Monitoring active structures in eastern Corinth Gulf (Greece): the Kaparelli fault.
The Gulf of Corinth in central Greece is one of the most tectonically active and rapidly extending regions in the world. Surface topography and geomorphology are clearly associated with seismic activity along large normal faults (Armijo et al., 1996). Extension is mainly directed N-S (Clarke et al. 1998). The southern side of the Gulf of Corinth is bound by a series of major north-dipping normal faults, forming a complex asymmetric half graben (Jackson et al., 1982). There are E-W striking normal faults with antithetic dip, i.e. to the south; however, they are visible at the northern edge of the Gulf.
In February-March 1981 a sequence of three earthquakes with magnitudes greater than 6.3 struck the eastern Gulf of Corinth (Hubert et al., 1996). North-dipping surface breaks were noted the morning after the first two events on the southern side of the Gulf (Perachora Peninsula) and south-dipping ruptures appeared on the northern side of the Gulf (Kaparelli region) as a result of the third event (Figure 1; data from Jackson et al., 1982). In both areas seismic motion occurred along basin-bounding faults bringing in contact Mesozoic limestone and alluvial deposits as well as colluvium.
Focal mechanisms of small and shallow earthquakes (data from Ambraseys and Jackson, 1990) also show normal faulting with the active fault plane dipping at about 45[degrees] for faults at the eastern end of the Gulf of Corinth including Kaparelli. Recently, three trenches have been excavated across the Kaparelli Fault (Pavlides et al., 2003; Kokkalas et al., in press). Their stratigraphic record shows at least three events during the Holocene period, with the 1981 event included. The estimated mean slip rate is 0.28 mm/yr. Colluvial tectonostratigraphy and analysis of displacements on key horizons suggests surface rupturing events in the order of 0.7-1 m.
The Kaparelli Fault consists of three segments, two of which were ruptured in 1981 (Jackson et al., 1982). The two-ruptured segments form left-stepping en echelon geometry, while the third north-western segment of the fault did not rupture (Figure 2). The fault segments are clearly expressed at the surface by nearly continuous scarps. The footwall elevation is 600 m and lithology is composed of hard, Mesozoic limestone. The hanging wall block forms a small basin and contains approximately 200 m of fluvial-terrestrial deposits of Pleistocene age as well as Holocene alluvium.
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2. MONITORING THE ACTIVITY OF THE KAPARELLI FAULT
In order to measure the kinematics of deformation in 3-D the Greek group together with the Polish and Slovak groups, have established a dense GPS network and two (2) extensometers. The details of installation of instruments and project rationale are given in Drakatos et al. (2005) and Cacon et al. (2005). In summary, the Slovakian and Greek groups have begun monitoring the Kaparelli fault area since May 2003. On May 26, 2003 they installed an extensometer of TM-71 type (Kostak and Cruden, 1990). A second instrument was installed on July 15, 2004. In addition, a dense GPS network has been installed within a collaboration of Greek and Polish groups, comprising six (6) stations. The network was deployed on November, 11-12 2003 and its first measurement took place in May 2004. The network was expanded on March 31, 2005. Five (5) new stations were installed inside the Asopos rift valley (see Figure 4 below). Both sub-networks were measured during May 2005 and May 2006.
3. MONITORING FAULT MOTION USING TM-71 INSTRUMENTS
The 2003 installation of observation point Kapa1 was done at the trench Kap2 that was excavated across the fault (Figure 2; Pavlides et al., 2003). The TM-71 is fixed to the fault (north side) and to a construction built on the hanging wall (south side) by steel bars. The 2004 installation of Kapa2 was done against a free surface of the fault plane, about 2 km to the west of locality Kap2. The position of the X, Y, and Z axes of the instrument corresponds respectively to: (X) the direction perpendicular to the fault or fault-normal motion, (Y) direction parallel to fault strike or the strike slip motion along the fault surface and (Z) vertical direction or the dip slip component of motion. The instrument readings are recorded on a nearly monthly basis by NOA. Our observations for the site Kapa1 span a period of 3 years so it is possible to make a preliminary interpretation (Figure 3). From November 2003 the course in Kapa1 X-axis shows variations [+ or -]0.25 mm which can be attributed to seasonal temperature variations upon rock and is normal with this type of measurement (Drakatos et al., 2005). An extreme was observed between May and August 2003, (+0.5 mm) followed by a back-movement of -0.85 mm. This occurred in the initial period of measurement when a concrete block settlement after its creation inside the trench may be present. Since 2004 we observe continuous opening that is in agreement with fault kinematics. Displacement along Z-axis is relatively stable. Along the axis Y (horizontal; along the fault strike) observed motion is dextral oblique slip of -0.6 mm between June and November 2004 followed by relative stability. It is interesting to see that a similar pattern was observed in the rotation plots (Figure 3c) for the planes XY (the horizontal plane) and XZ (the vertical plane). Records show change at the same time in the second half of 2004, after which stability re-established. This dextral shear motion is consistent with the slip vector model of Roberts and Ganas (2000) that predict oblique slip vectors at normal fault tips. At Kapa2 the observation period is only two years (X-axis; Figure 3b) and the record needs more time to be interpreted.
