The application of electrical resistivity tomography ...

Journal of Applied Geophysics 74 (2011) 69–80

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Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o

The application of electrical resistivity tomography and gravimetric survey as useful tools in an active tectonics study of the Sudetic Marginal Fault (Bohemian Massif, central Europe) Petra Štěpančíková a,⁎, Jiří Dohnal b, Tomáš Pánek c, Monika Łój d, Veronika Smolková c, Karel Šilhán c a

Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, Dpt. Engineering Geology, V Holešovičkách 41, Prague 8, 182 09, Czech Republic Faculty of Science, Charles University in Prague, Inst. Hydrogeology, Engineering Geology and Applied Geophysics, Albertov 6, Prague 2, 128 43, Czech Republic c Faculty of Science, University of Ostrava, Dpt. of Physical Geography and Geoecology, Chittussiho 10, Ostrava, 710 00, Czech Republic d University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Dpt. of Geophysics, al. Mickiewicza 3, 30-059 Kraków, Poland b

a r t i c l e

i n f o

Article history: Received 7 August 2010 Accepted 28 March 2011 Available online 12 April 2011 Keywords: Electrical resistivity tomography Gravimetric survey Active tectonics Trenching Sudetic Marginal Fault Bohemian Massif

a b s t r a c t This paper presents the results of two separate geophysical investigations undertaken across the Sudetic Marginal Fault zone in the Bohemian Massif. This fault zone represents one of the most important tectonic features in central Europe. The first, preliminary, investigation used electrical resistivity tomography (ERT) to define the exact position of the main fault at two localities (Kamenička & Bílá Voda). Both profiles revealed a resistivity gradient that divided a complex of crystalline rocks with high resistivities from sedimentary deposits with low resistivities. From these data, sites were selected for palaeoseismic trenching. This trenching provided detailed information regarding the surveyed structures and lithologies. The second, more extensive, investigation used electrical resistivity tomography and gravimetric survey in order to extend the previously recognised features both laterally and to depth. The fault is displayed by the presence of an expressive resistivity gradient, even on profiles undertaken in more homogeneous lithologies. Finally, the most intensive investigations were conducted at Bílá Voda, where highly detailed ERT was undertaken on the floor of the trench. From this, it was possible to trace approximately logged geological structures to far greater depths and assess sedimentary thicknesses. A gravimetric survey was also undertaken adjacent to the trench. From modelling the gravitational effects of the fault zone, it is shown that the fault dips about 75° NE and that the vertical offset of Miocene vs. bedrock on the fault is approximately 200 m. The presented study has demonstrated the value of undertaking two phases of geophysical exploration separated by a trenching investigation. It is considered that this methodology is particularly suitable for the study of active tectonics in analogous intraplate settings where fault slip-rate is low. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The identification and characterisation of active fault structures is of crucial importance in the recognition of potentially seismogenic areas. Without this information it is not possible to undertake the more detailed analysis of fault characteristics required for better Seismic Hazard Assessments. In the intraplate setting of central Europe, such fundamental work is frequently hindered because active fault movement is comparatively slow and faults are seldom exposed at the surface due to the extensive vegetation coverage. The combination of slow moving faults and moderate climate leads to a situation where potential fault scarps are rapidly eroded because the overall denudation rate

⁎ Corresponding author. E-mail addresses: [email protected] (P. Štěpančíková), [email protected] (J. Dohnal), [email protected] (T. Pánek), [email protected] (M. Łój), [email protected] (V. Smolková), [email protected] (K. Šilhán). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.03.007

generally exceeds the rate of displacement. To counter these problems, we have incorporated electrical resistivity tomography and gravimetric surveying into a study of active tectonics in central Europe. The study is focused on the NW–SE striking Sudetic Marginal Fault (SMF) zone. This fault zone is one of the most pronounced tectonic structures in central Europe. It controls the mountain front of the Sudetic Mountains for a length of 130 km (Fig. 1) (Badura et al., 2007; Štěpančíková et al., 2010). Faults can be studied by through a broad spectrum of geophysical methods. These tend to differ with regard to physical principles, penetration depth, and resolution. The identification of deep structures up to several kilometres below the surface can be conducted through seismic reflection, gravity survey, magnetotelluric, and extremely low frequency methods (e.g. Bielik et al., 2002; Cardozo et al., 2005; Ortuño et al., 2008; Suzuki et al., 2000; Zhao and Xu, 2010). Shallow subvertical discontinuous structures up to 100 m below the surface are effectively located through resistivity profiling or electrical resistivity tomography, very low frequency methods, shallow seismic refraction, and microgravity survey (e.g. Badura et al., 2002; Caputo et al., 2003, 2007; Nguyen et al.,

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P. Štěpančíková et al. / Journal of Applied Geophysics 74 (2011) 69–80

Fig. 1. Topographic situation of the morphologically distinctive Sudetic Marginal Fault (SMF) that divides the Sudetic Mountains from the Sudetic Foreland (SRTM, 90 m resolution). White double arrow denotes the study area while single arrows indicate the SMF trace.

