Microseismicity and focal mechanisms at the western termination of the North Anatolian Fault and

18 Pages·1999·3.44 MB·English

Geophys.J.Int.(1999)137,891–908
Microseismicity and focal mechanisms at the western termination of
the North Anatolian Fault and their implications for continental
tectonics
Denis Hatzfeld,1 Maria Ziazia,2 Despina Kementzetzidou,1
Panayotis Hatzidimitriou,3 Dimitris Panagiotopoulos,3 Kostas Makropoulos,2
Panayotis Papadimitriou2 and Anne Deschamps4
1LaboratoiredeGe´ophysiqueInterneetTectonophysique,IRIGM-CNRS,BPP53X,38041GrenobleCedex9,France.
E-mail:[email protected]
2DepartmentofGeophysics,UniversityofAthens,Ilissia,15784Athens,Greece
3GeophysicalLaboratory,AristotleUniversity,BP352–01,54006Thessaloniki,Greece
4Ge´osciences-Azur,250av.AlbertEinstein,06560Valbonne,France
Accepted1999January29.Received1998October10;inoriginalform1997December3
SUMMARY
For seven weeks, a temporary network of 68 seismological stations was operated in
Central Greece, in the region of Thessaly and Evia, located at the western termination
of the North Anatolian Fault system. We recorded 510 earthquakes and computed 80
focalmechanisms.SeismicactivityisassociatedwiththeNE–SWdextralNorthAegean
Fault,orwithveryyoungE–W-strikingnormalfaultsthatarelocatedaroundtheGulf
of Volos and the Gulf of Lamia. The important NW–SE-striking faults bounding the
Pilion,orthebasinsofLarissaand Karditsa,are notseismically active,suggestingthat
itiseasiertobreakcontinentalcrust,creatingnewfaultsperpendiculartotheprincipal
stresses, than to reactivate faults that strike obliquely to the principal stress axes
Key words: continental deformation, faulting, microearthquakes, seismotectonics.
and requires extension of approximately that amount within
INTRODUCTION
the Aegean(Noomenet al.1995).
Continental deformation is commonly more complicated TheNorthAegeanregionisaVectedbyintensedeformation.
than plate tectonics predicts, for it is not limited to narrow The seismicity is relatively high and defines a limited number
boundaries between rigid lithospheric plates, but aVects wide of active features (Fig.1). The most prominent is the North
areas with horizontal dimensions much larger than the litho- Aegean Trough, which formsone ofthe three branches ofthe
sphere thickness (England & Jackson 1989). The strength of western continuation of the North Anatolian Fault (Lyberis
continentallithosphereappearstobelessthanthatofoceanic 1984; Jackson & McKenzie 1988; Taymaz et al. 1991). In
lithosphere, in part because most continental regions have contrast, in the southern Aegean (Peloponnese, Sea of Crete,
beenaVectedbyprevioustectonicevents.Crustalblocksmove DodecaneseandCycladesislands),extensionof~1.0–1.5cmyr−1
relativetooneanothergenerallyinacomplexkinematicscheme, occurs nearly parallel to the Hellenic Trench (Noomen et al.
and the origins of the forces that drive the crustal blocks still 1995, Robbinsetal. 1994;Kastens etal. 1995).
remain a matter of debate. To what extent does the upper Severalgeodynamicalmodelshavebeenproposedtoexplain
crust passively respond to the motion of a ductile lower the deformation and the kinematics of the North Aegean.
crust and upper mantle (e.g. England & McKenzie 1982; Some models imply one (McKenzie 1972) or two (Le Pichon
Molnar 1988; Lamb 1994), or to forces applied to its edges et al. 1995) small rigid lithospheric plates that move relative
(e.g.Tapponnier1977; Nuretal. 1986)? to Eurasia. Others include crustal blocks of smaller size (e.g.
