BRIEF INFORMATION ON THE RECORDS

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BRIEF INFORMATION ON THE RECORDS OF THE ITSAK-EPPO ACCELEROMETRIC NETWORK IN SSW AEGEAN SEA OF THE EARTHQUAKES OF SEPTEMBER 2012 On September 12, 2012 (03:27 GMT) a strong earthquake occurred off the

1 BRIEF INFORMATION ON THE RECORDS OF THE ITSAK-EPPO ACCELEROMETRIC NETWORK IN SSW AEGEAN SEA OF THE EARTHQUAKES OF SEPTEMBER 2012 On September 12, 2012 (03:27 GMT) a strong earthquake occurred off the southwestern coasts of Crete Island. According to the web page of the Seismological Station of the Thessaloniki University ( ) it was a shallow earthquake with epicentral coordinates Ο N Ο E and focal depth 6.5km. The moment magnitude was Μ W =5.3. The epicenter is located close to the island of Gavdos. The earthquake was felt at Crete island with maximum macroseismic intensity (Ι ΕMS =V) observed at Iraklion according to information collected by the Euro-Mediterranean Seismological Center, which was based on citizens reports at various sites. The map in figure (1) ( earthquake.php?id =284984#map) shows the geographical distribution of the sites. The star represents the earthquake epicenter. Τhe color and size of the circles correspond to the macroseismic intensity value and the number of observations according to the legend of the map. Fig. 1. Geographical distribution of the macroseismic intensities according to the citizens reports. (source EMSC). 1

2 The earthquake occurred at the SSW part of Hellenic Arc at the boundary of the Hellenic Trough. The Hellenic Arc is one of the most seismologically active areas along the convergence zone between Eurasia and Africa. More than 60% of the seismic activity of the European area is released in the Aegean area which is part of the Eastern Mediterranean lithosphere. The deformation is expressed with the nucleation of earthquakes with magnitude up to M~8.3 (Papazachos, 1990, 1996). This deformation is the result of the subduction of the Eastern Mediterranean lithosphere under Aegean along the Hellenic Arc (Papazachos and Delibasis, 1969; Papazachos and Comninakis, 1970, 1971; Le Pichon and Angelier, 1979), the westwards motion of the Anatolian plate along the North Anatolia Fault (McKenzie, 1970, 1972) and the its extend in the North Aegean along the North Aegean trough. Fig. 2. Locations of the ITSAK-EPPO network of accelerographs used in the present report for the data analysis due to earthquake off SW Crete island. (See text for more details). The red squares in the map in figure (2) show the geographical distribution of the CMG-5TDE accelerographs in the southern Aegean area. The yellow triangle represents the location of the Etna (18 bits) accelerograph at Cythera Island with GPS timing system. The hexagon and the black circles represent the locations of the SSA-2 (12 bits) and the QDR (11 bits) strong motion instruments which have internal timing system. These instruments are working in trigger mode with a preselected level and are linked to ITSAK-EPPO premises by telephone dial-up telemetry and they were not triggered by the earthquake. The same symbols were used in the map of figure (3) to show the locations of the instruments which recorded the ground motion of the September 21, 2012 and were used for the data analysis. The red star in the map of figure (2) depicts the epicenter of the September 12, 2012 earthquake and the beach ball shows the fault plane solution determined by the GFZ Potsdam. The two stars in figure (3) represent the epicenters of the two earthquakes listed in Table (I). The white circles in the two maps show the epicentral distances of 100 and 200 km. The faults of the strong shallow earthquakes (Papazachos 2

3 et al., 2001) are also shown. Table (I) shows the corrected peak values of the ground acceleration (cm/sec 2 ) velocity (cm/sec) and displacement (cm) of the recordings of thee CMG-5TD accelerographs shown in figure (2). The symbol (*) in Table (I) denotes that the peak displacement values are smaller than 0.01 cm. The recordings were processed and displayed using the software ViewWave (Kashima, 2005). The acceleration time histories were filtered by band pass 2 nd order filter ( Hz). The graphs can be displayed by clicking on the station name appeared in Table (I). TABLE I. Results of the analysis of the acceleration records of the September 12, 2012 earthquake off SW Crete Island. SITE NAME DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm ΑGIOS ΝΙΚΟLAΟΣ (AGN1) Δ=150 km NS-comp EW-comp Z-comp AREOPOLIS (ARE2) Δ=251 km NS-comp EW-comp Z-comp ΑRΚΑLΟCHORI (ARK1) Δ=109 km NS-comp EW-comp Z-comp CHANIA (CHN1) Δ=69 km NS-comp EW-comp Z-comp CHANIA CHN2) Δ=69 km NS-comp EW-comp Z-comp GORTINΑ (GOR1) Δ=80 km NS-comp EW-comp Z-comp