2005, 2007; Similox-Tohon et al., 2006; Suski et al., 2010; Suzuki et al., 2000; Tsai et al., 2008; Valenta et al., 2008; Vanneste et al., 2008). In specific cases, the application of ground penetrating radar yields positive results with a penetration depth of up to 10 m (e.g. Alasset and Meghraoui, 2005; Anderson et al., 2003; Chow et al., 2001; McClymont et al., 2009; Reicherter and Reiss, 2001; Slater and Niemi, 2003; Vanneste et al., 2008). Finally, zones of weakness associated with faults that have greater infill porosity and less water saturation may also be discovered through emanometry survey in certain specific geological contexts, i.e. by measuring of volume activities cA 222Rn and cA 220Rn, (Gruntorád and Mazáč, 1994). In active tectonics research, determining the location of the fault through geophysical surveying forms only the first stage of investigation. Further critical information is derived from more detailed geological studies of micro- and macro-structures within the specific fault zone. These studies may comprise morphotectonic mapping, paleoseismological investigations such as trenching, or further geophysical surveying at a higher resolution (e.g. Anderson et al., 2003; Buddensiek et al., 2008; Caputo et al., 2004; Colella et al., 2004; Slater and Niemi, 2003; Štěpančíková et al., 2010; Vanneste et al., 2008). Our research represents a new case study and yields some new methodological aspects first, through the utilisation of detailed ERT measurement along the length of the trench floor (to some extent done also by Nguyen, 2005) and, second, through the simultaneous application of ERT and (micro) gravimetry. 2. Tectonic setting of the study area The NW–SE trending SMF separates the mountains of the Sudetic Block (Lugicum Zone) to the southwest from the hills of the ForeSudetic Block (Silesicum Zone) to the northeast (Fig. 2a). The Sudetic Block is composed of various metamorphic and magmatic rocks of

Proterozoic and Palaeozoic age (Skácelová et al., 1992; Žáček et al., 1995). The down-thrown Fore-Sudetic Block is composed of the Žulová granite pluton of late Variscan age (Pecina et al., 2005). Further to the southeast, this pluton is covered by metamorphosed rocks of Devonian age. The adjacent Vidnava Basin, a part of Paczków Graben, has been filled with Miocene strata reaching thicknesses of up to 680 m. The substratum beneath the sedimentary basin is of Early Palaeozoic age (e.g. Badura et al., 2004; Frejková, 1968). The study area is located between the towns of Javorník (Czech Republic) and Bardo (Poland). Close to the village of Bílá Voda, the geology reflects this morphostructural setting. The lithologies of the Sudetic Block comprise fine-grained orthogneisses, granodiorites, and micashists with lenses of amphibolites, and marbles of the Stronie Fm (Skácelová et al., 1992). The lithologies of the adjacent Fore-Sudetic Block comprise fluvial to limnic sediments of Miocene age. These deposits actually form part of the Paczków Graben, but are rarely displayed on geological maps. Surficial cover includes fluvial, colluvial, and mixed sediments of Quaternary age of the thicknesses of up to 10 m. The mixed sediments incorporate aeolian and erratic glacigenic materials (Skácelová et al., 1992). The strike azimuth of the SMF in the vicinity of the study area varies around 130°, as inferred from the linear trace of the mountain front. However, in the investigated locality the mountain front makes a left stepover with a width of ca. 1 km and is crossed by the transverse fault of Bílá Voda with a strike azimuth of around 70° (Skácelová et al., 1992) (Fig. 2b). The Sudetic Marginal Fault has been generally assumed to be a steeply-dipping normal fault (e.g. Oberc, 1977) although a minor horizontal component has been inferred (Mastalerz and Wojewoda, 1993; Skácel, 2004). The fault zone is thought to pre-date the early Variscan. Since the Permian, the main fault has experienced alternating vertical movements (Skácel, 2004). The fault was reactivated during the Alpine Orogeny (Badura et al., 2003). Along much of its course, this


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Fig. 2. a) Geological map of the SMF zone with location of historic earthquakes with assessed epicentral intensities (modified from Badura et al., 2007) b) More detailed geological scheme of the studied region (modified from Czech Geological Survey, 1998).

reactivation was thought to have occurred as normal faulting (Aleksandrowski et al., 1997). More recent neotectonic movements have yet to be fully elucidated despite considerable efforts (e.g. Štěpančíková et al., 2010 and references therein) and a number of differing opinions exist with regard to the present stress regime (cf. Badura et al., 2007; Nováková, 2009; Štěpančíková et al., 2008). Despite the relatively clear morphological expression of the mountain front, epicentral intensity of recorded local historic earthquakes was estimated to reach only I0 = 4–7 (MSK) (Guterch and Lewandowska-Marciniak, 2002). Historic earthquakes close to the present study area include events recorded in 1562, 1615, 1786, and 1877 (Guterch and Lewandowska-Marciniak, 2002; Kárník et al., 1958; Pagaczewski, 1972). However, recent paleoseismological studies of the SMF have revealed prehistoric earthquakes. These have been estimated to have a minimum moment magnitude of M 6.3 and slip rate of 0.03 mm/yr (Štěpančíková et al., 2009, 2010). 3. Research design The active tectonics study was undertaken in three stages: • The first phase of the project aimed to use electrical resistivity tomography to locate the trace of the fault in order to select the most suitable sites to be trenched.

