides (e.g., Papazachos and Delibasis, 1969; Papaza-
chos and Comninakis, 1971; Taymaz et al., 1991; Stiros,
1993) it is the two sources mentioned here that best describe
the Saronikos area.
The first observation worth mentioning is that the east–
west-oriented fault between the islands of Aegina and Sa-
lamis is also marked by significant seismic activity of shal-
low events. The seismicity can be followed onshore into east
Corinthia, delineating an active fault of more than 30 km
length. Such a fault can produce events on the order of mag-
nitude M
s
6.5 (see also Kiratzi et al., 1985). The fact that
historic catalogs (e.g., Papazachos and Papazachou, 1997;
Ambraseys and Jackson, 1990) do not report events from
this offshore area (see Fig. 3) is due to bias of observations
based on land-locked locations only.
Significant activity was also identified at the eastern
coast of Aegina, along a northeast–southwest-trending lin-
eament, mapped by Papanikolaou et al. (1988). At the vol-
canic areas of Methana and Aegina, on the other hand, we
did not map shallow seismicity worth mentioning. Although
many foci locations below the Saronikos Gulf are subcrustal
and associated with the subducting oceanic slab below the
Hellenides (see also Leydecker, 1975; Makropoulos, 1978;
Makris and Roever, 1986), we could not associate the seis-
mic activity with magma movements or hydrothermal pro-
cesses in the volcanic zone. This is probably due to the in-
adequate density of the local network, which did not permit
the identification of events of so small magnitude and vol-
canic noise.
Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 925
RMS Distribution Within the Network
0
20
40
60
80
100
120
140
160
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
RMS (sec)
Number of Events
RMS Distribution Outside the
Network
0
10
20
30
40
50
60
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
RMS (sec)
Number of Events
ERH Distribution Within the Network
0
20
40
60
80
100
120
140
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
ERH (Km)
Number of Events
ERH Distribution Outside the Network
0
20
40
60
80
100
120
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
ERH (Km)
Number of Events
ERZ Distribution Within the Network
0
20
40
60
80
100
120
140
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
ERZ (Km)
Number of Events
ERZ Distribution Outside the Network
0
20
40
60
80
100
120
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
ERZ (Km)
Number of Events
Figure 4. rms distribution within and outside the Saronikos Gulf seismic network.
Of interest is the area of Mount Parnis where the 1999,
M
s
5.9 Athens Eventoccurred. We observed a great con-
centration of microearthquakes close to the 1999 epicentral
location of the mainshock, indicating that the aftershock ac-
tivity is still ongoing and the stresses responsible for this
event have not yet been completely relaxed. Also the area
of Spata, in southeastern Attica is seismically active along a
fault of northeastsouthwest orientation, extending offshore
in the southern Evoikos Gulf (see Figs. 2, 6c, and 7).
Outside the network we recorded high seismicity in
three areas (see Fig. 2). One is the northeastern part of the
Corinth Basin, toward Thiva. This area is well known to
seismologists for its intense activity and, in particular, the
destructive earthquake sequence of 1981 that severely dam-
aged this region. The second area is the northeastern part of
central Evia (Fig. 2), north of Kymi as mentioned previ-
ously. This coastal part of the island and its immediate in-
terior are seismically very active. We could not dene the
faults precisely, since the network is located at a distance of
80 to 100 km. The lateral velocity variations in the crust and
sediments are intense and cause signicant systematic errors
in locating the hypocenters. Assuming a lateral homoge-
926 J. Makris, J. Papoulia, and G. Drakatos
Cumulative Magnitude Distribution
Within the Network
1
10
100
1000
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
Magnitude (ML)
logN(M)
Cumulative Magnitude Distribution
Outside the Network
1
10
100
1000
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
Magnitude (ML)
logN(M)
Limit Magnitude of
Com
p
leteness: ML 1.7
Limit Magnitude of
Com
p
leteness: ML 2.3
Figure 5. Frequency-magnitude distribution
within and outside the Saronikos Gulf seismic net-
work.
neous velocity model and reducing the three-dimensional
structure to one dimension only, location uncertainties in-
crease and errors may easily become systematic.
The third region we want to refer to is that to the east
and northeast of the island of Skyros (see Fig. 2). Here the
high seismicity recorded is associated with the aftershock
sequence of the M
s
5.8 Skyros Earthquakeof 2001. A
precise denition of the associated active faults is also not
possible for the reasons stated previously.
