GreDaSS_2.0: Seismogenic Source GRIS112 - East Stratoni Fault

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COMMENTS The East Stratoni Fault is the easternmost segment of the three ones comprising the Stratoni-Varvara CS (GRCS110). Segmentation is strongly supported by morphotectonic analyses of Michailidou (2005), Chatzipetros et al. (2005) and Michailidou et al. (2005). The western end of the fault segment forms a slight angular barrier with the West Stratoni Fault (GRGG110) while the eastern end is offshore. The total fault zone appears in many geological and morphotectonic maps (I.G.M.E., 1978; Pavlides and Tranos, 1987; Michailidou, 2005). The September 29, 1932 Hierissos earthquake produced coseismic ground ruptures near to the coast of Stratoni (Floras, 1933; Pavlides and Tranos, 1991). Maximum displacement was observed in between the fault segments of East and West Stratoni (GRGG112 and GRGG111, respectively) indicating that both fault segments were ruptured. The earthquake sequence relocation proposed by Pavlides et al. (2010) shows that the main event was initiated on the East Stratoni fault segment. Although field measurements by Pavlides and Tranos (1991) indicate a medium dipping angle on surface, the vertical earthquake distribution proposed by Galanis et al. (2004) implies a much steeper dipping angle all along the deepest part of the fault. Hence, an intermediate value is preferred. The maximum fault depth is inferred from the previous vertical earthquake distribution. Rake is based on kinematic indicators measured by Pavlides and Tranos (1991). Slip per event and Mw are based on analytical and empirical relationships (Kanamori and Anderson, 1975; Hanks and Kanamori, 1979; Wells and Coppersmith, 1994). The slip per event value refers to the slip produced by this segment alone. During the 1932 event, displacements of 1.6 m (Floras, 1933) and ca. 1.8 (inferred from photographic record of Georgalas and Galanopoulos, 1953) were observed along the fault scarp and near to Stratoni. These higher values are due to the reactivation of both three segments: the Varvara, West and East Stratoni Faults. According to the earthquake catalogue of Papazachos et al. (2009), on September 26 and 28, 1932, two strong aftershocks of M 6.0 and 5.7 occurred respectively near Stratoni, which after the relocation of the events proposed by Pavlides et al. (2010) are probably attributed to the West Stratoni Fault. A strong aftershock of M 6.3 also occurred on May 11 of the next year (triggered event?) located on the Gomati CS (GRCS260). OPEN QUESTIONS Although the maximum displacement was observed near Stratoni implying that this site should be around in the middle of the total fault zone extension, the eastern end of the East Stratoni Fault segment is restricted to the Elefthera Cape and Eleftheronesos Island due to lack of data. There is also one earthquake (Mw = 5.1) occurred on August 26, 1983 placed in the Akanthios Gulf (Papazachos et al., 2009). The Harvard CMT catalogue (Dziewonski et al., 1988) places the event 40 km southern inside Siggitikos Gulf suggesting a strike-slip focal mechanism. SUMMARIES Floras (1933) An investigation was carried out after the 1932 Hierissos earthquake. The fault scarp was observed emerging on the shore in Stratoni and continued westwards through the mines up to Stageira. The coast of Stratoni subsided 1.6 m and the sea permanently entered the land for 5-10 m. Maravelakis (1933) The author reported a precursory hydrogeological anomaly at the hot springs of Nea Apollonia, 7 hours before the 26 September 1932 Hierissos and 3 days before the 29 September, 1932 in lake Volvi (20-30 km NW from the Stratoni-Varvara CS) earthquakes. In more particular, the water and gas supplies were increased together with the temperature which arose many grades above the mean value (48° C). After the Volvi event, the piezometric level dropped 1 m down suddenly without coming back to its initial position. I.G.M.E. (1978) The Stratoni-Varvara CS is imaged on the 1:50000 scale “Stratoniki” geological map sheet. Pavlides and Kilias (1987) The authors carried out mesostructural analyses of several active faults in Chalkidiki region. Concerning the Stratoni Fault (corresponding to the Stratoni-Varvara CS of this database), field measurements on a striated surface located almost in the middle of the on-land fault’s length indicate N110° strike, 50° dip to SSW and 70°E pitch. The fault is imaged in one of the figures showing the structural map of the Chalkidiki region. Pavlides et al. (1990) According to field observations the fault trends almost E-W (N85° to N115°) and dips south (70°-85°). Many striated surfaces have been measured along fault’s strike and