Resistivity Distribution of Lembang Fault Based on Magnetotelluric Data

The Lembang fault located 15 Km north of Bandung City is one of the active faults situated on Java Island, Indonesia. The Lembang fault is an extension of the Cimandiri fault that can be one of the potential sources of earthquakes in Indonesia. In this study, the Magnetotelluric method was used to model the subsurface resistivity at the Lembang Fault area. The data acquisition was conducted in 8 points of measurement forming a line perpendicular to the Lembang Fault. The analysis was performed using 1D inversion of apparent resistivity and phase with the frequency range of 1 Hz to 320 Hz. The contrast apparent resistivity has observed in two sites around the fault. The inversion results show resistivity contrasts around the LMB02 point that is suspected to be the Lembang Fault. These results are consistent with geological data in the Lembang Fault area. Aside from the LMB02 point, there are also resistivity contrasts around the LMB05 point. However, from a geological perspective, it is not yet known whether the area around the LMB05 point represents a fault.


Introduction
Indonesia is located between three tectonic plates, which are the Australian, Eurasian, and Pacific plates.Oceanic plates are moving vertically through the Java Trench at a relative speed of 67 mm/year [1].According to Simandjuntak and Barber (1996) [2], this convergence is thought to cause an approximately north-to-south shift in horizontal compressive pressure, influencing the compaction of active faults in Java.Java Island is situated in the region where the Indo-Australian plate subducts beneath the Sunda region, on the northern side of the Java subduction zone.This happened 45 million years ago on the south side of the Eurasian plate [3].Faults develop as these plates move.One of the faults in West Java is The Lembang fault.
The Lembang fault is situated around 15 km north of West Java's capital city Bandung.The Lembang Fault is a normal fault type that is still active.It runs through the city of Lembang for 29 km from west to east.Volcanism is hypothesized to have an impact on the Lembang fault's activity in addition to tectonic forces since it alters the fault's mechanism of movement.The original creation of the Lembang fault was connected to the activity of the Sunda Mountains, according to Bemmelen (1949) and Tjia (1968) [4,5].Tjia added that the behavior of the Lembang fault exhibits a strike-slip movement rather than being entirely normal.This contradicts Bemmelen's assertion that the Lembang fault's movement is normal.However, Bemmelen's perspective on the Lembang fault is nearly identical to that of Nossin (1996) [6], who emphasized that because the Lembang fault is a volcanic structure that was created by gravity and its movement is normal.
To identify the presence of a fault, a geophysical method is needed.One of the methods is magnetotelluric, a technique for imaging the electrical conductivity and structure of the Earth [7].The resistivity structure under the Earth's surface is the physical parameter that we want to comprehend by using magnetotelluric.The frequency of the electromagnetic waves and the resistivity structure of the earth determine how far into the earth's interior electromagnetic waves can go.The magnetotelluric approach is one way to locate the Lembang fault.Tikhonov first put forth the fundamental theory of magnetotellurics in 1950, and Cagniard clarified it in 1953 [8].Both explained that information regarding the propagation of electromagnetic waves in the Earth's medium can be obtained by simultaneously measuring the elements of the electric and magnetic fields perpendicular to each other on the planet's surface.Because it makes use of sources from the Earth's magnetic field that change over time, magnetotellurics is referred to as a passive method.The frequency range of the natural fields produced by the Earth and employed in magnetotelluric research is 0.0001 Hz to 10 kHz [9].

