The 8 January 2006 Earthquake (M 6.7) Offshore Kythira Island
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The 8 January 2006 Earthquake (Mw 6.7) Offshore Kythira Island, Southern Greece: Seismological, Strong-motion, and Macroseismic Observations of an Intermediate-depth Event Konstantinos I. Konstantinou, Ioannis S. Kalogeras, Nikolaos S. Melis, Moissis C. Kourouzidis, and George N. Stavrakakis Konstantinos I. Konstantinou, Ioannis S. Kalogeras, Nikolaos S. Melis, Moissis C. Kourouzidis, and George N. Stavrakakis Institute of Geodynamics, National Observatory of Athens INTRODUCTION In this article we take advantage of a multitude of available observations to give a detailed report on this most recent large On 8 January 2006 at 11:34 GMT (13:34 local time), a strong intermediate-depth earthquake. First, we describe the temporal earthquake with a moment magnitude of 6.7 occurred in and spatial distribution of the mainshock-aftershock sequence southern Greece, off the eastern coast of the island of Kythira. and summarize all available moment tensor solutions reported The epicentral coordinates as estimated by the European by various agencies. Then, we present preliminary analysis of Mediterranean Seismological Centre (EMSC-CSEM, http:// strong-motion recordings in an effort to check the relationship emsc-csem.org were 36.31°N, 23.24°E, and the focal depth was between the shaking caused by such an event and the influence of 60 km. The shock was felt in a spatially extended area that cov- both attenuation and local geological conditions. Macroseismic ered Greece, Italy, Turkey, Egypt, Cyprus, Israel, Syria, Jordan, data collected from the whole of Greece also are included and and Lebanon. Despite the large magnitude of the earthquake, utilized toward understanding the regional intensity attenua- the reported damage was not extensive mainly due to the inter- tion pattern. Finally, we give an overview of the implications of mediate focal depth. No casualties were reported and the struc- such an event in terms of regional seismotectonics and seismic tural damage to buildings was limited to the islands of Kythira hazard. and Antikythira and to the city of Chania, western Crete. Furthermore, landslides and/or rockfalls were reported only at SEISMOLOGICAL OBSERVATIONS the village of Mitata (Kythira island), where the main square and the road were damaged due to a landslide. The National Observatory of Athens, Institute of Geodynamics ▲ Figure 1. Main features of tectonic origin in the broader Aegean region (modified from Papazachos and Papazachou 1997). Intermediate-depth earthquakes in this region are related (NOAIG) is responsible for the operation of the Greek to the southern Aegean subduction zone, which is referred to national seismic network (also known as the HL network, The mainshock as well as several of its aftershocks were Several institutes/agencies have reported moment-tensor as the Hellenic arc (figure 1). The deformation that takes place http://bbnet.gein.noa.gr) and the monitoring of earthquake recorded by the HL network, and our analysis started by manu- solutions for the mainshock using either teleseismic (Harvard along this arc is caused by the motion and collision of the south- activity in the country (figure 2). The network consists of 22 ally picking P- and S-phase arrival times of these events for the CMT, USGS, CPPT) or regional (ETH, INGV, AUTH) ern part of the Eurasian plate with the African plate at a rate stations equipped with broadband (up to 20–30 s) three-com- period between 8 January 2006 and 20 May 2006. These arrival waveform data (figure 3). All of these solutions exhibit a pre- of 40–60 mm/year. The basic picture of the regional kinemat- ponent seismometers recording continuously at a sampling times subsequently were inverted using HYPO2000 (Klein dominantly reverse slip component, even though the exact ics has not changed much since the early work of McKenzie interval of 50 samples per second. An additional seven stations 2002) assuming a velocity model and VP/VS ratio of 1.78 fault strike varies among the reported mechanisms from E–W (1972, 1978) but has been studied in more detail recently using are equipped with broadband instruments (up to 120 s) oper- based on the results of the tomographic study for the Aegean to NE–SW. This perpendicular/oblique direction to the trench GPS data (LePichon et al. 1995; McClusky et al. 2000; Nyst ated by the GEOFON and MEDNET groups in collaboration Sea region published by Papazachos and Nolet (1997) (table also is consistent with the orientation of the epicentral locations and Thatcher 2004, among others). The continuation of the with NOAIG. The data are transferred via dedicated telephone 1). The inferred mainshock location (36.21 N, 23.42 E, 65 km) reported earlier. Waveform data recorded by the HL network Hellenic arc is interrupted by active fault zones with a perpen- lines in near real-time to a collection center in Athens where appears to be consistent with the one reported by EMSC- also were inverted for