4 SITE NAME DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm GYTHIO (GTH2) Δ=249 km NS-comp EW-comp Z-comp HERAKLEIO (HER2) Δ=105 km NS-comp EW-comp Z-comp HERAKLEIO (HER3) Δ=102 km NS-comp EW-comp Z-comp KASSOS (KSS1) Δ=249 km NS-comp EW-comp Z-comp KISSAMOS (KIS1) Δ=78 km NS-comp EW-comp Z-comp LEONIDIO (LEO1) Δ=276 km NS-comp (*) EW-comp (*) Z-comp (*) NAXOS (NAX1) Δ=270 km NS-comp (*) EW-comp (*) Z-comp (*) NEAPOLIS (NPS1) Δ=203 km NS-comp EW-comp Z-comp (*) PALAIOCHORA (PLC1) Δ=54 km NS-comp EW-comp Z-comp

5 SITE NAME DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm PETALIDI (PET1) Δ=302 km NS-comp EW-comp Z-comp PYLOS (PYL1) Δ=312 km NS-comp EW-comp Z-comp RETHIMNO (RTH1) Δ=61 km NS-comp EW-comp Z-comp SANTORINI (SFIR) Δ=206 km NS-comp EW-comp Z-comp SFAKIA (SFK1) Δ=40 km NS-comp EW-comp Z-comp SITEIA Δ=185 km NS-comp EW-comp Z-comp As one can see from Table (II) the strongest PGA values were recorded at Sfakia. The values recorded at Herakleio are higher than those recorded at Chania and Rethimno which are located at closer epicentral distances. Specially the difference between Chania and Herakleio supports the evidence of one degree intensity higher at Herakleio tha in Chania as it is shown in the map of figure (1). The higher values at Santorini (inner part of the Hellenic Arc) compared to those at Neapolis (site locate along the axis of the Hellenic Arc) can be attributed to the influence of the local site conditions which is in agreement of the H/V fourier amplitude spectral ratios. Figure (3) shows the plot of the geometrical mean of the peak horizontal components of acceleration (Table II) as function of the epicentral distance. The curve that fits to the data has the form: log. = log( Dist) PGA G MEAN where PGA G.MEAN is the geometrical mean of the peak horizontal components of acceleration in cm/sec 2 and Dist is the epicentral distance. in km. The two dashed lines stand for the 95% confidence interval. 5

6 Fig. 3. Dependence of the geometrical mean of the peak the horizontal components of acceleration listed in Table (I) The map in figure (3) shows the distribution of the macroseismic intensities, theoretically determined, assuming point source. This map was compiled by KOERI (TR) within the framework of a bilateral project between KOERI and ITSAK. The calculation of intensities does not consider any site effects influence which may explain the differences with the map in figure (1) and the instrumental data. Fig. 3. Theoretically determined macroseismic field of the earthquake of September 12, (http: 6

7 On September 21, 2012 two earthquakes with moment magnitude Mw =5.0 and 4.7 occurred at the SW Aegean Sea at the boundary of the Hellenic Trough. Their source parameters as determined by the GFZ Potsdam are shown in Table (II). Table (II). Source parameters of the earthquakes of at SW Aegean (GFZ-Potsdam). ORIGIN TIME :47:41.0 UTC :39:17.8 UTC MOMENT MAGNITUDE 5.0 (Mw) 4.7 (Mw) EPICENTER E N E N DEPTH 23 km 41 km Table (III) gives the parameters of the fault plane solutions of the two earthquakes as they were determined by GFZ-Potsdam. ζ, δ and λ Table (III). Fault plane solutions of the two earthquakes in SW Aegean of September 21, (source: GFZ-Potsdam). FAULT PLANE Ι FAULT PLANE ΙΙ Ρ-AXIS Τ-AXIS ζ δ λ ζ δ λ ζ δ ζ δ SEISMIC MOMENT :47:41.0 UTC *10 16 Nm :39:17.8 UTC *10 16 Nm The fault plane solutions in Table (III) show that the first (08:47) earthquake was due to a strike slip rupture. Similar mechanism small-to-moderate magnitude earthquakes were determined in this region by Hatzfeld et al. (1990). Lyberis et al. (1982) determined strike slip movements in this region. The second (15:39) earthquake was nucleated due to a thrust rupture which is in agreement with the stress field in the area (Papazachos et al., 1999). The representation of the faults of the strong shallow earthquakes in the map of figure (4) are after Papazachos et al. (2001). The red squares in the map in figure (4) show the geographical distribution of the CMG-5TDE accelerographs in the southwestern Aegean area. The rest symbols are the same as in the map of Figure (2) ie. The yellow triangle represents the location of the Etna accelerograph at Cythera Island and the black circles represent the locations the QDR (11 bits) strong motion instruments which were not triggered by 7

