RECENT FINDINGS INTEGRATED IN SEISMIC HAZARD ASSESSMENT : THE CASE STUDY OF THE DURANCE FAULT

Cushing M. 1, O. Bellier 2, P. Volant 1, H. Aochi 1, S. Baize 1, C. Berge­Thierry 1 1 Institute for Radiological Protection and Nuclear Safety, France 2 CEREGE – UMR 6635 ­ Aix­Marseille III University, France

Abstract

In France, seismic hazard assessment for nuclear facilities is guided by a specific regulation based on a deterministic approach. It was recently revised (RFS2001­01­ Berge­Thierry et al. this issue [1]) to take into account for scientific improvements such as paleoseismology, site effects and other geological topics. This contribution presents the state of the art of academic studies : seismic lines re­interpretations, geomorphological analysis, paleoseismicity trenching, subsurface geophysiscal investigations, cosmogenic nuclides dating and geodetic and seismological surveys, carried on the Middle Durance Fault Zone (MDFZ). Their results can now be integrated to better assess the seismic hazard of neighboring nuclear facilities and its related uncertainty.

1. Introduction

Estimation of earthquake potential along slow active faults in Europe and in particular for the Provence area has required an integrated effort of several research groups over the last 10 years. Results from such multidisciplinary studies are now taken into account in the regulatory texts for the assessment of the seismic ground motion for nuclear facilities.

The most crucial information resulting from these studies and useful for DSHA associated with a significant fault source such as the MDFZ are :

Ø Identification of paleoearthquakes through paleoseismological studies. This information allows to better estimate the maximum magnitudes and the associated return periods, Ø Identification of the precise position of the fault, its possible segmentation pattern and the potential implication of basement faults. This improved knowledge of the fault trace and 3D geometry has a direct impact on the estimate of the site­source distance which is an input for ground motion prediction.

The case study of the Durance fault is presented here to illustrate the multidisciplinary approach performed to characterize the fault system and its potentiality. 2. The source “Middle Durance Fault Zone ­ MDFZ”

The MDFZ is a segmented fault system, which separates two different structural domains. It is about 70 km long and is composed of about 5 segments, each of them being between 8 and 20 km long. It is geometrically connected with the East­West trending active faults (Guignard et al., 2004 [2], Chardon & Bellier, 2003 [3]) like the Trevaresse Fault which produced the destructive 1909 Lambesc earthquake (Baroux et al., 2003 [4]). The fault is located along a remarkable depth gradient of the crustal basement, which is at 3 km depth to the East of the fault and at 10 km depth tot the West This fault system has been active through all important tectonic event since the Paleozoic. The series of 4 historical moderate earthquakes since the XVI th century (similar M5­5.5 events occurred in 1509, 1708, 1812, 1913) shows its seismic activity. .There also exists some geological evidences of Quaternary deformed levels. Furthermore, this Fault displays evidence for a possible recent paleoearthquake in a site named Valveranne, located near the town of Manosque (Cushing et al., 1997 [5], Sébrier et al., 1997 [6]).

Figure 1: Historical and instrumental seismicity of the Provence area

3. Improving the knowledge of the Surface fault trace Seismic hazard assessment requires locating seismogenic sources in order to determine source­target (site) distance useful for determining ground motion. Therefore, the surface trace, the underground geometry of the fault system and its segmentation are all primary information. The MDFZ was not precisely known before the 90’s along its presumed surface trace. The Middle Fault has some blind fault characteristics. Only a few outcrops, each of them being a few hundreds meters long, show the fault contact. Neither geological maps nor field surveys allowed a precise mapping of the fault trace. It is only with Digital Elevation Models and geophysical analyses that it was possible to propose a trace of the fault. Trenching was performed by different research teams in order to locally confirm this fault trace and perform paleoseismological investigations.

A specific study was performed by IRSN in order to improve the “subsurface” trace knowledge of the fault. This was achieved with a compilation of seismic lines obtained from the French petroleum companies TOTAL and ELF. Seven migrated lines were interpreted by Bove, (1996,1998 [7] ). Complementary published or confidential work was also used. Due to the target depth of the seismic profiles, the upper part of the seismic lines does not image the fault. Nevertheless, the projection up to the surface allowed us to propose the “best” possible surface trace of the fault. These local traces were compared with the proposed trace by Baroux (2000) [8] based on a morphological approach.

Combining the two above mentioned studies with knowledge of field evidences (local fault outcrops) and several studies performed in the past two decades (Villeger, 1983 [9], Combes, 1984 [10], Terrier, 1991 [11], Ghafiri, 1995 [12], Cushing et al., 1997 [5], Sebrier et al. 1997 [6], Baroux, 2000 [8], Lebatard 2003 [13], Guignard et al., 2004.[2]), a first synthesis of the whole fault surface trace was obtained in 2003 (Cushing & Bellier, 2003) [14] (Figure 2).