• The second phase involved trenching across the fault in order to derive information on the fault itself and on the physical characteristics of its associated lithologies. The preliminary results of this investigation have been presented (Štěpančíková et al., 2009) with a more detailed publication to follow (Štěpančíková et al. in prep). • The third phase aimed to extend the knowledge acquired during the first two stages. As both the geophysical signature of the fault and the physical characteristics of its associated lithologies were known, it was then possible to more precisely interpret further ERT and gravimetric surveys. This paper focuses on the geophysical data derived in the first and third stages. It emphasises the importance of the research design as a basis for accurately interpreting geophysical data. 4. Methods The presented study was carried out in the northwestern part of Czech segment of the SMF close to the municipalities of Bílá Voda and Kamenička (Fig. 3). Potential localities for palaeoseismic trenching were chosen, based on DEM analyses and geomorphological fieldwork. The position of the fault is suggested only by general morphology, i.e. the prominent rectilinear foot of mountain front, as a fault scarp has not been preserved. Moreover, the lack of available


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Fig. 3. Location of the geophysical profiles on topographic map with contour lines of 2 m interval. SMF – Sudetic Marginal Fault.

fault outcrops due to dense vegetation led us to use geophysical methods for specifying the position of the main fault for potential trenching sites. Geophysical profiles were located according to the supposed occurrence of different types of bedrock on both side of the SMF, considerable thicknesses of younger sediments, and other practical considerations. The presence of differing bedrock on either side of the fault allows identification of its trace without ambiguity using geophysical methods. The presence of considerable thicknesses of younger sediments is necessary for palaeoseismic analysis. Other practical considerations include site access for trenching and obtaining the requisite permission from landowners. The more detailed trenching results are however not included in this paper. 4.1. Electrical resistivity tomography (ERT) The electrical resistivity tomography (ERT) method was undertaken with several different electrode spacing and electrode arrays (Wenner–Schlumberger, dipole–dipole). Measuring was initially performed with electrodes positioned on the surface, and later on the trench floor. In the first phase, ERT with a configuration of Wenner–Schlumberger and electrode spacing of 2 m was performed at two localities: Kamenička (Profile A) and Bílá Voda (Profile C) in order to specify the position of the fault trace (Fig. 3). The shorter electrode spacing of 2 m was used to obtain more detailed information on the superficial part of the resistivity section for future trenching. Profile A was 78 m in length with an azimuth of 40°, whilst Profile C was 190 m in length with an azimuth of 40°. Surveying was undertaken using the Resistar RS-100. The maximum depth of penetration defined according to DOI was 16 m (Oldenburg and Li, 1999). Primary data were processed through a 2-D inverse method, using the program Res2DInv (Loke and Barker, 1995), into resistivity cross sections that included topography. The presented figures are the results of smoothness-constrained

inversion; these outputs best fit the situation observed in the trenches (unlike those of robust inversion e.g.). The validity of these results is supported by the low RMS error of around 2.5% for the 3rd iteration. In the third, post-trenching phase, the Profiles B, C1, and E were measured by a georesistivimeter ARES (GF Instruments, Ltd). Sounding was performed with various electrode spacing and configurations according to the purpose and position of the particular section (Fig. 3). Data from these profiles were also processed through a 2-D inverse method in the program Res2DInv. Resistivity tomographies for the Profiles B & E were acquired in order to specify the fault trace continuation in more distal localities. An electrode spacing of 5 m was chosen in order to get more information about the deeper parts of the fault previously identified during the pre-trenching phase (Profile C). Profile B was measured using two variants. Profile B1 was 315 m in length with an azimuth of 32°. The electrode spacing was 5 m. Profile B2 was 126 m in length with the same azimuth of 32° and overlapped Profile B1. The electrode spacing was 2 m, in order to get more details from the fault zone. Profile E was 235 m in length with an azimuth of 45°. The electrode spacing was 5 m. Both Profiles B & E were surveyed at the topographic surface. In contrast, Profile C1 was surveyed on the floor of Trench C, which was 1 m wide and 2–3 m deep. Profile C1 was 48 m in length with an azimuth of 42° (Fig. 3). The electrode spacing was 0.8 m, in order to obtain a higher resolution for extrapolating the geology logged within the trench to greater depths. Resistivity tomographies for Profile C1 were measured both with Wenner–Schlumberger and dipole–dipole electrode arrays in order to compare the different electrode configurations. The real geometry was not a half space and the calculated resistivities are, therefore, lower than the real resistivities due to rarefaction of electrical current lines. Nevertheless, the basic distribution of resistivity structures should be preserved because of the constant “geometry” along the whole of Profile C1. Moreover, the validity of Wenner–Schlumberger ERT section of Profile C1 is supported by good


agreement with ERT section of “identical” part of the real half space Profile C. The resulting inversion models obtained from Res2DInv have from a very low RMS error of 1.4% to 7.1% for the 5th iteration.

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(1966). The program was used together with the GM-SYS Profile Modelling Program by Geosoft. 5. Results

