Lake Water Depletion Linkages with Seismic Hazards in Sikkim, India: A Case Study on Chochen Lake

Abstract

After the 2011 earthquake, lake water depletion has become a widespread issue in Sikkim, especially in regions classified as high to very high seismic zones, where many lakes have turned into seasonal water bodies. This study investigates Chochen Lake in the Barapathing area of Sikkim’s Pakyong district, which is facing severe water seepage and instability. The problem, intensified by the 2011 seismic event and ongoing local construction, is examined through subsurface fracture mapping using Vertical Electrical Sounding (VES) and profiling techniques. A statistical factor method, applied to interpret VES data, helped identify fracture patterns beneath the lake. Results from two sites (VES-1 and VES-2) reveal significant variations in weathered and semi-weathered soil layers, indicating fractures at depths of 17–50 m (VES-1) and 20–55 m (VES-2). Higher fracture density near VES-1 suggests increased settlement risk and ground displacement compared to VES-2. Contrasting resistivity values emphasize the greater instability in this zone and the need for cautious construction practices. The findings highlight the role of seismic-induced fractures in ongoing water depletion and underscore the importance of continuous dewatering to stabilize the swampy terrain.

1. Introduction

The Sikkim region, situated in the seismically active northeastern Himalayas, lies along the convergent boundary between the Indian and Eurasian tectonic plates, making it highly prone to earthquakes. This tectonic collision zone is characterized by intense crustal deformation, active faults, thrusts, and folding, which contribute to frequent seismic activity ranging from microtremors to moderate and strong earthquakes. Classified under seismic zone IV of the Bureau of Indian Standards (BIS) seismic zoning map, Sikkim is one of the most earthquake-prone areas in India. The devastating 6.9 Mw Sikkim earthquake of 18 September 2011 highlighted the region’s seismic vulnerability and its cascading environmental impacts. Notably, detailed investigations at key sites such as Poison Lake, Nagi Lake, and Dhap Pokhari revealed [1,2,3] that formerly perennial lakes in high to very high seismic zones have developed extensive fracture networks beneath their beds since the 2011 event. These fractures facilitate increased seepage, leading to seasonal drying and gradual lowering of water levels. The spatial coincidence of recent earthquake epicenters near these lakes supports the hypothesis that ongoing seismicity, including frequent microtremors, actively promotes subsurface fracturing and water loss. In contrast, lakes in the northern parts of Sikkim, classified as low-seismic-hazard zones according to microzonation studies [4], remain largely perennial. This stark difference underscores the linkage between seismic-activity-induced fracturing and the transformation of perennial lakes into seasonal water bodies, emphasizing the need for stringent building codes, continuous monitoring, and disaster preparedness measures to mitigate seismic and environmental risks in the region.

A study on Chochen Lake, located in the picturesque Bara Pathing area of Pakyong, stands as a vital natural reservoir, serving as a lifeline for local residents. This serene body of water, depicted in Figure 1, not only replenishes groundwater aquifers and springs but also contributes significantly to the thriving tourism activities in its surrounding vicinity. Unfortunately, in recent times, the lake has undergone a distressing transformation, succumbing to desiccation attributed to the seepage of its once pristine waters. The primary factors contributing to this concerning phenomenon are multifaceted. One potential cause lies in human interventions, particularly the construction of a boundary wall and leisure structures in the lake’s immediate vicinity. These human-made additions, while intended for recreational purposes, may have inadvertently disrupted the natural integrity of the lakebed, leading to seepage through compromised joints or fractures. Alternatively, seismic activity, notably the 2011 Sikkim earthquake, emerges as another plausible catalyst for Chochen Lake’s dwindling water levels. Discussions with the local community and on-site investigations suggest that the drying-up process commenced in the aftermath of this seismic event. The seismic activity may have induced subsurface fractures, altered the hydrogeological dynamics of the lake, and contributed to the persistent seepage issues. The consequences of Chochen Lake’s desiccation are not confined solely to the depletion of its water reservoir. The transformation of this once-vibrant lake into a swampy and marshy landscape, now adorned with flourishing vegetation, paints a poignant picture of environmental upheaval. This shift not only jeopardizes the delicate balance of the ecosystem but also has implications for the tourism industry that thrived on the allure of the pristine lake.

