Basement-Controlled Urban Fracturing: Evidence from Las Pilas, Zacatecas, Mexico

Abstract

The formation of fractures in urban areas is typically related to construction processes, natural ground settlement, and material quality. In valleys, the distribution of ground fissures is associated with aquifer overexploitation and basement faulting. However, where the soil layer is only a few meters thick or absent, the influence of basement structures remains poorly understood. We hypothesize that urban fractures develop parallel to major basement faults. To test this, we applied a simple structural geology technique to systematically measure extension axes, from street fractures, throughout the town of Las Pilas. These axis orientations were then compared with those calculated for normal faults of Las Pilas Complex. Street fractures are generally about 1 cm thick, with lengths ranging from 0.51 to 1 m and occasionally reaching up to 3 m. They occur within streets 2 to 4 m wide, typically appearing as a single fracture within a 1–2 m wide fracture zone. Based on these characteristics, the fractures do not represent a significant hazard. Measurement results indicate that urban fractures primarily extend in an NE-SW direction. This is consistent with the orientation of the minimum principal stress axis (3) of the regional San Luis-Tepehuanes fault system, thereby supporting our hypothesis.

1. Introduction

Land subsidence, which is the sinking or settling of the ground surface, has significant effects, including the formation of cracks and scour. These features affect agricultural, industrial, infrastructure, and urban areas [1]. This phenomenon is most thoroughly studied in valleys, where overexploitation of aquifers generates a mass deficit which promotes compaction of the sedimentary fill. Consequently, ground fissures align themselves with the underlying orientation of basement faults [2,3,4,5,6,7].

Subsidence monitoring is commonly carried out using techniques such as extensometers, geopositioning, precision topography, satellite images, InSAR, and Lidar, among others [6,8,9]. The formation and propagation of ground fissures costs millions of dollars annually in infrastructure and building repairs [1,10,11].

Given the critical nature of this issue, the United Nations Educational, Scientific and Cultural Organization (UNESCO) has organized symposia and working groups on it since the 1960s—the current effort in the “International Land Subsidence Initiative” (https://www.landsubsidence-unesco.org/). At a local level, several state governments systematically monitor subsidence, such as the State Government of Aguascalientes [12] or the Arizona Geological Survey [13].

Mitigation impacts and crack-control strategies have been studied for several decades (e.g., [14,15,16]). Therefore, understanding and addressing this problem is essential, as subsidence is a hazard capable generating risk areas. In this context, subsidence and ground fissures are continuously monitored in agricultural, industrial, and urban settings. Despite these efforts, significant challenges arise when the spatial scale narrows and the focus shifts to minor urban fractures affecting houses and infrastructure.

When the scale changes and attention turns to urban areas developed on young or advanced young mature landscapes with shallow soils, subsidence processes tend to be very slow. In such environments, fractures that form in walls and streets are often associated with ground settlement, the natural configuration of the urban landscape, vehicle traffic, or poor-quality materials and construction practices. Moreover, the lack of interest in these features may stem from their small dimensions, typically only a few meters long and a few millimeters wide. As a result, they are often not considered structurally related to the basement faults, nor are they recognized as a social issue.

Escalona-Alcázar et al. [17] proposed the hypothesis that basement faulting also influences the formation and distribution of fractures in sidewalks, streets, and walls. The orientation of fractures observed in urban infrastructure tends to be subparallel to major basement faults [17,18,19,20,21].

To provide further evidence supporting this hypothesis, we conducted a geological survey in the Las Pilas community, Zacatecas, located on the western edge of the Sierra de Zacatecas (Figure 1). The area lies within the morphogenetic region classified as “Erosion and Pluvial Accumulation” [22]. The dominant geomorphic agent in this region is wind, while water erosion is mild to moderate along the creeks [23,24]. The Arroyo Las Pilas forms the northern boundary of the community (Figure 2) and is oriented NW-SE with nearly perpendicular tributaries. Water erosion is scarce because most streams are intermittent [22]. Slopes along the main channel range from 6° to 15°; whereas in the central and southern areas slopes are less than 6° and appear to align with a normal fault.

