Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece by J

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 Ms 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 Ms 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 Ms 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 a cluster of events ranging from 3.8 to 4.5 in November 1997 of Athens have long been considered aseismic (Sieberg, [see the National Observatory of Athens (NOA) bulletins], 1932). This was mainly attributed to the fact that the his- did not attract public interest. It was therefore assumed that torical center of the City, built on schist of high rigidity, is a detailed examination of the microseismicity of the Athens not vulnerable to earth tremors. However, since Athens has area was unnecessary. been constantly expanding, densely urbanized areas have en- The 1999 event, however, clearly demonstrated that the croached into seismogenic regions and Athens has become low rate of seismicity alone is not a safe criterion to assess vulnerable to earthquakes. Today, according to estimates of the seismic potential of a region. Only a detailed tectonic Papadopoulos and Arvanitides (1996) the seismic risk of analysis and mapping of active faults and soil conditions Athens is among the highest in Greece. This was verified contribute toward a reliable seismic risk assessment. Had dramatically by the Ms 5.9 earthquake of 7 September 1999, such studies been undertaken before 1999, they certainly which was the costliest natural disaster of Greece in recent would have revealed the potential risk of the active tectonic history. structures that ruptured during the 7 September 1999 seismic Prior to this event and during the instrumental period of crisis. Greek seismology, no significant earthquake activity had Public awareness and the need for a thorough under- been reported. The strongest shocks (Ms 6.0) to affect Athens standing of the regional seismic potential after the 7 Septem- were located in the Thiva-Oropos zone at a distance of more ber 1999 event motivated the Earthquake Planning and than 50 km from the historic center of the city (Papazachos Protection Organization of Greece to support an onshore- and Papazachou, 1997). Events of rather small magnitude offshore microseismicity survey in the region of the Saron- recorded from time to time in the vicinity of Athens, like ikos Gulf, whose purpose was to identify active faults on- those of 3 April (Ms 4.2) and 4 December (Ms 3.5) 1965 and and offshore and to define their seismic potential. A local

