Structural analysis of S-wave seismics around an urban sinkhole ; evidence of enhanced subrosion in a strike-slip fault zone

In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the subrosion, we carried out several shear wave reflection seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement, in the form of a NW–SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks (<100 m in size), around which steeply-dipping normal 5 faults, reverse faults, and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead the groundwater is flowing along the faults which serve as conduits. A near-surface fault zone located in soluble rocks can enhance subrosion in two ways: (1) tectonic movements accompanied by strain variations , :::: and ::::: forms ::::::: cavities :: in ::: the ::::::: Permian ::::::: deposits :::::: below ::: ca. :::: 60 m :::::: depth. :::: Mass :::::::::: movements ::: and ::: the :::::::: resulting :::::: cavities : lead to the formation of small fault blocks and a fracture network that increases the 10 rock permeability and creates fluid pathways, and (2) the faults can serve as groundwater conduits. Also note, ::::::: sinkholes :::: and ::::::::::::::: subrosion-induced :::::::::: depressions. ::::: Since ::: the :::::::: processes ::: are :::: still :::::::: ongoing, the ::::::::: occurrence :: of ::::::: another ::::::: sinkhole :::::: cannot :: be ::::: ruled :::: out. :::: This :::: case :::: study ::::::::::: demonstrates :::: how ::::::: S-wave ::::::: seismics :::: can :::::::::: characterize : a :::::::: sinkhole, ::: and ::::::: together ::::: with :::::::: geological :::::::::: information :::: can :: be :::: used :: to ::::: study ::: the :::::::: processes :::: that :::: result ::: in ::::::: sinkhole ::::::::: formation, :::: such :: as : a ::::::::::: near-surface :::: fault :::: zone :::::: located ::: in :::::: soluble ::::: rocks. :::: The more complex the fault geometry and the more interaction between faults, the more fractures are generated and the more prone 15 to sinkholes occurrenceis the area :: an :::: area :: is :: to ::::::: sinkhole ::::::::: occurrence.

and the type of the overburden (e.g., soft sediments or solid rock). The two main subrosion features that can evolve close to the surface are collapse or depression structures. The former occurs if the overburden is thin enough, the latter is due to slow dissolution (Smyth, 1913;White & White, 1969;Beck, 1988;Martinez et al., 1998;Yechieli et al., 2002;Waltham et al., 2005;Gutiérrez et al., 2008). Sinkholes can cause damage to buildings and infrastructure, and may even lead to life-threatening situations, if they occur e.g. in urban areas. 5 Subrosion is a barely-understood phenomenon. To determine the causes and the main controlling factors of the sinkhole formation in the urban area of Schmalkalden, a number of investigations were conducted on behalf of the Thuringian State Institute for Environment and Geology (TLUG), including investigation of possible man-made underground cavities, boreholes, micro-gravimetry, 2D compression wave (P-wave) reflection seismic, and hydrological investigations (e.g., chemical composition of the four aquifers) (Schmidt et al., 2013). 10 The P-wave reflection seismic was unsuccessful to image the first ca. 30 m below surface due to a relatively poor resolution, but for instance shear waves (S-wave) are able to image the near-surface in high resolution (Dobecki & Upchurch, 2006;Krawczyk et al., 2012;Polom et al., 2016a;Wadas et al., 2016). Interpretation of near-surface faults and structures from the surface down to ca. 100 m depth is important for understanding the local geology and the subrosion structures and processes in general. Therefore Leibniz Institute for Applied Geophysics (LIAG) carried out 2D S H -wave reflection seismics in this area. 15 2 Study area 2.1 Geological evolution Schmalkalden is located in southern Thuringia in Germany. The deeper bedrock below the research area consists of metamorphic gneiss and micaceous shale, which were deformed during the Variscan Orogeny. Due to erosion of the overburden during the Upper Carboniferous and the Lower Permian, WNW-ESE and NE-SW striking valleys were formed (Wunderlich, 1995).
In the study area the Zechstein Formation is followed by terrestrial sediments of the Triassic; the Calvörde and the Bernburg Formations of the Lower Buntsandstein (Fig. 1). Because of intense erosion due to fault movement, mostly since the Upper Cretaceous, which also lead to the uplift of the Thuringian Forest, these formations are also the youngest in the region, except for some Quaternary deposits.
Altogether, ::::::::::::::::::::::::::: Wunderlich (1997b) distinguished : six tectonic phases between the Lower Carboniferous and the Tertiaryhave 5 been distinguished, of which the last two are the most important for this work. From the Upper Permian to the Lower Cretaceous the area of Schmalkalden was subject to an extensional stress regime and from the Upper Cretaceous to the Tertiary the area was dominated by a compressional stress regime (Wunderlich, 1997b).