TheAegeanregion(Figs1and2)isoneofthemostrapidly McKenzie & Jackson 1983; Taymaz et al. 1991) that interact
deforming regions in the continental domain (Jackson 1994; over aviscoplasticsubstratum(e.g. England etal. 1985).
Papazachos & Kiratzi 1996). Although located between the Insomemodels,thegeodynamicsoftheAegeanisgoverned
two major lithospheric plates of Eurasia and Africa, whose bythemotionofArabiaandTurkeyrelativetoEurasia,which
1cmyr−1convergentmotionisapproximatelyN–S(McKenzie implies a synchronous motion for both the North Anatolian
1972, 1978; De Mets et al. 1990), the Hellenic Trench moves Fault andthe Aegean.However, the extensionthataVectsthe
southwestwards at about 3.5cmyr−1 with respect to Eurasia Aegean region began a least 15Myr ago and therefore it is
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892 D. Hatzfeld et al.
Figure1. General sketch of the Aegean showing seismicity and geographical names. White dots mark locations of earthquakes of magnitude
greaterthan4.5from1963to1996locatedbytheNEIS.ThewhitearrowshowsthemotionofAegearelativetoEurope.Theboxsurroundsthe
areastudiedhere.
earlier than the Upper Miocene start of motion along the
SEISMICITY AND TECTONICS
NorthAegeanTrough(Angelieretal.1982;Mercieretal.1989;
Jolivetetal. 1994).Therefore,the geodynamicsof the Aegean The North Aegean Trough follows the western continuation
seemstobeindependentof(oronlyweaklydependenton)the of the dextral strike-slip North Anatolian Fault, which in
movement of Anatolia (Le Pichon et al. 1995). McKenzie & western Turkey, before entering the North Aegean Sea, splits
Jackson(1983)proposedthattheNE–SWright-lateralstrike- intothreebranches(Taymazetal.1991).Thenorthernbranch
slip motion on the North Aegean Trough is taken up on a is the most seismically active(Figs 1 and 2) and is associated
wide zone of normal faulting that is responsible for block withanimportantbathymetricfeatureconnectedtotheSaros
rotations.Pavlides&Caputo(1994)suggestedthattheNorth and Sporades basins with a depth of 1500m (Lyberis 1984),
Anatolian Fault propagated southwestwards into the Aegean. whichwerereactivatedduringPliocenetime(Dinter&Royden
Armijo et al. (1996) proposed that this fault is deeply rooted 1993). In contrast, the southern branch is less developed, the
in the lithosphere and that its propagation is responsible for associated basins are not as deep, and the amount of motion
the recent increase of tectonic activity in the pre-existing rifts along the fault zone is probably smaller (Mascle & Martin
of the North Evia Basin, the Gulf of Evia and the Gulf of 1990).
Corinth.ThisincreaseofactivityintheGulfofCorinth1Myr AllthreebranchesoftheNorthAegeanFaultstopabruptly
agoissynchronouswithachangeindirectionoftheextension at the Saros and Northern Evia basins. No well-defined
that is documented in most of the Northern Aegean region tectonicstructureresponsibleforthetransferofmotiontothe
such as the Mygdonia graben (Mercier et al. 1983), western GulfofEviaandtheGulfofCorinthhasbeenrecognized.The
Macedonia (Pavlides & Mountrakis 1987) and Thessaly Sarosand Eviabasinsare bounded,east ofPilion andeast of
(Caputo&Pavlides1993). Evia, by very important NW–SE-striking faults that were
Inthiscontext,westudiedtheseismotectonicsofthewestern activeduringPliocenetime(Caputo1990;Roberts&Jackson
termination of the North Aegean Trough and its relationship 1991; Armijoetal. 1996).
with the tectonics of continental Greece in order to address The tectonics of continental Greece underwent changes
the questions related to the tectonics (active faulting) and in late Cenozoic time that varied from region to region.
kinematics (both in space and in time) of this area and infer In Thessaly, after the orogenic compressive episode that
itspossibledynamics. aVectedwesternGreeceduringtheEarlyMiocene,twodiVerent
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Figure2. Mapshowinglower-hemisphereprojectionsoffocalmechanismscomputedinthenorthwestAegeanregionforteleseismicdata.Black
andgreyquadrantsincludecompressionalfirstmotionsformechanismsmodelledbyTaymazetal.(1991)andHarvardCMTsolutions,respectively.