8 MINISTRY of DEVELOPMENT, COMPETITIVENESS INFRASTRUCTURE TRANSPORT AND NETWORKS the earthquake. The two yellow stars in the map of figure (4) represent the epicenters of the two earthquakes listed in Table (III) and the beach balls show the fault plane solutions determined by the GFZ Potsdam (Table IV). The white circles in the two maps show the epicentral distances of 100 and 200 km. The faults of the strong shallow earthquakes (Papazachos et al., 2001) are also shown. Table (IV) shows the corrected peak values of the ground acceleration (cm/sec2), velocity (cm/sec) and displacement (cm) of the recordings of thee CMG-5TD accelerographs shown in figure (4). The recordings were processed and displayed using the software ViewWave (Kashima, 2005). The acceleration time histories were filtered by band pass 2nd order filter ( Hz). The graphs can be displayed by clicking on the station name appeared in Table (IV). Fig. 4. Geographical distribution of the ITSAK-EPPO network of accelerographs used for the data analysis due to September earthquakes at SW Aegean. The representation of the faults of strong shallow earthquakes (Papazachos et al., 2001), and the fault plane solutions (GFZ-Potsdam) of the two earthquakes are also shown. 8

9 TABLE IV. Results of the analysis of the acceleration records of the September 21, :47GMT M=5.0 earthquake at SW Aegean Sea. SITE NAME EPICENTRAL DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm AREOPOLIS (ARE1) 158 km NS-comp EW-comp Z-comp CHANIA (CHN1) 132 km NS-comp EW-comp Z-comp CHANIA (CHN2) 133 km NS-comp EW-comp Z-comp GYTHIO (GTH2) 167 km NS-comp EW-comp Z-comp KISSAMOS (KIS1) 100 km NS-comp EW-comp Z-comp LEONIDIO (LEO1) 214 km NS-comp 0.07 EW-comp 0.10 Z-comp 0.05 NEAPOLIS (NPS1) 145 km NS-comp EW-comp Z-comp PALAIOCHORA (PLC1) 98 km NS-comp EW-comp Z-comp

10 SITE NAME EPICENTRAL DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm PETALIDI (PET1) 199 km NS-comp EW-comp Z-comp RETHIMNO (RTH1) 170 km NS-comp EW-comp Z-comp SFAKIA (SFK1) 143 km NS-comp EW-comp Z-comp TABLE V. Results of the analysis of the acceleration records of the September 21, :39 GMT M=4.7 earthquake at SW Aegean Sea. SITE NAME EPICENTRAL DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm AREOPOLIS (AARE1) 129 km NS-comp EW-comp Z-comp CHANIA (CHN1) 110 km NS-comp EW-comp Z-comp CHANIA (CHN2) 111 km NS-comp EW-comp Z-comp GYTHIO (GTH2) 195 km NS-comp EW-comp Z-comp

11 SITE NAME EPICENTRAL DISTANCE ACCELERATION cm/sec 2 VELOCITY cm/sec DISPLACEMENT cm KISSAMOS (KIS1) 77 km NS-comp EW-comp Z-comp LEONIDIO (LEO1) 179 km NS-comp EW-comp Z-comp MEGALOPOLIS (MGP1) 213 km NS-comp EW-comp Z-comp NEAPOLIS (NPS1) 108 km NS-comp EW-comp Z-comp PALAIOCHORA (PLC1) 87 km NS-comp EW-comp Z-comp PETALIDI (PET1) 176 km NS-comp EW-comp Z-comp 0.09 * RETHIMNO (RTH1) 152 km NS-comp EW-comp Z-comp SFAKIA (SFK1) 128 km NS-comp EW-comp Z-comp It can be observed on the basis of the values appeared in Tables (IV) and (V) the recording acceleration values were low with the highest values recorded at Sfakia and Kissamos. 11

12 Figure (5) shows the plot of the geometrical mean of the peak horizontal components of acceleration (Tables IV & V) as function of the epicentral distance for the wo earthquakes. The curves which fit to the data ar: log PGA log PGA G. MEAN G. MEAN = = *log( Dist) *log( Dist) : : 39 where PGA G.MEAN is the geometrical mean of the peak horizontal components of acceleration in cm/sec 2 and Dist is the epicentral distance. in km. The two lines stand for the data of the two earthquakes according to the legend. 10 EQ :47 EQ :3947 Geometric Mean Horizontal PGA Epicentral Distance (km) Fig. 5. Dependence of the geometrical mean of the peak the horizontal components of acceleration values listed in Tables (IV & V). 12

