DISS 3.2.GreDaSS: Seismogenic Source GRCS810 - North Aegean Basin (NAB)

Source Info Summary

Commentary

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References

COMMENTS This source is one of the longest structures in the North Aegean Sea defining the southern margin of the North Aegean Basin. The fault zone is well expressed morphologically, as it has formed a deep and steep NW-dipping slope, clearly imaged in detailed bathymetric maps (Maley and Johnson, 1971; IOC, 1981; Papanikolaou et al., 2002). It comprises at least three segments, with two of them well recognized due to past seismic activity; these are the NAB segments A and B that produced the August 6, 1983 and January 18, 1982 earthquakes respectively. The fault zone is also clearly imaged in the seismic reflection profiles indicating significant cumulative displacements and especially the deformation of the sea-bottom sediments (Brooks and Ferentinos, 1980; Ferentinos et al., 1981; Mascle and Martin, 1990; Roussos and Lyssimachou, 1991; Laigle et al., 2000; Papanikolaou et al., 2006). However, it kinematic behaviour is not constant along its strike (Papanikolaou et al., 2006; Armijo et al., 2003; Flerit et al., 2004). According to Papanikolaou et al. (2006), this structure is the result of oblique opening produced by the combination of i) dextral shear along the northeast-southwest structures of the basin due to the westward escape of Anatolia and ii) southwestward pull due to the retreat of the Hellenic subduction zone. Thus, it is suggested by the same authors that the gradual increase of opening and deepening of the basin from northeast to southwest is the result of a gradual southwestward increase of the normal character of the faults. This could imply that the geometry of the fault zone does not remain constant, but its dip, from sub-vertical to the northeast becomes quite shallower towards southwest as the seismic profiles suggest (Laigle et al., 2000). Nevertheless, there are some evidences of the geometry of the northeastern part of the fault zone from the various focal mechanisms of the two aforementioned earthquakes. Concerning slip-rate, only indirect information is available through basinal or in situ extension rates estimated either from seismic profiles (Laigle et al., 2000; Koukouvelas and Aydin, 2002) or more synthetic studies (Armijo et al., 2003; Flerit et al., 2004; Papadimitriou and Sykes, 2001). The maximum expected magnitude is based on the estimations of Papanikolaou and Papanikolaou (2007). OPEN QUESTIONS The geometry and kinematics of this fault zone is rather complex and still not well defined. Segmentation at the northeastern part is evident from two strong events, in contrast with the southwestern part where significant historical or instrumental seismicity is absent. The gradual geometric and kinematic change of the fault zone’s behaviour is not enough to define whether the whole zone can rupture at once. Is this change responsible for the absence of significant earthquakes? There are many, prior to the two aforementioned, events that could possibly be associated with one of the two already recognised segments of the fault zone. However, their uncertainty is too high to make direct associations, since many other seismogenic sources are in close distance. There are also suggestions from some authors (e.g. Papanikolaou et al., 2006) that there are many secondary basinward faults that deform the sea-bottom sediments. SUMMARIES Maley and Johnson (1971) During the summer of 1968, a bathymetric survey of the Aegean Sea was conducted by the U.S. Naval Oceanographic Office in order to interpret the sea-floor geomorphology. A structural-tectonic map is produced which shows the presence of a NE-SW-trending marginal fault at the southern side of the NAB. Brooks and Ferentinos (1980) Air gun and sparker profiling data were carried out at the western part of the NAB, in the Sporades sub-basin, from which the authors describe the tectonic features of the southern margin of the basin as follows: the southern flank of the basin is morphologically narrow and steep and it is characterised by major faulting. Air-gun records from the marginal zone of the basin reveal a steep reflector interpreted as the deeper part of a listric fault plane dipping under the basin. Dziewonski et al. (1983) According to the CMT solution, the following source parameters are available for the January 18, 1982 (Mw = 6.6) earthquake: Depth = 10 km, Scalar Moment = 8.61E+25 dyne*cm. The nodal plane of the ‘best double couple’ solution has 241° strike, 57° dip and -176° rake. Dziewonski et al. (1984) According to the CMT solution, the following source parameters are available for the August 6, 1983 (Mw = 6.6) earthquake: Depth = 10.1 km, Scalar Moment = 1.17E+26 dyn*cm. The nodal plane of the ‘best double couple’ solution has 228° strike, 89° dip and -168° rake. Lybéris (1984) The author investigated the