4.2. Gravimetric survey 5.1. Pre-trenching ERT: Profiles A & C Moreover, a gravimetric survey was carried out in order to verify the methods used to determine the fault trace. The qualitative– quantitative interpretation was based on the gravity anomaly distribution and known geological conditions. The surveyed profile was situated along the trench at Bílá Voda C. As the trench crossed the SMF, the exposed fault position and detailed geology was known for a length of 50 m. In addition, the bulk densities of individual rocks collected from the trench were determined in a laboratory and used for the detailed interpretation of the section close to the fault. The profile was 1800 m in length with an azimuth of 50° (Fig. 3). The midpoint was located on the main boundary fault, where it also overlapped the ERT profiles. This length ensured that all the gravitational effects generated by the fault zone could be observed. Stations were placed at a range of intervals of between 2.5 and 100 m. In the central part of the profile (Stations 860–940), the stations were spaced 2.5 m apart. Farther out, they were spaced every 10–20 m. At the distal ends of the profile, the stations were spaced every 50– 100 m. In addition, the stations were geodetically positioned. The gravity observations were recorded using the gravimeter Scintrex CG3. The survey was carried out using a method of indirect points. All gravity observations were connected to one common base point. The error of output relative value of the gravity did not exceed 0.01 mGal. The corrections for non-geological gravitational anomalies introduced in all observation points (Fajklewicz, 2007; Torge, 1989) included free-air, Bouguer, and gravity terrain correction. In calculations of Bouguer and terrain corrections, the average bulk density of surface rocks was assigned a value of 2.3 Mg m−3. Gravity modelling was performed using the 2.5D modelling software ModGraw. This software was created in 2000 by S. Porzucek from the Department of Geophysics, AGH University of Science and Technology, Kraków, Poland. The software is based on the algorithm described by Hanemann and Militzer (1985). It is a function that calculates the gravity effect of a two-dimensional body with an arbitrary crosssectional geometry and a finite extent along the y-axis, i.e. it is 2.5D modelling. ModGraw was successfully tested by its author to solve the problem of calculating the gravity effect of a rectangular prism. The gravity effect of a rectangular prism was first described by Nagy

ERT on Profile A (Kamenička, Fig. 4) and Profile C (Bílá Voda, Fig. 5) reveals a sub-vertical zone with a pronounced resistivity gradient that represents the main fault of the SMF. This zone divides rocks of highly contrasting resistivity. The resistivities along Profile A (RMS error = 2.5%) span a range of 20–1900 Ω m. The substratum is represented by conductive material (tens of ohm m) in the NNE, in contrast to material of markedly high resistivities (over 1000 Ω m) in the SSW (Fig. 4). The substratum is covered by a thin less conductive (100–500 Ω m) layer in the NNE, and a by thicker layer of higher resistivities (500–1000 Ω m) in the SSW. The boundary between both types of rocks inclines steeply to the NE and is displayed by an intense resistivity gradient. Based on these results, it was recommended that a trench was sited between Stations 7 and 38. The resistivities along Profile C span a range of 20–700 Ω m (RMS error = 2.3%). An intermediate-resistive (100–400 Ω m) superficial part of the section is nearly continuous, except for a conductive (30– 80 Ω m) sector between Stations 55 and 74 (Fig. 5a). The substratum resistivities differ markedly in the northeastern, central, and southwestern sectors of the profile. The northeastern and, especially, central sectors are formed by conductive material of less than 150 and 100 Ω m respectively. Resistivities of the southwestern sector are generally higher (N150 Ω m). The boundary between these sectors is again shown by an expressive resistivity gradient, with a distinctive resistivity minimum in the northeast. The boundary is sub-vertical inclining steeply to the NE, and represents the main fault. Based on these results, it was recommended that a trench was dug between Stations 120 and 176. 5.2. Trenching: Profiles A & C As previously mentioned, the results of the trenching form the basis of a forthcoming paper (Štěpančíková et al. in prep.). Nonetheless, it is possible to provide a more detailed account of Profiles A & C based on the results of the trenching. Trenches A and C were dug at the localities of Kamenička and Bílá Voda, and overlap ERT Profiles A & C. They confirmed that the expressive resistivity gradient

Fig. 4. Model resistivity of electrical resistivity tomography (ERT) at the locality Kamenička Profile A, Wenner–Schlumberger electrode array with simplified geology exposed in Trench A (black outline). Resistivity in ohm m; vertical exaggeration = 1; iteration 3; RMS error = 2.5%; the distance in meters corresponds with the name of stations in the text. The main fault of the SMF zone in the trench coincides with the expressive resistivity gradient around Station 18.

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Fig. 5. Model resistivity of ERT at the locality Bílá Voda Profile C, Wenner–Schlumberger electrode array with logged geology in Trench C. a) Resistivity in ohm m; vertical exaggeration = 1; iteration 3; RMS error = 2.3%. The SMF lies within the expressive resistivity gradient between Stations 143 and 145. b) Geology in the trench 1 – Late Pleistocene colluvium; 2 – Miocene lacustrine deposits, 3 – fault zone with tectonic breccia and fault gauge, 4 – micashists, 5 – granitic aplite, 6 – gneisses, 7 – Holocene colluvium.