Complementary investigations were also performed such as subsurface ground penetrating radar, high resolution seismic reflection, morphotectonics analysis, cosmogenic nuclides analysis (Siame et al., 2004) .

Figure 2 : Surface trace of the MDFZ (left). Uncertainties in the trace of the fault (right). dotted strips showing the range of uncertainty on the order of 500m on either side of the fault trace.

4. Potentiality of large earthquakes of the MDFZ The MDFZ is now admitted to be a significant source capable of producing large earthquakes associated with surface rupturing i.e. Mw > 6.0­6.5.

4.1 Paleoseismological investigations using multidisciplinary approach : research of paleoseismological evidences, quantification studies The Valveranne paleoseismological evidence was discovered because of the 1992 field survey located in the topographic depression carved within the Tortonian sands and lacustrines beds. This outcrops is the only observation site in the area where recent activity has been demonstrated (Ghafiri, 1995 [12], Sébrier et al., 1997 [6]). At this site, a paleoseismological trench showed that a torrential debris­flow (26 800+/­6100 BP) is deformed by a West­vergent kink fold due to movement on a presumed east­dipping fault. This deformation is unconformably covered by a 9123 +/­190 years old colluviums. Since the corresponding displacement is of the order of 1.2 m, the observations suggest that this deformation occurred during a seismic event of relatively high magnitude (6.4­6.9 Mw). However, the Valveranne, may be associated with an antithetic secondary fault or bedding slip (Siame et al., 2004). It cannot be regarded as a major surface expression of the MDFZ which clearly dips towards the West as it can be observed from seismic lines.

Direct trenching without geophysical and geomorphic investigations, in countries of low strain rate remain hazardous. The better way to locate and eventually quantify active fault surface deformation is to carry out some multidisciplinary approaches coupling geomorphic and subsurface geophysics at different scales, and subsequently, trenches. Taking into account this fact, the following institutions : IRSN and OrsayTerre, Pau University, and more recently CEREGE AIX­Marseille Universities have joined forces to perform geophysical studies on some places of the supposed trace of the MDFZ. Some of this work was first methodological (Baroux, 2000). Ground Penetrating Radar (GPR) and high resolution seismic profiles were carried out on 3 target sites (Valveranne, La Forestière and la Brillanne). The first two were successful to identify possible shifted recent markers.

Geological interpretation of one seismic line (TOTAL VL 86 O) showed the possibility of a main “staircase shape” fault emerging in the vicinity of the lower terrace edge East of Manosque. In spite of its significant anthropogenic and erosional contributions, this slope has been interpreted as a possible geomorphic relief associated with cumulative deformation of the eastern branch of the MDFZ (Cushing et al. 1997 [5]; Baroux, 2000 [8], Siame et al., 2004 [15]).

More recently electrical tomography was performed in the framework of E.U.project S.A.F.E. EVG1­2000­22005. The results were encouraging showing a clear physical contrast (Baroux, 2000[8], Siame et al, 2004 [15], Nguyen et al., 2004 [16]). This was interpreted as a “tectonically” offset of the Weathering Zone Limit (WZL). Finally, a trench cutting the terrace edge was dug and logged. The first results did not success to identify near surface faulting. This was likely due to the very recent age and great thickness of the alluvial cone which is probably younger than a few hundreds years old . This was proved by the discovery of man made tiles at the base of the 3.50 m deep trench. Older formations were not reached by the digging. Some complementary investigation with drilling are planned in order to identify geoelectrical markers observed on the electrical panel.

4.2 Useful parameters or data for seismic hazard assessment in the surrounding of MDFZ The above mentioned multidisciplinary studies reveal information that can be used as input data for seismic hazard assessment. This concerns particularly all input data for potentiality assessment (i.e. magnitude, return periods, …). The main useful items are listed below. 4.2.1. Surface segmentation The mapping of the MDFZ allows a surface segmentation of the fault zone to be proposed (fig 2).This segmentation, may be used for estimating magnitudes and return periods using scaling laws (e.g. Wells & Coppersmith, 1984) as Baroux (2000) [8] and Siame et al. (2004) [15] – see Table 1 .

4.2.2. 3D segmentation IRSN gathered information from seismic lines, geological data (i.e; boreholes data), and published geological cross sections in order to image the 3D structural pattern of the MDFZ (fig 3). All along the MDFZ, the structural pattern shows the “staircase” shape with two kinds of fault. These are probably deep basement rooting fault (eastern fault) emerging in the Durance Valley or, in the southern part near the limit between Mesozoic and molassic Valensole mio­ pliocene conglomerates. On the West side, listric shape faults are rooted in Triassic evaporites. These faults allow the decoupling between the Mesozoic and Tertiary cover over the crystalline basement. Alpine (S.L. structural inversion created bending anticlines like Manosque Forcalquier Anticline. This study allows a simplified 3D segmentation to be proposed. It may be used for geomorphic modeling (Baroux, 2000) [8]), dynamic (Aochi, 2004 [18]) or kinematics simulations (Baumont et al., this issue [17] in figure 2) in order to better predict rupture behavior.