Water seepage commonly arises from subsurface fractures [5], which are breaks in rock formations caused by concentrated stress near cracks. These fractures exhibit heterogeneity and represent a form of discontinuity within the rock body. In the Sikkim region, these fractures are also a consequence of frequent microtremors and earthquakes. Although there may not be observable movement or displacement along these fractures, they play a pivotal role during earthquakes by significantly amplifying seismic intensity. This phenomenon has cascading effects, triggering the activation of pre-existing landslides, causing damage to infrastructure such as roads and buildings, and facilitating water seepage from lakes. In the context of Chochen Lake, these subsurface fractures, induced by seismic activity, likely contribute to the desiccation observed in recent times. Studies on tsunami and earthquake hazards along the southern coast of Peru [6] have shown how seismic events act as powerful forces that not only reshape the geological structure but also influence the hydrogeological dynamics of the landscape. Similarly, in Sikkim, seismic activity has impacted the stability of natural lakes like Chochen Lake. This highlights the need to understand these geological processes carefully to reduce the negative effects of earthquakes and protect the long-term stability of these fragile ecosystems.

Surface and subsurface rock fractures are of paramount importance to geologists and hydrologists. Surface fractures allow rainwater and runoff to seep in, while subsurface fractures, defined by their orientations, control deeper water movement and play a crucial role in enabling the movement of groundwater across diverse areas. If ignored, these fractures can threaten tunnels, reservoirs, dams, and roads, underscoring the need for detailed investigation and understanding.

Consequently, the investigation of the fracture system in subsurface rocks holds great significance for geophysicists and geologists [7]. Electromagnetic and electrical methods were used to map subsurface resistivity and assess the geological conditions [8,9,10]. Geophysical methods like GPR, seismic refraction, microgravimetry, and ERT are used to detect and map subsurface features such as cavities, voids, and fractures [11,12,13,14,15,16,17,18]. A study [19] in Tarimba (Goiás, Brazil) highlighted the effectiveness of ERT and VLF-EM in detecting a filled collapse sinkhole. A study in the Gunung Kidul district (Indonesia) used filtered VLF-EM data to identify a subsurface river in a karst region [20]. Similarly, in northwest Kermanshah City (Iran), VLF-EM data were analyzed using the Karous–Hjelt method and a custom inversion technique to locate a cave [21], demonstrating the method’s ability to detect karst features. Other research highlighted VLF-EM’s effectiveness in tracing groundwater flows contaminated by landfill leachate [22], where Fraser-filtered data, current density pseudosections, and 2D resistivity models revealed conductive zones linked to contaminated water moving through interconnected fractures.

The electrical resistivity method is widely recognized as an accessible and cost-effective geophysical tool for mapping the depth, location, and orientation of subsurface fractures. Electrical resistivity imaging has been applied for decades across diverse fields, including hydrogeology, mining, petroleum exploration, civil engineering, and archaeology. It is particularly effective in studying complex geological settings [23,24] and has also proven valuable for shallow subsurface investigations and environmental studies [25,26,27,28], as well as archaeological and soil surveys [29,30]. Among near-surface imaging techniques, electrical resistivity tomography (ERT) stands out as one of the most commonly used methods in civil engineering [31,32,33], enabling both 2D and 3D resistivity mapping [34,35,36]. Prior research has highlighted its application in identifying bedrock structures [37,38,39,40], detecting cavities and sinkholes [41,42], supporting geotechnical site investigations [43,44], assessing slope stability [45,46], and determining unknown bridge foundations [47,48]. The electrical resistivity method has proven highly valuable for detecting fracture zones in rock formations, as highlighted in earlier studies [49,50]. For instance, it has been successfully applied in Morocco’s Etherachidian Basin to explore groundwater resources [51]. Similarly, in Kadarta village, Hungary, this geophysical technique effectively identified fracture zones within carbonate rocks [52,53]. Horizontal resistivity profiling, visualized through contour plots, offers crucial insights into fracture locations. Fractures often contain loose sediments, water, or air, which produce resistivity contrasts relative to the surrounding material. In contour plots, closely spaced lines near these zones can indicate the presence of fractures [53]. To achieve greater accuracy in pinpointing these features, additional investigations such as Vertical Electrical Sounding (VES) are recommended. Using the Schlumberger array for VES enables deeper current penetration, making subsurface resistivity variations more evident at greater depths. Moreover, numerical techniques like factor analysis of VES data provide an added layer of confirmation for fracture mapping. This statistical approach has successfully delineated fracture aquifers in granite terrains, such as in Nalgonda district (Andhra Pradesh), India [54]. The integration of these methods will provide a clearer understanding of the spatial extent and properties of fractures within geological formations can be achieved.

