ACTIVE FAULTS HAZARD ON OFFSHORE PIPELINES: CASE OF THE SUBMARINE GAS PIPELINE ROUTE ACROSS THE SOUTH EVOIKOS GULF, CENTRAL GREECE

Christoforos P. Metaxas

Earthquake Planning and Protection Organization, Greece. [email protected]

Major faults that could constitute a serious seismic hazard in the area of the Stamata-Alivery routing of the 20’’ H.P.N.G. pipeline offshore the South Evoikos Gulf are the Lefkadi and the Oropos faults, the bordering faults of the South Evia neotectonic basin. The offshore tracks of these major normal faults, as it proved by submarine geophysical survey, cross the designate gas pipeline rout as at the southern margin of the Gulf and at the it’s northern margin. An amount of historical strong seismic events of magnitude M>6.0, are associated with these fault zones, the potential of which to create the maximum expected strong event is about Mmax = 6.8. The Lapsed Rate of the active Lefkadi fault is about 89-111%. So, the fault could be in the last study of its seismic cycle and the probability an earthquake with M>=6.0 to occur at pipeline crossing with Lefkadi active fault, during the pipeline operational life (50 year), are very high. For designate pipeline route the Operating Design Earthquake (ODE) having 70 years return period is an event with M=5.4-5.5. The Maximum Design Earthquake (MDE) with 975 years return period is an event with magnitude 6.5-6.6. The Permanent Ground Displacement Hazard Curves are calculated from two fault sources for sites at the pipeline-fault crossings. The 51% and 5% chance of exceedance of Permanent Ground Displacements are 9-10 cm and 75-80 cm respectively for ODE (M= 5.4-5.5) and MDE (M=6.5-6.6) events. Probabilistic Seismic Hazard Analysis has been performed for the designate pipeline route area. The Total Seismic Hazard Curves displaying the values of peak ground acceleration (PGA), velocity (PGV) and 5% damped spectral acceleration (Sa) and velocity (Sv) for spectral period T = 0.3 sec, for rock conditions have been calculated for three sites along the pipeline route. The major characteristics of the PSHA results are that the calculated strong ground motion values are higher than ones proposed for the pipeline route area by Greek Seismic Design Code (2000). The explanation could be based on the fact, that the site – surface fault track distance, but not the site – epicenter distance is the major parameter defining the ground motion values and related damages of infrastructure. Thus, the calculated strong ground motion values appear to be lower than real values, which could be recorded in field at pipeline fault crossings with the Lefkadi and Oropos faults during the expected earthquakes of M= 5.4-5.5 (ODE) and M=6.5-6.6 (MDE), as it have taken place during the Northridge, 1994 (USA) and Kobe, 1995 (Japan) earthquakes.

