Localization and characterization of the Zhangdian-Renhe fault zone in Zibo city, Shandong province, China, using electrical resistivity tomography (ERT)

    Localization and characterization of the Zhangdian-Renhe fault zone in Zibo city, Shandong province, China, using electrical resistivity tomography (ERT) T. Zhu, J. Zhou, H. Wang PII: DOI: Reference:

S0926-9851(16)30536-5 doi: 10.1016/j.jappgeo.2016.11.016 APPGEO 3143

To appear in:

Journal of Applied Geophysics

Received date: Revised date: Accepted date:

24 March 2016 23 October 2016 15 November 2016

Please cite this article as: Zhu, T., Zhou, J., Wang, H., Localization and characterization of the Zhangdian-Renhe fault zone in Zibo city, Shandong province, China, using electrical resistivity tomography (ERT), Journal of Applied Geophysics (2016), doi: 10.1016/j.jappgeo.2016.11.016

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ACCEPTED MANUSCRIPT Localization and characterization of the Zhangdian-Renhe fault zone in Zibo city, Shandong province, China, using electrical resistivity

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T. Zhu1,* J. Zhou1 H. Wang2

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tomography (ERT)

Key Laboratory of Seismic Observation and Geophysical Imaging, Institute of Geophysics,

China Earthquake Administration, Beijing, China, 100081

Shandong Institute of Earthquake Engineering, Jinan, China, 250014

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* Corresponding to T. Zhu: Email: [email protected]; Tel: +86-10-68729112

Abstract

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A 2D ERT survey is performed along 10 cross-sections intersecting with the trace of

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Zhangdian-Renhe fault zone, a Quaternary active normal fault zone going from south

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to north across Zibo city, Shandong province, China. During the survey, the Wenner- array with the strongest anti-electrical disturbance ability is adopted, and some ways to improve signal-to-noise ratio (SNR) of apparent resistivity data are

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performed. The reconstructed resistivity tomograms illustrate that Zhangdian-Renhe fault zone in Zibo city consists of 4 NW-striking normal faults which are the west branch (F1), the secondary fault of west branch (F1-1), the secondary fault of east branch (F2-1) and the east branch (F2). Fault F1 has NE apparent dip direction and 67~75 apparent dip angle, and fault F2 SW and 60~63. The two faults are the main faults of Zhangdian-Renhe fault zone and form a graben. Subsequent geologic drilling records prove our inference. Our results present an important basis for the definition of seismic fortification level and new city planning in Zibo city.

Keywords: Fault; Geophysical survey; Electrical resistivity imaging; City planning;

ACCEPTED MANUSCRIPT Earthquake safety

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1 Introduction

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It is of importance for earthquake protection and disaster reduction as well as urban

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planning to detect clearly the subsurface geological structure and tectonics in an urbanized area. The detection of a near-surface buried (active) fault is one of the most important tasks, because active faults are not only the origin of earthquake occurrence

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but also responsible for earthquake disaster (e.g., Dai et al., 2011; Ozawa et al., 2011;

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Løvholt et al., 2012). In recent years, near-surface geophysical prospecting has become a standard tool for the study of seismically active faults in a variety of

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geological and tectonic contexts (e.g., Demanet et al., 2001; Gwendolyn et al., 2005;

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Vanneste et al., 2008). So far, the most frequently geophysical tools applied to locate and characterize near-surface fault traces have thought to be surface-based seismic

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and georadar reflection techniques as well as ERT (e.g., Eberhart-Phillips et al., 1995; Meghraoui et al., 2000; Demanet et al., 2001; Gross et al., 2002; Green et al., 2003;

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Vanneste et al., 2008). Seismic and georadar methods can provide more detailed images of the subsurface than ERT, but they tend to be more sensitive to culture noise. The noise caused by vehicles, a main noise source in an urbanized area, makes it impossible to perform seismic and georadar reflection surveys under the condition of no road closure. In addition, the application of georadar methods is limited to poorly conductive surficial environments (e.g., Annan, 2005) and high-resolution seismic reflection data generally fail to adequately image the immediate subsurface mainly due to static problems and the interference with source-generated noise (e.g., Steeples, 2005). Compared with them, despite its inherently lower resolution and more ambiguous quantitative interpretation, ERT has the advantages of relative simplicity,

ACCEPTED MANUSCRIPT efficiency, non-destruction, low cost as well as its limited sensitivity to urban noise, and can have an effective depth of investigation of several hundred meters, so this

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technique has become to be one of the most powerful geophysical tools for the

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detection of a buried fault in an urbanized area (e.g., Suski et al., 2010).