Conventional resistivity surveys are typically conducted on the Earth’s surface using specific electrode arrays to generate apparent resistivity sounding curves, profiling data, or pseudo-sections. These outputs provide a qualitative representation of vertical and horizontal variations in subsurface resistivity. This method has long been applied in groundwater exploration, civil engineering projects, and environmental studies. Over the past decade, significant advancements in computerized data acquisition systems and the development of 2D and 3D inversion software have revolutionized the approach. As a result, resistivity imaging or tomography has emerged as a highly effective and widely adopted exploration technique. Numerous studies have demonstrated that by collecting a large volume of spatially distributed data and applying advanced 2D or 3D inversion algorithms, it is possible to generate accurate and detailed resistivity models of the subsurface [55,56,57,58,59,60,61,62,63].

Electrical resistivity is strongly influenced by the conductivity of subsurface materials and their relative proportions within a given volume of earth. Standard electrical techniques enable detailed mapping by continuously collecting profile-oriented data through either multi-electrode systems [64] or various mobile/pulled configurations [65]. In many cases, a 1D model with lateral constraints is adequate for quasi-layered sedimentary settings [66]. However, geological processes such as neotectonics, glaciotectonics, or other structural disturbances can disrupt sub-horizontal stratification, making 1D formulations insufficient for accurately representing the geophysical structure. In such scenarios, a 2D approach is required to provide a more robust representation of complex subsurface layering [67].

In the present study, multiple methods have been employed, including Vertical Electrical Sounding (VES) using the Schlumberger array, horizontal resistivity profiling with the Wenner configuration, and numerical analysis through the factor method. The key aims of this research are outlined as follows:

(a) To analyze the subsurface lithology of Chochen Lake using Vertical Electrical Sounding (VES) data, with a focus on the Schlumberger configuration for detailed geological interpretation.

(b) To accurately identify and map the location and depth of subsurface fractures in Chochen Lake by integrating horizontal resistivity profiling with the Wenner array method.

(c) To evaluate the feasibility of treating identified fractures to mitigate seepage-related water losses and enhance the hydrological stability of Chochen Lake.

This study seeks to improve understanding of the subsurface conditions of Chochen Lake, accurately identify the location and depth of fractures, and assess potential measures to reduce water seepage. The findings aim to support sustainable management and conservation of Chochen Lake, ensuring its role as a critical water resource for the Bara Pathing region in Pakyong.

1.1. Study Area

Sikkim, in northeastern India, lies between the latitudes of 27°05′ N 28°08′ N and the longitudes of 88°10′ E and 88°55′ E. It shares borders with Nepal to the west and Bhutan to the east. Covering about 7096 km², the state’s hilly terrain spans elevations from 240 m to 8484 m above mean sea level. Sikkim is divided into six districts: Gangtok, Gyalshing, Mangan, Namchi, Pakyong, and Soreng. Water scarcity is a major issue, especially in the southern and western regions, where people depend on springs and lakes for domestic and agricultural use. Forests cover approximately 3380 km², and nearly 44% of the area comprises scrub, alpine pastures, and permanent snow [68,69].