Faults
Thuringia is crossed by several major NW-SE striking faults (Wunderlichet al., 1997a;Wunderlich, 1997b;Andreas & Wun-10 derlich, 1998). Schmalkalden is located to the south of the Stahlberg Fault Zone (SFZ) in this area. The SFZ is downthrown to the southwest and raises basement rocks to the northeast. However, this was accompanied by dextral strike-slip movement, as can be seen from the jogs in the fault trace ( Fig. 1). Together with the Viernauer Fault (VFZ) to the south, the SFZ and the VFZ formed a dextral strike-slip fault zone. The Heßleser Fault Zone (HFZ) connects the two major faults at an acute angle of 30°. We interpret this as a Riedel R-shear (for definition of a Riedel shear, see Woodcock & Fischer (1986)), also dextral 15 in movement (Fig. 1). Most probably this movement took place during the Upper Cretaceous/Early Tertiary inversion phase in Europe (Tanner et al., 1998;Littke et al., 2008;Kley & Voigt, 2008;Tanner & Krawczyk, 2017). The southeastern part of the HFZ crosscuts the town of Schmalkalden. The fault zone contains several smaller fault branches that strike NW-SE (Bücking, 1906;Böhne, 1915;Krzywicki, 1937).

Seismic interpretation of S1
10 S H -wave reflection seismic profile S1 of ca. 350 m length, was carried out north of the sinkhole ( Fig. 4). At ca. 10 m to 25 m depth, a continuous reflector with high amplitudes is traceable throughout the entire profile (Fig. 6). The strong impedance contrast represents the boundary between the Triassic sandstones of the Calvörde Formation (suC) and the Permian claystones of the Fulda Formation (z7). This reflector, which can be found in all S H -wave reflection seismic profiles, was used as a marker horizon. In contrast, the area beneath shows a mostly discontinuous reflection pattern with no remarkable reflector, but instead, 15 lateral amplitude variations are observed due to strongly fractured strata within the seven Zechstein Formations. In

Seismic interpretation of S2
S H -wave reflection seismic profile S2 of ca. 400 m length, was carried out south of the sinkhole (Fig. 4). The marker horizon which represents the base of the Calvörde Formation is clearly visible at ca. 10 m to 15 m depth and is traceable throughout the entire profile (Fig. 7). The Permian deposits below show the same discontinuous and disrupted pattern as in profile S1 and numerous fractures were imaged. In the Staßfurt Formation, which includes to the subrosion horizon, a large LRZ is observed between 175 m and 300 m 5 profile length at ca. 70 m to 100 m depth below the depression.

Seismic interpretation of S3
S H -wave reflection seismic profile S3 of ca. 370 m length (including a 30 m gap around the sinkhole), was carried out from south to north, passing the sinkhole (Fig. 4). The reflection pattern is similar to S1 and S2.
The flat-lying, mostly continuous reflectors of the Quaternary and the marker horizon of the Triassic Buntsandstein can be 10 precisely identified (Figs. 8). The reflection pattern of the Permian is discontinuous due to vertical displacements of reflectors.
Several near-surface normal and reverse faults were identified with fault offsets of ca. 10 m to 20 m.