1. TECTONIC SETTINGS AND ACTIVE FAULTS The area of the designate pipeline route belongs to the zone, where the Pelagonian zone upthrusts the metamorphic formations of the Attico-cycladic zone (Vergely, 1984, Papanikolaou et al., 1999). Various tectonic phases, from alpine napping to the opening of neotectonic basins, have led to a complicated fracturing of the area. After the alpine movements, the main geodynamics of the broader area, as of the whole Aegean area, is characterized mainly by a tensional regime causing normal faults with remarkable displacements in relief. There are three phases in the neotectonic history of the area (Lemeille, 1977, Mercier et al., 1989): (a) NE-SW tensional regime during the Pliocene, (b) A short period of NW-SE compressional regime during the Upper Pliocene-Lower Pleistocene and (c) N-S tensional regime from the Middle Pleistocene to date The last period is responsible for the opening of the Evoikos Gulf as tectonic graben (rift) and has caused earthquakes of large magnitudes in ancient and recent times. The main neotectonic structures of the broader area, which could be capable to produce strong earthquakes, could be responsible for seismicity and consequently for seismic hazard on the under study pipeline route area, are the post-alpine neotectonic basins and related to them active faults (Figure 1). The Southern Evoikos Gulf is a shallow basin less than 250 m deep that separates Attica from the southern Evia and was formed in Late Pliocene. The thickness of the Plio-Quaternary sediments within the Gulf does not exceed 150 m, except for the southeastern area, where they are 250 m thick (Papanikolaou et al., 1988). The main faults that affect the Pre-Neogene and recent geological formations of the South Evia Basin (under study pipeline route area) are normal fault oriented WNW-ESE to NW- SE. The dominant structure of the South Evoikos Basin, as of the whole under study area, is a major detachment fault (see Figure 1) separating the metamorphic units of Attico-cycladic zone towards the east, from non-metamorphic units of Pelagonian zone towards the west. Moreover, it separates the E-W trending faults in the western part (as Lefkadi fault, Oropos fault, etc) from the NW-SE faults in the eastern part of the under study area, as well as obviously responsible for changing the strike of South Evoikos Basin from E-W to NW-SE. At 1987 the Department of Submarine Geology of IGME in collaboration with Stanford University of USA conducted in the Southern Evoikos Gulf the sub-sea survey using seismic reflection method SPARKER, 300-500 Joules (Perissoratis et al., 1989). Two active faults, bordering the South Evoikos basin towards the North and towards the South have been clearly distinguished. In 1989 Papanikolaou et al published the Submarine Neotectonic Map of the Southern Evoikos Gulf, based on the results of submarine surveys of the Greek National Center of Marine Researches (NCMR). This map also displays two active faults bordering the basin. In February 2006, Geopro GmbH, Hamburg, Germany performed on behalf of HCMR (for Project of the Asprofos S.A.), Athens, Greece a multi-component seismic survey in order to delineate the stratigraphic elements and tectonic structures between the Grammatiko and Aliveri in the Gulf of South Evia along the designated pipeline route. The results of this survey are shown in Figure 2, where we also can clearly see the major bordering faults of the South Evoikos basin. According to (Rondoyanni et al., 2007), two antithetic normal faults with WNW-ESE direction outcrop onshore in the Avlida region and in the plain of the river Lilas, on opposite sides of the South Evoikos Gulf. The Avlida fault, having a northeastern dip, outcrops to the South of village Avlida and affects the Neogene’s deposits, as well as the Pleistocene. The fault continues offshore (we call it as Oropos Fault after Papanikolaou et all, 1999, and Papanikolaou & Papanikolaou, 2007), passing to the north of the village Dilesi, as detected into the recent sediments, by seismic reflection profiles in the Gulf (Papanikolaou et al., 1989, Perisoratis et al., 1989, Perisoratis and van Andel, 1991, Geopro, 2006) and consist of two segments – of WNW-ESE direction and NW-SE direction (see Figure 1). It has a total length more than 40 km (~25 km of the WNW-ESE segment and ~ 20 km of the NW-SE segment), and forms the southern margin of the Evoikos Gulf. The southern Evoikos gulf is a shallow basin less than 250 m deep and the Oropos fault has a throw of 250m, implying that Oropos fault zone is a very recent and active structure.

Figure 1: Tectonic scheme and strong (M>=6.0) earthquakes epicenter map of the under study area Black solid lines represent active neotectonic faults (barbs towards the subsided block). Gray dashed line is the location of designate offshore pipeline route Stamata-Aliveri. Stars depict the location of strong (M>=6.0) earthquakes epicenters, and triangles the location of the calculation sites.

The antithetic fault (we call it after Rontoyanni et al., 2007, as Lefkadi Fault), dipping to the South, affects the alluvial deposits of the plain of the river Lilas. It continues offshore, for a total length of 35 km, to the South of the villages Eretria , Amarinthos and more to East (Papanikolaou et al., 1989, Perisoratis et al., 1989) and forms the Northern margin of the E-W segment of South Evoikos Gulf. The quantitative interpretation of 22 geoelectrical soundings, based on a horizontal stratification model, showed the existence of a sub vertical electrical discontinuity disturbing the Quaternary deposits, which is directly related with the position of the concerned fault (Rontoyanni et al., 2007). The NE margin of the NW-SE elongated segment of the Southern Evoikos Gulf is formed by another fault, which seems to not being active, as the results of submarine seismic survey show (Papanikolaou et al., 1989, Geopro, 2006). Taking into account the thickness of the syn-rift Holocene and Upper-Pleistocene sediments in offshore seismic profiles (Perissoratis et al., 1989), an average value of the faults slip-rate of the order of 1 mm/yr for the last 150.000 years have been estimated (Perisoratis and van Andel, 1991). This slip rate is smaller than that of the faults delimiting the graben of the North Evoikos Gulf, which is of the order of 3mm/yr, according to Philip (1974). Therefore, within the South Evoikos Basin the two antithetic faults of Oropos and Lefkadi, may be considered currently active, according to the geological and geophysical data, and their evolution have to be connected very closely with the seismic activity of the area. Comparison of the surface expressed neotectonic faults structure with subsurface deep faults tectonic structure, derived from qualitative gravity field interpretation (Metaxas et al., 2001, Metaxas, 2005), shows in generally correspondence of surface and subsurface structures, i.e. the evolution of the neotectonic structures during the Neogene’s - Quaternary period was controlled by the reactivation, in accordance with the overall current geodynamic regime, of the Alpine structures, impressed in the Paleozoic – Mesozoic basement of under study area.