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Zhangdian-Renhe fault zone is a Quaternary active normal fault zone going across Zibo city from south to north and makes the city probably facing the threat of Ms 6.0 earthquake (Wang et al., 2010). Therefore, it is very important and necessary to define

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clearly its location and characteristics for the earthquake prevention and disaster

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reduction and new city planning. Nevertheless, the fault zone is buried in the subsurface of Zibo city and its location has been only inferred from structural and

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geomorphic surface observations (Wang et al., 2010). In order to locate it more

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precisely, a 2D ERT survey is carried out across the fault zone. We report the results in detail in this paper. The results are proven to be correct by subsequent geologic

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drilling records.

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2 Regional setting

The administrative territory of Zibo city hosts ~4,530,000 people. This city is located to southeast of Beijing (Fig. 1a) and in the transitional zone between mountainous highlands in the central and plain in the northern Shandong province (Fig. 1b). Taking the Qihe-Guangrao fault as a boundary, the south of Zibo city locates in the central Shandong uplift area consisting of the basement rock series of Taishan Group and Neoarchean-Paleoproterozoic granite and the mainly Cambrian and Ordovician overlying strata, while the north in Jiyang depressed area completely covered with Quaternary strata.

ACCEPTED MANUSCRIPT The Quaternary overburden is composed of Holocene, Upper and Middle Pleistocene strata in our study area. Its thickness ranges from several meters in the

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south and east to about 100 m in the north and west. The Holocene stratum mainly

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consists of miscellaneous fill, brown-yellow and brown silt soil and silt clay. The

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Upper Pleistocene stratum is composed of brown-yellow silt clay and silt soil, and includes sand laminae and sand and detritus lens in some places. The Middle Pleistocene stratum is mainly composed of brown-yellow, brown-red and brick-red

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silt clay, clay and medium-coarse sand. The underlying bedrock is mainly composed

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of strong and medium weathered sandstone, medium weathered mudstone and conglomerate.

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The Tanlu, Yanshan-Bohai, Liaokao and Cangdong fault zones, Bohai seismic

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tectonic zone, Luxi (the west of Shandong province) uplift, and the NW- and

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NE-striking active fault zones in Jidong (eastern Hebei province) -Bohai depressed area form the complex active tectonic pattern in and around Zibo city (Wang et al.,

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2008). These fault zones, to a certain degree, have been active in Quaternary period and dominate the mid-strong earthquake activity in this region. NNW-striking TanLu and NW-striking Yidu fault zones have important effects on the seismic hazard in Zibo city. There were 6 earthquakes with magnitude above Ms 7.0 along Tanlu fault zone, 5 Ms 4.0-6.0 earthquakes along Yidu fault zone, and B.C. 70 Anqiu Ms 7.0 earthquake in their joint portion. Zibo city and its surroundings also locate in the northwestward distorted part of aeromagnetic, gravity and geothermal gradient zones where strong earthquakes occur frequently (e.g., Ye et al., 1980; Liu et al, 1996). Zhangdian-Renhe fault zone is a main branch of Yidu fault zone; it is inferred that the fault zone has the length of about 50 km, and the strike of 335, the dip direction

ACCEPTED MANUSCRIPT of 255 and the dip angle of 70-85 from structural and geomorphic surface observations; its southeastern segment has ever been active in Late Pleistocene and

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northwestern segment in Middle Pleistocene, i.e., its active era from the new to the

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old from southeastern to northwestern segment, based on analysis of Quartz

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micro-appearance in the fault gouges and radiocarbon dating; and the fault zone, as mentioned above, probably makes Zibo city facing the threat of Ms 6.0 earthquake from analysis of seismic hazard (Wang et al., 2010). There was a Ms 5.0 earthquake

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in 1730 close to the northern end of Zhangdian-Renhe fault zone.

3 ERT surveys: measuring lines, equipment and methology

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Ten ERT measuring lines are arranged along the inferred Zhangdian-Renhe fault zone

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from structural and geomorphic surface observations (Wang et al., 2010) in Zibo city (Fig. 2. Only the ERT-inferred locations are marked for simplicity) and intersect with

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the strike at angles larger than 45. There are not metal pipelines and electrical substations on the area where ERT measuring lines go across.

