A Detailed Seismic Zonation Model for Shallow Earthquakes in the Broader Aegean Area
Total Page：16
File Type：pdf, Size：1020Kb
Nat. Hazards Earth Syst. Sci., 16, 55–84, 2016 www.nat-hazards-earth-syst-sci.net/16/55/2016/ doi:10.5194/nhess-16-55-2016 © Author(s) 2016. CC Attribution 3.0 License. A detailed seismic zonation model for shallow earthquakes in the broader Aegean area D. A. Vamvakaris1, C. B. Papazachos1, Ch. A. Papaioannou2, E. M. Scordilis1, and G. F. Karakaisis1 1Geophysical Laboratory, School of Geology, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece 2Institute of Engineering Seismology and Earthquake Engineering (ITSAK), P.O. Box 53, 55102, Thessaloniki, Greece Correspondence to: D. A Vamvakaris ([email protected]), C. B. Papazachos ([email protected]) Received: 30 August 2013 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 25 November 2013 Revised: 29 August 2015 – Accepted: 6 December 2015 – Published: 18 January 2016 Abstract. In the present work we propose a new seismic Mt, for a typical time period (T D 50 yr), revealing signif- zonation model of area type sources for the broader Aegean icant spatial variations of seismicity levels within the study area, which can be readily used for seismic hazard assess- area. The new proposed seismic zonation model and its pa- ment. The definition of this model is based not only on seis- rameters can be readily employed for seismic hazard assess- micity information but incorporates all available seismotec- ment for the broader Aegean area. tonic and neotectonic information for the study area, in an at- tempt to define zones which show not only a rather homoge- neous seismicity release but also exhibit similar active fault- ing characteristics. For this reason, all available seismologi- 1 Introduction cal information such as fault plane solutions and the corre- sponding kinematic axes have been incorporated in the anal- Even before the first steps of modern Seismology, the reli- ysis, as well as information about active tectonics, such as able assessment of seismic hazard was traditionally consid- seismic and active faults. Moreover, various morphotectonic ered as one of the most important tasks of Seismology. Espe- features (e.g. relief, coastline) were also considered. Finally, cially since the pioneering work of Cornell (1968) it was ob- a revised seismic catalogue is employed and earthquake epi- vious that this assessment depends on several factors, among centres since historical times (550 BC–2008) are employed, which perhaps the most important, and often more poorly un- in order to define areas of common seismotectonic charac- derstood, is the seismic source model. The compilation of a teristics, that could constitute a discrete seismic zone. A new reliable seismic source model requires the use of high qual- revised model of 113 earthquake seismic zones of shallow ity (and density) data from various disciplines such as Geol- earthquakes for the broader Aegean area is finally proposed. ogy (for the location and configuration of potentially active Using the proposed zonation model, a detailed study is per- faults), Geophysics and Geodesy (for the definition of seis- formed for the catalogue completeness for the recent instru- mic sources not readily observed at the Earth’s surface), Ac- mental period. tive Tectonics (for the rate and type of deformation) and His- Using the defined completeness information, seismicity torical Seismology (for the elaboration of historical records parameters (such as G–R values) for the 113 new seismic of strong past earthquakes). zones have been calculated, and their spatial distribution was Modern studies on probabilistic seismicity and seismic also examined. The spatial variation of the obtained b values hazard assessment at any scale are usually based on sev- shows an excellent correlation with the geotectonic setting eral procedures and algorithms, which typically require the in the area, in good agreement with previous studies. More- study area to be divided into seismogenic source zones. In over, a quantitative estimation of seismicity is performed in several cases, this corresponds to the definition of a set of terms of the mean return period, Tm, of large (M ≥ 6.0) earth- sources (e.g. sets of polygons for areal seismic sources) quakes, as well as the most frequent maximum magnitude, over the seismically active region which is examined. In the case of areal sources, the polygons that are often em- Published by Copernicus Publications on behalf of the European Geosciences Union. 