Surface Geology Analysis on the Relationship between Fault Creep and Overpressure in Grobogan, Central Java, Indonesia

The Grobogan Regency is a significant location for oil and gas exploration due to its proximity to active oil and gas fields. Geologically, this area is intersected by the Kendeng thrust fault and exhibits several surface manifestations of mud volcanoes near the fault. However, there have been limited studies examining the activity of the Kendeng thrust fault and its relationship to overpressure and earthquakes, which could affect oil and gas exploration and natural disaster mitigation in the region. This paper focuses on utilizing surface geological mapping data and Interferometric Synthetic Aperture Radar (InSAR) data to analyze the relationship between fault creep and overpressure manifestations in Grobogan, Central Java. While continuous research on fault creep has been conducted since the 1960s on the San Andreas fault in the United States, there has been a lack of continuous research on onshore fault creep in the Northeast Java Basin or Kendeng Basin, particularly in the Grobogan area. This region, known for its abundance of active mud volcano manifestations, is predominantly characterized by marl and clay lithology. It represents a warm basin environment that is prone to overpressure and ideal for fault creep occurrences.To investigate further, fluid, gas, and mud samples were collected from the mud volcanoes, along with rock samples from the surrounding area, for comprehensive laboratory analysis. The integration of these sample analyses aimed to determine the correlation between the activity of the Kendeng thrust fault creep and the subsurface overpressure conditions. While this study observed several indications of creeping in the vicinity of InSAR anomalies, the exact location of the creeping fault plane could not be definitively determined. The Kendeng thrust fault exhibits the three main factors necessary for initiating fault creeping. The fault segment’s extremely slow creeping may be linked to the activity of the mud volcanoes, as indicated by the results of fluid, gas, and rock sample analysis. The findings of this research hold potential for informing the development of oil and gas fields, as well as enhancing the mitigation of natural disasters associated with oil and gas exploration and earthquakes in the region.


Introduction
The Onshore Northeast Java Basin, also known as the East Java Basin, has experienced several past earthquakes.Within this region, the Kendeng thrust fault plays a significant role as it separates the Kendeng Zone from the Rembang Zone [1].In fact, there have been at least 10 major earthquakes in the vicinity of the Kendeng thrust fault zone (Figure 1), resulting in numerous casualties and substantial damage to infrastructure [2].Recognizing the seismic potential of this area, it is crucial to investigate the activity of the Kendeng thrust fault, particularly its surface and subsurface manifestations of fault creeping.To undertake this study, the Grobogan area was chosen due to its intricate geological characteristics and the presence of mud volcano manifestations on the surface.The research primarily focused on observing the surface geology surrounding the Kendeng thrust fault in Grobogan and evaluating its activity (refer to Figure 1).The onshore region of East Java can be categorized into four distinct zones, characterized by their morphology, physiography, and major structural domains.Starting from the northernmost part, we have the Rembang zone, followed by the Kendeng zone, the Central Volcanic Mountains zone, and finally, the Southern Mountain zone [1].The Kendeng Thrust fault marks the boundary between the Kendeng zone and the Rembang zone (refer to Figure 2).Java exhibits three major structural patterns that govern its structural behavior: the NE-SW Meratus pattern, the E-W Java/Sakala pattern, and the N-S Sunda pattern.The Kendeng thrust fault, situated in Central and East Java, aligns with the East-West (E-W) trend associated with the Java/Sakala pattern.Surface physiography reveals a transition in lithology from the southern to the northern part of Java.The southern region (Southern Mountains Zone and Central Mountains Zone) is predominantly composed of volcanic rocks, while the Kendeng Zone is characterized by clay, and the Rembang Zone is dominated by sandstone.Furthermore, the onshore section of the Northeast Java Basin is recognized as an overpressure zone [3][4][5][6].To provide a comprehensive overview, a cross-sectional and regional stratigraphic column was constructed, illustrating that the Kendeng Zone's lithology consists of clay and marl from the Kalibeng formation, the Kerek formation, and the Pelang formation [7].