This study focuses on Chochen Lake in Bara Pathing, Pakyong district (Figure 1). Once a lake, it has now become a swampy wetland. Located at 27°15′33″ N and 88°42′03.54″ E, the site spans about 9000 m² at an elevation of 1370 m above mean sea level. Bara Pathing lies 30 km southeast of Gangtok and 10 km east of Pakyong town.

1.2. Geological Settings of the Study Area

Sikkim, in northeastern India’s eastern Himalayas, lies in seismic zone IV (high risk). Chochen Lake (Figure 1(II)), at 1370 m elevation, sits on a NE-SW trending ridge of metamorphosed rocks, quartzite, mica schist, and gneiss, of the Chungthang Formation, part of the Central Crystalline Gneissic (CCG) group. These schistose rocks show significant weathering, with dominant schistosity planes striking NE-SW. The Chungthang Formation, extending N-S, adjoins the Kanchenjunga Gneiss Formation, marked by banded migmatite, garnet–biotite gneiss, and mica schist (https://bhukosh.gsi.gov.in, accessed on 6 August 2021) [70]. Regional lineaments near the study area also trend NE-SW (Figure 1(II)).

2. Methodology

The electrical resistivity of rocks varies significantly due to intrinsic properties such as mineral composition, permeability, porosity, and the nature and salinity of pore fluids. These factors make resistivity a complex parameter, yet they provide critical insights into subsurface conditions. The analysis of resistivity anomalies helps in the detection of zones where values deviate from surrounding materials, so geologists can identify distinct strata, fracture zones, or potential seepage paths.

In this study (Figure 1B–E), resistivity surveys were conducted using the Aqua-meter CRM-500, electrodes, GPS, and other field tools to investigate subsurface features and potential seepage zones in fractured bedrock. The Aqua-meter CRM 500 is a 40 W electrical resistivity meter, which provides a selectable current range of 5–500 mA and a maximum voltage output of 500 V (1000 Vpp), allowing subsurface investigations up to ~600 m depth. With a 10 MΩ input impedance, high precision (±0.05 mV), ±2% accuracy, and a 6-second reading cycle, it is ideal for fracture mapping and seepage-zone assessment in complex geological settings. A Wenner 2D profiling method, selected for its high resolution and superior sensitivity to lateral resistivity variations, was employed to map horizontal discontinuities and delineate seepage-prone areas. These zones were identified as densely contoured regions in the resulting resistivity plots. To complement this, Schlumberger Vertical Electrical Sounding (VES) was utilized for its greater depth penetration and ability to capture vertical resistivity variations and layer thicknesses. This combination allowed for the estimation of fracture dimensions and the flow direction of seepage water, providing a comprehensive assessment of the subsurface hydrological behavior and fracture-controlled seepage dynamics in the study area.

Data Acquisition

The geo-electrical profiling survey was conducted using the Wenner electrode array configuration (Figure 2). Two profiling lines were established in the study area, with current electrodes (C1, C2) spaced 15 m apart and potential electrodes (P1, P2) spaced 5 m apart, based on the required depth of investigation. The alpha-Wenner setup was applied, yielding a geometrical factor (G) of 43.96, as determined by the standard relation for this configuration.

$$G=2\pi a$$

(1)

where a = spacing between successive electrodes.

Both profiling lines were aligned in a northeast-to-southwest orientation, with Line 1’s starting point set as the origin (0, 0). This reference point, located near the lake’s right corner adjacent to the viewpoint tower, was selected for its stability and served as the fixed baseline for the entire geophysical survey. Due to site constraints, the two survey lines were positioned 25 m apart, with Line 1 and Line 2 originating at coordinates (0, 0) and (0, 25), respectively. Spacing beyond 25 m was not feasible within the available area. For consistency, the inline direction was defined as the Y-axis and the crossline direction as the X-axis. Each profile line extended 98 m in length, while the crossline spanned 40 m across the survey area.