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Measurements are performed with the DCX-1 resistivity meter (Its technical specifications can be found at the website http://www.cgeg.com.cn.) which consists of a main controller and an electrode converter and can be connected to a linear array of 120 electrode nodes, with a 8 m of maximum spacing. The instrument belongs to a centralized measurement system and its input impedance is larger than 80 M. The resistivity meter is able to automatically perform the pre-defined sets of measurements according to the type of array selected and provides direct reading of input current, potential difference, electrode location and apparent resistivity. In the field stainless steel electrodes are used to set up a 2D linear electrode array which is connected to the DCX-1 resistivity meter by means of two strings of heavy-duty

ACCEPTED MANUSCRIPT seismic-like cable with 64 output each. Each electrode is positioned using a handheld Garmin eTrex Vista GPS whose horizontal positional accuracy is about 4 m. The

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datum used is WGS84. The measuring parameters used are listed in Table 1.

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The electrical environment in an urbanized area often has some sources of

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electrical noise, including stray currents from industrial power and buried conductors. These electrical conditions require an array with strong anti-electrical disturbance ability in order to acquire high SNR apparent resistivity data. In an electrically

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homogeneous and isotropic half-space, the relationship among apparent resistivity a,

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the potential difference V between two potential electrodes and the current I can be

(1)

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written as follows:

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where K is the geometric factor with respect to electrode spacing and can be

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expressed as

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where A and B denote current electrodes, and M and N denote potential electrodes.

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We call 1/K the intensity coefficient. If an electrode array has a larger intensity coefficient, it has stronger anti-disturbance ability. A few commonly used electrode arrays and their geometric factors are presented in Fig. 3. Among them, Wenner- and pole-pole arrays have the minimum geometric factor K = 2a (a representing electrode spacing), i.e., maximum intensity coefficient, so they have the strongest anti-disturbance ability. The necessity of remote electrodes makes pole-pole array impractical in an urbanized area, therefore Wenner- array is adopted in this study. In addition, some ways are performed to improve the SNR of apparent resistivity data during the data acquisition. Firstly, the earthing resistance of each electrode is

ACCEPTED MANUSCRIPT ensured as low as possible. For a larger one, it is connected to a parallel electrode or/and watered with saltwater. For the electrode on a cement or/and asphalt pavement,

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a hole is drilled through the cement or asphalt layer and a longer electrode (longer

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than the thickness of cement or asphalt layer) is used. Secondly, V must be larger

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than 20 mV and in the meantime I must be larger than 100 mA for each measurement. At last, in order to effectively suppress random electrical disturbance, the power supply cycle of 2.7 s – 3.6s and 50 stack folds (i.e., an apparent resistivity value is the

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average value of 50 ones measured in a power supply cycle.) are adopted after

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experiment.

The recorded apparent resistivity data are processed using RTomo software which

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is designed by Geogiga Technology Corporation

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(http://www.geogiga.com/cn/index.php) in order to determine a vertical resistivity section as a function of a true depth from a pseudo-section. At first, an initial forward

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model is set up based on the apparent resistivity data measured in field in the form of a pseudosection, a contour diagram in which the apparent resistivity values are

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assigned to a predefined location according to the array type used. The model has a computation domain which is 10 times as large as imaging domain, i.e., the area of pseudosection, in order to avoid edge effects. The resistivity cells have equal height and width within the imaging domain. The height and width increase by 1.7 and 1.5 times, respectively, out of this imaging domain; Secondly, a forward modelling subroutine is performed by finite element method to calculate apparent resistivity via the initial forward model; Thirdly, the residual apparent resistivity is calculated by subtraction of calculated from measured ones; At last, an inversion routine is performed to calculate an adjustment based on the residual apparent resistivity to modify and improve the initial forward model by solving a least-squares equation.

ACCEPTED MANUSCRIPT These steps are repeated until reaching an acceptable agreement between measured and calculated data. The imaging depth is calculated by AB/3 (Feng et al., 2004).

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Subsequently, the inverted vertical resistivity tomogram is reconstructed using

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software of golden surfer.