Conclusions
The Saronikos local seismic network, despite its limited
operation of 45 days, produced important results permitting
several conclusions on the existence and location of active
faults previously unknown or inaccurately mapped. The
shallow seismicity within the network is, to a great extent,
associated with the deformation and opening of the Corin-
thiakos Gulf, which is one of the most rapidly deforming
areas of Greece. This is particularly true for the eastwest
fault extending from Corinthia across the Saronikos Gulf,
between the islands of Salamis and Aegina. The deep, sub-
crustal seismicity, extending to a minimum of 100 km depth,
is obviously linked to the interaction of a subducted oceanic
Ionian Sea lithosphere with the mantle below Corinthia, the
Saronikos Gulf, and western Attica. The subducting litho-
sphere is limited to the north by the volcanic area of Li-
chades, in the northern Evoikos Gulf. Deep seismic events
reported by the Greek Seismological Services and our ob-
servations indicate that this is the northernmost limit of the
subducting slab. All the remaining seismicity is crustal. It is
controlled to the east, in the central and north Aegean Sea,
by transtension and normal faulting along the Corinthia Rift
due to rearrangement of crustal blocks escaping the com-
pression along the western Hellenides.
Use of a local velocity model obtained by a controlled-
source seismic experiment (Makris et al., 2002) signicantly
increased the location accuracy. The seismicity dened by
observations of the seismological network of Greece delin-
eates active faults by the concentration of large-magnitude
events observed over long periods of time. Local networks
can signicantly improve the picture by detecting and better
locating small-magnitude events over short periods of time.
This permits rapid assessment of the seismic hazard of an
area. The necessity for an accurate three-dimensional veloc-
ity model for the territory of Greece cannot be stressed
enough. It is mandatory if we wish to more accurately dene
the active faulting and provide the engineering community
with the reliable seismic hazard parameters needed.
Acknowledgments
We are grateful to two anonymous reviewers for their useful com-
ments, which contributed to the improvement of our article. We acknowl-
edge the assistance provided by GeoPro-Hamburg for the eld operations
and the instruments used in the network. We thank Captain C. Handras and
the crew of the R/V AEGAIO for their support during the marine opera-
tions. We specially thank Mr. D. Becker and Mr. S. Stoyanov from GeoPro-
Hamburg for the data processing and preparation of gures. Our eld
operations were nanced by the Earthquake Planning and Protection
Organization of Greece.
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Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 927
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928 J. Makris, J. Papoulia, and G. Drakatos
Figure 7. Location of events recorded within the Saronikos network. A minimum
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GeoPro GmbH
20457 St. Annenufer 2
Hamburg, Germany
(J.M.)
Institute of Oceanography
Hellenic Centre for Marine Research
19013 Anavyssos
Attica, Greece
(J.P.)
Geodynamic Institute
National Observatory of Athens
11810 Thission
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... However, several aspects provide sufficient motivation for a dedicated investigation into the Methana volcano. From a hazard point of view, the neotectonic basin constituting the Saronic Gulf area is considered seismically active [5], and the active fault systems therein were considered preferential paths for present-day geothermal fluid leakage and, as such, potential sites for magma uprising [3]. Thermal springs are indeed clear manifestations of volcanic geothermal energy in Methana, and gas exhalations suggest a mixture between a dominant hydrothermal component and mantle-derived fluids [3]. ...
... Additional to the surface seismicity, very deep (80-100 km) sporadic micro-events took place on a wider area extending from Methana towards Hydra Island. These deep micro-events may be attributed to the NE-subduction zone of the Ionian oceanic plate that reaches very deep in this area[5,54,59,60].Appl. Sci. ...
... Additional to the surface seismicity, very deep (80-100 km) sporadic microevents took place on a wider area extending from Methana towards Hydra Island. These deep micro-events may be attributed to the NE-subduction zone of the Ionian oceanic plate that reaches very deep in this area[5,54,59,60]. ...