[FIGURE 4 OMITTED]
4. MONITORING REGIONAL DEFORMATION USING GPS INSTRUMENTS
The motion of crustal blocks in central Greece can be described in a first approximation by a combination of translation and rotation (Goldsworthy et al., 2002). By combining the displacement vectors obtained at points of successive GPS campaigns during 2004-2006 and later, we will compute the velocity field over a distance of 30 km (E-W). The network was measured for the first time in May 2004 using the ASTHECH receivers of the Polish group. In particular stations ERIT, KAPA and TAPS (Figure 4) were observed using ASHTECH Zxtreme instruments and ASH701975.01A antennas while the remaining stations were observed using Z-XII instruments and ASH700718B antennas. Station AGTR (Figure 4) was used as the base station. The initial network was expanded during 2005 with the installation of five (5) more stations. All NOA stations were measured using Leica 1200 receivers.
The GPS observations from 2005 and 2006 campaigns were processed in ITRF2000 (Altamini et al., 2002) frame (each daily session separately) using as fiducial points the IGS Italian Permanent Station Matera (MATE) and Greek stations: IGD1 in 2005 and NOA1 in 2006. The Bernese GPS Software v. 4.2 (Hugentobler et al., 2001) was used and special data processing strategy was developed for the local precise network with the following issues and assumptions (Bosy and Kontny, 1998; Bosy et al., 2003):
* Global ionosphere model CODE for a phase ambiguity resolution,
* Antenna phase centres calibrated according to the NGS,
* Troposphere model--the Niell mapping function without a priori model; residual atmosphere zenith delays were estimated for one-hour intervals,
* Precise ephemeris computed by the Centre of Orbit Determination in Europe (CODE) to determine satellite positional data.
All KAPNET positions were resolved with Root-Mean-Square residuals less than 5 mm (horizontal plane; Figure 5). The antenna parameters of the 2005 campaign are given in Table 1. Table 2 presents the ITRF 2000 coordinates of all stations. Figure 5 shows the RMS (Root Mean Square) residuals of the station solutions.
Our first results include coordinates and RMS errors of KAPARELLI local network points for two years 2005 and 2006. They were generated by connecting the daily solutions using the ADDNEQ program (Brockmann, 1996; Hugentobler et al., 2001). Computed ITRF2000 geodetic coordinates ([phi], [lambda] h) referring to their respective epochs as well as their RMS errors are provided in Table 2. The accuracy of determined coordinates ([phi], [lambda] and h) representing un-weighted RMS values of coordinate residuals taken from each year (comparison of station coordinates from each session with respect to the combined solution in mm) is shown in Figure 5. The un-weighted RMS values computed from repeated observations give more realistic accuracy of stations coordinates. The organisation and data processing of repeated satellite GPS standards provide conditions for detecting horizontal movements of the stations at the <5 mm level.
a) Dextral shear motion at Kapa1 was indicated. This is found in agreement with non-uniform strain patterns along active normal faults (Roberts and Ganas, 2000). Besides, till now, the TM71 instruments along the Kaparelli Fault show seasonal effects (Figure 3) compatible with seasonal temperature variation.
b) Three (3) similar instruments have been installed in the Perachora peninsula by Maniatis et al. (2003; Figure 1). Given that Holocene fault slip rates at these localities differ by a factor of five (5) or even more it is important to compare 3-D strain measurements in order to estimate interseismic strain accumulation along the two fault zones. Another issue concerns the identification of instrument-induced errors on our measurements and their estimation. The advantage that TM-71 has no electronics parts is counterbalanced by the infinitesimal deformations caused by climatic fluctuations (precipitation, temperature). For example a seasonal effect along the X-axis is visible in the 16-year data series from the Simitli graben in SW Bulgaria (Dobrev and Kostak, 2000).
c) The data processing of repeated satellite GPS standards provide conditions for detecting horizontal movements at our stations at 2-4 mm level. Taking into account the fault movement rate of about 0.5 mm/y, indicated by TM-71 (Figure 3), significant displacement across Kaparelli can be detected by GPS after 4-5 years of observations.