Seismic interpretation of S4
S H -wave reflection seismic profile S4 of ca. 190 m length, was carried out northwest of the sinkhole (Fig. 4). The Quaternary and the Triassic deposits were identified using the marker horizons and the stratigraphy of borehole 01/2010, which was projected onto the seismic line (Fig. 9). In the northeast and the southwest, within the discontinuous and displaced Zechstein 10 Formations, faults with vertical offsets of 5 m to 10 m were imaged, which are probably the same as those seen in S1, since S4 runs parallel to the western part of S1. In the same areas two almost bowl-shaped structures can be identified down to ca. 20 m depth, but they are not as good as visible as the depressions of S1 and S2.  Fig.4. The profile was surveyed from south to north across the sinkhole area leaving out the sinkhole itself. Just like in profiles S1 and S2 steep normal and reverse faults can be seen, but no bowl-shaped structure.
Instead the southern sinkhole margin is visible as a transparent area (blue circle) with near-surface reflectors dipping to the direction of the sinkhole.
In the Zechstein Formations z2 to z3 a large LRZ is observed between 60 m and 110 m profile length at ca. 60 m to 90 m depth.

Geological interpretation
Combining the information of the geological map, the seismic profiles and the boreholes give the following interpretation.
The subsurface below Schmalkalden has been affected by tectonic movements since at least the Mesozoic, when a NW-SE 5 striking, dextral strike-slip fault zone containing the SFZ and the HFZ formed. The latter crosses the subsurface of the town of Schmalkalden. In the area of the sinkhole, the strike-slip fault created a zone of reverse, normal and strike-slip faults, as imaged by the seismic profiles (Fig. 10).
We observe local thickening of the Triassic Calvörde Formation, which can only be accounted for by syn-tectonic sedimentation during the Triassic. These normal faults generated additional accommodation space for the terrestrial fluvial sediments. The stratigraphic units are explained in Fig.4. The profile was surveyed north of the sinkhole parallel to S1(1). The normal faults seen in S1 were also identified in S4 and two almost bowl-shaped structures (red circles) are visible in the southwest and the northeast.
This was part of the multi-phase tectonic deformation which has been recorded in southern Thuringia (Wunderlich, 1997b).
The extensional stress regime during the Upper Permian to the Lower Cretaceous and the compressional stress regime during the Upper Cretaceous to the Tertiary produced the dextral strike-slip zone, and the various strike-slip faults within it. The fine mosaic of fault blocks is mainly attributed to this latter phase.

Fault inventory 5
The complex 3-D structure of the faults is difficult to decipher with 2-D seismic lines, even if they are densely spaced. Nevertheless, large (5 m to 10 m) displacements of the reflectors were found at several locations along the profiles, and they are interpreted as near-surface normal and reverse faults, e.g., below the western margin of the depression structure, steep, northeast-dipping reflectors are visible in profile S1 at a depth of 50 m to 100 m. The borehole 05/2011 proved the existence of a fault in this zone, which was interpreted as a northeast-dipping normal fault. (apparent angle because they are measured in a 2-D section). This means the seismic profile is close to the true dip direction of the faults, because otherwise the faults would not be so steep. Consequently, the faults strike roughly NW-SE, similar to the strike of the HFZ (Figs. 4 and 10). There is a tendency for normal faults on the southwest end of the profiles to dip northeast and vice versa. Note also that a few faults are outside of the presumed fault zone.
Interestingly, reverse faults cannot be followed from one seismic section to another (e.g. S1 and S4, Fig. 10), meaning that 5 either the strike length of the fault is less than the distance between seismic sections (e.g. less than 50 m), which is most unlikely, or that a reverse fault in one profile correlates with a normal fault in another profile. The significance of this is described in detail in the next section.