Article
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The Methana volcano in Greece belongs to the western part of the Hellenic Volcanic Arc, where the African and Eurasian tectonic plates converge at a rate of approximately 3 cm/year. While volcanic hazard in Methana is considered low, the neotectonic basin constituting the Saronic Gulf area is seismically active and there is evidence of local geothermal activity. Monitoring is therefore crucial to characterize any activity at the volcano that could impact the local population. This study aims to detect surface deformation in the whole Methana peninsula based on a long stack of 99 Sentinel-1 C-band Synthetic Aperture Radar (SAR) images in interferometric wide swath mode acquired in March 2015–August 2019. A Multi-Temporal Interferometric SAR (MT-InSAR) processing approach is exploited using the Interferometric Point Target Analysis (IPTA) method, involving the extraction of a network of targets including both Persistent Scatterers (PS) and Distributed Scatterers (DS) to augment the monitoring capability across the varied land cover of the peninsula. Satellite geodetic data from 2006–2019 Global Positioning System (GPS) benchmark surveying are used to calibrate and validate the MT-InSAR results. Deformation monitoring records from permanent Global Navigation Satellite System (GNSS) stations, two of which were installed within the peninsula in 2004 (METH) and 2019 (MTNA), are also exploited for interpretation of the regional deformation scenario. Geological, topographic, and 2006–2019 seismological data enable better understanding of the ground deformation observed. Line-of-sight displacement velocities of the over 4700 PS and 6200 DS within the peninsula are from −18.1 to +7.5 mm/year. The MT-InSAR data suggest a complex displacement pattern across the volcano edifice, including local-scale land surface processes. In Methana town, ground stability is found on volcanoclasts and limestone for the majority of the urban area footprint while some deformation is observed in the suburban zones. At the Mavri Petra andesitic dome, time series of the exceptionally dense PS/DS network across blocks of agglomerate and cinder reveal seasonal fluctuation (5 mm amplitude) overlapping the long-term stable trend. Given the steepness of the slopes along the eastern flank of the volcano, displacement patterns may indicate mass movements. The GNSS, seismological and MT-InSAR analyses lead to a first account of deformation processes and their temporal evolution over the last years for Methana, thus providing initial information to feed into the volcano baseline hazard assessment and monitoring system.
... The region is characterized by moderate seismicity, with the northern and western Gulf margins exhibiting highmagnitude historical and recent earthquakes, associated with the presently active N-S extensional tectonic regime (Papazachos and Kiratzi 1996;Drakatos et al. 2005). Shallow seismicity in the Saronikos Gulf is attributed to the extensional stresses and the deformation of the upper crust while deeper seismicity, at sub-crustal levels, is related to the subducting oceanic slab at depths of 100-170 km, beneath Corinth, the Saronikos Gulf, and Attica (Makropoulos and Burton 1984;Papazachos et al. 2000;Makris et al. 2004). ...
... The southern part of the gulf is characterized by Pliocene age E-W extension, expressed as N-S to NW-SE striking normal faults (Papanikolaou et al. 1988;Papanikolaou et al. 1989;Hatzfeld 1999). Active NE-SWoriented faults occur in the NE Saronikos Gulf (Makris et al. 2004), while active NE-SW-oriented normal faults occur along the western coast of Peloponnese (Hatzfeld 1999). Similar NE-SW normal faults are also mapped along the entire Saronikos Gulf (Papanikolaou et al. 1989). ...
... At both the northern and the southern margins of the Salamina basin, active deformation along a NE-SW direction is observed. Major faults align with the onshore NE-SW tectonic lineaments of the Piraeus-Athens region and the Parnitha and Hymettus mountains (Papadimitriou et al. 2002;Makris et al. 2004;Papanikolaou et al. 2004), as well as with the offshore major NE-SW dextral shearing strike slip zones (Mascle and Martin 1990;Sakellariou and Tsampouraki-Kraounaki 2019). Minor synsedimentary faulting is observed within Salamina basin, and it is restricted below marker Z (Fig. 4a), implying a recent stable phase for the last 130 ka. ...