Fault creep refers to the gradual movement of an aseismic fault in the upper part of the Earth's crust between two earthquakes, relieving significant stresses on the fault.It can also occur as an after-slip event, involving daily or yearly shifts following an earthquake.The study of earthquake creep originated around 1960 in the United States, specifically focusing on the San Andreas fault and its observable effects on physical infrastructure [8][9][10].Changes in structures such as buildings, roads, or exposed rock formations were measured using basic creep meter tools, and these methods have since undergone further refinement [11][12][13].This assessment has been applied to actively creeping faults worldwide, including locations like Italy, Taiwan, and other countries [14][15][16][17][18][19][20].
Fault creep can be attributed to three primary factors: the geological characteristics of the region, the existence of overpressure fluids, and the geometry of the fault [21][22][23][24][25][26][27][28][29].To monitor the speed of fault plane creep, a commonly employed tool is the creep meter.Initially, this instrument was deployed to observe the activity of creep on the San Andreas fault in the United States [30,31].Presently, there exist various models of creep meter tools, each utilized based on the specific objectives of individual studies.Typically, this tool consists of two iron stakes, an invar rod, a protective cover, and a centrally positioned sensor [18,19].
In the field of geology, three primary types of major faults are recognized: normal faults, strike-slip faults, and reverse or thrust faults, classified according to the Anderson fault classification system.Notable examples of fault creep can be observed in major faults worldwide, such as the Alto Tiberina Normal Fault in Italy, the strike-slip San Andreas Fault in the United States, and the Chishang Fault in Taiwan [14,16-17, 28, 32, 33].By employing a creep meter, it is possible to monitor the creep behavior of a measured fault [39].Generally, three types of creep patterns are observed across most sites: steadystate creep, episodic creep, and episodic creep accompanied by a steady-state background creep.As fault creep represents the movement of an aseismic fault between significant earthquakes, the creep meter may provide insights into the cyclic occurrence of major earthquake events, allowing for prediction or approximation of earthquake cycles.The study of the creeping activity of the Kendeng Thrust fault has not been thoroughly explored, particularly through surface geological observations.Similar to other well-known creeping faults, the presence of mud volcanoes in close proximity to the Kendeng Thrust fault indicates an obvious manifestation of overpressure conditions in the subsurface.This research aims to elucidate the relationship between the overpressure conditions in the region and the creeping activity of the Kendeng Thrust fault, potentially associated with a series of significant earthquakes in the past.Given that a significant portion of the Indonesian population resides in Java, particularly in East Java and Central Java, this research focuses on the Kendeng Fault.According to the Central Bureau of Statistics, Java Island alone accommodates at least 56.1% of the Indonesian population [40].Therefore, the impact of this research holds considerable importance for Indonesia.

Methodology and Research Procedures
The methodology and research procedures primarily focus on surface geological mapping and observation.Field mapping was conducted in the Grobogan region, specifically targeting areas near the Kendeng Thrust fault zone.This region is well-known for its complex of mud volcanoes and its 4 proximity to the Kendeng Thrust fault zone.The Google Earth application was also utilized to identify potential geological features of interest on the surface.According to the regional geological map, the Kendeng thrust fault runs through the southern part of Grobogan.Various samples were collected in this area, including rock, mud, fluid, and gas, and were carefully preserved before being sent to laboratories.The rock samples obtained from the field were cut and prepared as thin sections for further analysis at the Geology Department of the Bandung Institute of Technology (ITB).Thin section analysis enables the examination of mineral composition, texture, and fabric of rocks and minerals in thin slices.This provides valuable information about rock and mineral properties, facilitating the interpretation of geological history and processes.The gas, fluid, and mud samples from each mud volcano were also collected and prepared for laboratory analysis.The gas samples were sent to the Coal and