920 Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 921 seismic network was deployed consisting of eight ocean bot- ginal faults. The Eastern Saronikos gulf is dominated by tom seismographs (OBS) and 28 land stations (Fig. 1). This northwest–southeast faults of relatively small throws and al- network operated for 45 days. ternating shallow basins and plateaus (Papanikolaou et al., Both marine and land stations were equipped with 1988). SEDIS seismic recorders of GeoPro, Hamburg (Makris and Several volcanoes and volcanic outcrops of Plio- Moeller, 1990; GeoPro technical reports). These instruments Quaternary age are also situated in the Gulf and are part of are capable of recording six seismic channels on hard disc the western active Aegean volcanic arc. The westernmost with 10-GB capacity. Onshore, an integrated Global Posi- group of volcanic outcrops are those of the Loutraki-Sousaki tioning System (GPS) receiver provides coordinates and tim- area. They are locally more than 70 m thick (Freyberg, ing at each location. Marine stations are housed in a 17-inch 1973). The peninsula of Methana consists mainly of a large glass sphere and record seismic signals directly on the sea volcanic dome and volcanic rocks of Pliocene and Quater- floor. A gimbal-mounted three-component geophone system nary age which are widely exposed (Gaitanakis and Dietrich, and a deep sea hydrophone are used for this purpose. The 1995). The most recent eruption is dated around 250 B.C. instruments are autonomous and can be acoustically ad- and was desrcibed by Strabo (Stothers and Rampino, 1983). dressed by a surface-mounted system. Timing is provided To the northwest they appear to truncate all but the upper- by an internal quartz clock. By measuring the quartz tem- most 100 m of Quaternary sediments (Papanikolaou et al., perature continuously and GPS time at the beginning and end 1988). Volcanic rocks are also found to the south of the of the survey, time-drift corrections are obtained for each island of Poros (Fytikas et al., 1986). Clearly, volcanic ac- instrument. tivity in the Gulf is extensive, recent, and very close to Ath- ens. It is poorly understood and its hazard has never been Regional Tectonics assessed. From displacements at faults and the distribution of sed- The region of Attica as presented in the 1:50,000 scale iments it is shown that the Saronikos Gulf is more active in geological maps of the Institute of Geology and Mineral Ex- the west than in the east. This has also been verified by the ploration of Greece (IGME) (1989), belongs to an autoch- microseismic activity, as will be presented subsequently. thonous metamorphic basement of Paleozoic-Mesozoic age, The most seismically active area in the vicinity of Ath- consisting of marbles and schists (see also Jacobshagen, ens is the Corinthiakos Gulf, which exhibits deformation 1986). Remnants of a “tectonic cover” have been identified typical of the extension that dominates central Greece and in several localities. A major tectonic boundary, dominating the Peloponnese (Jackson et al., 1982; Collier et al., 1992; the area, separates the mountains of Parnis to the northwest Armijo et al., 1996), and an annual extension rate on the from those of Pendeli and Immitos to the southeast. These order of 1 to 7 mm. About 20% of this extension is linked two are characterized by metamorphic rocks, mainly marbles to east–west-striking normal faults of eastern Corinthia (Pa- and schists belonging to the Cyclades Massif, which dip to pazachos and Kiratzi, 1996). The continuation of this fault the northwest, below the nonmetamorphic limestones of system to the east was also mapped by high-resolution, northwestern Attica. Normal faults along the Parnis moun- single-channel reflection seismic profiles (Papanikolaou tain are considered to be the eastward extension of the Cor- et al., 1988) offshore in the Saronikos Gulf. A significant inthiakos Gulf fault system and are of roughly east–west part of the observed seismicity coincides with this system of orientation. During historic times and the more recent period faults, as will be discussed subsequently. 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 Microseismicity aseismic (Galanopoulos, 1960, 1961; Makropoulos et al., 1989; Papazachos and Papazachou, 1997). Within 45 days of field observations we recorded 739 The Saronikos Gulf is a neotectonic basin, divided by a microearthquakes above a threshold magnitude of ML 0.3 very shallow north–south-oriented platform into a western (Fig. 2), which were observed by at least six stations. and an eastern part. The islands of Methana, Angistri, Ae- Magnitudes were defined by the coda length of the seis- gina, and Salamis (see Fig. 1) are situated on this platform. mograms, calibrated by using earthquakes recorded also by The western part of the Gulf consists of two basins. The the Seismograph Network of the NOA. For the magnitude Epidaurus Basin is of west-northwest–east-southeast orien- estimates, we derived a mean function from four stations tation and more than 400 m deep, whereas the Megara Basin having similar site response and applied this relationship to is of east–west orientation with relatively shallow depth, less the full temporary network (Fig. 3). than 250 m. The marginal extensional faults of the Epidaurus The locations of the hypocenters were obtained using Basin strike parallel to the bathymetry and have a 350-m HYPOINVERSE software (Klein, 1989). HYPOINVERSE throw, forming a symmetric graben. The Megara Basin is allows the application of spatially varying local velocity formed by east–west to east-northeast–west-southwest mar- 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 using a local network of densely spaced stations is obvious. between Kythnos and Eratini, crossing the Saronikos Gulf Even a short period of observations is enough to resolve from southeast to northwest (Makris et al., 2002). However, active structures that a regional network would require tens to avoid local geological complexities, we adopted a mean of years to identify due to the different threshold of com- velocity model, as presented in Table 1. pleteness. Within the network most travel-time residuals after lo- Within the network and its immediate vicinity we location have an rms 0.2 sec. This corresponds to an epicentral cated 306 events (Fig. 7). Most of them are within the upper km, depending on the local 15 km of the crust, whereas only a few were found at depths 2ע accuracy of the order of velocity model used, whereas the hypocentral depths are af- exceeding 30 km. The distribution of the foci is illustrated fected by an error twice this value. Outside the network, in Figure 8. The southwest–northeast oriented profile (Fig. errors are significantly higher (Fig. 4). 8) extends from eastern Peloponnese (point A) crossing the The frequency-magnitude distribution of the recorded Saronikos Gulf to central Evia (point B). Hypocenters con- events is shown in Figure 5. Within the network the thresh- tained in a 100-km-wide zone, parallel to line AB, were pro- old magnitude of completeness is ML 1.7. Outside the net- jected in the surface defined by AB and the z axis extending work this increases to ML 2.3. to 40 km depth. Their distribution shows some distinct zones To demonstrate the advantage of using local networks of foci concentration that can be associated with known over regional ones in assessing the seismic activity in this faults. At 90 km (see Fig. 8) the hypocenters are located at area, we present in Figure 6 a comparison between historic the “Aegina fault” as presented in Figures 2 and 7. The linear events (Fig. 6a), instrumentally recorded events published distribution of the foci along the fault extends to a depth of by the NOA regional network (Fig. 6b), and events recorded more than 15 km and marks a deep fracture of the thinned by this temporary local network (Fig. 6c). The advantage of 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 micity shows that all faults around the Parnis epicenter of Salamis, which were reactivated by the “Athens Event” of 7 September 1999 are still active and that the activity extends 7 September 1999. They do not seem to have a deep exten- through the complete crust. sion and probably originate at the thrust front between the At 45-km distance to the northeast we cross an active Cyclades Massif and west Attica. In the Parnis area, 30 km seismic zone that is associated with the Chalkis area (see further northeast, we encounter the Fili fault zone. The seis- Fig. 2). The Psachna region nearby is presently very active 924 J. Makris, J. Papoulia, and G. Drakatos