Figure3. MapofthetemporaryseismologicalnetworkinstalledinThessalyandLocridduringthesummerof1992forsevenweeks.Thelocations
ofhistoricalearthquakes(Papazachos&Papazachou1997)andofearthquakeswithmagnitudesgreaterthan4.5(NEIS)areshownbyboxesand
circles,respectively.Thesmokedpaperanddigitalseismologicalstationsareshownbyopenandfilledtriangles,respectively.
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894 D. Hatzfeld et al.
extensionphasesareobserved.Thefirstextensionphase(Late upgoingrays.Amongthe80focalmechanisms,22(categoryA)
Miocene–Pliocene), possibly due to post-orogenic collapse, had three quadrants sampled and the azimuth of both planes
trendedNE–SW(Mercieretal.1976,1979;Caputo&Pavlides was constrained to within 10°, 27 (category B) had one plane
1993) and created the two important basins of Larissa and constrainedtowithin10°andtheothertowithin20°.Forthe
Karditsa. The second extension phase, trending N–S, started remaining 31 (Category C), we relaxed the criteria regarding
in the Middle Pleistocene and activated many E–W-striking thequadrantsandthereforethefaultplanesvariedbyasmuch
faults. The later episode is certainly the less well developed, as 30° but they still gave an indication of the type of faulting
with extension of only ~1mmyr−1 (Caputo 1995, 1996; (strike-slip,normalor reverse).
Caputo & Pavlides 1993). To the south, the tectonics are
dominatedbylargefaultsstriking WNW–ESEthatdip either
RESULTS
totheNEortotheSW,andareresponsiblefortheformation
of important basins such as the Gulf of Evia and the Gulf Withatimespanofonlysevenweeks,thefewhundredevents
of Corinth (Mercier et al. 1976; Roberts & Jackson 1991). that we located may not be representative of the long-term
Microtectonicobservationssuggestthatmotionisdiachronous deformation. Their virtue, however, lies in the fact that their
onthesamefault(Philip1976).DuringthePliocene,extension locations are much more accurate than those recorded only
trendedNE–SWasinThessaly,butduringtheQuaternarythis byteleseismicorregionalstations.Toensurethatourdatawere
orientationchangedtoNNW–SSEwithasmallcomponentof representative, we smoothed our locations and mechanisms
strike-slipmotiononthe faults. over a scale consistent with regional tectonics and compared
ThereisstrongevidencethatcontinentalGreeceunderwent our data with those of stronger (historical and teleseismically
extension with a direction variable in time. Most of the computed) events in the area. The magnitudes of the earth-
importanttectonicstructuresareduetotheNE–SW-trending quakes ranged from 0.4 to 4.5 with a maximum number of
Pliocene extension which generated basins bounded by faults events around magnitude 2.0. The depths of the earthquakes
striking NW–SE, but this extension changed to N–S or rangedbetween thesurfaceand 34km, butthegreat majority
NNW–SSE during the Quaternary, and it is probably still were locatedatadepthshallower than15km.
active,asattestedbyseismicity.Thischangeintheorientation The lack of subcrustal seismicity, even over such a short
is also seen on a few islands of the Northern Aegean Sea period of time, concurs with the inference that the northern
(Mercier et al. 1989; Pavlides & Tranos 1991) which are extentofHellenicsubductionlieslargelysouthofournetwork.
crossedby largestrike-slip-motionactivestructures. During a previous microearthquake study, we located some
events at depths greater than 120km beneath southern Evia
(Hatzfeld et al. 1990). Reliable teleseismically located inter-
DATA
mediateseismicityalsostopsbeneathsouthernEvia(Hatzfeld
Duringthe summerof1992(fromJuly1untilAugust25),we &Martin1991).Therefore,itislikelythatintermediate-depth
operated a temporary network of portable stations covering seismicity related to the Hellenic slab does not extend to the
southern Thessaly, Locrid, northern Evia and the islands of northofEvia.