represents the main fault of the SMF zone (Figs. 4, 5). This divides two blocks of different lithology: a complex of crystalline rocks with high resistivities on the southwestern up-thrown side from lacustrine clayey deposits of Miocene age with low resistivities covered by Quaternary colluvium of slightly higher resistivity on the northeastern down-thrown side. In Trench A (Kamenička), the resistivity gradient between Stations 17 and 32 corresponds with extensive zone of loosened granodiorites (Fig. 4). The 2 m wide water saturated fault zone of the SMF, which is located in the trench around Station 18, is limited by dark fault gouge. On the northeastern down-thrown side, a conductive substratum corresponds to fine-grained clayey sand sediments of Miocene age. The sediments are covered by less conductive 1.5 to 2.5 m thick colluvium of Quaternary age from the onset up to Station 24. The colluvial sediments are interrupted where there is a forestry road. The shallow resistivity maximum (Stations 24 to 28) and subsequent minimum (Stations 28 to 32) has resulted from construction material for the road. The non-conductive substratum upslope of Station 32 corresponds to relatively intact granodiorites. The tomography suggests that the wide zone with the resistivity gradient dips approximately 70 –80° NE, while the dip of the main fault measured in the upper exposed part of Trench A is 60° NE. In other trenches and outcrops the dip of the main fault is around 75° NE. In Trench C (Bílá Voda), the geology was more differentiated (Fig. 5b). The main marginal fault with strike and dip of 135°/75°NE had characteristics suggesting strike–slip faulting such as a flower structure within the 4-m wide zone of tectonic breccia and fault gouges displayed both on the trench walls and on the floor, and completely different lithologies on both sides without any piercing points. The youngest faults of this flower structure cut the colluvium deposited during the late Pleistocene, which overlies the warped Miocene sediments in the down-thrown block. Although the mid to coarse colluvium in the trench does not display clear discrete sedimentological units that could be ascribed to individual events, the late Pleistocene deposits close to the fault are obviously faultrelated since they include clasts of the breccia (Štěpančíková et al., 2009; in prep.). However, the whole body of the late Pleistocene

colluvium exposed in the trench shows more-or-less homogenous resistivities and therefore it was not possible to detect fault-related colluvial wedges. The youngest documented movement, which even displaced the geliflucted layers deposited during the Late Glacial, is most probably related to a strike–slip fault with a reverse component out of the flower structure. Therefore, the latest faulting must have occurred after their deposition (Štěpančíková et al., 2009). The Holocene colluvium in Trench C has apparently not been displaced. In Profile C, the SMF is located between Stations 142 and 146 (Fig. 5b). It lies within the expressive resistivity gradient structure (40 to 200 Ω m/ 5 m) that dips approximately 75°. This correlates with the dip of 75°NE measured in Trench C. Warped deposits of Miocene age, logged at the bottom of the trench around Station 141, correlate with the deeper conductive “substratum” detected by ERT between Stations 131 and 142. The low resistivities of these sediments result both from high clay content and higher water content. The Miocene sediments are covered by the late Pleistocene colluvium described above, represented by the layer of intermediate resistivities. This colluvium is, in turn, overlain by stony Holocene colluvium with a slightly higher resistivity (Fig. 5b). Generally higher resistivities in the southwestern part of the profile (uplifted block) correspond to the complex of crystalline rocks excavated. The identified rocks included fractured and folded phyllonites–micashists (Stations 145–154), granitic aplite (Stations 154–160; a lenticular body of higher resistivities), and gneiss (Stations 160-end). The subjacent resistivity minimum between Stations 145 and 170, reaching down to a depth of 8 m, reflects the presence of fractured crystalline rocks saturated by water in the upper part of crystalline complex. 5.3. Post-trenching geophysical surveys 5.3.1. Post-trenching ERT: Profile C1 This profile was undertaken on the floor of Trench C with the aim of extrapolating structures and lithologies to greater depths. A specific aim was to determine the thickness of the fault-related colluvium adjacent to the fault zone between the boundary fault and warped Miocene sediments as this might give an idea of the rate of fault


activity. High-resolution tomographies were performed using both Wenner–Schlumberger and dipole–dipole electrode configurations with an electrode spacing of 0.8 m (Fig. 6). Results for both electrode arrays show reliable RMS errors reaching 3.9% and 7.1% for Wenner– Schlumberger and dipole–dipole configurations respectively. The higher RMS value for dipole–dipole section is likely to be due to the lower signal-to-noise ratio of the array. They displayed relatively similar subsurface resistivity structure. The northeastern sector (Stations 0–23.2) displays an overall conductive Miocene substratum (b100 Ω m). This is, on the first 10 m of profile, overlain by b3 m thick layer of Quaternary colluvium with a relatively high resistivity (N200 Ω m). A pronounced sub-vertical boundary (Station 23.2) corresponds to the SMF but the geometry of this structure differs somewhat according to the electrode configuration. The dipole– dipole profile shows rather vertical structure, whilst the Wenner– Schlumberger profile displays the same structure as distinctly inclined to the NE. The southwestern part of the sector (Stations 23.2–48) reveals a relatively complex resistivity structure. The prevailing nonconductive medium (200 to N1000 Ω m) represents a crystalline substratum (micashists, gneisses, and granitic aplite) penetrated by several low resistivity anomalies. A distinct sub-vertical conductive zone (b80 Ω m) protrudes to the surface between Stations 31 and 34. It is the expression of a narrower fault zone logged in Trench C (Fig. 5b, Station 155). Its position is shifted laterally by ~2 m on the dipole–dipole tomography in respect to the Wenner–Schlumberger section. 5.3.2. Post-trenching ERT: Profiles B1, B2, & E The measurements for these profiles were undertaken at the surface to determine the fault trace continuation in more distal localities, where only gentle surface depressions and foothills suggest its occurrence. In turn, this would also verify the geomorphologic approach. Moreover, we also aimed to obtain information on the approximate fault geometry at greater depths, to assess the thickness