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 lithosphere are uplifted and stretched westward. The crustal deformation is very complex involving rotation of blocks, normal faulting, and crustal fragmentation. Significant westward 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 Figure 3. Calibration functions used to define lo- is also suggested on Figure 8. cal magnitudes by the coda-duration method. The av- eraged values were derived from functions defined at four different location of comparable recording con- Correlation of Seismicity with Tectonic Elements ditions and therefore influenced by similar site effects. Within the Saronikos Network To correlate the seismicity with the tectonic elements Table 1 we used two sources of tectonic information: one is the Seis- Local Velocity Model Used in the Location of Events motectonic Map of Greece, published by IGME (1989) for (Makris et al., 2002) the onshore areas, and the other is the submarine neotectonic

Velocity Vp (km/sec) Depth (km) map of Saronikos (Papanikolaou et al., 1988). Although extensive literature exists on the regional deformation of the 4.7 0.0 5.7 3.0 Hellenides (e.g., Papazachos and Delibasis, 1969; Papaza- 6.8 7.0 chos and Comninakis, 1971; Taymaz et al., 1991; Stiros, 8.1 17.0 1993) it is the two sources mentioned here that best describe the Saronikos area. The first observation worth mentioning is that the east– and has hosted several Ms 4.5 events. Another 35 km to the west-oriented fault between the islands of Aegina and Sa- northeast we encounter the Kymi fault zone with intense lamis is also marked by significant seismic activity of shal- crustal seismicity. This system of faults separates the north- low events. The seismicity can be followed onshore into east ern part of the island of Evia from the Skyros basin and Corinthia, delineating an active fault of more than 30 km truncates the entire crust. Large-magnitude events have been length. Such a fault can produce events on the order of mag- reported from this zone (Makropoulos and Burton, 1984; nitude Ms 6.5 (see also Kiratzi et al., 1985). The fact that Papazachos and Papazachou, 1997). Finally, around the historic catalogs (e.g., Papazachos and Papazachou, 1997; Skyros area, 65 km to the northeast off the coast, we en- Ambraseys and Jackson, 1990) do not report events from counter a seismically active region with a diffuse distribution this offshore area (see Fig. 3) is due to bias of observations of foci. This area is notorious for its intense seismicity and based on land-locked locations only. has hosted several large earthquakes (see, e.g., Makropoulos, Significant activity was also identified at the eastern 1978; Makropoulos et al., 1989). The large distance of the coast of Aegina, along a northeast–southwest-trending lin- Saronikos network from the island of Skyros and the limited eament, mapped by Papanikolaou et al. (1988). At the vol- period of monitoring does not permit us to associate the canic areas of Methana and Aegina, on the other hand, we recorded microseismicity with the existing faults. A special did not map shallow seismicity worth mentioning. Although study with a local network is needed because the position many foci locations below the Saronikos Gulf are subcrustal and size of the active faults around Skyros are not yet clearly and associated with the subducting oceanic slab below the mapped. Hellenides (see also Leydecker, 1975; Makropoulos, 1978; In general, below Peloponnese and particularly the Sa- Makris and Roever, 1986), we could not associate the seis- ronikos gulf, the foci are located much deeper than below mic activity with magma movements or hydrothermal pro- Attica, Evia, and the Aegean domain (see Fig. 8). This phe- cesses in the volcanic zone. This is probably due to the in- nomenon points to the existence of two regional stress fields adequate density of the local network, which did not permit that dominate the deformation of the Hellenides and the Ae- the identification of events of so small magnitude and vol- gean region. Deeper seismicity, at subcrustal levels, as ex- canic noise. Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 925