tectonic regime of the North Aegean suggesting a prevailing extensional with strike-slip stress field that is active since the uppermost Miocene. Papazachos et al. (1984) This paper includes a seismological investigation of the January 18, 1982 (M = 7.0) earthquake sequence. They provide both the horizontal and vertical aftershock distribution of the event and computed the focal mechanisms based on first motion polarities. According to the vertical aftershock distribution, the main aftershocks cluster reaches a maximum depth of approximately 17 km, while the nodal planes suggested by focal mechanism have strike, dip and rake = 325°, 50° and 53° (nodal plane 1), and 073°, 70° and 43° (nodal plane 2), respectively. Rocca et al. (1985) First motion fault plane solution of the August 6, 1983 (Ms = 6.8) Northern Aegean earthquake and spatial distribution of its aftershocks, which occurred in the central part of the Northern Aegean Trough, show that the main seismic fault is a right-lateral strike-slip fault with a very small thrust component. The fault plane parameters are 048° strike and 83° dip while the slip vector parameters are 48° trend and 2° plunge with a right-lateral sense of motion (equal to a rake of 178°). The suggested length and width is 46 and 8 km, respectively. Ekstrom et al. (1987) The provided source parameters, using the CMT technique, for the August 6, 1983 earthquake, are: Depth= 10.1 km, Scalar Moment = 1.2E+26 dyne*cm. The nodal plane suggested by the ‘best double couple’ solution has 228° strike, 86° dip and -165° rake. Ekstrom and England (1989) The authors present the moment tensors of moderate and large earthquakes (Mw = 5.0) determined from digital seismograms of the Global Digital Seismic Network (GDSN), using the centroid-moment tensor (CMT) method. The parameters of the August 6, 1983 earthquake (also published in the Global CMT Catalogue), are: 229° strike, 81° dip, -174° rake, 10 km focal depth and 1.16E+19 Nm scalar moment. Martin and Mascle (1989) In the seismic reflection profiles conducted in the area where the current source is located, flower structures are clearly detected. Kiratzi (1991) The suggested deformation rate in the northern Aegean area is associated with NS extension of about 9 mm/a and EW contraction at a rate of 15 mm/a. Kiratzi et al. (1991) The source parameters of the August 6, 1983 earthquake are determined by body wave inversion: Ms = 6.8, Mo = 12.1E+25 dyn*cm, strike = 050°, dip = 76°, rake = +177° and depth = 9km. The average slip of 45.5 cm is calculated by taking into account the Mo and a down-dip width equal to twice the focal depth. Roussos and Lyssimachou (1991) This paper is an interpretation of seismic profile data conducted south from the Mount Athos Peninsula. Based on the presence of both positive (towards northeast) and negative (towards southwest) flower structures, the authors suggest the presence of an extending, active, dextral strike-slip zone of over 100 km length, showing both transpressive and transtensional behaviour respectively, and having a cumulative strike-slip displacement on the order of 5-7 km. Taymaz et al. (1991) The authors studied the tectonic regime of the North and Central Aegean Sea. 19 focal mechanisms have been determined by using the body wave inversion. Among these focal mechanisms, the January 18, 1982 (Ms = 6.9) source parameters are provided. The suggested nodal plane has the following attributes (strike, dip, rake): 233° ±5, 62° ±5, 187° -10/+7. The seismic parameters suggest Mo = 7.32E+18 Nm, Ms = 6.9 and focal depth of 7 ±1 km. According to the authors, the source orientation is well constrained by waveforms at nodal stations and by first motion polarities. The NE-SW-striking nodal plane is selected due to the aftershock distribution which is elongated to the same direction. Similarly, the August 6, 1983 (Ms = 6.9) source parameters are also provided. The suggested nodal plane has the following attributes (strike, dip, rake): 047° ±8, 83° ±6, 180° ±10. The seismic parameters suggest Mo = 1.94E+19 Nm, Ms = 6.9 and focal depth of 7 ±1 km. According to the authors, the event was well recorded at all azimuths, and there is excellent coherence between waveforms at adjacent stations on the focal sphere. The NE-SW-striking nodal plane is selected due to the aftershock distribution which is elongated to the same direction. Jackson et al. (1992) The source parameters that derive from the CMT waveform inversions for the January 18, 1982 (Ms = 6.9) earthquake, are: 233° strike, 62° dip and 187° rake, while for the August 6, 1983 (Ms = 6.9) earthquake are: 047° strike, 83° dip and 180° rake. Laigle et al. (2000) The authors analyze three multi-channel seismic profiles that image a normal fault to at least 10 km depth towards the western part of NAB. According to their interpretations, fault is active and recent, forming a scarp at sea bottom and crossing the Quaternary deltaic