Discussion
The fact that steep normal and reverse faults occur side by side, as seen in the four seismic sections, could be due to two mechanisms, both related to strike-slip faults. Either some of the normal faults that were generated under extension during the deformation phase from the Upper Permian to the Lower Cretaceous, were inverted by later compression during the deformation phase from the Upper Cretaceous to the Tertiary (Wunderlich, 1997b). For instance fault bends may switch from 5 transtensional to transpressional systems or vice versa, if the original fault bend was at a low angle relative to the maximum horizontal stress (Tikoff & Teyssier, 1994;Legg et al., 2007). Alternatively, if there are jogs along strike of a strike-slip fault, this will produce constraining and releasing bends (Cunningham & Mann, 2007), which will cause, after movement, reverse and normal faults, respectively, with similar strike to the strike-slip fault, along the strike of the strike-slip fault (Crowell, 1974;Christie-Blick & Biddle, 1985;Gamond, 1987, Fig. 11).

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The high fault density and the complex fault geometry in the research area ( Fig. 10) did not allow to make a direct spatial correlation of the faults, e.g. connecting the faults that were identified in two 2D seismic profiles. Only a high-resolution 3D shear-wave reflection seismic survey could deliver more or less unquestionable spatial correlations, but such a technique is still in development. Nonetheless, we were able to identify the 2D fault geometries and displacements from the 2D reflection seismic profiles. 15 The presence of a fault or a fault zone is not the only condition that has to be fulfilled for the occurrence of a sinkhole like the one in Schmalkalden. Faults can be classified as open or sealed faults. A fault seal due to clay smear or mineralization could hamper subrosion because it reduces fluid pathways (Caine et al., 1996;Evans et al., 1997;Ngwenya et al., 2000). On the other hand an open fault can serve as a fluid pathway.
During displacement along a fault especially the hanging wall undergoes deformation caused by the fault morphology 5 and the resulting strain variations. The variations in strike of secondary faults are the direct result of these strain variations (Lohr, 2008). These small strike variations can be observed in Schmalkalden, but besides the faults and fractures visible in the seismic sections subseismic scale deformation will have occurred too. Displacements along faults can result to a high fracture density around and between faults, creating a damage zone, which has the potential to increase the fluid flow due to enhanced permeability. The key factors for the increase in fracture density are the change in mechanical rock properties, 10 interactions between faults and a change in fault geometry (Bolas & Hermanrud, 2003;Gartrell et al., 2004;Leckenby et al., 2005;Eichhubl et al., 2009;Kim & Sanderson, 2010;Ziesch, 2016). The S H -wave reflection seismic profiles carried out in this study identified a complex local and regional fault system with a dense fracture network, which enables the groundwater to circulate through the evaporites and therefore it enhances subrosion due to an increase in permeability. Different joint sets were observed in outcrops in the vicinity of the sinkhole: (1) steep joints and fracture with a NW-SE striking, (2) flat joints and 15 fractures with a NW-SE striking, (3) NE-SW striking joints and fractures, (4) young NNE-SSW striking joints and (5) young NNW-SSE striking joints (Schmidt et al., 2013).
The sinkhole of Schmalkalden is located at the meeting point of three groundwater catchment areas, the first is to the north and belongs to the Gespringe Spring. The second is ca. 200 m south of the sinkhole and belongs to the confluence of the Schmalkalden River (Fig. 4) and the Stille River. The third is to the west and belongs to the Mittelschmalkalde River. 20 Four groundwater levels are found in the Quaternary gravel, the Lower Triassic sandstone, the Leine Carbonate (z3) and the Paleozoic bedrock. The latter consists of deep thermal, mineralized water, which is undersaturated with regard to sulfates. This is an indicator of a short residence time and a high flow rate, and the more undersaturated the water, the more sulfates can be dissolved. The main groundwater level situated in the Zechstein Formations actively leaches the soluble Permian deposits :: of ::: the ::::: Werra :: to ::::: Leine :::::::::: Formations :::::: (z1-z3). This Zechstein water ascends along faults and fractures and mixes with water from upper 25 groundwater levels, since no widespread vertical separation of groundwater levels is available due to tectonics and subrosion (Henke, 1983;Schmidt, 1995). The groundwater runoff follows the morphological gradient, and at the steep faults and the intersections of faults the artesian-confined groundwater can migrate upward and leach the soluble Permian deposits. Tracer test revealed a flow rate of 100 m h −1 to 150 m h −1 (Schmidt, 1995), which is a typical value for fractured and karstic aquifer (Ravbar et al., 2012).