Skiathos,Skopelos and Alonissos.The purpose of this was to Theshallowseismicityisnotuniformlydistributedthrough-
study the seismicity and focal mechanisms of the area and outtheregion.WeobserveaclusterassociatedwiththeNorth
theirconsistencywiththesurfacetectonicstructures,especially Aegean Trough, where the strongest event (magnitude 4.5)
for northern Evia and Locrid (Fig.3). The network consisted occurred during the experiment. We observe clusters also
of 68 stations, 62 of them recorded on smoked paper using around the Gulf of Volos, the Pindus boundary, the Gulf of
1Hz vertical seismometers, and six of them Lennartz digital Evia and the Gulf of Corinth, which is located outside our
recorders connected to 1Hz 3-D seismometers. The average network.NotetheslightdiVerencebetweenlocationsfromthe
spacing between stations was less than 15km in order to International Seismological Center (NEIS) (Fig.3) and our
ensureaccurateestimationsofthefocaldepthsforcrustalevents locations (Fig.4),especiallyaround Evia andeast Locrid.
andtoallowagoodcoverageoffirst-motionpolaritiesonthe In the following, we describe our results in detail, region
focal sphere. All internal clocks were calibrated periodically byregion.
with a DCF radio telemetered time code, and we believe the
time to be accurate to within 0.1s. For the strongest events
TheNorthAegeanTroughandtheSporadesBasin
we also used the permanent stations of the University of
Thessaloniki. This region lies outside our network. Because the strongest
More than 9000 P+S phases allowed us to locate 510 event(Md>4.5)andtherelatedaftershockswerealsorecorded
earthquakes recorded by more than four stations (Fig.4). by the permanent network of the Aristotle University of
Details of the procedure used to determine the velocity Thessaloniki, we could determine more precise locations than
structure and the Vp/Vs ratio, to locate the earthquakes and would have been possible with our network alone and we
to select the most reliable events are discussed in Hatzfeld computedfault planesolutions forsomeoftheseevents.
et al. (1990). The velocity structure for the Northern Aegean The seismicity is distributed in two main clusters (Figs 5
Sea diVers from that for continental Greece (Table 1). The and 6). The northern cluster lies between the Sporades Basin
computedVp/Vsratiois1.78.Amongtheearthquakesthatwe and the Saros Trough. Relative to the surrounding high-
located, uncertainties in epicentre and depth for 318 of them bathymetryareas(LePichonetal.1985),thistroughisactually
werelessthan5km,andfor246lessthan2km.Themagnitude averynarrowanddeep(greaterthan6000m)basinfilledwith
wascalculatedusing thecoda duration(Lee& Lahr1972). Neogene sediments and constitutes the easterntermination of
Focal mechanisms were computed for earthquakes that the Sporades Basin (Le Pichon et al. 1985). Seismic activity
were recorded with more than eight first-motion polarities of followsthedirectionofthisnarrow,deepbasinandisconsistent
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Microseismicity around Volos (Greece) 895
Figure4. Mapshowingthe510epicentresthatwelocatedwithourtemporaryseismologicalnetworkduringthesummerof1992.
with the longer-term seismicity mapped with NEIS data. The the southern termination of the Pilion peninsula. Therefore,
focalmechanismscomputedforthemagnitude4.5earthquake thestrike-slipeventsareunlikelytoberelatedtotheimportant
of July 23 (#127) and its aftershocks (#128, 130, 131, 132, normal fault that bounds the Pilion area seawards and
142,152)show strike-slip faulting. However, noneofthe fault strikesNW–SE.