4.2.3. Basement implication Seismic lines analysis cannot show whether or not the fault is active. One seismotectonic hypothesis is that the MDFZ could act as an active left lateral ramp isolating autochthonous domain to the east from the floating sedimentary series to the west. (Champion et al., 1999 [19], Chardon & Bellier, 2003 [20]). In this case, this raises the question whether or not basement faults are active. The answer is not known. The potentiality, in term of maximal magnitude, of the MDFZ depends on this answer. One way to reach it is to accurately locate microtremors generated by the MDFZ. Therefore IRSN maintains a local seismic network to monitor the MDFZ microseismicty. Using a specific code coupled with a 3D velocity model, accurate location of microseismicity will soon be available (P. Volant et al. (2000) [21], P. Volant et al. (2003) [22]).

4.2.4. Estimating return period and magnitude of larger events From the paleoseismological evidence of Valveranne, to the permanent GPS survey, passing through old geological markers displacement and geophysical markers offset, there are many methods which allow the estimation of the long­short term velocity on the MDFZ. Values obtained from theses studies may be useful to know whether or not the case of active fault rupturing has to be taken in account. Table 1 summarizes recent evaluations of the velocities, paleomagnitude or potential magnitude and estimations of possible associated return periods . It is now commonly admitted the fault velocities in the south­East of France and more precisely on MDFA is of order of about 0.1 mm/y (it was not the case 10 years ago). This is also coherent with integrated GPS monitoring (Nocquet et al., 2003 [23]). Taking this into account, the return period of a major earthquake on the whole system of MDFZ may be of the order of a few thousands of years. Table 1 : Evaluation of large earthquake potential using different methodologies Author/year Method/marker Velocity, Return Period (R.P.) Benedicto, 1996 [24] Offset of the Miocene base 0.08­0.14 (v) mm/y Cushing et al., 1997 [5] Stream offset 0.05­0.22 (h) Baroux, 2000 [8] Cumulative deformation since 10 0.11 – 0.23 (v) My 0.04­0.013 (Valveranne) Scaling laws RP ~2 – 3 Ky for Mw ~6.4 (for the total length) Ghafiri, 1995 [12] Paleoseismological trench 0.07­0.13 (v) one event between 25 and 9 ky Siame et al. 2004 [15] Geomorphic and cosmic ray 0,03 mm/y (along slip dominantly reverse) exposure Mw 6.3­6.7 RP 13000­23000 y (on one segment) Champion et al., 1999 [19] Geomorphic analysis, cumulative < 0.1 mm/y deformation Calais et al., 2001 [26] Permanent GPS monitoring 1.3 ±0.8 H. Aochi et al. [18]. Dynamic modelling No return period.P. prediction/ Mw preferred 7.1­6.8 with segmentation scenario

Figure 3: 3D representation of seismic lines and geological cross sections used to establish the 3D velocity model (left); Serial cross sections on the MDFZ established from seismic lines and geological data. Figure 4 : Proposed MDFZ segmentation

MANOSQUE

5. Feed back of studies performed on MDFZ : characterizing seismic motion The knowledge improvement of the Durance Fault segmentation and geometry now allows the fault system to be taken into account as an identified source capable of producing significant earthquakes. The multiple and sometimes independent approaches lead to an estimation of fault potentiality in term of magnitude, fault (to the segments resolution) velocities and estimated return period for large earthquakes. All the parameters (geometry, segmentation, earthquakes characteristics, …) are useful to seismic hazard assessment. These parameters can be used whether for DSHA (using RFS 2001­01, see Berge­Thierry et al, this issue [1]) or PSHA as input hypothesis for logic­trees methodology.

6. Remaining uncertainties Although knowledge on this fault system has been improved, uncertainties still remain and may influence ground motion prediction for deterministic and probabilistic hazard assessment. For example: Ø some data (microseimicity, field data) show the possible proximity of an active fault, but no evidence of faulting is pointed out on seismic line, Ø Segment dipping, influencing source­site distance, Ø Segmentation definition which depends on an expert judgment, Ø Exact fault position, which influences ground motion prediction, is not known with a sufficient degree of precision. Only this last uncertainty can be quantified with some degree of confidence by taking into account the resolution of the methods used to map the fault trace. This was done on the MDFZ map (Figure 2) which shows uncertainties “strips” along the presumed fault position. These uncertainties may be mitigated with complementary investigations.

6. Conclusion The numerous studies performed during the last decade : seismic lines re­interpretations, geomorphological analysis, paleoseismicity trenching, subsurface geophysical investigations, cosmogenic nuclides dating and geodetic and seismological surveys, have allowed the seismotectonics behavior of the Durance Fault to be better constrained. This includes estimates of the fault geometry and segmentation, paleoseismicity, short and long term velocities. This new data has now been integrated to improve the seismic hazard assessment of neighboring nuclear facilities. Uncertainties still remain and these studies have also raised new questions which must be answered.

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