909 FIGURES56 D. A. Vamvakaris et al.: A detailed seismic zonation model for shallow earthquakes in the broader Aegean area 910 911 Figure 1 912 nental lithospheric system, at the convergence boundary be- tween Africa and Eurasia. Due to this setting, the Aegean hosts more than 60 % of the total European seismicity. This seismicity is a result of the deformation induced from three main geodynamic sources: (a) the northward motion of the African lithosphere, resulting in the subduction of the East- ern Mediterranean lithosphere under the Aegean (Papaza- chos and Comninakis, 1970; Le Pichôn and Angelier, 1979), (b) the westward motion of the Anatolia/Turkey microplate along the North Anatolia faults and its continuation in the Northern Aegean (McKenzie, 1970, 1972), and (c) the an- ticlockwise rotation of the Apulia/Adriatic microplate and its collision with Eurasia along the Adriatic, Albania and Epirus (NW Greece) coast (Anderson and Jackson, 1987). These geodynamic phenomena are responsible for the com- plex seismotectonic setting of this region, which results in the generation of a large number of very strong shallow and intermediate-depth events, with magnitudes up to M D 8.3 and M D 8.0, respectively (Papazachos, 1990). 913 Several researchers have worked on the seismic zona- tion problem for the broader Aegean area during the last Figure 1. Main seismotectonic features of the study area (broader four decades (Papazachos, 1980, 1990; Hatzidimitriou et al., Aegean Sea region). Local stress field is depicted by open arrows, 1985; Papazachos and Papaioannou, 1993; Papaioannou and while the southern Aegean Benioff zone isodepths (white solid Papazachos, 2000; Slejko et al., 2010). In a first attempt, Pa- lines), the southern Aegean volcanic arc and major basins within pazachos (1980) used the distribution of earthquake epicen- the Aegean Sea are also presented (modified from Papazachos et al., 1998; Karagianni et al., 2005; Mountrakis et al., 2012). tres, the strike and type of every significant known seismic fault, azimuth of P and T stress axes provided by the avail- able focal mechanisms and b parameter values of G–R rela- 36 tion. In the following years, additional information was taken ployed may have a complex geometry, reflecting the com- into account by several researchers for seismic source def- plexity of major tectonic trends and typically encompass ar- initions, such as geological data, geomorphological settings eas within which seismicity may be considered rather ho- and the attenuation pattern observed in macroseismic isoseis- mogeneously distributed in space and stationary in time. In mals. most cases, the geometrical definition of the seismic sources More recently, Papaioannou and Papazachos (2000) pro- is followed by the estimation/adoption of a statistical model posed a model for the south Balkan area containing 67 seis- for the magnitude distribution of the earthquakes within mic zones related with shallow earthquakes. At the same each source, which is parametrized usually by a Gutenberg– time, Papazachos et al. (2001) studied all recent geological Richter type frequency–magnitude relation (Gutenberg and and geophysical data in combination with seismological in- Richter, 1944). formation and determined 159 active faults (rupture zones), Published studies on the compilation and adoption of related with the known strong (M ≥ 6.0) shallow earthquakes proper seismic sources model include various types of from 550 BC up to 2000 for the broader Aegean area. Based sources, such as point sources (e.g. catalogues of earth- on this seismic source model, Papaioannou (2001) proposed quakes), linear sources (e.g. faults) or areal type of sources a hybrid model consisting of faults and shallow seismic zones (polygon-type active regions, with known tectonic and seis- at the same time and used in the compilation of the 2001 re- mic activity parameters). In several cases seismic zonation vision of the Greek seismic code. Recently, SEAHELLARC models have been used for seismicity or earthquake forecast- Working Group (Slejko et al., 2010) developed a new seismo- ing studies. A first global zonation attempt was performed by genic zonation in order to evaluate and compute seismic and Flinn and Engdahl (1965), who proposed a seismic regional- tsunamis hazard and risk for SW Peloponnesus (SW Greece). ization scheme that was refined later (Flinn et al., 1974) and The seismic zonation models have been incorporated in revised in 1995 (Young et al., 1996). the seismicity analysis, either in a qualitative or a quanti- In the present work we focus our attention in the broader tative manner. The initial attempts were rather qualitative, Aegean Sea region (Fig. 1). This area includes the whole mostly based on the geographical distribution of earthquake Greek area, Albania, FYROM, as well as significant seg- epicentres. For the Aegean area several such studies have ments of Bulgaria and western Turkey. From a geotectonic been presented for shallow (Galanopoulos, 1963; Makropou- point of view the Aegean belongs to the Eurasian conti- los, 1978; Comninakis and Papazachos, 1978;