Study Field
This study was conducted in the Lembang fault region in the Lembang districts, of West Java, Indonesia.Lembang district's elevation ranges from 1312 m to 2084 m.Tangkuban Parahu's summit is the point that is the highest.Lembang district, which has a total size of 9556 ha, is located at coordinates of -6.81169° south latitude and 107.6175° east longitude.
The subsidence of Java Island's oceanic crust, Java's middle-arc volcanism, and volcanic activity in the last two to three million years all had an impact on the geological events that took place in the Bandung Basin [27].According to Marjiyono and Karmawan (2008), the Lembang fault is categorized as an active fault and is a source of vibrations that cause earthquakes to be felt [28].This is proven by the cliff heights from east to west, which vary from about 450 m at the eastern end (Maribaya, G. Pulusari) to about 40 m at the western end (Cisarua) and disappeared at the northwestern tip of Padalarang.The location of the Lembang fault is shown in Figure 1.
There are roughly five geological units that are directly connected to the Lembang fault, including: a) The Qvu Unit, which has been defined as the byproduct of an ancient, insoluble (volcanic) volcano made up of breccia, lava, and lava, is the oldest geological unit connected to the Lembang fault.b) The Qyt unit is described as pumice tuff composed of solid basalt fragments, bombs, hollow lava, volcanic rocks, and tuffaceous sands.c) The Qyl unit is a recent lava flow that primarily originated from Mount Tangkuban Parahu and Mount Tampomas.The majority of this lava is basalt and has many pores.d) The Qyd unit is defined as a sandy tuff with breccia, worn lava, coarse hornblende crystals, and volcanic structures.e) The igneous rock blocks known as Qc, which are found between andesite-basalt, breccia, tuff sandstone, and tuff plates, are thought to have formed from ancient volcanically generated mountain ruins.This unit is the youngest one connected to the Lembang fault.
Mount Tangkuban Perahu and Mount Burangrang's eruptions contributed to the formation of the Lembang Fault.The surface of Lembang moved lower due to a cavity formed by volcanic material that was thrown from the volcano.increased in the south and somewhat decreased in the north [5].The Lembang fault's slip plane, which has a cliff (straight slope) with a wall pointing north, was created as a result of this tectonic process.
As seen in Figure 2, the data acquisition for this study included 8 measurement points.At these measurement points, the elevation of each point varies, ranging from 900 to 1500 m.It is hoped to ascertain the structure and presence of the Lembang fault.

Magnetotelluric Method
Magnetotelluric is a passive electromagnetic method that measures natural variations of electric and magnetic vector fields at the Earth's surface to map subsurface electrical conductivity/resistivity structure [29,30].The formulation of the magnetotelluric approach is based on the electrodynamic phenomena described by Maxwell's equations.Maxwell's equations are a set of fundamental equations that govern all macroscopic electromagnetic phenomena [31].Through the following first-order differential equations [32], Maxwell's equations analytically explain the relationship between the electric and magnetic field components and the physical properties (conductivity, permittivity, and permeability) of the medium under investigation.The surface impedance tensor is produced by comparing the electric and magnetic fields at the earth's surface.The earth's surface can then be used to determine apparent resistivity, which is derived from the surface impedance tensor [33].
The impedance tensor Z, which is the ratio of the electric field vector and the magnetic field vector perpendicular to each other on the earth's surface, is a crucial component of the magnetotelluric approach.A medium's impedance indicates how resistant it is to being inducted by electromagnetic fields.The impedance parameter of the medium can be expressed mathematically as a function of the ratio of the field components as with [34]  ⃗ =  0  − (6) So, the impedance becomes To calculate the apparent resistivity, we take the absolute value of the impedance.
The apparent resistivity value in the magnetotelluric method is calculated using this formula.