The community is located about 1 km from the eastern boundary of the overexploited Calera aquifer. Consequently, no subsidence monitoring techniques have yet been applied in Las Pilas. In addition, basement rocks outcrop along Arroyo Las Pilas and within the community, resulting in poorly developed soils.

The objective of this study is to provide further evidence supporting the hypothesis that basement faulting influences the formation and distribution of fractures in urban infrastructure.

2. Geologic Framework

The study area is located on the western edge of the Sierra de Zacatecas, which is cut by two major fault systems, both composed of normal faults (Figure 1). The older system trends WNW-ESE to NW-SE and is part of the San Luis-Tepehuanes Fault System (SLTFS). This system formed during the Paleocene-Early Eocene, with intermittent reactivation during the Late Oligocene to Early Miocene, as well as from Pliocene to the Quaternary [25,26,27].

The second fault system is associated with the development of the Basin and Range tectonic province, and is active from the Early Oligocene to the present [28]. Faulting within this province trends NNW-NNE [28]. The Sierra de Zacatecas is situated within this province and forms part of a horst (Figure 1).

The stratigraphic sequence of the Sierra de Zacatecas has been described in several papers [29,30,31,32,33,34]. In the study area, the outcrops belong to the Las Pilas Complex, which is subdivided into rock-type and fabric groups [32]. Figure 3 shows the lithological units present in the Las Pilas community.

The Las Pilas Complex is composed of lava flows with compositions ranging from basalt to andesite, intruded by a dioritic laccolith [32]. Although massive structure predominates, other structures, such as pillowed or deformed, occur in small outcrops. Where the deformation is dominant, the rocks exhibit foliation and intense, non-preferential fracturing [32]. Finally, a portion of a dioritic laccolith is exposed at the NE corner of the study area (Figure 3).

Reverse faulting within Las Pilas Complex exhibits a complex deformation pattern, with fault orientations varying from NW to NE [32,34]. At the end of the Mexican Orogen [35], stress reorganization promoted the development of the WNW-ESE to NW-SE San Luis-Tepehuanes Fault System [25,27], which cut across central and northern portions of the Sierra de Zacatecas.

Since the Early Oligocene, extensional deformation has prevailed, leading to the formation of the Basin and Range tectonic province [28]. Within this province, the Sierra de Zacatecas is a horst bounded by normal faults that strike approximately N-S, with local variations toward NNE and NNW (Figure 1 and Figure 3). In the adjacent basins, basement normal faults display a domino-style configuration [5,7,28].

3. Materials and Methods

Because our hypothesis requires supporting evidence to be either confirmed or refuted, no standardized methodology exists for collecting necessary field data; the only applicable procedures are those proposed by [17,20]. These methods involve applying straightforward structural geology techniques to measure urban fractures in the same way that conventional structural data is recorded. Fractures were measured using a Brunton-type compass, a 5-m measuring tape, and a Garmin eTrex GPS unit with a positional error of ±3 m. The base map was generated from a Google Earth image (Corel Draw 2019, the license ends in DXX56) at a scale of 1:20,000. The coordinate system used was UTM Zone 13 N, datum WGS84. The types of fractures are illustrated in Figure 4 [17,19,20], and the measurements obtained for each structure are summarized in

Table 1. Data measured in each type of structure.

Structure	Number of Data
Street fracture	393
Sidewalk fracture	60
Wall fracture	5
Sinking	23

.

The Las Pilas community covers an area of approximately 0.3 km², which is sufficient to obtain comprehensive data across the entire urban zone. The observed urban fractures and their corresponding measured parameters for each type are detailed below:

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Street fractures (Figure 4a) are the predominant structural features. For fractures that deviate from a straight line, the azimuth is determined from the feature’s start point to its endpoint, which defines the fracture’s minimum length. The extension azimuth is perpendicular to the fracture’s strike, oriented toward the side exhibiting downward displacement or, in the absence of visible displacement, toward the downslope direction. The fracture zone width corresponds to the affected area within the street, such as the concrete slab or asphalt paving. The fracture aperture (width) is recorded as the maximum open dimension of the break. The fracture was georeferenced by its starting point according to the walking direction.