planes is aligned with the direction of the deep bathymetry
or basement. In general, one plane strikes almost E–W and
TheGulfofVolosandsouthernThessaly
dips north, whilst the other strikes N–S and dips west. The
orientation of the E–W-striking plane is well determined Most earthquakes in this region occurred either north of the
because of the clear diVerence in polarities at stationslocated Gulf of Volos or on the western border of the Karditsa Basin
on the mainland. For the strike-slip events, the mechanisms (Figs 7 and 8). We located no seismic activity that can be
diVer only slightly from those computed for large earth- clearly related to the Larissa Basin or to the Pilion and its
quakes in the North Aegean Trough (Fig.2) (Taymaz et al. edges. The clear normal faults that dip eastwards and bound
1991). theSEedgeofthePilionwereinactiveduringourexperiment.
The southern cluster lies at the western border of the AroundtheKarditsaBasin,seismicactivityislocatedatthe
Sporades Basin, just north of Skopelos, where the deepest westernedge,whereastrong(Ms7.0)earthquakeoccurredin
basin is located. The bathymetry is 1000m but the basement 1954 and was associated with N–S extension (Papastamatiou
is more than 10000m deep (Le Pichon et al. 1985). More & Mouyaris 1986). Most of this seismicity is restricted to the
precisely we can observe two subclusters. The first is located Pinduschain,whosewesternsideisalsoquiteactive(Hatzfeld
at the southern edge of the basin, with no clear shape, et al. 1995). The seismicity reveals no clear relation to the
suggesting a single active fault. The second is located just NW–SE-strikingfaultsystemthatboundstheKarditsaBasin,
northofthe islands ofSkiathos and Skopelos andcrosses the and we could not compute focal mechanisms because these
NW–SE-striking faults inferred from the bathymetry (Lyberis earthquakeswere locatedoutsidethe network.
1984).Wecouldnotdeterminemechanismsforthefirstcluster North of the Gulf of Volos, the seismic activity is concen-
becauseitislocatedoutsideournetwork,andtheearthquakes trated in two clusters. The first is clearly associated with the
were not strong enough to be recorded by the permanent Nea Anchialos Fault system, which strikes E–W, dips south-
stations.TheearthquakeslocatednorthofSkiathos(#119,121, wards and bounds the northern edge of the Almyros Basin.
125, 159, 396, 440) clearly show strike-slip motion with one This region experienced several strong earthquakes during
planestrikingNE–SW,approximatelyparalleltothestrikeof historical time in 743 and 1773 according to Papazachos &
the North Aegean Trough, and the other NW–SE, which Papazachou (1997), and in 1980 when a magnitude Ms 6.5
diVersbyabout30°fromthestrikesofnormalfaultsbounding event occurred. The foreshocks and aftershocks of the latter
the Pilion to the east. This seismicity pattern continues west event are well aligned with the Nea Anchialos Fault and the
towardstheGulfofVolos,butthestyleoffaultingprogressively focal mechanismindicatesaN–S-trendingextensionalongan
changestonormalfaulting(#24,25,141,171,425,448)across E–W-striking fault (Papazachos et al. 1983). The distribution
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Table1.
of the microearthquakes confirms that the Nea Anchialos seismicity to any of those structures. Nevertheless, the Nea
Fault continues eastwards across the Pilion as proposed by PagasaeFaultseemstobethemostplausiblebecausethedipof
Caputo (1996). A N–S-trending section (Fig.9a) across the the structure fits the distribution of the earthquakes located at
faultshows south-dippingactivitydown to10km. about5kmdepth(Fig.9b).