After acquiring the profiling data, resistivity contours were generated using Surfer software 22.1. Zones with closely spaced contour lines, indicative of potential seepage paths or fractures, were identified for further investigation. To validate these anomalies, Schlumberger sounding surveys were conducted at the selected locations using the factor method.

Two Vertical Electrical Soundings (VESs), labeled VES-1 and VES-2, were carried out using the Schlumberger array formation. These were strategically positioned along Wenner profiling Lines 1 and 2, near the zones of tightly clustered contours identified in the Wenner areal plots. The VES surveys were critical in confirming fracture presence and determining their depth.

In the Schlumberger method, current was injected into the subsurface through electrodes C1 and C2, while potential differences were measured using electrodes P1 and P2. The potential electrodes remained fixed as the current electrodes were symmetrically extended about the midpoint. For larger current rod spacings, the potential rod separation was also increased to maintain adequate voltage readings, keeping the ratio of current-to-potential rod spacing (L/l) within 3 to 20.

3. Results and Discussion

3.1. Electrical Profiling Survey

Table 1. Configuration of electrical profiling at Chochen Lake, conducted using Wenner method.

Sl. No.	Distance of Real Time Data Acquisition Point (m)	Resistivity (Ωm)
		Line 1	Line 2
1	0	252.14	606.02
2	7	190.28	499.26
3	14	183.06	232.04
4	21	150.40	119.63
5	28	151.03	99.85
6	35	170.81	83.52
7	42	169.87	64.68
8	49	236.12	54.32
9	56	307.09	44.58
10	63	277.26	45.53
11	70	216.97	56.20
12	77	160.45	70.02
13	84	146.63	76.30
14	91	169.24	102.67
15	98	212.26	151.97

summarizes the outcomes of the electrical profiling performed using the Wenner method.

Seepage and fracture zones were identified based on anomalies in apparent resistivity relative to the surrounding material. A contour map of the resistivity numbers, generated using the software Surfer (Figure 3(i)), highlighted two distinct areas, labelled A and B, where closely spaced contours indicate potential seepage or fracture zones.

Location A has been identified as a major fracture zone, distinguished by its extremely low resistivity values (less than 60 Ωm), which are indicative of enhanced conductivity and the presence of saturated or highly fractured materials. In this area, the resistivity contours exhibit numerous concentric closed loops aligned predominantly from the northeast to the southwest, particularly concentrated along the downslope portion of the lake. Such a pattern suggests significant subsurface structural disruptions that likely facilitate increased water infiltration and seepage. In contrast, Location B represents a comparatively minor fracture zone, as evidenced by the presence of smaller closed contours, primarily concentrated towards the upslope region of the lake. This area appears relatively drier than Location A, pointing towards limited groundwater movement and reduced seepage potential. The observed contrast between these two zones underscores the spatial variability in fracture intensity and hydrogeological behavior, which may be critical in understanding water retention, drainage patterns, and the overall stability of the lake system.

3.2. Electrical Sounding Survey

Two Vertical Electrical Soundings using Schlumberger array configurations were carried out over different seepage locations as indicated in Figure 3(iv), namely VES-1 and VES-2. Corresponding data are shown in

Table 2. VES at Chochen Lake employing the Schlumberger method.