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of deposits on the hanging wall of the fault, and eventually to assess the vertical offset. 5.3.2.1. Profile B1. Profile B1 was surveyed with an electrode spacing of 5 m. It displays the general resistivity structure with a reliable RMS error (2.6%) on both sides of the fault up to depths of about 50 to 60 m (Fig. 7a). The distinct resistivity gradient at Station 100 corresponds to the sub-vertical SMF and marks the boundary with the less conductive southwestern part of the profile. The northeastern part of the section (up to Station 105) reveals a generally conductive substratum, interpreted as Miocene sediments (b80 Ω m). This is overlain by an intermediate resistivity layer (100 to 200 Ω m), which increases in thickness away from the main fault. The intermediate and higher resistivities may represent Quaternary colluvial deposits. It is evident, by analogy to the situation near Trench C, that the substratum of the sector upslope of Station 100 on the uplifted block consists of crystalline rocks. The overall continuity of the high resistivity zone (300 to 1000 Ω m) is disrupted in its central part (around Station 180) by another sub-vertical relatively low resistivity structure (100– 200 Ω m) that may correspond to a parallel fault. 5.3.2.2. Profile B2. Profile B2 was surveyed with an electrode spacing of 2 m and therefore of greater resolution (Fig. 7b). It displays a low RMS error (1.4%) allowing good interpretation of subsurface structures. This profile overlapped Profile B1 from Stations 60 to 186, and reached a maximum depth of 20–25 m. The main fault manifests itself as a prominent curved gradient structure (Station 92 to 102) that separates the intermediate resistivity sedimentary deposits in the northeast (up to Station 100) from the high resistivity crystalline rocks on the southwestern part of the profile. The latter high resistivity sector of the profile (Stations 102 to 186) is interrupted between Stations 136 to 144 by a sub-vertical structure with a relatively lower resistivity (150–200 Ω m). The supposed Quaternary colluvium overlying the Miocene sediments probably has a lower

Fig. 6. Model resistivity of ERT at the locality Bílá Voda carried out at the floor of the Trench C. a) Wenner–Schlumberger electrode array; vertical exaggeration = 1; iteration 5; RMS error = 3.9%, b) dipole–dipole electrode array; vertical exaggeration = 1; iteration 5; RMS error = 7.1%.

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Fig. 7. Model resistivity of ERT at the locality Bílá Voda, Wenner–Schlumberger electrode array. a) Profile B – electrode spacing 5 m; vertical exaggeration = 1; iteration 5; RMS error = 2.6% b) detailed Profile B1 with electrode spacing 2 m; vertical exaggeration = 1; iteration 5; RMS error = 1.4%. Expressive resistivity gradient around Station 100 coincides with shallow depression on the surface that lies within the supposed trace of the SMF.

Fig. 8. Model resistivity of ERT at the locality Bílá Voda Profile E, Wenner–Schlumberger electrode array with simplified geology exposed in Trench E (black outline). Resistivity in ohm m; vertical exaggeration = 1; iteration 5; RMS error = 2.0%; 1 – orthogneisses, the closer to the fault the more loosened, 2 – hydrothermally bleached crushed orthogneisses.

thickness in comparison with its thickness shown on Profile C and exposed in Trench C (Fig. 5b). 5.3.2.3. Profile E. Profile E at Bílá Voda (RMS error = 2%) shows two sectors with different resistivity patterns (Fig. 8). This profile reaches a maximum depth of 40 m. The northeastern part of the profile (up to Station 155) reveals a more complex structure and overall higher resistivities, whereas the southwestern part (Stations 155–225) displays a less complicated resistivity structure characterized by more conductive substratum. The northeasternmost section consists

of three distinct resistivity layers that up to Station 110 dip gently to the NE, while between Stations 110 and 155 the layers are subhorizontal. The subsurface (down to ~15 m depth) between Stations 0 and 100 is characterized by a layer with relatively high resistivity (200 to 400 Ω m). This layer wedges out towards the centre of profile. Beneath this is a relatively conductive (10 to 60 Ω m) layer with a thickness of about 10 m, which may correspond to more fractured and weathered water-saturated rocks. At the base, intermediate resistivity material (100 to 150 Ω m) of probably relatively intact rocks occurs. These structures are cut by a sub-vertical structure between Stations

Fig. 9. Distribution of Bouguer anomaly in the surveyed profile and gravity modelling. Localization of the fault zone is around Station 900. a) topographic cross section; b), c) – models with bad fitting in the fault zone that anticipate gravitational effects of the fault zone being vertical (b) to slightly inclined (~ 83°) (c); d) model with good fitting in the fault zone that shows fault dip ~ 75° towards the NE and the vertical offset on the fault ~ 200 m (Miocene strata vs. bedrock with crystalline rocks). RMS of fitting is 0.012 mGal. The rectangle shows the sector selected for further detailed modelling.


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Fig. 10. Detailed gravity model of geological structures in the surroundings of the fault zone using a greater density of stations (RMS fit: 0.009 mGal) (see Section 4.2). The model shows position of the faults and various bodies of different densities. Geology corresponding with that exposed in the trench: 1 – Late Pleistocene colluvium; 2 – Miocene lacustrine deposits, 3 – fault zones with fault gauge, 4 – micashists, 5 – granitic aplite, 6 – gneisses.

