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Fault Population Recognition Through Microseismicity in Mygdonia Region

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Fault Population Recognition Through Microseismicity in Mygdonia Region Boll. Geof. Teor. Appl., 56, 357-366 Aleardi Bollettino di Geofisica Teorica ed Applicata Vol. 56, n. 3, pp. 367-382; September 2015 DOI 10.4430/bgta0153 model space shows that for high Vp/Vs ratios the eigenvectors associated with the Vs-related Fault population recognition through microseismicity parameter (Rs and RJ) span the null-space of the inversion kernel. This fact, combined with the in Mygdonia region (northern Greece) observation of the resolution matrices, highlights that the determination of the Vs contrast (or the S-impedance contrast) for shallow sediments or at sea bottom becomes a hopelessly non-unique problem in the case of high Vp/Vs values. Finally, I observe that when increasing the Vp/Vs C. GKARLAOUNI1, E. PAPADIMITRIOU1, V. KARAKOSTAS1, A. KILIAS2 AND S. LASOCKI3 values the error propagation from data to model space becomes more and more severe. The same 1 Geophysics Department, School of Geology, Aristotle University, Thessaloniki, Greece happens to the cross-talk between Rp and Rd, making their independent estimation impossible. 2 Therefore, it emerges that linear AVA inversion is not suitable to investigate under- Geology Department, School of Geology, Aristotle University, Thessaloniki, Greece 3 Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland consolidated or overpressured sediments that are usually characterized by very high Vp/Vs ratios. In those cases, it is likely that non linear and wide-angle inversion approaches are needed. (Received: September 17, 2014; accepted: February 27, 2015) REFERENCES Aki K. and Richards P.G.; 1980: Quantitative seismology: theory and methods. WH Freeman, San Francisco, CA, USA, vol. 1 and 2, 557 and 373 pp. ABSTRACT Seismicity in the area of Mygdonia basin (northern Greece) is lately characterized Aster R.C., Borchers B. and Thurber C.H.; 2005: Parameter estimation and inverse problems. Elsevier Academic Press, by a lack of strong events (M≥5.0) on the seismogenic faults known to have been London, England, 296 pp. repeatedly activated in the past. Only small to moderate magnitude earthquakes Ayres A. and Theilen F.; 1999: Relationship between P- and S-wave velocities and geological properties of near-surface sediments of the continental slope of the Barents Sea. Geophys. Prospect., 47, 431-441. (M≤4.8) have occurred since 2000 in the area. Therefore, microseismicity is the only Castagna J.P., Swan H.W. and Foster D.J.; 1998: Framework for AVO gradient and intercept interpretation. Geophys., available information to be exploited for the identification of active faults and hazard 63, 948-956. assessment, since it may occur everywhere, as well as onto the major hazardous faults. De Nicolao A., Drufuca G. and Rocca F.; 1993: Eigenvalues and eigenvectors of linearized elastic inversion. Geophys., Earthquakes recorded between 2000 and 2014 by the Hellenic Seismological Network 58, 670-679. were relocated by using the available P- and S-seismic wave arrivals in order to improve Drufuca G. and Mazzotti A.; 1995: Ambiguities in AVO inversion of reflections from a gas-sand. Geophys., 60, 134-141. location accuracy. The VP /VS velocity ratio which was found equal to 1.78, along with Grion S., Mazzotti A. and Spagnolini U.; 1998: Joint estimation of AVO and kinematic parameters. Geophys. Prospect., 46, 405-422. station time delays, were all included in the Hypoinverse computer program. In the Mazzotti A.; 1990: Prestack amplitude analysis methodology and application to seismic bright spots in the Po Valley, case of strong spatial or temporal clustering the HypoDD algorithm was employed Italy. Geophys., 55, 157-166. for further improving location accuracy. The relocated seismicity was used to outline Mazzotti A.; 1991: Amplitude, phase and frequency versus offset applications. Geophys. Prospect., 39, 863-886. the geometrical properties of the local fault population and identify seismogenic faults Mazzotti A. and Zamboni E.; 2003: Petrophysical inversion of AVA data. Geophys. Prospect., 51, 517-530. deprived of a clear surface expression. Cross-sections made perpendicular to the main Ostrander W.; 1984: Plane-wave reflection coefficients for gas sands at non-normal angles of incidence. Geophys., 49, axis of the seismicity alignments, shed more light to the local fault network. 1637-1648. Riedel M. and Theilen F.; 2001: AVO investigations of shallow marine sediments. Geophys. Prospect., 49, 198-212. Key words: microseismicity, relocation, Wadati, Mygdonia graben. Rutherford S.R. and Williams R.H.; 1989: Amplitude-versus-offset variations in gas sands. Geophys., 54, 680-688. Tarantola A.; 2005: Inverse problem theory and methods for model parameter estimation. Soc. Ind. Appl. Math., Philadelphia, PA, USA, 342 pp., doi:10.1137/1.9780898717921. 