Sr. No.	Electrode Spacing		Geometric Factor, GF (K)	VES-1 (Line No.-1)		VES-2 (Line No.-2)
	AB/2	MN/2		R (Ω)	ρa (Ωm)	R (Ω)	ρa (Ωm)
1	2	1	4.71	33.3	156.84	20.8	97.97
2	4	1	23.55	6.54	154.02	1.91	44.98
3	6	1	54.95	3.54	194.52	0.738	40.55
4	8	1	98.91	2.26	223.54	0.468	46.29
5	10	1	155.43	1.49	231.59	0.362	56.27
6	12	1	224.51	0.996	223.61	0.306	68.70
7	14	1	306.15	0.69	211.24	0.269	82.35
8	16	1	400.35	0.51	204.18	0.245	98.09
9	18	1	507.11	0.421	213.49	0.227	115.11
10	20	1	626.43	0.35	219.25	0.218	136.56
11	20	2	310.86	0.729	226.62	0.423	131.49
12	22	2	376.8	0.63	237.38	0.401	151.10
13	24	2	449.02	0.566	254.15	0.38	170.63
14	26	2	527.52	0.512	270.09	0.361	190.43
15	28	2	612.3	0.474	290.23	0.343	210.02
16	30	2	703.36	0.44	309.48	0.329	231.41
17	32	2	800.7	0.404	323.48	0.318	254.62
18	32	3	531.18	0.639	339.43	0.475	252.31
19	34	3	600.26	0.564	338.55	0.46	276.12
20	36	3	673.53	0.531	357.64	0.447	301.07
21	38	3	750.98	0.499	374.74	0.435	326.68
22	40	3	832.62	0.468	389.67	0.394	328.05
23	42	3	918.45	0.44	404.12	0.405	371.97
24	44	3	1008.46	0.412	415.49	0.388	391.28
25	46	3	1102.66	0.38	419.01	0.369	406.88
26	46	4	824.25	0.518	426.96	0.496	408.83
27	48	4	898.04	0.474	425.67	0.47	422.08
28	50	4	974.97	0.442	430.94	0.44	428.99

.

The apparent resistivities plotted against AB/2 were analyzed on a log-log scale using IPI2Win software 3.0.1 to qualitatively interpret the field curves (Figure 3(iv(a,b)). This software facilitated a detailed assessment of subsurface conditions, allowing insights into the number, width, and resistivity of individual layers. In all log-log plots, fracture signatures are evident, marked by scattered data points deviating from the expected trend. These deviations, highlighted with dotted blue circles, indicate zones of anomalous resistivity associated with fractured lithologies.

The sounding data revealed pronounced fluctuations and abrupt shifts in resistivity values, reflecting the influence of the underlying lithological units, primarily mica schist and gneiss. Mica schist, characterized by its foliated structure and higher susceptibility to weathering, tends to exhibit lower and more variable resistivity due to enhanced water retention and fracture development. In contrast, gneiss, with its relatively massive and competent nature, displays comparatively higher resistivity but also shows localized anomalies where fractures disrupt its integrity. The evident scatter of data points within the blue-circled regions (Figure 3(iv(a,b)) underscores the critical role of these lithologies in modulating the electrical response. These variations highlight the heterogeneous condition (heterogeneous conditions refer to subsurface variations in lithology, moisture content, and structural features that influence electrical resistivity) of the subsurface, where the interplay between fracture density and lithological composition governs the resistivity behavior. Understanding these patterns is essential for identifying and characterizing fracture zones, thereby providing crucial insights into the subsurface geological framework and its influence on seepage dynamics in the study area.

3.3. Numerical Study of VES Data

The factor method is a key statistical approach used to analyze variability among correlated observed variables by expressing them in terms of fewer underlying, unobserved variables known as factors. In this study, the method is applied to apparent resistivity data to enhance interpretation.

The computation involves calculating the ratio of the apparent resistivity at a given AB/2 spacing (representing the distance between the current electrode and the center point) to the cumulative apparent resistivity of all preceding spacings. This ratio, termed the “factor,” is determined using the following formula:

$$\mathrm{F}{\rho }_a^n={\displaystyle \frac{{\rho }_a^n}{{\sum }_{{\rho }_a^1}^{{\rho }_a^{n-1}}.}}$$

(2)

Essentially, the factor serves as a quantitative indicator that defines the relationship between the apparent resistivity measured at a particular electrode spacing and the cumulative resistivity values obtained up to that distance. This parameter plays a critical role in analyzing and interpreting variations within the apparent resistivity dataset. Highlighting subtle patterns and trends, it offers valuable insights into the distribution and behavior of subsurface materials. Moreover, the factor helps to distill complex and interrelated resistivity observations into a more manageable form, enabling geoscientists to better characterize and differentiate the underlying geological structures. This, in turn, facilitates a more comprehensive and precise understanding of subsurface conditions, which is vital for applications such as groundwater exploration, fracture zone identification, and geotechnical assessments.