Article
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The Saronikos Gulf hosts the northwestern end of the South Aegean Active Volcanic Arc. It is affected by extensional back-arc tectonism in a prominent N-S direction, while older E-W rifting is also manifested. Recently acquired swath bathymetry and high-resolution seismic profiles led to the precise morphological mapping of five Quaternary continental shelf basins, along with their neotectonic delineations and their cross-correlation to the onshore active fault zones. Aegina and Methana basins, in the southeast Saronikos Gulf, display subsidence along ENE-WSW and NW-SE axes, and continuous marine sedimentation throughout the Upper Quaternary. Salamina basin in the northeast Saronikos Gulf is a shallow, relatively stable sedimentary basin, intermittently connected to the SE and NW Saronikos Gulf through the Aegina-Fleves and the Aegina-Salamina plateau respectively. In the western and most active region of Saronikos Gulf, the onshore E-W–trending faults of Kechrees and Agios Vassileios extend eastwards until the Salamina-Aegina plateau. The Megara basin is WNW-ESE oriented, possibly affected by the N-S Korfos fault valley, which connects the Megara and Epidavros basins. E-W–trending faults tilt the Quaternary sedimentary blocks, in compatibility to the N-S extensional tectonic regime of the adjacent Gulf of Corinth. Prograding clinoforms in the Megara and Epidavros basins reveal an Upper Quaternary-age intermittent connection to the open sea. Mass transport deposits and volcanic flows in-between the Upper Quaternary sediments of the Epidavros basin manifest the 500–400 ka tectonic phase that affected both the NE Peloponnese coast and the active N-S and NW-SE to WNW-ESE striking faults of the Epidavros basin margin.
... Although the volcanic hazard of this volcanic system is considered to be low (Vougioukalakis and Fytikas 2005) it has never been studied in depth. The Saronikos gulf area, to which Methana belongs, is a neotectonic basin considered to be seismically active (Makris et al. 2004) and active fault systems have also been recognised on the peninsula. These tectonic lineaments are the paths for present geothermal fluid leakage and are also potential sites for magma uprise. ...
... The area around Methana was characterized in recent times by extensional tectonic (Makris et al. 2004). In the geological map of Methana (Dietrich and Gaitanakis 1995) the authors evidence many fault systems. ...
... 25°-50°) directions. Makris et al. (2004), through microseismicity studies of the Saronikos gulf, attribute most of the seismic activity in the area to W-E and SW-NE tectonic structures. ...
Article
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An extensive geochemical survey on the fluids released by the volcanic/geothermal system of Methana was undertaken. Characterization of the gases was made on the basis of the chemical and isotopie (He and C) analysis of 14 samples. CO2 soil gas concentration and fluxes were measured on the whole peninsula at more than 100 sampling sites. 31 samples of thermal and cold groundwaters were also sampled and analysed to characterize the geochemistry of aquifers. Anomalies referable to the geothermal system, besides at known thermal manifestations, were also recognized at some anomalous degassing soil site and in some cold groundwater. These anomalies were always spatially correlated to the main active tectonic system of the area. The total CO2 output of the volcanic system has been preliminary estimated in about 0.2 kg s~ . Although this value is low compared to other volcanic systems, anomalous C02 degassing at Methana may pose gas hazard problems. Such volcanic risk, although restricted to limited areas, cannot be neglected and further studies have to be undertaken for its better assessment.
... The Argive basin contains observed and inferred normal faults (Fig. 2), some traditionally considered as active and others as probably active faults (Papastamatiou et al., 1960;Tataris et al., 1970;Papanastassiou et al., 1993;Papanikolaou et al., 1994; European Center on Prevention and Forecasting of Earthquakes and Protection Organization [ECPFEPO], 1996;Georgiou and Galanakis, 2010). Within a distance of 150 km, the basin is surrounded by active source zones (Papadopoulos and Kijko, 1991;Papazachos and Papaioannou, 1993;Papaioannou and Papazachos, 2000;Makris et al., 2004;Makropoulos et al., 2012), including the Patras-and-Corinth continental rift (PCCR) system passing into the dextral Kephalonia-Lefkada transform fault system in the Ionian Sea in the north, the oceanic trench of the Hellenic subduction zone (HSZ) offshore western and southern Peloponnese with its downgoing slab extending underneath the Argive basin, and the Iria-Epidaurus sinistral transform (IEST) fault system located in the southeast Argolis Peninsula (Fig. 1). ...
... Late Neogene-Quaternary sedimentary basins of the Peloponnese, including the Argive basin, show a much lower seismic activity than the surrounding source zones; nevertheless, seismicity within the Argolis Peninsula includes some crustal earthquakes and events down to 150 km depth in the downgoing slab (Lyon-Caen et al., 1988;Papazachos et al., 1988;Hatzfeld et al., 1989;Papadopoulos and Kijko, 1991;Hatzfeld and Martin, 1992;Hatzfeld et al., 2000;Makris et al., 2004). ...
