Subsurface structures identification at Bandung Basin based on gravity data

One of the metropolitan areas in Indonesia, Bandung Basin, is also known to have a high risk of disaster due to geological conditions and fault structures. In order to identify subsurface structures and fault locations, a study was carried out using the gravity method. By utilising TOPEX satellite data, subsurface modelling is performed using the forward modelling method. Based on the modelling results, the Bandung Basin is known to be dominated by young volcanic products such as tuff and lava with a density of 2.2 to 2.65 gr/cm3. However, sedimentary rocks with a density of 2.4 to 2.7 gr/cm3, such as sandstone, clay, and breccia, dominate in the western region of the basin. It is also known that there are indications of the presence and types of faults that cross the basin area based on the residual anomaly of gravity and also 2D modelling


Introduction
Bandung Basin is a large basin surrounded by volcanic highlands in West Java, Indonesia.This area was formed during the Middle-Late Quaternary period, namely 125 kilo year B.P. (Before Present), due to the volcanic activity of the giant volcano known as the Sunda volcano [1].The eruption of the giant volcano around the late quaternary collapsed it, causing the formation of a caldera, which later became Mount Tangkuban Perahu [1][2][3].
The eruption's impact also affects the formation of regional sedimentation and geological structures in the Bandung Basin Area.One of them is the initiation of the Lembang fault [1,3,4].The Lembang Fault, which separates the basin and the mountain ranges in the north, is one potential cause of earthquakes.One of the seismic activities around the Lembang fault in 2011 was an earthquake with a magnitude of 3.4 and a depth of 6 km that caused extensive damage to adjacent buildings [5].Besides the Lembang Fault, two other fault zones, Cimandiri and Baribis Kendeng, pass through this area and can potentially be earthquake sources.Both have seismic activity, which is currently causing earthquakes in West Java.Such as an earthquake with a magnitude of 5.6 in Cianjur caused by Cimandiri Fault activity and an earthquake with a magnitude of 3.8 in Kuningan caused by the Baribis Kendeng Fault activity.These three active faults are indicated to be horizontally displaced by 0.5 to 2 cm each year [6].
Due to these active faults, the Bandung Basin area cannot be spared from potential earthquake threats.Although the Bandung Basin area is a metropolitan area, it has the third highest population in Indonesia after Jakarta and Surabaya [7].The Bandung Basin area is also a part of Indonesia's National Strategic Area from an economic standpoint.The rapid development of urbanisation in this area is directly proportional to the increase in population growth.It is predicted that the Bandung Basin area will reach a population of 30 million by 2030 [8].Therefore, a study is needed to determine the location of faults and the geological structures in the Bandung Basin area.
In this study, we present subsurface structure models based on the gravity method and forward modelling.Gravity data modelling can identify the diversity of subsurface structures in the basin area through differences in rock density values.The result indicates the presence and types of faults related to its subsurface structures that cross the basin area.

Geological Setting
Bandung Basin is in the Bandung zone, an intermontane depression that is shaped from Pelabuhan Ratu to the east through northern Bandung.The middle area of the basin is about 660-675 m above sea level.Late Tertiary and Quaternary volcanic plains surround this region [9].As shown in the geological map (Figure 1), Bandung Basin is dominated by Quaternary volcanic rocks comprised of andesite to dacitic lava, breccia, agglomerate, tuff, lava, and intrusive igneous rocks.The western part of the basin is dominated by Tertiary sediments consisting of sandstone, claystone, and limestone, with young alluvium and fluvial sediments derived from volcanic deposits scattered in the middle of the basin.Similar to the western part, the southern part of the basin contains tertiary-aged volcanic rocks in the form of sandstone and tuffaceous breccia [10].The Bandung Basin is controlled by the straightness pattern of a west-east trending fault structure, while in the west and east, it is bounded by the north and south trending faults [17].There are three fault zones in the Bandung Basin area.The first zone, the Cimandiri Fault Zone, is a horizontal to oblique fault, extending from southwest to northeast, from Pelabuhan Ratu to Padalarang [18].The second one is the youngest fault in Java, formed in the Plio-Pleistocene period, the Baribis Fault Zone.Baribis is a thrust fault with a relatively west-east direction stretching from Purwakarta to the Baribis area in the Kadipaten-Majalengka [18].The last one is the Lembang Fault Zone in the northern part of Bandung.The Lembang Fault is a west-east trending fault, which stretches from the south of Tangkuban Perahu-Lembang Maribaya to the western slopes of Mount Manglayan [19].

Method
In this study, we used satellite data obtained from TOPEX, which can be accessed from the website https://topex.ucsd.edu/cgi-bin/get_data.cgi.TOPEX gravity data provided by the Scripps Institution of Oceanography, University of California San Diego, USA, is used as gravity anomalies in the form of FAA (Free Air Anomaly) data with geographical position and topographical data for each measuring point in ASCII -XYZ format.After doing some literary study, we reduced the gravity data with terrain correction (  ).Terrain correction is carried out to estimate the topographic shape around the gravity measurement point because mass is located between the observation point and the spheroid plane at a certain height, influencing the gravity value [20].Then we calculated the density average using the Parasnis method so that we can calculate the Bouguer correction ( = 0.04192ℎ).According to Rasimeng et al. (2020), the Parasnis approach is based on the Bouguer anomaly equation with the assumption that the Bouguer anomaly value is zero [21]: From the equation above, it can be simplified in the form of a linear equation  =  +  with density  as the gradient .
After reducing the gravity data with some corrections, we get the Complete Bouguer Anomaly (CBA).To gain a better understanding of the shallow structures in the study area, the complete Bouguer anomaly data was separated into regional (  ) and residual (  ) anomalies using the moving average method.The Moving Average method averaged the CBA anomalies to get regional anomalies so the residual anomalies could be calculated.Purnomo et al. (2013) state that the Moving Average method has the following mathematical equation [22]: So the residual anomalies: Where the n value is equal to −1 2 , with N is window width.From the residual anomalies, we made a 2-D gravity model using the forward modelling method.Then, from the gravity models, we analyse and interpret the subsurface structures.

