Geophysical investigation of the 2022 Cianjur earthquake: Uncovering a new active fault

. An earthquake disaster struck the Cianjur area on November 21 2022, with a main earthquake measuring 5.6 Mw. Until April 2023, BMKG recorded that aftershock occurred 448 times with a magnitude range of 1 – 4.2 Mw. This event occurred due to underground movement caused by a fault. The fault that has been identified in the Cianjur area is the Cimandiri fault, however, the epicentre of the earthquake and the location of the Cimandiri fault line are approximately 20 km away. BMKG indicated that there was a new fault that was the cause of the earthquake. This research aims to determine subsurface conditions with indications of active faults in the areas affected by the 2022 Cianjur earthquake. Data acquisition was carried out at Cibulakan and Firdaus Mosque using the GPR method. The GPR method used is GSSI – SIR 3000 with a 270 MHz frequency antenna. Data processing was carried out using GPRPy software. Based on the processing that has been carried out, there are indications that the fault stretches perpendicularly in a north – south direction at a depth of approximately 2 m below the ground surface. Apart from that, the rock that makes up the area is volcanic rock.


Introduction
The November 21, 2022 earthquake in Cianjur, measuring 5.6 Mw, resulted in 635 casualties, displacing about 41,166 families and causing extensive building damage [1,2].Earthquakes stem from subsurface movements associated with active faults.While the known Cimandiri fault lies 20 km from the epicentre, the Meteorology, Climatology and Geophysics Agency (BMKG) suspects a new fault, the Cugenang fault, due to its significant impact in the Cugenang District.Earthquake can be triggered ground motion in several areas, one of which was Cijedil Village, one of the reasons is Cijedil Village is composed of old volcanic products that have experience weathering [3].Ground motion is the movement of a mass of soil or rock from its initial state, either horizontal or vertical [4].
Cibulakan Village, within this district, sustained substantial damage, including road fissures and the complete destruction of Firdaus Mosque.This study employs Ground Penetrating Radar (GPR) with a 270 MHz frequency antenna to investigate the existence of the Cugenang faults.

Methodology
The subsurface structure of the land can be determined using several geophysical methods, one of which is the GPR method.The GPR method is a research method that uses electromagnetic wave signals with a high frequency of 10 -1000 MHz to detect and map shallow surface cracks [5][6][7].The GPR method technique is to use Electromagnetic Subsurface Profiling (ESP), which utilizes the reflection of electromagnetic waves emitted by the ground surface through the antenna [8].
Electromagnetic waves propagate very quickly, within nanoseconds when transmitting and receiving of waves [9].The maximum depth can be determined using the GPR is 30 m with low conductivity.Conductivity affects the propagation of wave signals, if the greater of the conductivity, the less penetration the emitted waves will have.This is because the energy from electromagnetic waves is attenuated.The shorter the electromagnetic waves emitted, the better the resulting depth resolution [10].

Principle of GPR Method
The GPR equipment consists of a signal generator, a transmitter signal (transmitter), and the receiving signal (receiver).The signal will be transmitted below the ground surface which hits an object and the waves will be reflected on the ground surface which will then be read by the receiver in the form of a subsurface radargram [11].The following is the mechanism of the GPR method which is shown in figure 1.The GPR system has a transmitter circuit with an antenna connected to the pulse source and a receiver circuit with an antenna connected to the signal processing unit.The transmitter circuit creates electrical pulses that have the form of Pulse Repetition Frequency (PRF), time, and energy.The depth range of the transmitted signal can be known in time expressed in t (), the wave propagation speed is expressed in v (   ⁄ ), and the depth is specified in h () [12].The equation can be written as follows: Fig.

Electromagnetic wave
The GPR method is a method that uses electromagnetic waves with a microwave spectrum.Electromagnetic waves are vibrations that can propagate in space and time and are emitted in various directions in the radar working system.If a wave is emitted and hits an object, the wave will be reflected back and received by the object.Information below the ground surface obtained from reflections will be displayed by the radar layer [13].The basic concept of electromagnetic waves is Maxwell's equations.This is because Maxwell's equations describe the mathematical nature of natural laws on the basis of electromagnetic phenomena.Maxwell's equations, that is [14]: •  = 0 (2) ×  =  0  + 0  0  ⃑⃑⃑⃑⃑ The equation of electromagnetic waves moving in vacuum with the speed of light is determined by three material properties, there are the electrical permittivity (), magnetic permeability (), and electrical conductivity () [15].