d) The large E-W faults of the Gulf of Corinth slip fast (1-2 mm/yr) and terminate towards the south of the Kaparelli Fault, while the large normal faults of the Parnitha region to the east slip at about five (5) to ten (10) times less (0.2-0.4 mm/yr; Ganas et al., 2004). It is interesting to map this transition in fault slip rates by using the GPS measurements of KAPNET. Our first results (Figure 5) point to an excellent quality of GPS data that will provide stable solutions. A second point concerns the total offset on the Kaparelli fault that is small (< 200 m), while the geological data suggest that it is segmented. So our GPS measurements will differentiate fault slip and strain accumulation among the segments. These observations will also shed light into fault growth processes as Kaparelli is an early phase in the development of large normal faults that involves the merging of two or more faults of differing strikes, rather than the steady lengthening of a single fault segment.
e) The results from the GPS observations will help us establish both the magnitude and direction of geodetic strain and compare it with the Holocene faulting record as published in Pavlides et al., (2003) and Kokkalas et al., (in press). The orientation of the strain axes will be also compared with the configuration of the rupture zones of both the 1981 and the 1999 earthquakes (Figure 4).
This research was primarily funded by COST Action 625. We are indebted to our COST colleagues for many stimulating discussions. The NOA GPS poles were constructed at the NESTOR workshop in Pylos. We acknowledge reviews by two anonymous referees and useful comments by B. Kostak, G. Stavrakakis, S. Pavlides, I. Koukouvelas, A. Gosar, L. Piccardi, E. Tondi and V. Spina. We thank our NOA colleagues Th. Vourakis, G. Mihalettos, I. Papastamatiou, M. Papanikolaou, and A. Pirentis for help during the field trips. Many thanks are due to E. Skassis for this help with GPS instrumentation. NOA1 data are available from http://www.epncb.oma.be/_trackingnetwork/siteinfo4 onestation.php?station=NOA1_12620M001
[FIGURE 5 OMITTED]
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Athanassios GANAS (1) *, Jaroslaw BOSY (2), Lubomir PETRO (3), George DRAKATOS (1), Bernard KONTNY (2), Marian STERCZ (3), Nikolaos S. MELIS (1), Stefan CACON (2) and Anastasia KIRATZI (4)
(1) National Observatory of Athens, Geodynamic Institute, Lofos Nymfon, Athens 118 10, Greece
(2) Institute of Geodesy and Geoinformatics, Wroclaw University of Environmental and Life Sciences, Grunwaldzka 53, 50-357 Wroclaw, Poland
(3) Geological Survey of Slovak Republic, Jesenskeho 8, SK-040 01 Kosice, Slovak Republic
(4) Department of Geophysics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
* Corresponding author's e-mail: firstname.lastname@example.org
(Received January 2007, accepted March 2007)