Result and Discussion
Bandung basin exhibits a CBA ranging from -94.49 mGal to 113.46 mGal, with high anomalies observed surrounding in the north to southwest part, while low anomalies are in the middle, especially in the basin area and mountain ranges (Figure 2).Low anomalies with circular negative anomalies are forming in the Tangkuban Perahu mountain.This could be because of uncompacted volcanic rock or new volcanic deposits.The negative anomalies are also shown in the western part of the basin which is related to the Rajamandala Formation that is dominated by Tertiary sediments.The Rajamandala Formation is distributed in the west of Bandung.This formation was formed between the Late Oligocene and the Early Miocene [23].
The CBA was analyzed to distinguish between residual and regional gravity anomalies.Residual anomalies are caused by shallow structures, while regional components are produced by larger, broader, and deeper structures.Shallow features produce anomalies with narrow and abrupt contour patterns, while the regional components exhibit consistent gradient fluctuations.

Figure 2. Complete Bouguer Anomaly (CBA)
In order to differentiate between residual and regional gravity anomalies, the CBA was analyzed using the moving average method.Shallow structures are the source of residual anomalies, they produce narrow and abrupt contour patterns.Whereas larger, broader, and deeper structures are the source of regional components that exhibit constant gradient fluctuations [24].
The residual anomaly of this region ranges from 43.51 mGal to -77.99 mGal as shown in Figure 3.The anomaly patterns that appear are more complex and characterize the topography of the area more clearly.At the northern part of the basin, there is a very low anomaly indicating the presence of Mount Tangkuban Perahu which is possibly caused by the presence of magma chamber in volcanoes associated with low density.Then in the southern part of the mountain, there is a high anomaly which shows a difference in density contrast.This difference in density can be an indication of the presence of faults.As we know from the geological map shown in Figure 1. between the basin and Mount Tangkuban there is a Lembang Fault.The west part of the map also shows low anomalies which is assumed to be the Rajamandala Formation in the form of sedimentary rock interspersed with volcanic igneous rock.So, there is a difference in density contrast which is also high to the west.If we look at the location of the fault zone that passes through the western region, there is a Cimandiri fault that passes through that area.Based on the residual anomaly, we made three slices to be modeled with the forward modeling method (Figure 3).

Slice A-B
The first one is the A-B slice; the model cross-section is made through Mount Tangkuban Perahu and also the Bandung Basin vertically, cutting the residual map from north to south or from point A to B. In the northern part of the models, there is a pyroclastic rocks layer with a density of 1.2 gr/cm 3 , then there are alluvial (1.9 gr/cm 3 ), tuff, and young volcanic with a density of 2.2 gr/cm 3 , then lava layer (density = 2.65 g/cm 3 ), Tertiary sediment (density = 2.4 g/cm 3 ), and the bottom are older -higher densities of igneous rocks (density = 3 g/cm 3 ).Based on the residual anomaly and the graphic of anomaly and distance, shown in the above model, the Lembang Fault is indicated in the northern part of the basin and described as a descending fault or normal fault because the anomaly on the north is lower than the southern part.

Slice C-D
The second cross-section showed low anomalies with a 2.1 gr/cm3 density as the Rajamandala Formation layer is composed of limestone and clay.Then in general, the model is composed of alluvial (1.9 gr/cm 3 ) and tertiary sediment (density = 2.4 g/cm 3 ).It also describes the layers with a density of 2.7 gr/cm 3 which are volcanic rocks and sediments such as sandstone, clay, and breccia, and in the bottom, there are igneous rocks (density = 3 g/cm 3 ).Slice C-D is passing the Cimandiri Fault.The Cimandiri Fault is widely indicated as a reverse fault because based on the residual anomaly there is a contrast of anomaly values, where the north is higher than the south.In addition, the existence of a normal fault in the northern part of the Cimandiri Fault is also described because there are density and anomaly contrasts where the southern part is relatively higher than in the north.

Slice D-E
The third cross-section is drawn horizontally across the basin region intersecting both slices A-B and C-D in the south.In general, the models present the layering of alluvial (density = 1.9 g/cm 3 ), lava (density = 2.65 g/cm 3 ), Tertiary sediment (density = 2.4 g/cm 3 ) and the bottom is igneous rocks (3 g/cm 3 ).In the left section, there are also volcanic rocks and sediments (density = 2.7 gr/cm 3 ) and the Rajamandala Formation with a density of 2.1 gr/cm 3 .

Conclusion
Based on the modelling results, the Bandung Basin is known to be dominated by young volcanic products such as tuff and lava with a 2.2 to 2.65 gr/ cm 3 density.However, sedimentary rocks with a density of 2.4 to 2.7 gr/cm 3 such as sandstone, clay, and breccia, dominate in the western region of the basin.The existence and types of faults that cross the basin area can be determined based on the residual anomaly of gravity.The Lembang Fault is a normal fault, as evidenced by the contrast of high and low anomalies in the northern Basin.The high and low anomalies in the western region also indicate the presence of the Cimandiri Fault, which is described as a reverse fault in the 2D model.It is also indicated that the northern part of the Cimandiri Fault has a normal fault since the anomaly in the south is comparatively higher than in the north.

Figure 3 .
Figure 3. Residual anomaly and slicing model location