DISS 3.2.GreDaSS: Seismogenic Source GRCS110 - Stratoni-Varvara

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COMMENTS The Stratoni-Varvara Composite Source is a ca. 37 km fault zone half of which is located offshore. It consists of at least three major fault segments. The Varvara (GRGG110) and the West Stratoni (GRGG111) fault segments form an angular barrier while the East Stratoni fault segment (GRGG112) follows the tectonically controlled coastline from the seaside. Beyond the Eleftheronesos Island the trace of the fault is not observable but according to the maximum displacement documented near Stratoni, the fault should continue more into the sea reaching approximately the same distance with the onland length. Segmentation is also strongly supported by morphotectonic analyses of Michailidou (2005), Chatzipetros et al. (2005) and Michailidou et al. (2005). The fault appears in many geological and morphotectonic maps from which location and strike derive. Field measurements from Pavlides and Tranos (1991) and a profile of vertical earthquake distribution provided by Galanis et al. (2004) indicate a steep dipping angle on surface and an even steeper dipping angle at depth. The same profile suggests a maximum depth of 16-17 km. According to geological observations and descriptions, the total Stratoni-Varvara CS was ruptured during the September 29, 1932 Ierissos earthquake. The coseismic ruptures followed closely the fault scarp and in some places the fault scarp itself produced vertical displacement that reached about 1.8 m (according to the photographic archive of Georgalas and Galanopoulos, 1953). Subsidence of 1.4 m and 1.6 m was also observed on the shore near to Stratoni according to Floras (1933) and accurate measurements from the navy (from Pavlides and Caputo, 2004), respectively. A large number of strong foreshocks and aftershocks has been recorded along the fault zone (Papazachos et al., 2009). On September 26 and 28, 1932 two strong foreshocks of M 6.0 and 5.7, respectively occurred near Stratoni. A strong aftershock of M 6.3 also occurred on May 11 of the next year located on the Gomati CS (GRCS260). The application of the empirical relationships of Pavlides and Caputo (2004) for the total extension of the fault zone indicates that the magnitude of the 1932 event coincides with the calculated magnitude of 6.9 (6.6 and 7.4 for the lower and upper envelopes, respectively). This implies that the 1932 earthquake reached the maximum expected magnitude. OPEN QUESTIONS Both ends of the fault zone are insufficient. The occurrence of an earthquake (Mw = 5.1) in the Gulf of Hierissos is also problematic. According to the Harvard CMT catalogue (Dziewonski et al., 1988) the focal mechanism shows an almost pure strike-slip movement in a region that is broadly recognised as extensional with a NNW-SSE direction. Another matter is that the epicentre is placed in the Siggitikos Gulf, 40 km southern than the epicentre calculated by Papazachos et al. (2009) and Vannucci and Gasperini (2003), where the presence of the North Aegean Trough justifies this kind of focal mechanisms. SUMMARIES Floras (1933) An investigation was carried out after the 1932 Ierissos 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 Ierissos 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. 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 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. 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 [Ierissos], 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 Ierissos 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 Ierissos 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.