Geothermal Mineral Resource Center Laboratory in Bandung.Gas analysis involved methods such as Titrimetric Analysis and Gas Chromatography Analysis.These analyses measure parameters including H2, He, O2, Ar, N2, CH4, CO, and CO2.Titrimetric analysis is a quantitative analysis technique based on volume measurement, while Gas Chromatography Analysis is commonly used for the separation and analysis of volatile compounds.The fluid samples were sent to the Water Quality Laboratory at the Faculty of Civil and Environmental Engineering (Bandung Institute of Technology) in Bandung.The fluid analysis followed the guidelines outlined in the Standard Methods for The Examination of Water and Wastewater 23 rd Edition 2017 (APHA).The fluid samples are pure water samples extracted from the mud.The analysis covers approximately 11 elements, including Boron (B), Calcium (Ca), Magnesium (Mg), Potassium (K), Sodium (Na), Chloride (Cl), Sulfate (SO4), Lithium (Li), Strontium (Sr), Barium (Ba), and Iron (Fe).Additionally, the laboratory measures the pH level of the water.The results of the gas and fluid analysis from the subsurface (mud volcanoes) will be utilized to determine the impact of dominant elements on lithology and their relationship to the activation of the Kendeng thrust fault creeping.Furthermore, the physical properties of the mud particles from each mud volcano were analyzed.The mud samples were sent to the Soil Mechanics Laboratory at the Faculty of Civil and Environmental Engineering, Bandung Institute of Technology.Some of the mud samples were also sent to Geological Survey Center laboratory in Bandung for Scanning Electron Microscope (SEM) analysis, the result will be in discussion (refer to Figure 9).Several methods were employed for the analysis, including Specific Gravity Analysis, Sieve Analysis, and Hydrometer Analysis.Specific Gravity Analysis helped determine the specific gravity value of the grains present in the mud samples.Sieve Analysis and Hydrometer Analysis were utilized to assess the grain size of the mud samples.This information will be crucial in establishing correlations with subsurface lithology and will be integrated with surface geological observations.It is widely recognized that lithology is one of the factors that can influence fault creep activity [27,28].The Kendeng Thrust fault exhibits a very slow creep rate of 2-3 mm per year, which is significantly slower compared to most other active creeping faults.This slow movement makes it challenging to observe the surface manifestations of creep.Relying solely on the regional geological map would prove difficult in identifying the creep planes on the surface.Hence, additional methods are required to support surface geological mapping efforts.One of the chosen methods is Interferometric Synthetic Aperture Radar (InSAR) modeling.InSAR is an established technology that evolved from Synthetic Aperture Radar (SAR) invented by Carl Wiley in 1951 [34].InSAR modeling serves as a valuable tool to aid in defining the possible surface manifestations of the Kendeng Thrust fault and complement surface geological mapping efforts.InSAR operates as a remote sensing technique that utilizes radar satellite imagery captured by satellites orbiting the Earth.Radar waves emitted by the satellites are reflected off the Earth's surface and captured by sensors on the satellite [35].The fundamental principle of InSAR involves combining two or more SAR images acquired from different positions to generate threedimensional (3D) positional information.Through SAR processing, multiple interferometric pairs are formed using SAR imagery, which allows for the extraction of distance and elevation information of objects on the Earth's surface based on phase differences.These phase differences can be converted into Line-of-Sight (LOS) shift values, providing an indication of surface shifts or deformations [36].
In a specific direction, the Line-of-Sight (LOS) in InSAR can detect displacements of points in particular locations (refer to Figure 3).InSAR measurement and modeling techniques are valuable tools for identifying displacements caused by various factors, including tectonic activity, earthquakes, volcanic eruptions, glaciology, oil and gas extraction, hydrology, and infrastructure monitoring [37].In the case of the Kendeng Thrust fault, its movement primarily involves vertical displacements.Theoretically, InSAR measurements can detect these vertical movements or displacements.Therefore, combining the results from laboratory tests, surface geological mapping, and InSAR modeling will allow for a comprehensive understanding of the relationship between Kendeng Thrust fault creeping and subsurface overpressure conditions.