1. Introduction Theilen F. and Pecher I.A.; 1990: Assessment of shear strength of the sea bottom from shear wave velocity measurements on box cores and in-situ. In: Hovem J.M., Richardson M.D. and Stoll R.D. (eds), Shear Waves in Marine Sediments, Kluwer Academic Publishers, Dordrecht, the Netherlands, pp. 41-49. Investigation of minor earthquakes (M≤4.0) can provide insight to seismicity processes or Ursenbach C.P. and Stewart R.R.; 2008: Two-term AVO inversion: equivalences and new methods. Geophys., 73, 31-38. seismic sources of large earthquakes before they occur. Small events are usually related to the Wang Y.; 1999: Approximations to the Zoeppritz equations and their use in AVO analysis. Geophys., 64, 1920-1927. same seismogenic processes as the fault population of larger earthquakes. Moderate (M≈4.0- Zoeppritz K.; 1919: Erdbebenwellen VII. Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, 5.5) and small earthquakes (M<4.0) are related to main or subsidiary faults, which, along with Mathematisch-Physikalische Klasse, 57-65. the main rupture zones, supplement the fault populations in active seismotectonic areas. The importance of acquiring precisely defined focal parameters of microseismicity is based upon the Corresponding author: Mattia Aleardi fact that when strong events are lacking, minor earthquakes is the only source of information Dipartimento di Scienze della Terra, Università when seeking for faulting and seismogenesis patterns (Bagh et al., 2007; Maggi et al., 2009; Via Santa Maria 53, 56126 Pisa, Italy Phone: +39 050 221; fax: +39 050; Tan, 2012). Microseismicity engagement for this purpose is encouraged by the deployment of email: [email protected] a dense and modern seismological network, which significantly contributed to lower magnitude 366 © 2015 – OGS 367 Boll. Geof. Teor. Appl., 56, 367-382 Gkarlaouni et al. Fault population recognition in Mygdonia region Boll. Geof. Teor. Appl., 56, 367-382 The main seismotectonic features of the Aegean (Fig. 1b) are the Hellenic Trench (thick red line with triangles), the North Aegean Trough (NAT) which accommodates the westward extrusion of the Anatolian plate into the Aegean, the Cephalonia transform fault (CTF) and the Rodos operating transform fault (RTF). In the past, a number of strong events with maximum macroseismic intensities equal to IX (1759 Anthemountas earthquake; 1902 Assiros earthquake) and X (1932 Ierissos earthquake) have occurred in the study area [Papazachos and Papazachou, 2003 (Fig 1a)]. However, the current seismotectonic pattern is described by low seismicity rate and sparse moderate magnitude events. However, the existing seismic sources are still posing a potential societal threat for the city of Thessaloniki and the surrounding urban areas. The determination of the active fault segments responsible for the recent seismicity in this back-arc tectonic basin as well as the variations of seismicity clustering in time and space were the motivations of this study. 2. Seismotectonic framework Mygdonia graben comprises a dense fault network that largely accommodates extensional tectonic processes, developed in the fast deforming back-arc Aegean area. The present form of the S-shaped Mygdonia basin comprises the impact of successive seismotectonic events, associated with extensional deformation and subsequent rotations of the stress field (Pavlides and Kilias, 1987). As a result, a complicated faulting network that incorporates NW-SE, NE-SW, E-W and NNE-SSW faults was formed, built on pre-existing structures. Most of these structures are strongly imprinted upon the relief and define the geomorphological barriers of the main or smaller elongated basins. The active faults which are verified by seismotectonic observations to be associated with seismic activity are mainly developed in an E-W direction, controlling the central part of the basin and the eastern part of Chalkidiki Peninsula (Tranos et al., 2003; Mountrakis et al., Fig. 1 - a) Digital elevation model for Mygdonia basin. Red lines correspond to important rupture zones, such as the 2006). The predominant feature is the E-W trending Thessaloniki-Gerakarou fault zone (TGFZ) E-W Thessaloniki-Gerakarou (TGFZ) normal fault zone, along with secondary fault segments described in the text and the semi parallel fault branches of Pilea-Peristera (PPF), Pefka-Asvestochori (PAsF) and (fault information gathered by Pavlides and Kilias, 1987; Tranos et al., 2003; Mountrakis et al., 2006). Green circles denote strong earthquakes (M≥6.5) and black dots depict earthquake epicentres with M≥4.0 since 1981. b) Simplified Asvestochori-Chortiatis (AChF) fault segments (Fig. 1a). The prolongation of TGFZ zone to map of the broader Aegean Sea with the dominant seismotectonic features.
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