Result and Discussion
After data acquisition, data processing was carried out to obtain sounding curves for each data point.Figure 3 is an example of the sounding curve of LMB02 and LMB03.At shallow depths, the curves show a sharp contrast in resistivity values where the resistivity at point LMB02 is substantially lower than at LMB03.Due to LMB02's vicinity to the Lembang Fault, this phenomenon is thought to be caused by a reduction in resistivity brought on by the fault's presence in the area.A useful geophysical indicator of subsurface geological characteristics can be provided by this variation in resistivity.In the Lembang fault region, the apparent resistivity ranges from 1 Ohm.m to 1000 Ohm.m.Field data errors often have low levels, especially for data with frequencies greater than 0.1 Hz.On the other hand, data with frequencies lower than 0.1 Hz show higher inaccuracies.Numerous causes can be identified for this occurrence.The Earth's subsurface consists of a variety of geological elements, each of which has unique electrical conductivity characteristics.Signals produced at the Earth's surface and transmitted via these subsurface layers are used in magnetotelluric studies.Deeper layers often have higher resistivity (lower conductivity), which makes it harder for electrical current to flow through them.As a result, impulses that penetrate these deeper levels gradually become weaker.
Attenuation also contributes significantly.Electromagnetic waves interact with a variety of geological elements as they travel through the Earth's subsurface, including layers with varied conductivity qualities, faults, fractures, and mineral deposits.As the signal travels deeper below the Earth, these properties cause signal attenuation, scattering, and dispersion.
The 1D Occam inversion method was then used to execute inversion on the resulting sounding results.A mismatch value of about 5% from the inversion results indicated a successful outcome.For resistivity or magnetotelluric problems, Occam inversion is renowned for its stability and often converges after five or six rounds [36].An illustration of the acquired inversion results at point LMB02 is provided in Figure 4.The outcomes of the 1D inversion models are used for interpretation in a 2D view inversion has been performed on all data points.These findings will show the properties of the subsurface around the Lembang Fault.As part of the interpretation process, resistivity and depth data are combined from various sounding stations.The results of the inversion are then used to guide interpretations.
The 2D view was conducted based on the 1D inversion results of magnetotelluric data in Transverse Electric or TE mode shown in Figure 5.The interpretation of several 1D inversion results revealed a directional trend from south to north, spanning approximately 15 km.In the 2D interpretation results, variations in resistivity values are depicted using colors ranging from purple, blue, green, and orange, to red.Low resistivity levels are represented by purple and blue, moderate resistivity values are represented by green, and high resistivity values are represented by orange and red.
The near-surface region often has a layer with a shallow depth (< 50 m) and low resistivity (< 100 Ohm.m).This layer is considered to be a soft rock-like sedimentary material.There is a zone with high resistivity values of about 1000 Ohm.m. at depths below 100 m in the southern section (up to point LMB-03).This zone, which is congruent with the geological structure of the region, which consists of unweathered old volcanic rock formations, including breccia, volcanic ash, and lava (Qvu), can be interpreted as more compact rock formations.While relatively low resistivity values (< 200 Ohm.m) predominate in the center of the research region, corresponding to the Qyd unit, which is made up of weathered volcanic ash, tuff sand, and coarse hornblende crystals, and layers of lapilli and breccia.
Based on the results of magnetotelluric modeling, the presence of the Lembang Fault is evident, characterized by contrast in resistivity around points LMB02 and LMB03.The resistivity contrast, with significantly high values to the north and low values to the south, suggests that the hanging wall is to the north and the footwall is to the south.This indicates the possibility that the Lembang Fault is a normal fault, where the northern block has undergone subsidence.In the TE mode interpretation results, there is also a drastic decrease in resistivity between points LMB06 and LMB05.Based on the geological structure map and cross-section, there is a fault structure between points LMB06 and LMB05, but it is yet to be determined whether this structure represents a fault or another geological feature.
The presence of a conductive zone (low resistivity) in the fault area can be explained as a fluid zone.This fluid zone is a result of faulting, which causes fractures and allows the entry of fluids into these fractures.The influx of fluids alters the rock structure around the fault zone [37].This interpretation provides valuable insights into the subsurface geological features, fault characteristics, and the potential presence of fluids within the Lembang fault system, contributing to our understanding of the geological dynamics in the region.Furthermore, for better interpretation of the field, using 3 Dimensional Magnetotelluric modelling needed to get more accurate results of the geological structure [38].

Conclusion
Data acquisition and model interpretation were conducted to delineate the Lembang fault structure.Data acquisition was carried out in the Lembang sub-district, with data acquisition points distributed from LMB01 to LMB08.This study focused on mapping the Lembang fault structure in the region.Based on the modeling results, the Lembang fault structure can be identified based on its resistivity values, which are characterized by a significant contrast in resistivity values.This Lembang fault is located around points LMB02 and LMB03.The resistivity-contrast, with high resistivity (around 1000 Ohm.m) in the northern part and low resistivity (around 100 Ohm.m) in the southern part, indicates the presence of softer rock types in the northern region, suggesting it has undergone subsidence.The existence of a low resistivity zone within the Lembang fault can be interpreted as a fluid-filled zone resulting from movement and fracturing.
These results highlight the value of thorough geological research in comprehending the complexity of fault systems like the Lembang fault.Such information is essential for accurate risk assessments and mitigation plans in earthquake-prone regions, adding to the body of knowledge in the seismology and geological sciences.

Figure 2 .
Figure 2. The location of the measurement sites.

Figure 3 .
Figure 3. a. Sounding curve at point LMB02, b.Sounding curve at point LMB03.Red-colored data represents Transverse Magnetic (TM) mode sounding results, while blue-colored data represents Transverse Electric (TE) mode sounding results.There is a difference in resistivity values at these two points.

Figure 4 .
Figure 4. Inversion results of 1D model for point LMB02 in a. TE mode and b.TM mode.The blue line is the result of normal method inversion, and the red line is the result of occam method inversion.

10thFigure 5 .
Figure 5. 2D view calculated from 1D inversion results of the TE mode from six sounding points.The dashed red line indicates the location of the Lembang Fault.It can be observed that there is a decrease in resistivity values from point LMB03 to LMB02, which corresponds to the location of the Lembang Fault.