2. SEISMICITY CHARACTERISTICS AND LAPSED RATES OF ACTIVE FAULTS

For seismicity analysis the borders of the region under study have been set approximately 50 km longitudinal and 70 km latitudinal away from the pipeline along both sides of the pipeline route. According to strong motion data, observed in the Greek area, the current seismic activity outside of this region cannot produce on the pipeline route ground acceleration exceeding 100 cm/sec2. Fore strong historical earthquakes with magnitude ranging between 6.0 and 6.4 (Figure 1) were recognized along the Lefkadi active fault that crosses the pipeline route at the northern margin of the South Evoikos Gulf. Three last events occurred in 1417, 1694 and 1785, show the ~200 year average recurrence time of strong (M>=6.0) events occurrence on the Lefkadi active normal fault. To the 198BC event likely could be addressed submerging by 1.5-1.8m of ancient harbor of Eretria happened 2300 years ago (Stiros et al., 1993). The Oropos active neotectonic fault has no so pronounced seismic history as the Lefkadi fault. Only one strong event of 1938, M=6.0 can be associated with this rupture zone. The maximum earthquake magnitude (Mmax) that can generate the fault (seismogenic potential) was calculated according to empirical relationship of Wells & Coppersmith (1994) using the value of surface rupture length of the fault. Seismicity rates of these faults have been calculated on the basis of the seismicity occurred inside the fault influence zones, the borders of which were determined using the faults geometry characteristics (fault orientation, length, dip, probable penetration at depth). Since there are large variations in recurrence times for each active fault, the concept of Lapsed Rate have been introduced (Itaba and Watanabe, 2001). To assess the Lapsed Rate of an active fault we have to know Lapsed Time (LT), which is the time interval from the latest major earthquake, occurred on the fault, and also Recurrence Time (RT), which is the mean recurrence interval of strong events (or events with particular magnitude of interest) for the fault. Therefore, LR is defined as from RT and LT of a fault as: LR = (LT/RT) * 100 [%] This methodology was applied to assess the Lapsed Rate for two major active faults crossing the designate pipeline route – Lefkadi and Oropos active faults (Table 1).

Table 1. Parameters of fault sources, and Lapsed Rates of the active seismogenic faults, at the faults crossings with designated pipeline route. Surface Seismicity parameters, calculated Recurrence Time Fault Related Rupture using seismicity associated with Slip- Laps-ed calculated for the Lapsed Seismic strong length, the fault influence zone rate Time, events M>=6.0 Rate, Source E/Q km b Mmin Seismicity Rate Mmax Mm LT, RT, LR, Μ  Μmin /year Year Year % 1417, M=6.4 Lefkadi 1694, Fault M=6.2 40 0.90 5.0 0. 04 6.8 1.0 222 200*/250**/230*** 89-111!! 1785, M=6.0 Oropos 1938, Fault M=6.0 35 0.85 5.0 0.03 6.8 1.0 69 280*** 24 * Recurrence Time calculated using Earthquake Catalogue, ** Recurrence Time derived from Slemmons & De Polo (1986) diagram using slip rate value, ***Recurrence Time calculated using seismicity rates.

Taking into account that completeness of the historical records for magnitudes M>= 6.0 covers time periods of a last few hundred (500-600) years (Papazachos et al., 2000), the 198BC earthquake was excluded from calculations of observed recurrence intervals of earthquakes with magnitude M>=6.0 for Lefkadi fault. The calculation performed show that the active Lafkadi fault could be in a last, pre-seismic and dangerous study of his seismic cycle and the probability an earthquake with M>=6.0 to occur at pipeline crossing with this active fault, during the pipeline operational life (50 year), is very high.

3. PROBABILISTIC SEISMIC HAZARD ANALYSIS

In present study, the Probabilistic Seismic Hazard Analysis (PSHA) for the under study pipeline route was performed using EZ FRISK program (Risk Engineering, 2005). The methodology used for the probabilistic seismic hazard assessment is well established in the literature (Cornell, 1968, Cornell & Vanmarke, 1969, Cornell & Merz, 1975; McGuire, 1976, 1978, McGuire, 1995, Risk Engineering, 2005- 2007, etc). 3.1 DEFINITION OF DESIGN EARTHQUAKES FOR DESIGNATED PIPELINE ROUTE.