the purpose of deriving moment-tensor dicular orientation relative to the boundary of the two plates they are stored for further analysis. This includes phase picking, CSEM (the difference between the two locations is 19 km). solutions for the mainshock and two of its largest aftershocks (Lyberis et al. 1982). One of these structures exists in the vicin- location, and local magnitude determination and, in cases when Figure 3 shows the locations that exhibit formal errors in the (see NOAIG solutions in figure 3). The inversion methodol- ity of Kythira island and during the 20th century produced the earthquake is larger than ML ~4, inversion of the broadband horizontal (ERH) and vertical (ERZ) directions smaller than ogy follows that of Randall et al. (1995) and also is described five strong earthquakes: in 1903 M( 7.5), 1910 (M 7.0), 1926 waveforms for moment tensor derivation. A detailed descrip- 4 km and rms residuals smaller than 1 s. Most aftershocks occur by Melis and Konstantinou (2006). It should be noted that the (M 7.2), 1937 (M 6.0), and 1932 (M 6.2) (Papazachos and tion of the network operation and near real-time data process- at depths shallower than the mainshock, while the orientation closest HL stations to the epicenter were clipped; therefore only Papazachou 1997). ing can be found in Melis and Konstantinou (2006). of the epicenter cluster is roughly NE–SW. stations at distances greater than 100 km were used in the inver- 544 Seismological Research Letters Volume 77, Number 5 September/October 2006 Seismological Research Letters Volume 77, Number 5 September/October 2006 541 ▲ Figure 1. Main features of tectonic region in the broader Aegean region. Triangles represent volcanic centers and open arrows repre- ▲ Figure 1. Main features of tectonic origin in the broader Aegean region (modified from Papazachos and Papazachou 1997). sent the stress regime (compressional, extensional, strike slip). NAF: North Anatolian Fault. TheThe mainshockmainshock as as wellwell as as several several of of its its aftershocks aftershocks were were SeveralSeveral institutes/agencies institutes/agencies have have reported reported moment-tensor moment-tensor recordedrecorded by by the the HL HL network, network, and and our our analysis analysis started started by by manu manu- - solutionssolutions for for the the mainshock mainshock using using either either teleseismic teleseismic (Harvard (Harvard allyally picking picking P- P- and and S- S-phasephase arrival arrival times times of of these these events events for for the the CMT,CMT, USGS, USGS, CPPT) CPPT) or or regional regional (ETH, (ETH, INGV, INGV, AUTH) AUTH) periodperiod between between 8 8January January 2006 2006 and and 20 20 May May 2006. 2006. These These arrival arrival waveformwaveform data data (figure (figure 3). 3). AllAll of of these these solutions solutions exhibit exhibit a apre pre- - timestimes subsequently subsequently were were inverted inverted using using HYPO2000 HYPO2000 (Klein (Klein dominantlydominantly reverse reverse slip slip component, component, even even though though the the exact exact 2002) assuming a velocity model and VP/VS ratio of 1.78 fault strike varies among the reported mechanisms from E–W 2002) assuming a velocity model and VP / VS ratio of 1.78 fault strike varies among the reported mechanisms from E–W basedbased on on the the results results of of the the tomographic tomographic study study for for the the Aegean Aegean toto NE–SW. NE–SW. This This perpendicular/oblique perpendicular/oblique direction direction to to the the trench trench SeaSea region region published published by by Papazachos Papazachos and and Nolet Nolet (1997) (1997) (table (table alsoalso is is consistent consistent with with the the orientation orientation of of the the epicentral epicentral locations locations 1).1). The The inferred inferred mainshockmainshock location location (36.21 (36.21°N, N, 23.42 23.42°E, E, 65 65km) km) reportedreported earlier. earlier. Waveform Waveform data data recorded recorded by by the the HL HL network network appearsappears to to be be consistent consistent with with the the one one reported reported by by EMSC- EMSC- alsoalso were were inverted inverted for for the the purpose purpose of of deriving deriving moment-tensor moment-tensor CSEMCSEM (the (the difference difference between between the the two two locations locations is is 1919km). km). solutionssolutions for for the the mainshock mainshock and and two two of of its its largest largest aftershocks aftershocks FigureFigure 3 3shows shows the the locations locations that that exhibit exhibit formal formal errors errors in in the the (see(see NOAIG NOAIG solutions solutions in in figure figure 3). 3). TheThe inversion inversion methodol methodol- - horizontalhorizontal (ERH) (ERH) and and vertical vertical (ERZ)(ERZ) directions smaller thanthan 4 ogyogy follows follows that that of of Randall Randall et et al. al. (1995) (1995) and and also also is is described described 4kmkm and rms residuals smallersmaller thanthan 1 1 s.