-

Sidewalk fractures (Figure 4b) share similar measurement parameters with street fractures; however, the sidewalk’s width is a uniform 1 m across the entire community. They were georeferenced in the same manner as the street ones.

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For Wall fractures (Figure 4c), the house front width is noted at the location of the most significant fracture, along with the fracture’s maximum thickness. If the fracture trace is irregular, the endpoints are used to determine its length and dip, with the extension azimuth being equated to the dip direction. In the case of vertical fracture, the extension azimuth is aligned with the walking survey path. This kind of fracture was georeferenced using the middle point of the wall where it is located.

-

The final category assessed was the subsidence of sidewalks and street blocks (Figure 4d), for which only the magnitude of vertical displacement (depth) was measured. The georeferencing point is the one with the maximum displacement.

Due to their prevalence, the analysis was restricted to fractures situated within streets and sidewalks. Excluded from the study were fractures induced by root intrusion, maintenance holes, drainage infrastructure, and fractures oriented perpendicular to the slope, as they form by downslope creep. Furthermore, fractures associated with building façade construction, specifically those at property boundaries or surrounding door and window apertures, were not considered.

The digital elevation model (DEM), used to calculate the digital slope model (DSM), was generated from the Zacatecas topographic map (F13B58) at a 1:50,000 scale [36]. The map featured contour lines spaced at 10 m intervals, and the subsequent calculations were performed using a 10 m grid resolution. Slopes were classified according to the official standard for evaluating geomorphological processes [37].

To assess the landscape morphology and its potential influence within the urban area, we employed the geomorphologic analysis proposed by [38]. The variables considered were dissection density and relief energy, both calculated following the methodology proposed by [39]. Specifically, because the only available topographic data are at a 1:50,000 scale [36], the study area was divided into 1 km × 1 km grid cells [38]. The following geomorphological parameters were calculated using topographic contours:

-

Relief energy is defined as the difference between the maximum and minimum elevations within each cell.

-

Dissection density is the sum of the length of all creeks within the cell [39].

Fault data were measured from mesostructures within the lava flows of the Las Pilas Complex. We employed the Right Hand Rule for systematic measurement, which dictates that if the plane dips toward the observer, the pinnule should point to the right. The parameters measured on the fault plane include azimuth, dip, slickenside rake, and displacement sense. The sense of fault displacement was determined following the criteria proposed by [40]. The kinematic analysis was performed using the Faultkin Ver. 8.3 software [41], following the approach of [42]. Faultkin calculates the paleostress axis from fault-field data and plots it on a stereographic equal-area projection [41]. Specifically, the cyclographic trace of the fault plane is plotted together with the rake and the pole to the plane. An auxiliary plane that passes through the rake and the pole is then used to determine the shortening and extension axes, which are positioned at 45° from the rake in accordance with the fault type [41,42].

Extension axes measured from street and sidewalk fractures were plotted in Stereonet Ver. 11.4 [41]. Using this software, the mean vector and standard deviation were calculated to determine the preferred extension trend within the study area. Stereonet calculates density contours based on the mean value, with intervals set at every two standard deviations. Although multiple intervals can be generated, the significance level is typically restricted to the first three [41].

4. Results

The fracture analysis focused exclusively on data collected from sidewalks and streets. Sink and wall fractures were omitted from the study because they accounted for only 6% of the total recorded structures (Table 1). Figure 5 illustrates the principal geometric characteristics of the sidewalk fractures. Most fractures measured 0.5–1 m in length, followed by those measuring 1–1.5 m (Figure 5a). The fracture thickness was predominantly within a 1 to 2 cm interval (Figure 5b). Within this zone, a single fracture was identified in the majority of the cases (65%), though two fractures occurred less frequently (23%) (Figure 5c).