Current seismic design philosophy for many critical facilities requires dual (two-level) design criteria, with a higher design level earthquake aimed at life safety and lower design level earthquake intended for economic risk exposure. The two design levels are commonly defined as “maximum design earthquake - MDE” (or safety evaluation earthquake) and “operational design earthquake - ODE” (or function evaluation earthquake), and have been employed in many pipeline projects. In a Probabilistic Seismic Hazard Analysis, the Maximum Design Earthquake is defined as an event with a small probability of exceedance during the life of the structure (e.g. an event with recurrence interval 975 year and probability of exceedance 3-5%). The Operating Design Earthquake is an earthquake event that can be reasonably expected to occur at least once during the design life of the structure (e.g. an event with recurrence interval of 70 year and probability of exceedance between 40 and 50%). The probability of exceedance represents the chance, expressed as a percentage that a more severe ground motion will occur within a specified exposure time expressed in number of years. In our case exposure time (operating life of the pipeline) is equal to 50 years. Approximate return periods for different probabilities of exceedance and annual rates of exceedance for 50-year Exposure Time of the pipeline are displayed in the Table 2. Relevant Magnitudes for corresponding events to occur on 2 Modeled as Seismic Sources Faults – Lefkadi fault at the northern crossing with sub-sea route, and the Oropos fault at the southern crossing with sub-sea route – are also presented in the Table 2.

Table 2: Approximate Return Periods for Different Probabilities of Exceedance, Annual Rates of Exceedance for 50-year Exposure Time and Relevant Magnitudes of two Major Faults (Lefkadi and Oropos) crossing the pipeline

Probability of Return Periods, Annual rates of Relevant Magnitudes on 2 exceedance for 50- Years exceedance, Modeled as Seismic year exposure time, 1/Return Period Sources Faults, % Lefkadi F/Oropos F 51 70 0,014 5.5/5.4 25 175 0,057 5.9/5.8 10 475 0,0021 6.3/6.2 5 975 0,0010 6.6/6.5 2 2475 0,0004 6.8/6.7

So, in our case the Operating Design Earthquake with 70 years return period is an event with M=5.4-5.5. The Maximum Design Earthquake with 975 years return period is an event with magnitude 6.5-6.6. 3.2. PERMANENT DISPLACEMENTS VS RECURRENCE INTERVALS AT THE PIPELINE- FAULTS CROSSING LOCATIONS.

Ground surface rupture is commonly associated with earthquakes of magnitude 5.5 and greater. The extent of the rupture and the permanent displacement across the fault generally increase with magnitude. Average displacement across the faults vary from approximately 1cm for Mw=6.0 to as much as 7.5 meter for Mw=8.0. Experience during earthquakes demonstrates that lifelines that cross active faults are vulnerable to damage from fault offset. The fault analysis process for permanent displacements computation and assessing of pipeline failure probability includes (Trufanic and Todorovska, 2005):  Calculation of fault activity rates (recurrence intervals for different magnitudes) using different activity models (seismic activity or slip-rate),

 Depending of Surface or Subsurface Fault Rupture Length, Mmax estimation, and estimate for maximum surface fault displacement MD for different magnitudes, using relatively correlations (i.e. from Wells and Coppersmith, 1994),  Construction of the hazard curves (Permanent Displacement vs Annual rate of Exceedance) for fault sources at the pipeline crossings for the 51%, 25%, 10%, 5% and 2% chance of exceedance of Peak Ground Displacements during the 50-year life period (50-years exposure) of designated structure (50- year Poissonian probability of exceedance). Poissonian probability of exceeding each displacement level over a 50-year period is shown in Figure 4. The Permanent Ground Displacement Hazard Curves (50-year Poissonian probability of exceedance) are calculated from two fault sources, Lefkadi and Oropos active faults, for sites 1 and 3 (see Figure 1) at the pipeline fault crossings. The 51% and 5% chance of exceedance of Ground Displacements are 9-10 cm and 75-80 cm respectively for ODE (M= 5.4-5.5) and MDE (M=6.5-6.6) events.

Figure 2: Seismic reflection profile and velocity model along the pipeline route (Geopro, 2006). Reflection picture interpretation is performed by the author of this paper: Thick solid lines – main border faults of the South Evia Basin, Lefkadi and Oropos faults. Solid lines – secondary faults.

Figure 3: Poissonian probability of exceeding permanent displacement level over a 50-year exposure period. The permanent displacement hazard curves (50-year Poissonian probability of exceedance) calculated for two fault sources at the pipeline crossing: black (A) – for Lefkadi Fault, grey (B) – for Oropos Fault. The 51% and 5% chance of exceedance are 9-10 cm and 75-80 cm for ODE (M= 5.4-5.5) and MDE (M=6.5-6.6) events respectively.