Article
Observations at Mycenaean archaeological sites of tilted and curved walls, broken pottery, and human skeletons led to the hypothesis that these sites in the Argolid, Peloponnese, Greece, were destroyed in large earthquakes between the late palatial (thirteenth century B.C.E.) and postpalatial (1200–1050 B.C.E.) periods. In particular, the destruction of Mycenaean palaces around 1200/1190 B.C.E. has often been attributed to a devastating earthquake. To test the Mycenaean earthquake hypothesis, this project focuses on the Argive citadels of Tiryns and Midea. With active and passive seismic measurements complemented by a gravimetric survey, we explored seismic site effects at these locations and calculated synthetic seismograms for potential earthquake sources to estimate intensities of ground motions inside and outside the citadels. The field work and results were supplemented by analysis of the individual damage descriptions and observations from the archaeological literature on which the hypothesis is based. Because of poor construction techniques and the associated site effects, the buildings in the Lower Town surrounding the citadel of Tiryns were more vulnerable than the structures within the Cyclopean palace walls, but evidence of an earthquake destruction stratum in the Lower Town has not yet been found. Although some of the observations from the two investigated citadels could be explained by seismic loading, alternative nonseismic causes could equally explain most observed damage. In some cases, the structural damage was clearly not caused by earthquakes. Simulated ground motions show that severe earthquake damage at Tiryns and Midea can be expected from activation of local faults in the Argive basin; however, palaeoseismic studies for such activity in and since the Late Bronze Age (LBA) are lacking. Our results indicate that the hypothesis of a destructive earthquake in Tiryns and Midea, which may have contributed to the end of the LBA Mycenaean palatial period, is unlikely.
... The study of such archives is critical for in-depth understand-playing modest activity [55]. Several volcanoes and volcanic outcrops of Plio-Quaternary age, being part of the western active Aegean Volcanic Arc, are situated in the gulf [56]. ...
Article
Full-text available
Coastal landscapes are sensitive to changes due to the interplay between surface and submarine geological processes, climate variability, and relative sea level fluctuations. The sedimentary archives of such marginal areas record in detail the complex evolution of the paleoenvironment and the diachronic biota response. The Elefsis Bay is nowadays a landlocked shallow marine basin with restricted communication to the open Saronikos Gulf. A multi-proxy investigation of a high-resolution sediment core recovered from the deepest part of the basin offered a unique opportunity to record the paleoenvironmental and aquatic ecosystem response to climate and glacioeustatic sea level changes since the Late Glacial marine transgression. The retrieved sedimentary deposits, subjected to thorough palynological (pollen, non-pollen palynomorphs, dinoflagellates), micropaleontological (benthic foraminifera, calcareous nannoplankton, ostracods), and mollusc analyses, indicates isolation of the Elefsis Bay from the Saronikos Gulf and the occurrence of a shallow freshwater paleolake since at least 13,500 cal BP, while after 11,3500 cal BP the transition towards lagoon conditions is evidenced. The marine transgression in the Elefsis Bay is dated at 7500 cal BP, marking the establishment of the modern marine realm.
... The post-seismic activity continued for a period of 13 months. Some researchers (Sebrier 1977;Mettos et al. 1988;Rondoyanni and Papadakis 1992;Jackson et al. 1982;Jackson 1994b;Tselentis and Zaradnik 2000;Makris et al. 2004) mentioned that the "Kakia Skala" fault is of great importance and possible to be approximately related with an earthquake of Ms = 6.5 Richter. Earthquakes of this size are likely to cause a significant movement along the crack and cause rockfalls. ...
Article
Because rockfalls cause significant hazard on human activities and infrastructure, the rockfall analysis is considered to be very important along roads and on other geotechnical works. Especially, in Greece owing to the complexity of geological structure, the tectonic characteristics, the climate and the high seismicity, this analysis is of great importance. This paper presents the topographical, geological, tectonic and seismic features and determines the Rock Mass Rating and Rockfall Hazard Rating System values of the tectonically disturbed limestones in the area of “Kakia Skala”, which is part of the “Patra–Athens–Thessaloniki–Evzoni (P.A.TH.E.)” motorway—one of the most major highways in Greece. For this reason, after geological and geomorphological investigations, bibliographic references and the characteristics of the road construction (tunnels, embankments, trenches, etc.), the most dangerous positions were identified (eight). Finally, the two most dangerous sites were selected for separate rockfall analysis. The RocFall software is used for the estimation of the unstable blocks trajectories and their kinematic characteristics. The appropriate support measure, with the right dimensions, is selected according to the kinetic energy of the falling rocks, and it is placed at the appropriate location.