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The well determination of water-bearing structures in man-made cylindrical samples (Hao et al., 2000) and of salt solution-bearing cracks in a cylindrical magnetite-quartzite rock sample (Hao et al., 2002) from the reconstructed resistivity

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tomograms by ERT indicates that, if a fault zone is buried in a rock formation and

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filled with ground fluid, ERT could locate and characterize the fault zone well. The resource of groundwater is abundant in Zibo city, Shandong province, China

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(Liu et al., 1999), which, in general, causes a fault zone filled with groundwater in our

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study area. A water-bearing fault zone often appears as a linear low resistivity feature while that the low resistivity zone is not due to a fault often lies in quite shallow

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region and appears as a horizontal extension or/and dumpling pattern in local regions, and its bottom depth in general is not deeper than that of strong weathered zone (Zhu

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et al., 2003). It is considered that electrical structure shallower than 10 m is often affected greatly by human activity such as irrigation, pumping and engineering activities. Accordingly, if a linear low resistivity zone extends upwards from the deep (in general deeper than 100 m) to the shallow in a resistivity tomogram, it can be interpreted as a fault, while a near-surface low resistivity zone that does not extend to the deep should not be interpreted as a fault. In addition, a fault in general has a spatial extension and same characteristics in a certain local region, which results in similar electrical structures near the fault, so a fault can be traced by finding the consistency of the electrical structures from different resistivity tomograms.

ACCEPTED MANUSCRIPT 4 Results and interpretation Ten ERT resistivity tomograms (Figs. 4, 6, 8 and 10) are analyzed to locate and

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characterize the Zhangdian-Renhe fault zone. Due to cement or/and asphalt

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pavements, the resistivity tomograms corresponding to measuring lines 5 and 9 (Figs.

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10c, g and h) has relative low resolution despite the ways, as mentioned above, are used to improve the SNR of apparent resistivity data in data acquisition. The numbers of iteration (NIs) are less than 5 and root mean square (RMS) errors are less than 5%

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for all profiles except for the measuring line 5 (Fig. 10c). For illustrating our method

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to infer a fault, here 3 tomograms corresponding to measuring lines 2, 3 and 10 (Figs. 4, 6 and 8) are taken as an example, two of which locate in the south and 1 in the

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north (Fig. 2).

4.1 Resistivity tomogram corresponding to measuring line 2

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This tomogram (Fig. 4) indicates that the overburden has the thickness of about 15 m (the zone shallower than the depth of green solid line L2) and it is divided into two

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layers by line L1 at about 5 m depth. The base of the overburden is marked by line L2 at about 15 m depth. Obviously, there is a linear low resistivity gradient zone at the horizontal coordinate of about 320 m. This zone extends continuously upwards from about 100 m depth to about 20 m depth, so it is interpreted as the west branch of Zhangdian-Renhe fault F1, that is, fault F1 goes across at the horizontal coordinate of about 320 m. The surface projection of its upper breakpoint (here refers to the top end of a fault plane) which is buried at the depth of about 20 m is at the horizontal coordinate of about 270 m, corresponding to the GPS coordinate (N364621.2, E118549.8). The apparent dip angle of fault F1 is about 74° and its apparent dip direction is eastward. The hanging side of fault F1 is heavily crushed and weathered.

ACCEPTED MANUSCRIPT The crushed zone is a water-bearing low resistivity zone shallower than 60 m. Its footwall side is relatively intact and even and has a higher resistivity value. In

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addition, there is a small fracture plane F11 with apparent dip direction of west at the

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horizontal coordinate of about 240 m. Faults F1 and F11 form a small horst.

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Four geologic boreholes ZK1-ZK4 with the spacing of 20 m, 10 m and 10 m (red solid lines in Fig. 4) are arranged according to this result. The geologic sections of boreholes ZK1-ZK4 (Fig. 5) reveal that only borehole ZK2 lacks the layers (6) and

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(7), so fault F1 goes across the area between boreholes ZK2 and ZK3. Its dip direction

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is eastward and the upper breakpoint is buried at about 12 m depth, which indicates

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the correct inference of the horizontal location of fault F1 in Figure 3

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4.2 Resistivity tomogram corresponding to measuring line 3 This tomogram (Fig. 6) indicates that the thickness of overburden (the zone shallower

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than the depth of green solid line L) is about 6 m. A striking linear low resistivity zone appears at the horizontal coordinate of about 500 m – 550 m and extends

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continuously upwards from about 120 m depth to about 10 m depth, so it is interpreted as the east branch of Zhangdian-Renhe fault F2. The surface projection of its upper breakpoint which is buried at the depth of about 10 m is at the horizontal coordinate of about 550 m, corresponding to the GPS coordinate (N36475.2, E118541.2). The apparent dip angle of fault F2 is about 63° and its apparent dip direction is westward. The footwall side of fault F2 is intact, even and slightly weathered, while its hanging side, ranging from the fault plane westwards to about 100 m, is heavily crushed and weathered. Four geologic boreholes ZK5 – ZK8 with the spacing of 20 m (red solid lines in Fig. 6) are arranged according to this result. The corresponding geologic sections (Fig.

