Seismotectonics of SE Aegean Inferred from Precise Relative Locations of Shallow Crustal Earthquakes
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JSeismol https://doi.org/10.1007/s10950-019-09881-8 ORIGINAL ARTICLE Seismotectonics of SE Aegean inferred from precise relative locations of shallow crustal earthquakes R. Andinisari & K. I. Konstantinou & P. Ranjan Received: 20 June 2019 /Accepted: 24 September 2019 # Springer Nature B.V. 2019 Abstract We relocated the seismicity in SE Aegean locations delineated faults along the Gulf of Gökova, that was recorded by both permanent and temporary SW of Nisyros, and Karpathos area. Based on the com- seismic networks during 2004–2018 in order to investi- parison of the resulting seismicity distribution with the gate its correlation with the active faults in this area. P regional stress field, it can be concluded that seismicity and S-phase travel times of best quality events were only occurred along faults with ENE-WSW strike in the used to estimate a minimum 1D velocity model with area north of Tilos and N-S strike in the area south of station delays by utilizing VELEST. This velocity mod- Tilos. Seismogenic layer thickness estimated from the el and station delays were then used to obtain absolute hypocentral depth distribution was found to vary be- locations of 3055 events, utilizing the probabilistic non- tween 12.1 and 15.4 km. Expected moment magnitudes linear algorithm NLLOC. The double-difference algo- of faults in this area were calculated by using their rithm was used along with catalog and cross-correlation geometrical properties and the seismogenic layer thick- differential times to relocate all the events, resulting in ness, yielding magnitudes in the range of 5.9–6.9. The 2200 precise relative locations with horizontal and ver- fact that most of the seismically active faults in SE tical uncertainties of less than 1.0 km. The precise Aegean lie offshore increases the probability that a major earthquake will be followed by a tsunami and calls for the close monitoring of seismicity in this area. Article Highlights • As many as 2200 crustal earthquakes in South East Aegean have been precisely relocated. Keywords Aegean . Greece . Seismotectonics . Crustal • – The estimated seismogenic layer thickness in the area is 12.1 earthquakes . Relocation . Seismic hazard 15.4 km. • Expected magnitudes of future earthquakes in South East Aegean vary between 5.9 and 6.9. Electronic supplementary material The online version of this 1 Introduction article (https://doi.org/10.1007/s10950-019-09881-8) contains supplementary material, which is available to authorized users. The Aegean region exhibits significant tectonic activity as a result of several geodynamic processes. One of R. Andinisari (*) : K. I. Konstantinou : P. Ranjan Department of Earth Sciences, National Central University, these processes is the subduction of the African litho- Chungli, Taoyuan, Taiwan spheric plate beneath the Aegean at the rate of about 0.9 e-mail: [email protected] cm/year (Reilinger et al. 2006;McCluskyetal.2000), forming the Hellenic subduction zone (Fig. 1). The P. Ranjan Taiwan International Graduate Program–Earth System Science African plate is subducting beneath the Aegean with (TIGP–ESS), Taipei, Taiwan an increasing degree of obliquity from the western to the eastern part with a maximum value of 40° to 50° J Seismol (Bohnhoff et al. 2005). The Aegean microplate is peninsula and on Kara Ada island, the Bodrum-Kos dragged in a southwestward direction at a rate of 3.5 earthquake also caused the death of 2 people in Kos cm/year due to the slab rollback of the African plate island. Based on these observations, it can be easily (Hollenstein et al. 2008; Nyst and Thatcher 2004; understood that SE Aegean is an area significantly ex- Reilinger et al. 2006; Rontogianni 2010). The SE Ae- posed to both seismic and tsunami hazards. gean in particular experiences significant extensional The main purpose of this work is to relocate the deformation (Rontogianni 2010) characterized by shallow crustal seismicity in SE Aegean in order to ENE-WSW trending normal faulting along the Gulf of investigate the geometry and segmentation characteris- Gökova (Kurt et al. 1999) as well as N-S normal faulting tics of active faults in this area. The resulting informa- to the north of Karpathos island. However, there is no tion will later be used to determine the seismogenic significant extensional deformation observed in the area layer thickness and assess the potential seismic hazard. around Rhodes island (Hall et al. 2009). The dominance First, a minimum 1D velocity model was estimated from of extensional regime in SE Aegean is also supported by the well-recorded earthquakes and was then utilized to GPS observations showing an increase in the velocity obtain absolute earthquake locations. The resulting ab- field towards the Pliny-Strabo trenches, relative to cen- solute locations were then compared to routine locations tral Aegean and Peloponnese (Reilinger et al. 2010). provided by NOA to assess their differences. Subse- Aside from that, the SE Aegean also hosts the active quently, precise relative relocation was performed and Nisyros caldera that produced several eruptions in the the resulting seismicity distribution was analyzed in the past and during 1995-1997 it also experienced a period interest of delineating active faults in the area, and for of unrest. the purpose of examining the correlation of the resulting From the point of view of historical seismicity, the seismicity distribution with the prevailing regional stress SE Aegean region has a rich record of large earthquakes. field. Finally, we combined historical seismicity infor- Based on the ISC-GEM catalog (Storchak et al. 2013) mation with the delineated faults to study several and the catalog of the National Observatory of Athens seismogenic sources in the area in terms of their poten- (NOA), as many as 15 earthquakes with moment mag- tial to generate strong earthquakes in the future. nitude varying between 5.5 and 7.3 have been recorded from the year 1911 up until 2017 (Table 1). Among these earthquakes, there were 3 events with moment 2Data magnitude of 6.4 or larger (Fig. 1). The first of these events is the Kos earthquake that occurred on 23 April The data that we used was recorded by permanent and 1933 with a moment magnitude of 6.4. This earthquake temporary networks deployed in the southern Aegean was felt in the islands of Kos and Nisyros, as well as in and SW Turkey. These networks are Hellenic Seismic broad areas along the Turkish coast. Other than causing Network (HL) (National Observatory of Athens, a tsunami that affected Kos island, this earthquake also Institute of Geodynamics, Athens 1997), the Hellenic caused the death of 200 people with 600 others injured Unified Seismic Network (HUSN), KOERI seismic net- (Papazachos and Papazachou 2003). The second large work (Boğaziçi University Kandilli Observatory and earthquake occurred on 9 February 1948 to the east of Earthquake Research Institute 2001), and the Karpathos with moment magnitude of 7.3 (Ebelling EGELADOS (Exploring the Geodynamics of et al. 2012). This earthquake destroyed 573 houses and Subducted Lithosphere Using an Amphibian Deploy- excited a tsunami that inundated the coast up to 1 km ment of Seismographs) network (Friederich and Meier (Papazachos and Papazachou 2003). The latest destruc- 2005). HL consisted of only 26 seismic stations which tive earthquake in the SE Aegean occurred on 20 Ju- were equipped with 20–120 s three-component broad- ly 2017 with magnitude of 6.6 and was located in band seismometers. This seismic network was merged Gökova graben between the city of Bodrum and the with other permanent seismic networks in Greece to island of Kos. Hence, this event will be referred to as form HUSN in 2008. From August 2004 to January the Bodrum-Kos earthquake hereafter. The rupture zone 2005, a series of events originated in the Gulf of Gökova of this earthquake could not be observed directly since it were recorded by HL and stations that belonged to was located offshore. Besides from generating a small KOERI. As many as 41 crustal events from the men- tsunami and causing heavy damages in Bodrum tioned period with moment magnitude between 4.0 and JSeismol Fig. 1 Map of south Aegean with the study area highlighted in the Inset map in the bottom right corner shows a detailed map of the dashed square. Solid orange lines represent active faults contained study area. GG and HP in inset map represent the Gulf of Gökova in the GReDaSS database (Caputo and Pavlides 2013). The ma- and Hisarönü Peninsula, respectively. Red stars in inset map genta lines show isodepth curves of earthquake hypocenters that indicate the location of 3 earthquakes with moment magnitude occurred along the Wadati-Benioff zone (Papazachos et al. 2000). 6.4 or larger that occurred in SE Aegean within the period of The thick black arrows represent the present-day plate motions. 1911–2017 The yellow stars indicate the locations of active volcanic centers. 5.5 were used in this study. We also included 30 other seismometers, 4 STS-2 seismometers, and also 7 1-Hz events recorded by KOERI and HL/HUSN for the peri- Mark seismometers). The EGELADOS network also od of March 2006 to November 2010. These events also included permanent stations of the GeoForschungsNetz occurred in the Gulf of Gökova with magnitudes rang- (GEOFON) network equipped with short period three- ing from 3.3 to 4.8. The inclusion of these events in our component seismometers and 1 Mediterranean Very analysis was based on the fact that they occurred in the Broadband Seismographic Network (MedNet) station. central and eastern part of the gulf where fewer events As many as 740 crustal events that occurred in our study were recorded in the following years. We obtained the area were recorded by this network during its functional waveforms of all these events and manually picked their period and their phases were also manually picked.