13 The values f the geometrical spreading factor are in good agreement with the value 2.418, which was determined for the area of Chania (Theodulidis 1999) based on scaling relations holding between macroseismic intensity and peak ground values of shallow earthquakes data located at the Hellenic Arc. The results of the present report are preliminary and may subject to revision if more detailed analyses will be carried out. The uncorrected data of the records in ASCII are available upon request. ACKNOWLEDGMENTS The network of ITSAK-EPPO was installed by the members of the (former) directorate of Engineerng Seismology and the staff of the Laboratory of ITSAK. Mrs. K. Konstantinidou, MSc IT personnel, has the responsibility for the 24h data transmition and the contacts with the staff of SYZEFXIS network. The local authorities supported the installation of the instruments Thessaloniki October 14, 2012 Dr. Christos A. Papaioannou Seismologist Ph.D. Research Director ITSAK-EPPO chpapai@itsak.gr DISCLAIMER This report was compiled by the Research Director of ITSAK-EPPO Christos A. Papaioannou Seismologist PhD. The accuracy and credibility of the results and the opinions expressed in this report are solely of him and do not necessary reflect those of the institute. 13

14 REFERENCES MINISTRY of DEVELOPMENT, COMPETITIVENESS INFRASTRUCTURE TRANSPORT AND NETWORKS Hatzfeld, D., G. Pedotti, P. Hatzidimitriou and K. Makropoulos (1990): The strain pattern in the western Hellenic arc deduced from a microearthquake survey. Geophys. J. Int., 101, Kashima T. (2005): ViewWave, Building Research Institute, Tsukuba, Japan. LePichon, X. and J. Angelier (1979): The Hellenic arc and trench system: a key to the neotectonic evolution of the eastern Mediterranean area. Tectonophysics, 60, 1-4. Lyberis, N., J. Angelier, F. Barrier and S. Lallemant (1982). Active deformation of a segment of arc, the strait of Kythira, Hellenic arc, Greece. 1. struct. Geol., 4, Papazachos, B.C. and C.B. Papazachou (1997): The earthquakes of Greece. Ziti Publ. Thessaloniki, Greece, 304 pp. Papazachos, B.C. and C.B. Papazachou (2003): The earthquakes of Greece 3 rd Edition. Ziti Publ. Thessaloniki, Greece, 286.pp. McKenzie, D.P. (1970): The plate tectonics of the Mediterranean region. Nature, 226, McKenzie, D., (1972): Active tectonics of the Mediterranean region. Geophys. J.R. astr. Soc., 30, Papazachos, B.C. (1990): Seismicity of the Aegean and surrounding area, Tectonophysics, 178, Papazachos, B.C. (1996): Large seismic faults in the Hellenic Arc. Analli di Geofisica,39, Papazachos, B.C. and N.D. Delibassis (1969): Tectonic stress field and seismic faulting in the area of Greece. Tectonophysics, 7, Papazachos, B.C. and P.E. Comninakis (1970): Geophysical features of the Greek Island Arc and Eastern Mediterranean Ridge. Com. Ren. des Seances de la Conference Reunie a Madrid, 1969, 16, Papazachos, B.C. and P.E. Comninakis (1971): Geophysical and tectonic features of the Aegean arc, J. Geophys. Res., 76, Papazachos, B.C., Ch.A. Papaioannou, C.B. Papazachos and A.S. Savvaidis (1999): Rupture zones in the Aegean region. Tectonophysics, 308, Papazachos B.C., D.M. Mountrakis, C.B Papazachos, M.D. Tranos, G.F. Karakaisis and A.S. Savvaidis, The faults of known strong earthquakes in Greece and surroundings from 5 th century AD until today. 2 nd Hell. Conf. on Earthq. Engin. & Engin. Seismology, Thessaloniki November 2001, Α, Theodulidis, N.P, (scientific responsible) (1999): Seismic Hazard Assessment and Seismic Scenarios for the City of Hania Crete Island (Greece). Rpt compiled for the EU project SEISMOCARE. 14

PRELIMINARY REPORT. The earthquake of January 8, 2013 at SE of Limnos Island, Northern Aegean, Greece I. Kalogeras, N. Melis & C.

PRELIMINARY REPORT. The earthquake of January 8, 2013 at SE of Limnos Island, Northern Aegean, Greece I. Kalogeras, N. Melis & C. PRELIMINARY REPORT The earthquake of January 8, 2013 at SE of Limnos Island, Northern Aegean, Greece I. Kalogeras, N. Melis & C. Evangelides At 16:16 local time (14:16 GMT) January 8, 2013 a moderate size