ACCEPTED MANUSCRIPT 7) suggest that the layer (4) between boreholes ZK6 and ZK7 shifts strikingly, so indicating that the fault F2 goes across the area between boreholes them. Its dip

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direction is westward and upper breakpoint is buried at about 4 m depth, which

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indicates the correct inference of the horizontal location of fault F2 in Fig. 6, but the

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buried depth of its upper breakpoint is slightly different (The reason is presented in section 5).

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4.3 Resistivity tomogram corresponding to measuring line 10

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This tomogram (Fig.8) indicates that the thickness of overburden (the zone shallower than the depth of green solid line L) is about 40 m. There are two low resistivity zones

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near the horizontal coordinates 400 – 500m and 800 – 1000 m, and both of which

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extend from the deep (about 150 m depth) to the shallow (about 50 m depth). However, only the latter has the similar electrical structures marked by A2, B2 and C2

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to those by A1, B1 and C1 in Fig. 6, so it is interpreted as fault F2 which goes across the measuring line 3 (Fig. 6) in the south and then northwestwards across the

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measuring line 10 (Fig. 8) in the north of Zibo city. The fault plane of fault F2 in Fig. 8 locates at the horizontal coordinate of about 1000 m. The surface projection of its upper breakpoint which is buried at the depth of about 40 m is at the horizontal coordinate of about 1060 m, corresponding to the GPS coordinate (N36509.0, E118234.1). The apparent dip angle of fault F2 is about 60° and its apparent dip direction is westward. Four geologic boreholes ZK9-ZK12 with the spacing of 30 m (red solid lines in Fig. 8) are arranged according to this result. The geologic sections (Fig. 9) suggest that boreholes ZK11 and ZK12 lack the layer (14), so fault F2 goes across the area between boreholes ZK10 and ZK11. Its dip direction is westward and upper

ACCEPTED MANUSCRIPT breakpoint is buried at about 50 m depth, which indicates the correct inference of the horizontal location of fault F2 in Fig. 8, but the buried depth of its upper breakpoint is

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about 10 m deeper than ERT-inferred depth (The reason is presented in section 5).

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Combined the results inferred from all tomograms (Figs. 4, 6, 8 and 10), the

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Zhangdian-Renhe fault zone is located and characterized (Table 2), and then the strike is defined (Fig. 2). The fault zone consists of 4 NW-striking normal faults which are the west branch (F1), the secondary fault of west branch (F1-1), the secondary fault of

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east branch (F2-1) and the east branch (F2). Fault F1 has NE apparent dip direction and

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67~75 apparent dip angle, and fault F2 SW and 60~63. They (Faults F1 and F2) are the main faults of Zhangdian-Renhe fault zone and form a graben.

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5 Discussion

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The existence of a Quaternary active fault is a serious threat to the earthquake safety of a city. In general, earthquake harder-hit area mainly concentrates in a narrow zone

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along an earthquake-producing fault (Xu et al., 2002). The Zhangdian-Renhe fault zone is a Quaternary active fault and could probably lead to Ms 6.0 earthquake (Wang

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et al., 2010), which is a potential threat to Zibo city. Therefore, it is very important to localize and characterize clearly the fault zone in Zibo city for defining a reasonable seismic fortification level or earthquake safety zone in new city planning. In this paper, ERT is used to effectively locate and characterize the fault zone. ERT-inferred dip direction and strike are consistent with and location is more precise than those from structural and geomorphic surface observations (Wang et al., 2010). Subsequent geologic drilling records indicate that our inferred horizontal locations of the Zhangdian-Renhe fault zone are correct. However, it must be noted that, in principle, the inversion of ERT is to solve an inverse problem of potential field, which results in ERT has a better resolution in horizontal direction than in depth. As a result, it is