RMS Distribution Within the Network RMS Distribution Outside the Network

160 60

140 50 120 40 100 80 30 60 20 40 Number of Events

Number of Events 10 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

RMS (sec) RMS (sec)

ERH Distribution Within the Network ERH Distribution Outside the Network

140 120 120 100 100 80

80 60 60 40 40 Number of Events

Number of Events 20 20 0 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 ERH (Km) ERH (Km)

ERZ Distribution Outside the Network ERZ Distribution Within the Network

140 120

120 100

100 80

80 60 60 40 40 Number of Events Number of Events 20 20

0 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

ERZ (Km) ERZ (Km)

Figure 4. rms distribution within and outside the Saronikos Gulf seismic network.

Of interest is the area of Mount Parnis where the 1999, Corinth Basin, toward Thiva. This area is well known to

Ms 5.9 “Athens Event” occurred. We observed a great con- seismologists for its intense activity and, in particular, the centration of microearthquakes close to the 1999 epicentral destructive earthquake sequence of 1981 that severely dam- location of the mainshock, indicating that the aftershock ac- aged this region. The second area is the northeastern part of tivity is still ongoing and the stresses responsible for this central Evia (Fig. 2), north of Kymi as mentioned previ- event have not yet been completely relaxed. Also the area ously. This coastal part of the island and its immediate in- of Spata, in southeastern Attica is seismically active along a terior are seismically very active. We could not deﬁne the fault of northeast–southwest orientation, extending offshore faults precisely, since the network is located at a distance of in the southern Evoikos Gulf (see Figs. 2, 6c, and 7). 80 to 100 km. The lateral velocity variations in the crust and Outside the network we recorded high seismicity in sediments are intense and cause signiﬁcant systematic errors three areas (see Fig. 2). One is the northeastern part of the in locating the hypocenters. Assuming a lateral homoge- 926 J. Makris, J. Papoulia, and G. Drakatos

areas of Greece. This is particularly true for the east–west Cumulative Magnitude Distribution fault extending from Corinthia across the Saronikos Gulf, Within the Network between the islands of Salamis and Aegina. The deep, sub-

1000 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 100 Saronikos Gulf, and western Attica. The subducting lithosphere is limited to the north by the volcanic area of Li- chades, in the northern Evoikos Gulf. Deep seismic events logN(M) 10 Limit Magnitude of Completeness: ML 1.7 reported by the Greek Seismological Services and our observations indicate that this is the northernmost limit of the subducting slab. All the remaining seismicity is crustal. It is 1 controlled to the east, in the central and north Aegean Sea, 2 0.0 0.4 0.8 1. 1.6 2.0 2.4 2.8 3.2 3.6 by transtension and normal faulting along the Corinthia Rift Magnitude (ML) due to rearrangement of crustal blocks escaping the com- pression along the western Hellenides. Use of a local velocity model obtained by a controlled- Cumulative Magnitude Distribution source seismic experiment (Makris et al., 2002) significantly Outside the Network increased the location accuracy. The seismicity defined by observations of the seismological network of Greece delin- 1000 eates active faults by the concentration of large-magnitude events observed over long periods of time. Local networks can significantly improve the picture by detecting and better 100 locating small-magnitude events over short periods of time.

Limit Magnitude of This permits rapid assessment of the seismic hazard of an logN(M) 10 Completeness: ML 2.3 area. The necessity for an accurate three-dimensional velocity model for the territory of Greece cannot be stressed

1 enough. It is mandatory if we wish to more accurately deﬁne the active faulting and provide the engineering community 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 with the reliable seismic hazard parameters needed. Magnitude (ML)