155 and 165. It was proposed that this represented the trace of the SMF since the place also coincided with a foothill. The southwestern section (Stations 155–225) consists of an apparently homogenous conductive substratum (b90 Ω m), overlain by a very thin surface layer with a high resistivity. The ERT results obtained along Profile E were not expected to produce a conspicuous resistivity gradient as the nearby lithologies only comprise crystalline rocks (orthogneisses and micashists). Nonetheless, a distinctive sub-vertical resistivity structure was encountered between Stations 155 and 165. Consequently, this section was also trenched in order to confirm the validity of the geophysical results. It was seen that the resistivity gradient did, once again, correspond to the SMF. The subvertical boundary corresponds to a 5 m wide zone of hydrothermally bleached and crushed orthogneisses related to the main fault, which were found on the both sides of the fault (Fig. 8). 5.3.3. Gravimetric survey In addition, gravimetric surveying was performed as a test to verify the method for defining fault location. The central part of the profile overlaps with ERT Profile C (Fig. 3). The Bouguer anomaly in Fig. 9 shows a considerable horizontal gradient in the central section of the profile with the overall amplitude of changes reaching a maximum of

3.35 mGal. The shape of the Δg curve characterises a density contrast that is related to a lithologic contact. This may represent an inclined fault zone. This supposition is confirmed as the density contrast coincides spatially with the position of the fault as exposed in the trench. To determine the position of the tectonic contact, gravity modelling was undertaken. The gravity model represents the distribution of geological structures with different average bulk densities. First, a general gravity model was created that aimed to locate the density contrast based on the gravitational effects recorded along the entire profile (Fig. 9). For the both sides of the fault, only an average value of density for the rock complex was determined as this was considered to be sufficient to identify the fault zone. Modelling the gravitational effects of the fault zone showed that the fault is located in the vicinity of Station 900. During modelling, the iteration method was used in order to produce a gravimetric model with a gravitational effect that would best fit with the Bouguer anomaly. The distribution of the Bouguer anomaly shows that the fault zone is not vertical, and this is confirmed by the modelling in Fig. 9(b,c,d). Different inclinations of the SMF were used to search for the minimum difference between the modelled and measured gravity distribution. The models show that the fault dips at about 75° NE (Fig. 9d). Furthermore, the vertical offset of bedrock vs. Miocene on the fault is ca. 200 m. In this gravity model, the zone below 200 m was approximated by a flat horizontal plate with average density of about 2.50 Mg·m− 3 cut by the fault. The accuracy of fitting (RMS fitting) the modelled curve to the Bouguer anomaly is 0.012 mGal. Further, more detailed, gravity modelling was undertaken in the vicinity of the fault zone using a greater density of stations (see Section 4.2) (Fig. 10). A preliminary gravity model of this profile sector was created using the fault location outlined in the initial modelling, the logged geology exposed in the trench, and measured bulk densities from the trench samples. The measured values of density provided a first approximation for the densities of the modelled gravity structures. Then trial and error, using various densities and inclinations, was used to fit the modelled and measured data. On the hanging wall, two bodies with average densities of 1.65 and 1.85 Mg·m− 3 were modelled. In addition, the fault zone was outlined with 2.10 Mg·m− 3 separating different rocks within the footwall. The footwall is composed of several structures dipping in the opposite direction with average densities ranging between 2.25 and 2.65 Mg·m− 3 (Fig. 10). Misfit between the calculated and the observed gravity anomalies is small (0.009 mGal). Horizontal continuation of the different lithologies and structures known from outcrops and geological investigations support the extension of the structures also to the greater depth. In addition, the results of the detailed gravimetry correlate well with ERT (see Fig. 11).

Fig. 11. Interpretation of ERT Profile C1 carried out at the floor of Trench C with Wenner–Schlumberger electrode array and electrode spacing 0.8 m. Extrapolation of the logged geology exposed in the trench to the depth based on ERT.


6. Discussion 6.1. The application of ERT Of particular significance is the detailed ERT Profile C1, undertaken on the floor of Trench C (Fig. 6). This profile used an electrode spacing of just 0.8 m in order to provide a unique high-resolution dataset. This enabled the logged geology to be traced to far greater depths (Fig. 11). Interestingly, similar results are obtained if the geology is simply extrapolated. Despite the fact that the measurement was performed inside the trench and the real geometry was not a half space (see Section 4.1), our results are in good agreement with ERT section of the real half space Profile C done within “identical” part on the surface (Fig. 5). Moreover, other evidences support the extrapolation of the logged geology to the depth: similar results from Profiles B1 and B2 done 100 m from Trench C (Fig. 7), gravity data along the trench, and horizontal continuation of geological structures confirmed by geological investigations. In the northeastern section of the profile, both tomographies (dipole–dipole and Wenner–Schlumberger) clearly demonstrate the northeasternward dip of the warped Miocene deposits observed in Trench C, which apparently continues to depth. The overall thickness of the overlying fault-related colluvium of late Pleistocene age adjacent to the fault is estimated to be 3.6 m based on Profile C1. As the trenching results show, these sediments were deposited during repeated faulting events (Štěpančíková et al., 2009). The SMF corresponds to resistivity gradient around Station 23, which is inclined to NE. Independent information (observation in Trench C and the results of gravity modelling) suggests that the Wenner– Schlumberger configuration is better able to constrain the fault dip. Furthermore, the resistivity section according to the dipole–dipole configuration is quite discontinuous. This is partly caused by the geometry itself and partly by the relatively higher RMS error. The part between Stations 24 and 33 corresponds to fractured and folded micashists, which at the higher depth (up to 6 m) show resistivity minimum probably due to water saturation. The depth of 6 m from the trench bottom correlates well with the same resistivity structure on Profile C shown at the depth of 8 m (see Section 5.2; Fig. 5). The central part of the elongated low resistivity anomaly described on Profile C is displayed between Stations 31 and 34 on Profile C1 more distinctly and corresponds with several closely-spaced faults that dip to the SW and are filled with fault gouge in the trench. At Station 40, the granitic aplite expressed by relatively higher resistivity is also limited by a fault. The southwesternmost sector corresponds to gneisses. In all cases, except for Profile E, the main fault of the SMF is displayed as a distinctive resistivity gradient structure with a width varying from a few metres to a few tens of metres. This width depends on the extent to which the crystalline rocks of the uplifted block are fractured and/or water saturated. Profile E shows the SMF as the subvertical structure, which represents crushed rocks within a 5 m wide fault zone. Orthogneisses occur on both sides of this zone. These results shows that ERT is able to specify the fault location even within more homogeneous lithologies composed of only metamorphic rocks. 6.2. The application of gravity survey The gravity survey could be tested with considerable precision due to the availability of the logged geology in Trench C and detailed information on densities of rock sampled in the trench and then measured in the laboratory. The survey was undertaken overlapping Profile C and, therefore, included Trench C. The data shows that the gravity method was able to determine the fault position with a considerable degree of accuracy (+/−1 m). It can, therefore, be applied more widely within the SMF zone provided that rock densities on either side of the fault differ. A detailed gravity model was proposed based on the logged geology, measured bulk densities of the