ACCEPTED MANUSCRIPT difficult to define precisely the depth of a target. For instance, the buried depth of fault F2 in Fig. 6 is about 10 m while actually geologic drilling records indicates about

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4.1 m (Fig. 7). In addition, this difference is also probably due to the erosion and

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reconstruction of groundwater to electrical structure. It is found that the presence of

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numerous cement or/and asphalt pavements is a dominant contributor to low SNR of the acquired apparent resistivity data. The measuring lines 5 and 9 go across many cement or/and asphalt pavements and the later also across a long cement bridge,

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which causes the low resolution of their tomograms (Figs. 10c, g and h). Accordingly,

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it needs the joint interpretation or inversion of the data from multiple geophysical surveys in order to locate and characterize a buried fault more precisely (e.g.,

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Gallardo，Meju，2003，2004).

6 Conculsion

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The following conclusions could be arrived at in this study. The Zhangdian-Renhe fault zone is composed of 4 NW-striking normal faults which are the west branch (F1),

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the secondary fault of west branch (F1-1), the secondary fault of east branch (F2-1) and the east branch (F2). Fault F1 has NE apparent dip direction and 67~75 apparent dip angle, and fault F2 SW and 60~63. The two faults (F1 and F2) are main ones and form the west and east boundaries of the fault zone.

Acknowledgements We thank the anonymous reviewer for his critical and constructive comments. This work was carried out as part of the project entitled “Urban Active Fault Surveying Project” (143623) funded by National Development and Reform Commission of China and “Active Faults Exploration and Seismic Hazard Assessment in Zibo City”

ACCEPTED MANUSCRIPT (SD1501) funded by Department of Science & Technology of Shandong Province,

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China.

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ACCEPTED MANUSCRIPT Ye H, Zhang W (1980) The characteristics of intraplate earthquake faults in North China and their relationship to the dynamical process in earth’s crust and

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uppermost mantle, Seismology and Geology 2(1): 27 – 38 (in Chinese)

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Zhu T, Feng R, Hao J (2003) Groundwater and faults in resistivity images. In

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Symposium-Imaging Technology, 316-321

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Proceedings of the 6th Society of Exploration Geophysics Japan International

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Fig. 1 (a) the relative location of Zibo city to Beijing, China, and (b) the modified geologic tectonics in and around Zibo city from Wang et al. (2010). ①- ⑧ indicate

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Shangwujing fault, Taishan mountain piedmont fault, Yuwangshan fault,

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Wangmushan fault, Zhangdian-Renhe fault, Shuanshan-Lijiazhuang fault, Yidu fault and Qihe-Guangrao fault, respectively. Faults ①-④ and ⑧ were active in the early

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and middle Pleistocene, ⑥ and ⑦ in the Holocene, and the northern part of fault ⑤ was active in the early and middle Pleistocene while the southern part in the late

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Pleistocene at the boundary of fault ④. The closed area by dot line is the administrative territory of Zibo city. The area covered by rectangle is our study area. The circle indicates an earthquake. The topographic data is from ETOPO1 (Amante and Eakins, 2009).

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Fig. 2 Layout of ERT measuring lines and the location of inferred Zhangdian-Renhe fault zone from ERT. F1 and F2 indicate the west and east branches. F1-1 and F2-1 indicate the secondary faults of the west and east branches.

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V a

a N K = 2a (a) Wenner  array M

a

B

A

na a B M N K = n(n+1)(n+2)a (b) Dipole-dipole array

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N K = 2a (c) Pole-pole array

na

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a A

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a

V

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na M N B K = n(n+1)a (d) Wenner-Schlumberger array

B

A

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Fig. 3. Sketch of four commonly used resistivity arrays and their geometric factors (K). Integer n is the depth factor.

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Fig. 4. The resistivity tomogram of measuring line 2 in Fig. 2. NI: 3; RMS error: 2.45%.

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Fig. 5. The geologic sections of boreholes ZK1-ZK4 marked in Figure 3. (1) Miscellaneous fill, (2) silt clay, (3) silt soil, (4) clay, (5) silt clay, (6) highly weathered mudstone, (7) highly weathered sandstone.

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Fig. 6. The resistivity tomogram of measuring line 3 in Figure 2. NI: 3; RMS error:

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electrical structures.

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2.36%. Marks A1, B1 and C1 are for tracing fault F2 by means of finding the similar

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Fig. 7. The geologic section of boreholes marked in Fig. 5. (1) Miscellaneous fill, (2) silt clay, (3) highly weathered sandstone, (4) moderately weathered sandstone.