... This was not the case for the offshore area of Saronikos Gulf, where the lack of nearby stations increased these uncertainties to a value of 5 km or larger. This is unfortunate, because this area is well known for its high-seismicity levels (Makris et al., 2004). The installation of additional stations at the islands of Saronikos and at the coast of Peloponnese would significantly increase the quality of absolute locations and enhance our knowledge on the existence of active faults offshore. ...
Article
Full-text available
The existence of active faults near large cities poses significant risk to the life and property of its inhabitants as well as to its public infrastructure. Here, we investigate the interplay between seismicity, active faulting, and interseismic strain accumulation within a radius of ∼50 km from the metropolitan area of Athens, the capital of Greece. We find that during the period 2011–2018, a total of 4722 earthquakes were recorded, the majority of which had local magnitudes <3.0 with only four events being of moderate magnitude (ML 4.1–4.3). Precise relative locations with horizontal and vertical errors of ≤1 and 2 km, respectively, were obtained for 2666 of these events using the double-difference algorithm. Earthquake relocation was compared to the surface traces of 31 active and 49 “less-active” normal faults drawn from high-resolution (∼5 m pixel size) digital elevation models and complemented by analysis of geodetic data from 30 permanent Global Positioning System (GPS) stations. Joint analysis of these datasets suggests that microseismicity mostly clusters along the “less-active” faults, whereas the faults associated with impressive postglacial scarps (indicating recent activity) and historic seismicity are mostly quiet. Interestingly, GPS data indicate that both fault types currently accumulate elastic strain that ranges from 0.5 to 2.3 mm/yr. Based on their estimated rupture area, more than half of the recorded faults (N=54) are capable of generating earthquakes with moment magnitudes between 6.0 and 6.6. Although some of these sources are characterized by impressive postglacial scarps, many others have long earthquake recurrence intervals (i.e., have not ruptured during the past ∼16 ka) and are associated with intense microseismicity and elastic strain accumulation, calling for future investigations on their seismogenic potential.
... Clusters are also identified close to Sousaki and in the NW termination of the Saronikos Gulf (Fig. 2b). Makris et al. (2004), using data recorded by a temporal local network, highlighted the existence of much deeper foci in the area, related to the subduction of the Tethys oceanic plate below Greece. Recent tomographic studies have shown that the slab depth in the area varies between 80 and 100 km Halpaap et al. 2018). ...
Article
Full-text available
The broader South Aegean area is characterized by the convergence of the African and Eurasian lithospheric tectonic plates at a rate of approximately 30 mm/year, resulting in a complex and intense deformation of the Aegean (McClusky et al. Geophys. Res. 105: 5695–5719, 2000). The Saronikos Gulf, situated in the NW termination of the Hellenic volcanic arc (Central Aegean Sea), is an area of special geophysical interest due to the variety of the recorded seismicity, regarding both the origin (tectonic, volcanic) and the focal depths. The presence of Plio-Quaternary volcanic centers and the complex seismicity distribution urge the need of detailed research on the geophysical properties such as seismic velocities, revealing possible presence of magmatic bodies or geothermal fluids. In the present study, more than 3000 manually revised events were located using a local 1-D layered velocity model. The main spatiotemporal clusters are concentrated along the Leuces and Methana-Poros E-W striking neotectonic fault zones. A local earthquake tomography (LET) was performed, using body-wave (P and S) travel time data, to investigate small to medium scale (~ 10 km) anomalies that can be related to local neotectonic or volcanic patterns. Major shallow ~ E-W to NW-SE trending discontinuities between positive (to the north) and negative (to the south) VP perturbations, anticorrelated to the respective VS image, are identified south of Aigina Island. These results are mainly related to the principal local stress orientation and fault zones (~ E-W to NW-SE strike), such as the Moni, Anghistri, and Leuces, whereas the respective Vp/Vs ratio values imply the presence of possible magmatic features which can be linked to the Plio-Quaternary NW edge of the Hellenic volcanic arc and hydrothermal activity ENE of Poros Island.