Measurement of GPR method
The GPR method is used to identify subsurface structures and map shallow surface fractures [7].Ground motion can be determined using the GPR method because it requires a high transmitter to determine the subsurface area of ground motion which consists of dielectric constant, magnetic contrast, active faults, and water content [16].In liquefaction, this method cannot be used optimally, because when liquefaction occurs, the soil has a high conductivity value, this is the main factor in the frequency used [17].
Data acquisition was carried out on Jalan Koleberes, Cibulakan Village and Firdaus Mosque, Cijedil Village.The following is a map of the research location.Figure 2 shows a map of the Cibulakan and Firdaus Mosque research locations.At each research location, data acquisition was carried out using 1 track.Data acquisition will be held on 29 and 31 August 2023.The GPR method using GPR GSSI -SIR 3000 devices with 270 MHz frequency antenna.The results obtained by the GPR method are stormy in the form of a radargram in the form of a 2D visualization shown on the monitor.Data processing using of software GPRPy.The steps for processing data using GPRPy as shown in Figure 3.

Result and analysis
Data acquisition was carried out on Jalan Koleberes as shown in figure 4(a).The measurement track length was 215 m with an orientation from west to east.On the antenna used, the maximum depth range is 8.5 m.The higher the frequency used, the shallower the penetration depth, and vice versa.Figure 4(a) shows layered subsurface material and high spatial correlation reflector differences and displays the presence of faults or faults as reflector offsets [18].The fault is located at a depth of 3.4 m at the 80 m track position and 3.2 m depth at the 90 m track position.This can be seen because the indication of a fault is characterized by a low dielectric value due to an increase in the air fraction compared to earth material and there is the same lithology but is experiencing damage or deformation [18].There is a curved line forming a green parabola starting at a depth of 3.3 m at a track position of 80 m which is a change in the propagation of electromagnetic wave signals in the layer near the surface which is expressed in the curvature of the layer boundary [19].
Data acquisition was carried out at the Firdaus Mosque as shown in Figure 4(b).The measurement path length was 47 m with an orientation from west to east.On the antenna used, the maximum depth range is 8.5 m.The higher the frequency used, the shallower the penetration depth, and vice versa.Figure 4(b) shows layered subsurface material and high spatial correlation reflector differences and displays the presence of faults or faults as reflector offsets [18].The fault is located at a depth of 4 m at a track position of 27 m and a depth of 4.2 m at a track position of 32 m.This can be seen because the indication of a fault is characterized by a low dielectric value due to an increase in the air fraction compared to earth material and there is the same lithology but is experiencing damage or deformation [18].There is a curved line forming a green parabola starting at a depth of 6.6 m at a track position of 12 m which is a change in the propagation of electromagnetic wave signals in the layer near the surface which is expressed in the curvature of the layer boundary [19].

Conclusions
This research was conducted to determine the existence of faults that have just been identified by BMKG and to map the existence of the Cugenang fault.In Cibulakan, faults are known at depths of 3.4 m and 3.2 m on tracks of 80 m and 90 m.Meanwhile at the Firdaus Mosque, the faults are at depths of 4 m and 4.2 m at track positions of 27 m and 32 m.Data acquisition was carried out perpendicular to the direction of the fault indication, namely west-east direction.So, it can be seen that the direction of the Cugenang fault is north to south.This research was conducted to determine the existence of the Cugenang fault which is the cause of the 2022 Cianjur earthquake.Thanks are given to the supervisors and examiners who have helped in writing this article.And then, Mr. Adrin Tohari and Mr. Achmad Fakhrus Shomim who have guided us in data acquisition

Fig. 2 .
Fig. 2. Data processing software of GPRPy.(a) Results of research data processing at Cibulakan, (b) Results of research data processing at the Firdaus Mosque.