some conjugate structures. After a mesostructural analysis, two main extensional directions have been recognised from which the most recent is N-S. Dziewonski et al. (1988) According to the Global CMT Catalogue the following data are available for the August 26, 1983 (Mw = 5.1) earthquake: Depth= 15 km, Scalar Moment = 6.41E+23. The fault plane suggested by the focal mechanism solution has 072° strike, 73° dip and -168° slip. The epicentre is located near Sithonia Peninsula (40.11° N, 24.04° E), 40 km southern than the epicentre calculated by Papazachos et al. (2009). Pavlides and Tranos (1991) and Tranos (1998) The authors carried out a morphotectonic mapping of the E-W trending Stratoni Fault (corresponding to the Stratoni-Varvara CS of this database). Field descriptions indicate a pure dip-slip structure, steeply dipping to the south, estimated to be active at least since Oligocene. Some E-W and NW-SE trending conjugate faults were also observed affecting the hanging-wall block. According to the authors the fault belongs to a complex fractured zone of recent crustal weakness, trending NW-SE to WNW-ESE, which probably behaves as a major left-lateral oblique-slip wrench fault zone. Papazachos and Papazachou (1997; 2003) According to the earthquake catalogue’s descriptions concerning the co-seismic effects induced by the 26 September 1932 earthquake, a 7 km-long open fissure appeared in the ground from Stratoni and westwards with a mean width of 2 m and depth of 10 m, following the mountain's foot in a E-W direction. A sea wave (tsunami) entered the land in Hierissos, Stratoni, Olympiada and Nea Roda, 4 or 5 times repeatedly and in a distance of 30 m. The sea level rose 2 m in the coastline between Hierissos and Stratoni. The largest intensity was observed in Hierissos (X). Two large aftershocks occurred, the first on the same day (21:26, M=6.0) and the second on 29 September (03:57, M = 6.2). Goldsworthy et al. (2002) According to the authors the Stratoni fault is close to a strike-slip faulting earthquake in 1983 whose location is an offshore extrapolation of the Kavala-Xanthi fault system (corresponding to Thrace CS-GRCS150 of this database). The fault is imaged on a regional morphotectonic map of the Chalkidiki broader area along with the focal mechanism of August 26, 1983 event taken from the Harvard CMT catalogue. Vannucci and Gasperini (2003; 2004) According to the database of Earthquake Mechanisms for European Area (EMMA) the following data are available for the August 26, 1983 (Mw = 5.1) earthquake: Depth= 15 km, Scalar Moment = 5.495E+16. The fault plane suggested by the focal mechanism solution has 072° strike, 73° dip and -168° slip. The location of the epicentre is near to the one calculated by Papazachos et al. (2009) and to the East Stratoni Fault. Galanis et al. (2004) The seismicity of the broader area of Mygdonia Basin has been relocated and many profiles are produced showing the vertical distribution. One N-S trending profile near to the western end of the Stratoni-Varvara CS (GRCS110) shows that the majority of the events occur until the depth of 16-17 km while the deepest ones reach the depth of 20 km. The authors suggest the existence of an almost vertical south-dipping fault. Pavlides and Caputo (2004) In order to produce empirical relationships the authors revised many historical and recent events with morphological expression, among which is the September 26, 1932 Hierissos earthquake. The co-seismic displacement of 1.80 m measured by Floras (1933) is accepted as maximum. There is also a reference to accurate measurements of the Greek Navy (Hydrographic Service, 1932) in the Akanthios Gulf and close to the fault indicating 1-1.40 m co-seismic displacement. The suggested average displacement is 0.30 m, while for the estimated geological fault length is larger than 25 km and possibly up to 35-40 km, of which ca. 15-20 km on land. Michailidou (2005), Chatzipetros et al. (2005) and Michailidou et al. (2005) A geological-morphotectonic mapping and morphotectonic quantitative analyses have been carried out along the Stratoni-Varvara CS. Results indicate a three-part segmented, ca. 12-15 km-long active structure that probably continues for approximately 15 km more offshore. It strikes Í 90-110°, dips 60-50° SSW and has a pitch angle of about 70-80° Å with a normal dip-slip sense of movement. Many secondary blind faults have been also traced from boreholes on the hanging wall, 100-300 m away and parallel to the main fault. Segmentation is based on morphotectonic analysis and geomorphologic indicators. The potential magnitude of the total fault zone ranges from 6.0 to 7.1 after using various empirical relationships of magnitude versus fault’s length.