The second cluster is located near the region of Velestino, TheNeaAnchialosandNeaPagasaefaultsareE–W-striking
whereanearthquakeofmagnitude6.8andtwoothersofmagni- antithetic normal faults. The better-constrained focal mech-
tudegreaterthan6.0occurredwithin12hrin1957(Papazachos anisms (#4, 209, 356), as well as the less well-constrained
& Papazachou 1997). No ground rupture was observed, and solutions(#61,208,215,421)associatedwiththeNeaAnchialos
there was no obvious evidence relating this earthquake to any Fault indicate E–W-striking normal faulting. We obtained a
fault,eventhoughtheRigheoFaultcouldbeagoodcandidate similar pattern for the focal mechanisms associated with the
(Caputo1990).Ourseismicitysplitsintotwoclustersthatalign Nea Pagasae Fault (#33, 37, 297, 383, 386, 389, 398). In
approximatelyE–W.BecausetherearemanyfaultsstrikingE–W conclusion,itseemsclearthatthisareaisunderN–Sextension
inthisareasuchastheVelestino,theRigheo,theVassilikaand along a set of E–W-striking parallel faults. It is worth noting
theNeaPagasaefaults(Caputo1990),itisdiYculttorelateour thatafewevents,locatedtotheeast,exhibitaslightcomponent
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Microseismicity around Volos (Greece) 897
Table1. (Continued.)
of strike-slip motion, which is consistent with the events does not show any seismicity associated with this fault. Due
recordedin southernPilion. to the impressive normal faults observed along Kallidromon
andKamenaVourlaforseveraltensofkilometres(Philip1976;
Lemeille 1977), this lack of seismicity can be interpreted as a
LocridandnorthernEvia
seismic quiescenceperiodbetween major earthquakes.
One of the major goals of our experiment was to investigate Important seismicity is located around the Lamia Basin,
the seismicity located south of the Gulf of Volos (Figs 10 the Renginion Basin and northern Evia. The microseismicity
and 11). This region experienced very strong earthquakes in showsadiVusepatternthatcannotbedirectlyassociatedwith
historical time, in 426 bc, in 1740 and more recently in 1894. anyspecificfaultsuchastheKamenaVourlaor Kallidromon
This most recent event, known as the Atalanti-Martinon faults, but seismic activity unquestionably occurs as deep as
earthquake, was composed of two shocks of magnitudes 6.5 15km.MinoractivityoccurredaroundtheAtalanti-Martinon
and 6.7, a week apart. Remnant surface breaks that are still Fault.TheearthquakeslocatedbeneathnortheasternEviaare
visibletodayshownormalfaultingwiththereactivationofthe unlikelytobeassociatedwithanyoftheknownsurfacefaults,
N300°-striking fault (Ambraseys & Jackson 1990). However, because the western Kandili Fault dips westwards and the
instrumentalseismicity(evenlocatedbytheregionalVOLNET easternEviaFaultdipseastwardsandthereforeawayfromthe
seismological network operated by the University of Athens) seismic activity(Fig.9c).
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Figure5. SeismicitymapofthenorthernAegeanseashowingthereliablylocatedearthquakeswithuncertaintiessmallerthan5kmandrecorded
by more than eight stations. The major faults are shown with heavy and thin lines for the Quaternary and Pliocene structures, respectively
(afterMercieretal.1976;Roberts&Jackson1991;Caputo&Pavlides1993).
Figure6. MapshowingfocalmechanismscomputedforthenorthernAegean.Solidquadrantsincludecompressionalfirstmotion;openquadrants
showdilatations.Blackquadrantsindicatecategory1mechanismswithtwoplaneswellconstrained;darkgreycategorytwomechanismswithone
planewellconstrained;lightgreycategory3mechanisms.Wenotethatthedextralstrike-slipmotionalongtheNorthAegeanTroughprogressively
transfersintopurenormalfaultingonE–W-strikingplanesandN–SextensionaroundtheGulfofVolos.
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Microseismicity around Volos (Greece) 899
Figure7. SeismicitymapofthereliablylocatedearthquakesinThessalyandtheGulfofVolos.NoearthquakesarelocatedalongtheNW–SE-
striking faults that bound the Pilion to the east and the Larissa Basin. The major faults are reported with the same convention as in Fig.4.