Figure 5. Frequency-magnitude distribution Acknowledgments within and outside the Saronikos Gulf seismic net- We are grateful to two anonymous reviewers for their useful com- work. ments, which contributed to the improvement of our article. We acknowl- edge the assistance provided by GeoPro-Hamburg for the field operations and the instruments used in the network. We thank Captain C. Handras and neous velocity model and reducing the three-dimensional the crew of the R/V AEGAIO for their support during the marine operations. We specially thank Mr. D. Becker and Mr. S. Stoyanov from GeoPro- structure to one dimension only, location uncertainties in- Hamburg for the data processing and preparation of figures. Our field crease and errors may easily become systematic. operations were financed by the Earthquake Planning and Protection The third region we want to refer to is that to the east Organization of Greece. and northeast of the island of Skyros (see Fig. 2). Here the high seismicity recorded is associated with the aftershock References sequence of the Ms 5.8 “Skyros Earthquake” of 2001. A precise definition of the associated active faults is also not Ambraseys, N. N., and J. A. Jackson (1990). Seismicity and associated possible for the reasons stated previously. strain of central Greece between 1890 and 1988, Geophys. J. Int. 101, 663–708. Armijo, R., B. Meyer, G. King, A. Rigo, and D. Papanastasiou (1996). Conclusions Quaternary evolution of the Corinth rift and its implications for the late Cainozoic evolution of the Aegean, Geophys. J. Int. 126, 1, The Saronikos local seismic network, despite its limited 11–53. operation of 45 days, produced important results permitting Collier, R. E., M. R. Leeder, P. J. Rowe, and T. C. Atkinson (1992). U- several conclusions on the existence and location of active series dating and tectonic uplift of upper Pleistocene marine sediments faults previously unknown or inaccurately mapped. The from the Corinth and Megara basins, Greece, Tectonics 11, 1159– 1167. shallow seismicity within the network is, to a great extent, Freyberg, B. V. (1973). Geologie des Isthmus von Korinth, Erlanger Geol. associated with the deformation and opening of the Corin- Abh. 95, 183 p. thiakos Gulf, which is one of the most rapidly deforming Fytikas, M., F. Innocenti, N. Kolios, P. Manetti, and R. Mazzuoli (1986). Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 927

Figure 6. A comparison among historical (Papadopoulos et al., 2000: 450 B.C. to 1889, light-gray circles; Makropoulos et al., 1989 and Papazachos and Papazachou, 1997: 1914– 1964, dark-gray circles and NOA bulletins 1965–1999: black circles), (a), instrumental readings from NOA (b), and presently recorded seismicity (c) in the Saronikos Gulf and adja- cent areas. 928 J. Makris, J. Papoulia, and G. Drakatos

Figure 7. Location of events recorded within the Saronikos network. A minimum of six records per event has been used to locate each: earthquake. Events with an rms value exceeding 0.5 sec have been discarded. Active faults identiﬁed by our observations are marked in red. A discussion of the map is presented in the text.

The Plio-Quaternary volcanism of Saronikos area (western part of the Kahle, H. G., M. V. Muller, A. Geiger, G. Danuser, S. Meuller, G. Veis, Aegean volcanic arc), Ann. Geol. Pays Hell. 33, 23–45. H. Billiris, and D. Paradisis (1995). The strain ﬁeld in NW Greece Gaitanakis, P., and A. D. Dietrich (1995). Geological map of Methana and the Ionian islands: Results inferred from GPS measurements, Tec- peninsula, scale 1:25 000, Swiss Federal Institute of Technology, tonophysics 249, 41–52. Zurich. Kiratzi, A. A., G. F. Karakaisis, E. E. Papadimitriou, and B. C. Papazachos Galanopoulos, A. G. (1960). A catalogue of shocks with Io Ն VI or M Ն (1985). Seismic source parameter relations for earthquakes in Greece, 5 for the years 1801–1958, Seismological Laboratory, Athens Uni- Pageoph 123, 27–41. versity, Athens, Greece, 1–119. Klein, F. (1989). User’s Guide to HYPOINVERSE, a program for VAX Galanopoulos, A. G. (1961). A catalogue of shocks with Io Ն VI for the computers to solve for earthquake locations and magnitudes, U.S. years prior to 1800, Seismological Laboratory, Athens University, Geol. Surv. Open-File Rep. 89-314. Athens, Greece, 1–19. Le Pichon, X., and J. Angelier (1979). The Hellenic arc and trench system: Institute of Geology and Mineral Exploration (1989). Seismotectonic map a key to the neotectonic evolution of the eastern Mediterranean area, of Greece, scale 1:500,000. Tectonophysics 60, 1–42. Jackson, J., J. Gagnepain, G. Houseman, G. King, P. Papadimitriou, C. Leydecker, G. (1975). Seismizitatsstudien im Bereich des Peloponnes auf- Souﬂeris, and J. Virieux (1982). Seismicity, normal faulting, and the grund von Prazisionsherdbestimmungen, Ph.D. Dissertation, Univer- geomorphological development of the Gulf of Corinth (Greece): the sity of Frankfurt, Frankfurt, Germany. Corinth earthquakes of February and March 1981, Earth Planet. Sci. Makris, J., and L. Moeller (1990). An ocean bottom seismograph for gen- Lett. 57, 377–397. eral use. Technical requirements and applications, in Proceedings of Jacobshagen, V. (1986). Geologie von Griechenland, Beitr. Regionalen Symposium “Europe and the Sea,” Hamburg, J. Hoefeld, A. Mitzlaff, Geologie der Erde, 363 p. and S. Polomsky (Editors), sponsored by the EC. Tectonic Deformation and Microseismicity of the Saronikos Gulf, Greece 929