In Formula (1), (n − 1) and n indicate the AB/2 positions. The above equation is the generalized equation for calculating factor value. The VES results were utilized to compute the factors, and the calculated data are presented in

Table 3. Demarcation of fractures through numerical factor (F) analysis of VES Data.

Sl. No.	AB/2 (m)	VES-1, ρa (Ωm)	F-1	VES-2, ρa (Ωm)	F-2
1	2	156.84	-	97.97	-
2	4	154.02	0.981981982	44.98	0.459134615
3	6	194.52	0.625757576	40.55	0.28369028
4	8	223.54	0.442311277	46.29	0.252258727
5	10	231.59	0.317717757	56.27	0.244855292
6	12	223.61	0.232805374	68.70	0.240162018
7	14	211.24	0.178396697	82.35	0.232142857
8	16	204.18	0.14632615	98.09	0.224395237
9	18	213.49	0.133471336	115.11	0.215086978
10	20	219.25	0.120929926	136.56	0.209994423
11	20	226.62	0.111508276	131.49	0.167109277
12	22	237.38	0.105088084	151.10	0.164527726
13	24	254.15	0.101809253	170.63	0.159545029
14	26	270.09	0.098199123	190.43	0.153565094
15	28	290.23	0.096086025	210.02	0.146812371
16	30	309.48	0.093476691	231.41	0.14105402
17	32	323.48	0.089354134	254.62	0.136019961
18	32	339.43	0.086067595	252.31	0.11864728
19	34	338.55	0.079042094	276.12	0.116071664
20	36	357.64	0.077383896	301.07	0.113396317
21	38	374.74	0.075259182	326.68	0.110510682
22	40	389.67	0.072779643	328.05	0.099932502
23	42	404.12	0.070357947	371.97	0.103016423
24	44	415.49	0.067582345	391.28	0.098243943
25	46	419.01	0.063841205	406.88	0.093021734
26	46	426.96	0.061148591	408.83	0.085511981
27	48	425.67	0.057450731	422.08	0.081328966
28	50	430.94	0.055001548	428.99	0.076443014

. Analysis of this data indicates the presence of fracture zones at different depths.

The log-log plot presented in Figure 3(iii(a,b)), commonly termed the factor plot, depicts the relationship between the calculated factor (vertical axis) and AB/2 (half the current electrode separation, horizontal axis), with both axes on a logarithmic scale. In these plots, the highlighted red boxes mark discontinuities or “broken segments” in the curve, which are diagnostic of subsurface fractures. Such disruptions indicate resistivity contrasts associated with heterogeneities in the geological media, typically resulting from fracture zones that enhance fluid movement and alter electrical properties.

In VES-1, multiple curve discontinuities are observed within the AB/2 range of 17–55 m, suggesting a high density of fractures within this depth interval. Similarly, VES-2 exhibits comparable anomalies beginning at depths greater than 20 m, extending to approximately 55 m, though with less pronounced deviations, implying relatively lower fracture intensity compared to VES-1. These AB/2 values, which approximate the investigation depth, provide a reliable indication of fracture occurrence and distribution within the subsurface.

The application of the factor method effectively delineates these fractured zones, offering critical insights into the structural controls on seepage and subsurface hydrodynamics in the Chochen Lake area. This approach underscores the utility of factor analysis in identifying vertical and lateral variability in fractured bedrock, thereby contributing to a robust characterization of the hydrogeological regime in seismically active terrains.

4. Interpretations

The integrated Wenner profiling and Schlumberger VES datasets (Figure 3(i,ii) and Figure 4) delineate two primary fracture (seepage) locations, A (downslope) and B, within Chochen Lake’s catchment. In the Wenner 2D profiles, pronounced lateral resistivity contrasts at offsets of ~50 m and ~75 m correspond to narrow zones of anomalously low resistivity (<100 Ωm), which could be high–porosity fracture networks facilitating preferential fluid migration. This interpretation aligns with established hard rock resistivity responses, wherein fluid-filled fractures sharply reduce apparent resistivity relative to intact host rock [71]. The spatial coincidence of these anomalies with surface seepage at A and B confirms their hydrogeological significance.