VF:Velestino; RiF:Righeon;BV: Vassilika; NPF:Nea Pagasae;TF: Tyrnavos; NAF:Nea Anchialos; AF:Ambelia.Seismicity is associated with
theNeaAnchialosFault,theVelestinoFaultsystemandtheLamiaBasin.Thelocationsofthecross-sectionsA–A∞andB–B∞arealsoshown(Fig.9).
Figure8. Map of the focal mechanisms computed for Thessaly and the Gulf of Volos (symbols as in Fig.5). Note the transition from dextral
strike-slipmotionaroundtheislandofSkiathostotheN–S-trendingpureextensionalongtheNeaAnchialosFault.
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(a) The focal mechanisms show two distinct patterns. Beneath
the Lamia Basin, west of the city of Lamia (#55, 57, 183,
265, 277) and close to the Kallidromon, the Kamena Vourla
faults and the Renginion Basin (#226, 329, 341, 342), mainly
normalfaultingalongE–W-strikingfaultsisobserved.However,
in northern Evia focal mechanisms (e.g. #99, 254, 279, 407)
indicatestrike-slipfaultingwithT-axestrendingapproximately
N–S. We have found no evidence that the same occurs along
the Atalanti-Martinon Fault (#180, 425). Therefore, although
alltheT-axesareorientedapproximatelyN–S,paralleltothe
regional extension (Mercier et al. 1989; Caputo & Pavlides
1993), the two diVerent types of focal mechanisms (normal
and strike-slip) suggest that individual blocks are limited by
inherited faults.
DISCUSSION
The pattern of microseismicity that we obtained from seven
weeks of recording shows some common features and some
diVerences compared to the instrumental seismicity map
(b)
provided by NEIS locations (Fig.1), the historical seismicity
(Fig.2, Papazachos & Papazachou 1997) and the most
important faults.
Onallmaps,weseeseismicactivityalongtheNorthAegean
Trough, around the Gulf of Volos and west of the Karditsa
Basin. In some places (e.g. the North Aegean Trough or the
Nea Anchialos Fault) the seismicity is associated with known
active(orveryrecent)faults.Insomeareas(e.g.Velestino-Nea
Pagasae system) activity is notclearly associated with specific
faults,andinonecase(theLamiaBasin)ourpreciselylocated
earthquakescannotbeassociatedwithanymappedfault.This
observationimpliesthatbrittledeformationisnotconfinedto
important individual surface faults, but is distributed over
broad areasor associated withhidden faults.
On the other hand, some important tectonic features are
not related to any seismic activity. This is especially true for
most of the large NW–SE-striking faults such as that to the
east of Pilio, in Locrid, or those bounding the Karditsa and
(c) Larisa basins where Pliocene activity has been documented
(Caputo1990).
We will describe in more detail a few regions of particular
interest.
TheeasternPilion
We didnot locate any earthquake along the NW–SE-striking
normal faults that bound to the east the Pilion region and to
thewesttheSporadesBasin(Fig.7).Also,wedidnotobserve
any significant seismic activity along the eastern coast of
Evia(Fig.10).Thisisalsotrueforinstrumentalandhistorical
seismicity. Therefore, these faults seem seismically inactive at
the present time, even though they were important active
featuresduringthePliocene.Accordingtomostoftheproposed
models of the geodynamic evolution of the Northern Aegean,
Figure9. Cross-sectionsacross(a)theAlmyrosBasin,suggestingthat the Sporades Basin is an important extensional feature
theNeaAnchialosFaultisactive, (b)theVassilika Basinand(c) the bounded to the west by an active fault (Taymaz et al. 1991;
GulfofEvia,whichdonotshowaclearpatternofseismicityassociated Dinter&Royden1993;Jolivetetal.1994;Armijoetal.1996).
withspecificfaults. However,noseismologicalevidenceofsuchaNW–SE-striking
active normal fault is found in the historical seismicity or in
ourdata.
In contrast, microseismicity is located in areas that were
inactiveoninstrumentalseismicitymaps(Fig.10).Inparticular,
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