Figure 8. Seismicity along a northeast-southwest-oriented proﬁle with all foci con- tained in a 100-km-wide and 40-km-deep block, projected on the surface shown in the ﬁgure. For further information, see text.

Makris, J., and P. Roever (1986). Struktur und heutige Dynamik der Lith- Papazachos, B. C., and C. Papazachou (1997). The Earthquakes in Greece, osphaere in der Agais, in Geologie von Griechenland, V. Jakobshagen Ziti Ed., Thessaloniki, 304 pp. (Editor), Gebruder Borntraeger, Berlin, 241–256. Rotstein, Y. (1985). Tectonics of the Aegean block: rotation, side arc col- Makris, J., J. Papoulia, G. Drakatos, and V. Karastathis (2002). Seismo- lision on crustal extension, Tectonophysics 117, 117–137. logical investigations in the Saronikos gulf, Athens, XXVII General Sieberg, A. (1932). Erdbebengeographie, Verlag von Gustav Fischer, Ber- Assembly of the European Seismological Society, Nice (Abstracts). lin, 102 pp. Makropoulos, K. C. (1978). The statistics of large earthquake magnitude Stiros, S. (1993). Kinematics and deformation of central and southwestern and an evaluation of Greek seismicity, Ph.D. Thesis, University of Greece from historical triangulation data and implications for the ac- Edinburg, 1–193. tive tectonics of Aegean, Tectonophysics 220, 283–300. Makropoulos, K. C., and P. W. Burton (1984). Greek tectonics and seis- Stothers, R. B., and M. R. Rampino (1983). Volcanic eruptions in the Med- micity, Tectonophysics 106, 275–304. iterranean before A.D. 630 from written and archeological sources, Makropoulos, K. C., D. Drakopoulos, and J. Latousakis (1989). A revised J. Geophys. Res. 88, 6357–6371. and extended earthquake catalogue for Greece since 1900, Geophys. Taymaz, T., J. Jackson, and D. McKenzie (1991). Active tectonics of the J. Int. 98, 391–394. north and central Aegean Sea, Geophys. J. Int. 106, 433–490. McKenzie, D. P. (1978). Active tectonics of the Alpine-Himalayan belt: the Aegean Sea and surrounding regions, Geophys. J. R. Astr. Soc. GeoPro GmbH 55, 217–254. 20457 St. Annenufer 2 Papadopoulos, G. A., and A. Arvanitides (1996). Earthquake risk assess- Hamburg, Germany ment in Greece, in Earthquake Hazard and Risk, V. Schenk (Editor), (J.M.) Kluwer, Dordrecht, The Netherlands, 221–229. Papadopoulos, G. A., A. Vassilopoulou, and A. Plessa (2000). A Catalogue of Historical Earthquakes for Central Greece: 480 B.C.–A.D. 1910, Institute of Oceanography Institute of Geodynamics, National Observatory of Athens, Publ. Hellenic Centre for Marine Research no. 11. 19013 Anavyssos Papanikolaou, D., V. Lykousis, G. Chronis, and P. Pavlakis (1988). A com- Attica, Greece parative study of neotectonic basins across the Hellenic arc: the Mes- (J.P.) siniakos, Argolikos, Saronikos and Southern Evoikos Gulfs, Basin Res. 1, 167–176. Geodynamic Institute Papazachos, B. C., and P. Comninakis (1971). Geophysical and tectonic National Observatory of Athens features of the Aegean arc, J. Geophys. Res. 76, 8517–8533. 11810 Thission Papazachos, B. C., and N. D. Delibasis (1969). Tectonic stress ﬁeld and Athens, Greece seismic faulting in the area of Greece, Tectonophysics 7, 231–255. (G.D.) Papazachos, B. C., and A. A. Kiratzi (1996). A detailed study of the active crustal deformation in the Aegean and surrounding area, Tectono- physics 253, 129–154. Manuscript received 9 October 2002.