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Fig. 8. The resistivity tomogram of measuring line 10 in Fig. 2. NI: 3; RMS error: 2.58%. Marks A2, B2 and C2 are for tracing fault F2 by means of finding the similar

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electrical structures.

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Fig. 9. The geologic section of boreholes marked in Fig. 5. (1) Miscellaneous fill, (2) silt clay, (3) silt soil, (4) clay, (5) medium-coarse sand, (6) silt clay-clay, (7) silt soil, (8) detritus, (9) clay, (10) silt soil, (11) clay with detritus, (12) conglomerate, (13) clay, (14) moderately weathered mudstone, (15) conglomerate, (16) highly weathered sandstone.

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Fig. 10. The resistivity tomograms of measuring lines (a) 1, (b) 4, (c) 5, (d) 6, (e) 7, (f) 8, (g) 9 (left) and (h) 9 (right) in Figure 2. (a) NI: 3; RMS error: 2.13%; Overburden: thickness of about 30 m, and low resistivity; Basement: intact; Fault: F1. (b) NI: 3; RMS error: 2.35%; Overburden: thickness of about 30 m, low resistivity and crushed; Basement: intact; Fault: F2 and F2-1. (c) NI: 6; RMS error: 6.96%; Overburden:

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and crushed; Basement: almost intact; Fault:

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4.21%; Overburden: thickness of about 30 m, high resistivity; Basement: almost intact;

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Fault: F2-1. (f) NI: 4; RMS error: 3.87%; Overburden: thickness of about 30 m, high resistivity and crushed; Basement: intact; Fault: F2. (g) NI: 4; RMS error: 4.78%; Overburden: thickness of about 40 m, high resistivity and crushed; Basement: almost

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intact; Fault: F1. (h) NI: 4; RMS error: 4.69%; Overburden: thickness of about 30 m,

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low resistivity and crushed; Basement: almost intact; Fault: F1-1.

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Table 1 parameters of measuring lines Length

Spacing

Max. depth

(m)

(m)

factor 30

EW

476

4

2

EW

908

4

3

EW

764

4

4

EW

476

5

NS

760

30

F2, F1-1

4

30

F2, F2-1

8

25

F1

8

F2, F2-1, 25 F1-1 25

F2-1, F1-1

952

8

25

F2

EW

952

4

30

F1-1

EW

1720

8

22

F2, F2-1

8

NW-SE

9

930

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NW-SE

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F1, F1-1

6

7

10

F1

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fault

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Direction No.

Target

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ML

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Table 2 Localization and characterization of Zhangdian-Renhe fault zone ML ADD

ADA

BDUB

SPUB

E

74

50 m

N364525.2 E1180652.5

20 m

N364621.2 E1180549.8

E

74

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No. 1

F1 67

60 m

9

E

75

40 m

3

W

63

63

50 m

N364932.6 E1180321.0

40 m

N365009.0 E1180234.1

61

8

NW

W

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NW

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N364845.7 E1180358.1

6

60

Geologic boreholes

W

72

20 m

N364620.9 E1180606.8

3

W

72

12 m

N364706.3 E1180528.6

9

W

75

30 m

N364940.4 E1180206.9

4

W

63

30 m

N364828.2 E1180416.5

6

NW

66

70 m

N36489.8 E1180338.3

7

NW

66

70 m

N364907.4 E1180313.0

10

W

65

40 m

N365009.5 E1180212.3

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Geologic boreholes

60 m

63

2

F1-1

N364705.2 E1180541.2 N364827.2 E1180421.3

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boreholes

N364941.8 E1180137.5

40 m

4

10

10 m

Geologic

N364833.9 E1180300.1

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F2

Remarks

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Fault

F2-1

Note: ML=measuring line, ADD=apparent dip direction, ADA=apparent dip angle, BDUB=buried depth of upper breakpoint, SPUB=surface projection of upper breakpoint

ACCEPTED MANUSCRIPT Higlights Zhangdian-Renhe fault zone is effectively defined by ERT.

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Zhangdian-Renhe fault zone consists of 4 NW-striking normal faults.

ERT-inferred Zhangdian-Renhe fault zone is proven by drilling records.

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ERT is a powerful tool for detecting a buried fault in urbanized areas.