To support our hypothesis, the extension axes measured in the sidewalks were plotted in a stereographic projection. The resulting mean value is 215°/00°, with an eigenvalue of 0.9514, indicating that the data are highly clustered around the mean (Figure 5d). This finding is further supported by the tight contour intervals, each 1σ (Figure 5d).

The geometric characteristics of fractures in the street pavement are presented in Figure 6. Typical fracture lengths range from 0.5 to 3 m (Figure 6a). The fractures were observed along streets varying from 1 to 4 m wide (Figure 6b), where the fracture zone is commonly 1–2 m wide and, less frequently, 2 to 3 m wide (Figure 6c). In most cases, only a single fracture is present (Figure 6d).

The fracture extension axes are plotted in Figure 6e, with azimuths ranging from 010° to 070° and corresponding antipodes from 200° to 250°. Although Figure 6e displays only a few visible dots representing the extension axes, it is important to note that overlapping occurs where multiple structures share the same extension azimuth.

Simple statistics were used to define the mean extension vector at 045°/00° with an associated eigenvalue of 0.9580. The tight clustering of the extension axes is also evident in the contour plot, which shows 1σ nearly identical intervals.

When comparing the trend of the street and sidewalk fractures, a parallelism is observed (Figure 7). The trend ranges from 310° to 320° in streets and from 290° to 310° in sidewalks. This parallelism is further supported by the mean extension vectors which are 215°/00° and 045°/00°.

The parallelism appears to be uninfluenced by the urban street layout (Figure 7), since both fracture types display a well-defined NW-SE azimuth (Figure 7), and their extension axes plot along the NE-SW direction (Figure 5 and Figure 6).

The principal geomorphic feature in the area is the Arroyo las Pilas (Figure 7), which trends NW-SE and defines the community’s northern boundary. The Digital Slope Model (DSM) was classified to evaluate geomorphological processes, drawing upon established methods [37,43]. Slopes less than 6° exhibit two preferred distributions: one forms a narrow zone along Arroyo las Pilas within the community, and the other covers most of the study area. Specifically, slopes between 3.01° and 6° form a trend parallel to the fault bounding the Calera Valley (Figure 7). Slopes ranging from 6.01° to 15° form a NW-SE band that parallels to the Arroyo las Pilas and extends beyond the community limits (Figure 7). Most of the urban area is situated within the footwall of the normal fault that borders the Sierra de Zacatecas, whereas the less urbanized area lies within the Calera Valley (Figure 7). Within the Sierra de Zacatecas, the main geological structures are NW-SE trending faults belonging to the SLTFS, a system to which the Arroyo las Pilas is parallel, thereby suggesting a structural control on its course.

To investigate a potential relationship between landscape with urban fracturing, we also analyzed dissection density (Figure 8a) and relief energy (Figure 8b). Although the 10 m contour interval restricts detailed analysis, it is sufficient for identifying general trends. Both variables were calculated using the geomorphologic model proposed by [38].

Dissection density is defined as the total length of creeks within a defined area [39]. In Figure 8a, the dissection density shows an NW-SE trend, running parallel to Arroyo Las Pilas, the slope distribution, and the observed urban fracturing. Values increase toward the NE near the Sierra de Zacatecas and decrease toward the SW into the Calera Valley.

Relief energy (Figure 8b) is the difference in elevation within the measurement area [39]. Relief energy can be interpreted as the ease with which exogenous processes act on the surface. Its distribution exhibits an N-S trend, running parallel to the normal fault that defines the western margin of the Sierra de Zacatecas. The highest values are found in the Sierra de Zacatecas and decrease toward the Calera Valley. Furthermore, in Figure 8b, the isolines of relief energy are parallel to the normal fault and appear to be related to the pattern of urban fracturing.

Our hypothesis suggests that basement faults influence the development of urban fracturing. If this is correct, the trends of basement faults and urban fractures, as well as their extension axes, should be parallel. To support this argument, we compare structural data from Las Pilas Complex with measurements taken within the study area and the nearby Hacienda Nueva (Figure 9).