4. TOTAL SEISMIC HAZARD

In the present study different attenuation equations from data base of EZ FRISK PC program were used for rock (hard rock) conditions. For Peak Ground Acceleration calculations, the attenuation equation proposed by Ambraseys et. al (1996) was used, which shows a good agreement with Margaris et. al (2002) equation performed for Greek seismotectonic environment. For PGV calculations the equation of Campbell (1997) was applied as more consistent with the Ambraseys et al (1996) equation. As fault seismic sources for the calculations, the zones of deep-seated Lefkadi and Oropos active seismogenic faults have been identified (see Table 1). The Total Seismic Hazard Curves displaying the values of peak horizontal acceleration (PGA), velocity (PGV) and 5% damped spectral acceleration (Sa) and velocity (Sv) for spectral period T = 0.3 sec, for rock conditions, and different annual frequencies of exceedance (recurrence intervals or return periods of the corresponding expected events) have been calculated for three sites (Table 3):  Site 1 – at the fault crossing with Lefkadi fault  Site 2 – in the middle of the designate pipeline route  Site 3 – at the fault crossing with Oropos fault

Table 3: Seismic Hazard Parameters, calculated for designate pipeline route Stamata – Aliveri.

Operational Design Greek Design Code Maximum Design Earthquake Earthquake Earthquake RT = 975 year Sites RT= 70 year RT = 475 year Ground Ground Ground Ground Ground Ground Velocity, Acceleration, g Velocity, Acceleration, g Velocity, Acceleration, g cm/sec cm/sec cm/sec PGA Sa, PGV Sv PGA Sa, PGV Sv, PGA Sa, PGV Sv, T=0.3 T=0.3 T=0.3 T=0.3 T=0.3 T=0.3 Site 1 0.250 0.61 4.60 65.0 0.50 1.30 6.20 115.0 0.62 1.60 7.00 125.0 Site 2 0.230 0.57 3.00 48.0 0.46 1.15 4.00 70.0 0.58 1.50 4.60 80.0 Site 3 0.240 0.58 3.60 59.0 0.48 1.20 5.00 86.0 0.59 1.55 5.50 100.0

For comparison the Seismic Hazard values for earthquake with RT=475 year, corresponding to Greek Seismic Design Code, 2000 (EAK 2000) event are displayed also in Table 3.

5. DISCUSSION OF RESULTS

Possibility of the permanent vertical surface rupture with amplitude about 1 meter to occur during the seismic excitation on the fault crossings with the pipeline route, cannot be excluded, the fact that have to be concerned in seismic design of pipeline. Areas of the probable surface ruptures expression in crossings with designate pipeline route during the strong earthquakes occurrence on the corresponding seismogenic fault (Lefkadi and Oropos) have to be accessed using precise geophysical and geological investigations. The Greek Seismic Design Code (EAK, 2000) proposes for the designate pipeline route area the value of 0.240g, which is about 50% lower than ones calculated in current study for RT=475 year event (Table 3). The explanation of high seismic hazard values calculated in the current study could be based on the fact, that the site – surface fault track distance, but not the site – epicenter distance, is the major parameter defining the ground motion values and related damages of infrastructure. It is concluded from international and Greek bibliography analysis (Gazetas, 1996) that in close vicinity (from 1km to 10 km) to the seismic fault the recorded strong ground motion values for M = 6.8 event (USA, Northridge earthquake, 1994) could reach for PGA – 940 cm/sec2, and for Sa (T = 0.4 sec) – 2800 cm/sec2. In the case of the M = 7.2 event (Japan, Kobe earthquake, 1995) the PGA values of 850 cm/sec2 and Sa (0.3 > T < 1.0 sec) values of 2000 cm/sec2 were recorded in close vicinity to the surface track of the seismogenic fault. Even for the moderate earthquake the recorded near the seismogenic fault strong ground motion values were found for PGA to be 450-600 cm/sec2 (Greece, Pirgos earthquake, 1993, M = 5.2). The PGA value of 750 cm/sec2 and Sa value of 1820 cm/sec2 (T = 0.32 sec) were recorded in close vicinity to the seismogenic fault during the Kalamata, Greece, 1986, M = 6.1 earthquake (Gazetas, 1996). It has to be noted also that a free-field recording of the strong Denali Fault Earthquake 2002 (M=7.9) shows a permanent displacement of 3.0 m, relatively low peak acceleration (0.36g) and very high (180 cm/s) peak velocity (Ellsworth et al., 2004). Thus, the calculated in the frames of the current study expected on the pipeline route strong ground motion values appear to be lower than real values, which could be recorded at field at pipeline fault crossings with the Lefkadi and Oropos faults during the expected design earthquakes of magnitudes M= 5.4-5.5 (ODE) and M=6.5-6.6 (MDE).

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