Table 1 Parameters of the KAPNET GPS antennas during the 2005 (DOY 130, 131) and 2006 (DOY 122, 123) observation campaigns. Height is in metres. 2005 130 131 POINT ANT. TYPE h-OBS h-RED POINT ACLA ASH700718B 0.8290 0.7654 ACLA AGTR ASH700936D_M 0.7640 0.7262 AGTR ERIT ASH701975.01Agp 0.7810 0.7380 ERIT KAPA ASH701975.01Agp 0.8080 0.7650 KAPA TAPS ASH701975.01Agp 0.8140 0.7710 TAPS VILI ASH700718B 0.8150 0.7514 VILI ALYK LEIAT502 0.7230 0.6672 ALYK ASO P LEIAX1202 0.7500 0.6880 ASOP DAFN LEIAX1202 0.7210 0.6590 DAFN DFNL LEIAX1202 0.7390 0.6770 DFNL KALI LEIAX1202 0.7230 0.6610 KALI 2006 122 123 POINT ANT. TYPE h-OBS h-RED POINT ACLA ASH700718B 0.8350 0.7714 ACLA AGTR ASH700936D_M 0.7640 0.7292 AGTR ERIT ASH701975.01Agp 0.7810 0.7380 ERIT KAPA ASH701975.01Agp 0.8070 0.7640 KAPA TAPS ASH701975.01Agp 0.8120 0.7690 TAPS VILI ASH700718B 0.8200 0.7564 VILI ALYK LEIAT502 0.6910 0.6910 ALYK ASOP LEIAX1202 0.6910 0.6910 ASOP DAFN LEIAX1202 0.6910 0.6910 DAFN DFNL LEIAX1202 0.6910 0.6910 DFNL KALI LEIAT502 0.6910 0.6910 KALI 2005 130 131 POINT ANT. TYPE h-OBS h-RED ACLA ASH700718B 0.8300 0.7664 AGTR ASH700936D_M 0.7640 0.7262 ERIT ASH701975.01Agp 0.7830 0.7400 KAPA ASH701975.01Agp 0.8070 0.7640 TAPS ASH701975.01Agp 0.8140 0.7710 VILI ASH700718B 0.8150 0.7514 ALYK LEIAT502 0.7230 0.6672 ASO P LEIAX1202 0.7500 0.6880 DAFN LEIAX1202 0.7210 0.6590 DFNL LEIAX1202 0.7390 0.6770 KALI LEIAX1202 0.7230 0.6610 2006 122 123 POINT ANT. TYPE h-OBS h-RED ACLA ASH700718B 0.8350 0.7714 AGTR ASH700936D_M 0.7640 0.7292 ERIT ASH701975.01Agp 0.7810 0.7380 KAPA ASH701975.01Agp 0.8070 0.7640 TAPS ASH701975.01Agp 0.8120 0.7690 VILI ASH700718B 0.8210 0.7574 ALYK LEIAT502 0.6910 0.6910 ASOP LEIAX1202 0.6910 0.6910 DAFN LEIAX1202 0.6910 0.6910 DFNL LEIAX1202 0.6910 0.6910 KALI LEIAT502 0.6910 0.6910 Table 2 The ITRF 2000 coordinates of the KAPNET stations Point [phi] [lambda] [o ' ''] [o ' ''] ITRF2000 EPOCH: 2005-05-10 23:59:45 ACLA 38 14 20.457830 23 10 1.655841 AGTR 38 12 29.119021 23 14 13.982700 ALYK 38 13 34.536100 23 2 42.984761 ASOP 38 17 28.851765 23 29 6.323049 DAFN 38 14 8.585989 23 25 7.455789 DFNL 38 15 31.194095 23 28 16.535688 ERIT 38 12 47.859336 23 20 21.377514 KALI 38 19 18.825944 23 25 14.223037 KAPA 38 13 52.699457 23 13 18.760112 TAPS 38 11 47.745238 23 8 45.103563 VILI 38 10 7.788711 23 18 11.234132 ITRF2000 EPOCH: 2006-05-02 23:59:45 ACLA 38 14 20.457462 23 10 1.656128 AGTR 38 12 29.118592 23 14 13.982988 ALYK 38 13 34.535574 23 2 42.985050 ASOP 38 17 28.851322 23 29 6.323437 DAFN 38 14 8.585571 23 25 7.456152 DFNL 38 15 31.193592 23 28 16.535760 ERIT 38 12 47.858913 23 20 21.377813 KALI 38 19 18.825917 23 25 14.223237 KAPA 38 13 52.699048 23 13 18.760358 TAPS 38 11 47.744646 23 8 45.103803 VILI 38 10 7.788286 23 18 11.234429 Point h RMS RMS RMS [phi] [lambda] h [m] [mm] [mm] [mm] ITRF2000 EPOCH: 2005-05-10 23:59:45 ACLA 473.2161 0.5 0.7 3.7 AGTR 578.4204 0.5 0.7 3.7 ALYK 474.9853 0.7 0.7 4.9 ASOP 321.9817 0.5 0.7 3.7 DAFN 533.3963 0.5 0.7 3.7 DFNL 357.5885 0.5 0.7 3.7 ERIT 474.0419 0.5 0.7 3.9 KALI 395.7988 0.5 0.7 3.9 KAPA 340.7526 0.5 0.7 3.7 TAPS 481.9591 0.5 0.7 3.9 VILI 694.3970 0.5 0.7 3.7 ITRF2000 EPOCH: 2006-05-02 23 ACLA 473.2449 0.5 0.5 3.1 AGTR 578.4606 0.3 0.3 2.0 ALYK 475.0196 0.4 0.5 3.0 ASOP 322.0029 0.3 0.4 2.3 DAFN 533.3879 0.4 0.5 2.7 DFNL 357.6028 0.3 0.4 2.1 ERIT 474.0702 0.3 0.4 2.1 KALI 395.8117 0.3 0.4 2.7 KAPA 340.7809 0.4 0.4 2.4 TAPS 481.9939 0.3 0.4 2.1 VILI 694.4191 0.3 0.3 2.0
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|Author:||Ganas, Athanassios; Bosy, Jaroslaw; Petro, Lubomir; Drakatos, George; Kontny, Bernard; Stercz, Maria|
|Publication:||Acta Geodynamica et Geromaterialia|
|Date:||Jan 1, 2007|
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