front on the northern slope of the trough controlled to the south by the North Anatolian Basin Fault Zone. Based on the orientations of three profiles, the authors constrained the true fault dip to an average 20° in the basement. The throw and age estimated from the geometry in the sedimentary rocks document, according to the authors, recent onset of the motion (much more recent than middle Pleistocene) that must have occurred at a high rate. The 0.8 km slip measured at depth on the seismic section indicates for the authors an extension rate possibly an order of magnitude larger than the 1 mm/yr rate if its onset was middle Pleistocene. Papadimitriou and Sykes (2001) A synthetic seismotectonic investigation has been carried out for the Northern Aegean Sea where nine major fault segments are defined. One of these corresponds to the total southern margin of NAB. Based on other existing data, like focal mechanisms and GOS measurements, the authors modelled this fault with the following geometric and kinematic parameters: 050° strike, 80° dip, 80 km length, 0-15 km depth, 12 mm/a slip-rate and right-lateral strike-slip movement. Koukouvelas and Aydin (2002) The authors suggest the existence of a large SW-NE trending fault zone, that is the NAB CSS (named Sporades-Limnos), that borders the south margin of NAB. In particular, the eastern part of the Sporades-Limnos fault zone, where the NAB segment A is located, includes three high-angle fault strands which offset the top of the pre-Tertiary basement by as much as 2.3 s two-way travel time (according to Lalechos and Savoyat, 1977). Based on profiles from several other authors, which show that fault scarps produced by these faults are commonly accompanied by slumping, it is suggested that the actual fault scarps rarely maintain their initial angle resulting in low-angle scarps and apparently in a broad fault zone. At the western part near to Sporades, a complex positive flower structure with a maximum width of 2 km offsets the top of the pre-Tertiary basement by as much as 0.7 s two-way travel time. A cumulative horizontal slip of 25 km for the total Sporades-Limnos fault zone is estimated, as well as a slip-rate of 13 mm/a including also the antithetic tectonic structures at the opposite side of the trough. It is suggested that the amount of deformation is similar to the one accommodated by the Ganos fault zone with which the NAB fault zone is physically linked. Papanikolaou et al. (2002; 2006) A morphotectonic investigation of the North Aegean Trough has been carried out based on a swath bathymetric survey (2002) and seismic profiles (2006). The resulting bathymetric map and the slope analysis suggest an active tectonic morphology where the NAB segment A lies. According to the authors, the dominant structure of the basin is a 160 km-long NE-SW-trending fault extending from Skopelos to Limnos, which borders the basin’s southern margin and accommodates more than 2 km of throw, probably representing the continuation of the North Anatolian Fault. Armijo et al. (2003); Flerit et al. (2004) The authors determined the localised deformation zones represented by dislocation elements which extend from the base of the lithosphere to the locking depth at the base of the seismogenic layer. Their models incorporate GPS data with geological and seismological constraints to provide a robust description of the present-day deformation of the Anatolian-Aegean region. The elements Nos 61 and 62, which roughly correspond to the NAB Fault Zone, represent two deformation zones with different behaviours: the former is an almost pure strike-slip structure with 20 mm/a of right-lateral slip-rate and 1 mm/a of contraction, while the latter one is practically a pure normal dip-slip zone with 11 mm/a opening. Vannucci and Gasperini (2003; 2004) According to the database of Earthquake Mechanisms for Mediterranean Area (EMMA) the following data are available for the January 18, 1982 (Mw = 6.5) earthquake: depth = 10 km, scalar moment Mo = 6.918E+18 Nm. The suggested nodal plane has a strike of 241, a dip of 57° and a rake of -176°. Similarly, for the August 6, 1983 (Mw = 6.6) earthquake: depth = 10 km, scalar moment Mo = 9.772E+18 Nm. The suggested nodal plane has a strike of 229°, a dip of 81° and a rake of -174°. Papanikolaou and Papanikolaou (2007) Several seismic hazard scenarios are examined concerning the seismic potential of the North Aegean Basin based on geological data. Detailed analysis of the bathymetry incorporated by seismic reflection profiles from previous surveys demonstrates a dominant 160km-long NE-SW trending structure consisting of three major fault segments and one branch. One of their earthquake scenarios includes the simultaneous rupture of all the three major segments, i.e. the total NAB Fault zone. The authors applied various empirical relationships of magnitude versus length and suggested a maximum expected magnitude of 7.6.