Normal faults were measured along Arroyo las Pilas, and we also used previously reported data [32]. In both communities, the normal fault trends are parallel, reflecting regional deformation in the Sierra de Zacatecas, which is governed by the NW-SE-trending San Luis-Tepehuanes Fault System (SLTFS) [32].

The dominant SLTFS trend is clearly observed in the sidewalk fractures (Figure 9a), the street fractures (Figure 9c), and the measured normal faults from both Las Pilas (Figure 9e) and Hacienda Nueva (Figure 9g). While the natural structures exhibit a slight dispersion, this variation remains centered around on the primary regional faulting trend of the SLTFS.

Regarding urban fracturing, we measured the extension perpendicular to the structure’s orientation, ideally following the downslope direction. However, this ideal measurement was not possible in flat areas, which explains the bidirectional orientation roses. The dominant extension fractures on sidewalks vary from 020° to 040° (Figure 9b), with a mean vector at 215°/00° (Figure 5d) or, since they are horizontal, at 035°/00°, consistently indicating NE-SW extension. On streets, the dominant extension is between 040° and 050° (Figure 9d), with the mean vector located at 045°/00° (Figure 6e).

For basement faults, the extensional paleostress axis (σ₃) of the fault set is at 018°/08° in the Arroyo las Pilas areas (Figure 9f), while in Hacienda Nueva it is 045°/15° (Figure 9h). The slight dispersion observed in the faults of the Las Pilas Complex may be attributed to its complex deformation history, which includes at least two compressional and three extensional tectonic stages [32,34]. Nevertheless, the compelling parallelism between the trends of urban zone structures and the direction of the extensional paleostress axis strongly suggests that basement deformation significantly influenced the deformation observed in the urban area.

5. Discussion

Land subsidence associated with the aquifer overexploitation and the structural control exerted by the basement on crack formation is a well-known and widely studied phenomenon [e.g., [7,8,9]]. The structural control of the basement and its relationship with aquifer overexploitation have been reported in the grabens of Aguascalientes, Querétaro, Loreto and Villa Hidalgo [9,12,20,44]. Although there is currently no land subsidence monitoring within the study area, regional studies in grabens associated with the Basin and Range tectonic province have used remote-sensing technologies, such as InSAR, to document subsidence rates of up to 6 cm/yr. This rate has been observed in the northern part of the Aguascalientes graben, near the border with Zacatecas [9,45,46,47].

The study area lies outside the Calera aquifer, which is known for overextraction of water [48,49]. This condition has led to the formation of ground fissures approximately 10 km away from the study area, specifically to the SW [48] and NW [50].

In the study area, no ground fissures have been reported as a result of aquifer overexploitation, mainly because the community is situated atop the Las Pilas Complex, where the sedimentary cover or soil is less than 2 m thick. For this reason, any fracturing observed in the community is not directly associated with this process; instead, we propose that it results from the underlying basement structure. Monitoring is typically not performed in areas where subsidence effects are barely perceptible or observed only in a few isolated locations, as in the study area.

Short-range structural issues are often overlooked in urban infrastructure planning and are frequently attributed to unequal building settlement. When residential and urban infrastructure is constructed, cracks in walls and pavements are usually associated with material quality, construction standards, natural settlement, and foundation weakness. Furthermore, because these fractures are often around 1 m long and have an average thickness of 1 mm to 1 cm, they frequently do not receive the necessary attention [18,19]. Similarly, fractures in sidewalks and streets, which often exceed 10 m in length and occur in multiple locations, are often attributed to material and construction quality, slope direction, and vehicle weight.

If our hypothesis is correct, then the faults act as conduits that facilitate the settlement of the urban infrastructure, rather than transmitting stress. This process could be as follows: the natural landscape is modified, often by ‘cut and fill’ processes, to create flat surfaces for construction. Once construction begins, there is a natural settlement due to factors such as weight, construction procedures, material quality, vibrations from vehicle movements, and seismicity (though not in the study area), among other causes.