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Groundwater table contour plans reveal a change in flow direction around the area of the HFZ (Fig. 12). To the north, the groundwater contour lines run from north to south with a flow direction from east to west and from west to east towards the Werra River. But in the area of the HFZ the contour lines run approximately east-west with a flow direction from north to south and from south to north towards the Schmalkalde River. This change in groundwater flow direction may be another reason for the occurrence of the sinkhole and can be correlated with the faults discovered in this study, because steep-dipping faults 35 are assumed to be barriers for horizontal groundwater flow perpendicular to the faults but serve as conduits for horizontal flow along the faults (Bredehoeft et al., 1992;Bense et al., 2003). The NW-SE striking fault branches of the HFZ hamper the groundwater flow coming from the east from the Thuringian Forest towards the Werra River; as a result the water flows from north to south along the faults towards the Schmalkalde River and thereby passes through the sinkhole area.
Several other studies regarding sinkhole distribution have shown the clustering of sinkholes along fault lineaments (Abelson 5 et al., 2003;Doctor et al., 2008) and the decreasing number of sinkhole occurrences with increasing distance from the fault (Hyland et al., 2006;Billi et al., 2007). A lineament of sinkholes can also be used to find hidden faults (Closson & Abou Karaki, 2009). This is the case in Schmalkalden since the only sinkhole in the urban area formed within the strongly fractured HFZ.
A single fault within the soluble rocks might not have influenced the groundwater flow direction and the upward migration of artesian-confined groundwater that much and might not have triggered a collapse due to subrosion. But since the soluble 10 rocks are located within a strike-slip fault zone with a typically strongly-fractured underground and increased permeability, as described above, the subrosion process is greatly enhanced.
ing area (Schmidt et al., 2013). This anomaly can be linked to the fault system which crosses the sinkhole area. The source for the gravimetric anomaly is located at a depth of 50 m to 100 m, which coincides with the subrosion horizon (Seidel & Serfling, 2010).

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In this study we used S H -wave reflection seismic to analyze subrosion features, and the link between faults, groundwater flow and soluble rocks.
Areas affected by tectonic deformation phases are prone to enhanced subrosion. The deformation of the fault blocks leads to the generation of a damage zone around and in between the faults with a dense fracture network, which enables the groundwater to flow through the subsurface and to leach soluble rocks. The more complex the fault geometry and the more interaction 20 between faults, the more fractures are generated e.g. subrosion in a strike slip-fault zone with steep normal and reverse faults will be enhanced more than in an area with just a simple normal or reverse fault.

Wadas for details.
Competing interests. The authors declare that they have no conflict of interest.
Katzschmann, Sven Schmidt and Ina Pustal from the Thuringian State Institute for Environment and Geology (TLUG) who provided the borehole information and helped improve the manuscript. :: We ::::: thank ::: two ::::::::: anonymous :::::: referees :: for :::: their :::::: helpful :::::::: comments. Appendix A: Shear-wave reflection seismic data Table A1 shows detailed information about the equipment, the acquisition parameters, and geometry used for the SH-wave reflection seismic survey carried out around the sinkhole of Schmalkalden. For detailed explanations regarding the equipment for the high-resolution shear wave reflection seismics, see Polom et al. (2010); Krawczyk et al. (2012Krawczyk et al. ( , 2013; Polom et al. (2013). 5 The data processing here was based on general processing procedures, as described by Krawczyk et al. (2012) and Pugin et al. (2013). Table A2 shows the processing steps applied to the SH-wave reflection seismic profiles. Most of the processing steps were carried out iteratively and were adjusted for each profile to get the best results. For detailed explanations regarding the processing algorithms see Hatton et al. (1986); Lavergne (1989); Baker (1999); Yilmaz (2001). Table A2. General processing sequence applied to SH-wave seismic reflection data. Most of the processing was carried out iteratively and each profile is individualized due to the differing data quality of the profiles.