Quantitative 1D inversion of VES 1 and VES 2 (Figure 4) reveals a stratified subsurface architecture: an upper silt–clay veneer (159–329 Ωm, thickness 0.68–1.17 m) underlain by a more conductive clayey horizon (9.98–68.9 Ωm; thickness 0.68–2.5 m), above the resistive bedrock interface (>200 Ωm) representing weathered-to-semi-weathered mica schist and gneiss. The progressive increase in surficial layer thickness upslope suggests active surface recharge and sediment trapping in low-energy depositional settings, characteristic of monsoonal alluvial processes. Below ~1.75 m (VES 1) and ~2.5 m (VES 2), the transition to high-resistivity bedrock indicates the depth of complete weathering, beyond which fracture controlled porosity dominates hydraulic connectivity.

Factor curve discontinuities on log-log plots, manifesting as “breaks” in the factor versus AB/2 trend between 17 and 50 m at VES 1 and 20 and 55 m at VES 2, provide a robust means of fracture detection [72]. These depth-correlated anomalies reflect discrete fracture sets with variable aperture and infill, which modulate the apparent resistivity through enhanced fluid saturation and altered lithological contrast. The greater fracture density and deeper penetration at VES 1 explain the intensified seepage observed at A, as well as the differential settlement recorded over a mere 25 m traverse. Such localized subsidence underscores the need to integrate geophysical findings with geotechnical monitoring to forecast progressive deformation in seismically active zones.

Given Chochen Lake’s gentle slope and resultant swampy terrain, maintaining a controlled groundwater table via continuous dewatering at A is essential to reduce effective stress and forestall further subsidence. We therefore advocate the exclusion of any heavy construction or surface loading within ±20 m of the identified fracture corridors (A and B) to prevent exacerbation of fracture dilation, pore pressure build-up, and consequent ground instability. This precautionary buffer, combined with periodic resistivity re-surveying, will support sustainable land use and safeguard the integrity of the lake basin in this high seismicity setting.

5. Conclusions

Major conclusions of the work are as follows.

• Seismic activity, particularly in high-hazard zones, plays a critical role in the formation of subsurface fractures, leading to increased lake water seepage and the transformation of perennial lakes into seasonal ones.
• The analysis of geo-electrical profiling and Schlumberger sounding data successfully identifies seepage and fracture locations at points A and B within Chochen Lake. Notably, location A emerges as a prominent seepage site, particularly in the downslope direction, underscoring its significance in understanding the dynamics of subsurface water movement in the lake.
• The Chochen Lake subsurface exhibits a stratified composition comprising layers of silt and clay, characterized by varying resistivity values and thickness. Of particular significance is the silt and clay mixture layer, influenced by surface flow recharge, which plays a crucial role in the dynamics of the lake depression. Below this layer, another distinct clay stratum introduces complexity to the subsurface, showcasing geological characteristics that differ from those of the overlying silt and clay mixture. The varying thickness of this clay layer adds to the overall heterogeneity of the subsurface, shaping the geological landscape and influencing factors such as water movement and seepage dynamics in the lake area.
• Vertical Electrical Sounding data indicate the presence of weathered to semi-weathered schist layers below specific depths at both the VES-1 (below 1.75 m) and VES-2 (below 2.5 m) sites. The presence of weathered to semi-weathered schist layers can impact factors such as permeability, stability, and water flow characteristics in the subsurface. Additionally, these layers may contribute to the overall heterogeneity of the geological formations, influencing the behavior of groundwater seepage patterns, and the potential for fractures in the studied area.
• Analysis of VES data at VES-1 and VES-2 reveals variations in the thickness of weathered to semi-weathered layers, indicating the presence of fractures. The excessive settlement at VES-1, located near the toe area of the slope, poses a risk of further displacement and emphasizes the need for cautious measures, such as avoiding construction activities near Chochen Lake, to mitigate potential damage and enhance regional stability.