Moreover, in a non-active tectonic setting, such as the study area, these causes typically drive the settlement of urban infrastructure. However, as we propose in the hypothesis, this is not a random process; instead, settlement follows the major fault systems that cut the basement. Since faults are planes of weakness, in a modified landscape featuring a backfill and urban construction, these weakness surfaces are the locations where settlement is eased (or facilitated). An aspect not considered in this hypothesis, and whose value is unknown, is the effect of the amount of backfill material on this process.

The faulting associated with the Basin and Range tectonic province is oriented from NW to NE [28]. Normal faults within this system bound the Sierra de Zacatecas, producing a domino-like structure in the valleys (or grabens). Conversely, this style of deformation has little effect within the mountain ranges, or horsts [32,51]. Although the normal fault that crosses the community has an almost N-S orientation, it does not appear to control urban fracturing.

In the study area, reverse faults cutting Las Pilas Complex are found in the NW and NE quadrants. The normal faults exhibit a clear NW-SE tendency, and, to a lesser extent, a NE trend, with the extension axis directed NE-SW. The orientation of the normal faults in the study area and those reported by [32] is parallel to the fractures observed in sidewalks, streets, and walls. Moreover, this parallelism is also seen in the slope distribution (6.01° to 15°) and the preferred WNW-ESE trend of the drainage pattern in the central and northern parts of the Sierra de Zacatecas [38]. The primary NW-SE trend of the slopes and drainage is structurally controlled by the SLTFS [27], which also controls the main vein-fault systems of the central and northern parts of the Sierra de Zacatecas [29].

The distribution of fracturing adjacent to a fault zone is intrinsically associated with the deformation zone of the main structure [52]. The amplitude (or width) of this deformed zone depends on the fault’s dimensions, the rock’s mechanical resistance, and the duration of the stresses [52]. In the study area, the parallelism between the extension axis of the faults (σ₃) and the extension of the urban fractures suggests a structural origin related to SLTFS, originating from basement deformation (Las Pilas Complex). This argument is further supported by the NW-SE trend of the Arroyo las Pilas, which runs parallel to the SLTFS. Furthermore, even though no specific fault was mapped along the creek due to the sedimentary cover, the faults within the surrounding deformation zone share the same trend. The urban fracture trend is also parallel to the SLTFS, suggesting that these fractures reflect the effect of the underlying deformation zone.

Based on the arguments presented, the roles of geology, basement structure, and geomorphology must be considered when determining the origin of urban fracturing. Understanding this complex relationship is crucial to addressing the root causes of fractures in urban infrastructure. Furthermore, the evidence presented supports our hypothesis, suggesting that urban fracturing provides valuable insights into the main basement structures underlying areas already covered by cities.

6. Conclusions

In this paper, we present structural geology and urban fracturing data as arguments to support our hypothesis. To collect this information, we used a simple structural geology technique to obtain urban fracture data, including azimuth, fracture length, thickness, fracture zone width, and extension axis. These data were subsequently compared with measurements obtained from known faults to perform a kinematic analysis. Based on geometric relationships, we found that the extension axis derived from urban fractures is nearly parallel σ₃ as determined from normal faults in the San Luis-Tepehuanes fault system.

Although the study area is not located in an active tectonic setting, the faults constitute a zone of weakness. When the landscape is modified via cutting or backfilling, this fault-related weakness zone favors the non-random settling of urban infrastructure, causing it to align with the fault’s preferred trend. This settling is not solely attributable to the weight of the buildings; other factors, including, but not limited to, construction quality, traffic vibrations or seismicity in tectonically active areas, may also promote the fracture development. These elements, combined with the slope direction, dissection density and relief energy distribution, may contribute to the observed structural failures.

The methodology proposed here provides a framework for diagnosing the influence of basement structures on urban infrastructure. This approach is scalable from small towns to large urban centers across diverse tectonic and geomorphological conditions. Such analysis is critical, as structural influences pose a significant threat to inhabitants. While this study provides a robust starting point, further research is required to fully validate the hypothesis.