Preprint
Full-text available
The broader South Aegean area is characterized by the convergence of the African and Eurasian lithospheric tectonic plates at a rate of approximately 30 mm/yr, resulting in a complex and intense deformation of the Aegean (McClusky et al. 2000). The Saronikos Gulf, situated in the NW termination of the Hellenic Volcanic Arc (Central Aegean Sea), is an area of special geophysical interest due to the variety of the recorded seismicity, regarding both the origin (tectonic, volcanic) and the focal depths. The presence of Plio-Quaternary volcanic centers and the complex seismicity distribution urge the need of detailed research on the geophysical properties such as seismic velocities, revealing possible presence of magmatic bodies or geothermal fluids. In the present study, more than 3000 manually revised events were located using a local 1-D layered velocity model. The main spatio-temporal clusters are concentrated along the Leuces and Methana-Poros E-W striking neotectonic fault zones. A Local Earthquake Tomography (LET) was performed, using body-wave (P and S) travel-time data, to investigate small to medium scale (~10 km) anomalies that can be related to local neotectonic or volcanic patterns. Major shallow ~E-W to NW-SE trending discontinuities between positive (to the north) and negative (to the south) VP perturbations, anticorrelated to the respective VS image, are identified south of Aigina Island. These results are mainly related to the principal local stress orientation and fault zones (~E-W to NW-SE strike), such as the Moni, Anghistri and Leuces, whereas the respective Vp/Vs ratio values imply the presence of possible magmatic features which can be linked to the Plio-Quaternary NW edge of the Hellenic Volcanic Arc and hydrothermal activity ENE of Poros Island.
Chapter
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A quantitative approach of the earthquake risk, in a long-term and large-scale sense, has been made for 23 distinct regions of Greece. The risk, Ri, in each region, i, has been determined as a convolution of earthquake hazard, buildings oldness coefficient, average population density and regional Gross Domestic Product per capita. A relative risk scale was adopted by assinging each region with Rri = Ri/min Ri. The final product is a map of large-scale, relative earthquake risk distribution in Greece, which allows the elaboration of strategic plans for the risk mitigation.
Article
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The validity of existing tectonic models for the area of Greece is examined in the light of the new recalculated parameters for earthquakes of the region (Makropoulos and Burton, 1981). Relocated hypocentral positions are extracted from the catalogue to form radial and vertical distance-depth cross-sections centred on a reference point near the mid-point of the Aegean Volcanic arc, and these are used to form a three dimensional topography of the base of earthquake occurrence below 60 km. Isodepth maps are extracted from this topography as both three and two dimensional map presentations. These maps reveal several significant features of deep-seated tectonic processes in the region. Isodepths exceeding 150 km are seen in the northwest Aegean, and these are more closely linked to the Sporadhes and Gulf of Thermaicos, rather than the North Aegean trough. The 150 km isodepths are also seen in the northeast Aegean straddling the Dardanelles; in the northeastern part of the Peloponnesus, Gulf of Saronikos and eastern Gulf of Corinth in the southern Peloponnesus towards Crete; and extending from north of Kos to south of Rodos. The largest extent of deepest activity is seen south of Rodos and this continues towards southwest of Turkey. The subduction zone of the Hellenic arc is clear, but smooth Benioff zones are not the norm, and these data show that structural complexity is more readily observed. It is concluded that none of the proposed tectonic models completely explain the observed activity over the whole area, and rather than propose yet another model places where further work is still particularly necessary are identified.
Article
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An earthquake catalogue for Greece is presented covering the period 1900–1985. It is based on a similar effort attempted a few years ago. The present version contains more than double the number of events, i.e. 4310 events compared with 1806 events, than in the previous papers. It also includes another 1711 events which took place in the region in an Appendix. The accuracy and completeness of the parameters of these shocks were insufficient for inclusion in the main catalogue without affecting the catalogue's homogeneity. For the new entries magnitudes are redetermined using Uppsala's reading as before, whereas the other parameters have been rechecked against local macroseismic information. The completeness of this catalogue varies according to the period of observation, but the lower threshold of completely reported magnitudes has been substantially decreased allowing for more detailed seismicity studies to be performed.