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rocks, and the distribution of Bouguer anomalies. In this study, the most important advantage of the gravity method was that it was able to accurately locate the fault zone. It was also used to assess the inclination of the fault. The value of the fault dip can be ascertained from gravity modelling providing that correctly selected density values are used. Modelling the gravitational effects of the fault zone showed that the vertical offset (bedrock vs. Miocene) on the fault is approximately 200 m. In addition, the fault dips at around 75° towards the NE. This dip is identical to that measured in the trenches, as well as that estimated from ERT (e.g. Figs. 4, 5). Therefore, even though the geophysical methods are based on different petrophysical parameters, it is possible to note a similarity between the physical (ERT) and geological (gravimetric) constructions created by both sets of geophysical methods. 7. Conclusions In order to get a detailed understanding of active fault structures, it is preferable to take advantage of more than one method. This study has demonstrated that electrical resistivity tomography (ERT) and gravimetric survey can be used as useful tools for determining precise position of a fault. This is especially the case in regions where a moderate climate has not favoured the preservation of fault scarps due to high erosion rates and where fault exposures are lacking due to soil cover and dense vegetation. During the first stage of this study, the Sudetic Marginal Fault (SMF) was successfully positioned by ERT. The fault was shown as an expressive resistivity gradient on all ERT profiles. It was assumed that the fault separates a complex of crystalline rocks in the footwall from Miocene sediments in the hanging wall. This contention was confirmed during the second stage by subsequent trenching, which also provided information on the exact location of geological structures and the physical properties of the associated lithologies. As a result, it was then possible to interpret the ERT profiles retrospectively with a far greater degree of accuracy. Generally, lower resistivities correspond to clayey sand Miocene deposits. These were covered by intermediate resistivities that correspond to Quaternary colluvium. The higher resistivities on uplifted block corresponded to crystalline rocks. The width of the gradient structure that represents the SMF varied from a few metres to a few tens of metres depending on the degree to which the crystalline rocks of the uplifted block are fractured and/or water saturated. In the third stage, post-trenching ERT profiles were surveyed in more distal localities where only gentle morphological features potentially related to faulting suggest occurrence of the fault. The profiles show similar results with a distinctive resistivity gradient or at least sub-vertical resistivity structure. By additional trenching, this sub-vertical structure was proved to be the SMF. This structure was even seen where similar lithologies occurred on the both sides of the fault. In addition, a unique feature of our study was the application of closely spaced ERT on the floor of the trench. This allowed us to extrapolate the geological structures logged in the trench to the greater depth and to assess the thickness of Quaternary colluvium next to the fault. Moreover, the gravimetric survey carried out within the profile along the trench localized the main fault with accuracy of +/−1 m. The gravity method was also able to assess the vertical offset on the fault (Miocene vs. bedrock) to about 200 m, and to model the fault dip as 75°. Although one should be cautious about inferring a dip from ERT, the results show that the dip shown by the ERT (70–80°) is in a good agreement with the dip modelled by gravimetry as well as the dip measured on the fault outcrops or in the trenches. Thus, with specific reference to the SMF zone in intraplate settings, it is shown that preliminary geomorphologic reconnaissance is necessary in order to select suitable localities for geophysical survey. Moreover, the research design consisting of three stages (preliminary ERT, trenching that validates the ERT, and post-trenching ERT and gravimetric survey) is shown to be very important for accurately interpreting geophysical data.

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The ERT investigations and gravimetric survey appear to be able to accurately define the location of the main fault and assess its dip, particularly where there are sufficiently contrasting resistivities and rock densities. Acknowledgements The research was supported by the Czech Science Foundation No. 205/08/P521, Ministry of Education of the Czech Republic: Project No. MSM 0021620855, and by the University of Ostrava Foundation SGS5/PrF/2010. The work has been elaborated within the Institute Research Plan of the Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, No. AVOZ30460519. We acknowledge Matt Rowberry for the substantial language improvement of the manuscript. Kris Vanneste and Frederic Nguyen are thanked for their critical reviews, which helped to improve the paper. References Alasset, P.-J., Meghraoui, M., 2005. Active faulting in the western Pyrenees (France): paleoseismic evidence for late Holocene ruptures. Tectonophysics 409, 39–54. 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