Result
The Grobogan region is home to at least 8 main mud volcanoes, namely Kesongo, Anak Kesongo, Crewek, Banjarlor, Bledug Kuwu, Detil, Cakringan, and Medang (refer to Figure 6D).These mud volcanoes are situated at distances ranging from 5 to 11 kilometers from the Kendeng Thrust fault.Among them, Crewek mud volcano stands as the closest to the Kendeng Thrust fault zone, at approximately 5 kilometers away.Samples were collected from each of these mud volcanoes and subsequently sent to specialized laboratories for analysis.The fluid samples extracted from the mud volcanoes were dispatched to the Water Quality Laboratory at the Faculty of Civil and Environmental Engineering, Bandung Institute of Technology.It was observed that all the fluid samples possessed a salty taste, and this was reflected by the significantly high levels of natrium and chloride content (as shown in Table 1).According to the Regulation of the Minister of Health Number 492 (2010), the chloride content should not exceed 250 mg/L, and the natrium content should be below 200 mg/L.Gas samples were also collected from each of the mud volcanoes and sent to the Coal and Geothermal Mineral Resource Center Laboratory in Bandung for analysis (refer to Table 2).The gas samples were obtained using a customized device, consisting of a funnel pipa connected to a hose, which was further connected to a plastic bottle filled with water.As the gas entered the bottle, it displaced the water, causing it to be pushed out.Upon analysis in the laboratory, it was revealed that carbon dioxide (CO 2 ) was the dominant gas component across all the samples, followed by nitrogen (N 2 ) and methane (CH 4 ).This research primarily focused on conducting geological field observations within the onshore Central Java province.In order to identify and delineate the creeping fault segments of the Kendeng thrust fault, InSAR modeling was utilized, covering the research area (refer to Figure 5 and 6).Both ascending and descending InSAR models were generated to assess potential ground movement associated with fault creep.Remarkably, these models produced similar anomaly results in terms of ground movement.The research clearly highlights the trend and continuity of the Kendeng thrust fault within the designated study area.Furthermore, it was noted that several mud volcanoes are located in the Grobogan area, in close proximity to the suspected fault plane.The predominant lithology in the area was found to be massive marl, which has undergone significant structural deformation (refer to Figure 4 and 6A).Based on thin section analysis, the rock was classified as mudstone according to Dunham's classification system from 1962 (refer to Figure 7B and 7C).The physical properties analysis of the mud samples collected from the mud volcanoes in Grobogan revealed that the specific gravity of the mud ranged from 2.65 SG to 2.72 SG.Additionally, the grain size analysis of the mud samples indicated a predominance of clay particles.This suggests that the mud source in the subsurface is also predominantly composed of clay lithology.Consequently, it can be concluded that the Grobogan region is characterized by a clay-dominated lithology extending from the surface down to the subsurface.This clay lithology plays a significant role in the occurrence of fault creeping in the area.

Discussion
One of the common characteristics observed in the example of three main creeping faults worldwide is the presence of mud volcanoes on the surface, indicating subsurface overpressure conditions.Each of these faults exhibits mud volcanoes in close proximity to the fault segment.Overpressure conditions or elevated pressure in the subsurface are among the factors contributing to fault creep.For instance, the Alto Tiberina Normal fault is surrounded by mud volcanoes, with the closest ones located in the northern part of the fault [41].The Pieve Santo Stefano mud volcano complex, connected to a deep CO 2 reservoir, is particularly notable [42].The Alto Tiberina Normal fault experiences creep at a rate of approximately 1.7 ± 0.3 mm/yr from the surface to a depth of 5 km.At a depth of around 7 km, the fault becomes locked, which correlates with earthquakes of magnitudes 6.5 and 6.7 [43].Similarly, the San Andreas Strike Slip (SAF) fault has mud volcanoes located in the southern part of the fault.The southern section of the SAF creeps at a rate of 2-4 mm/yr, while the central and northern parts exhibit higher rates ranging from 10 to 34 mm/yr [45].Notably, the central part of the SAF, particularly the Parkfield area, demonstrates the highest creep rate at 34 mm/yr [46].
To accurately map the displacement velocity along the San Andreas Fault, a combination of InSAR (Interferometric Synthetic Aperture Radar) and Global Navigation Satellite System (GNSS) time series was utilized [47].This modeling approach successfully delineated the fault plane and provided precise information on displacement velocity.Notably, the central section of the San Andreas Fault, particularly in the Parkway area, exhibited the highest velocity displacement ranging from 10 to 30 mm/yr.

A B D C
Menawhile, the Chishang Thrust fault, on the other hand, demonstrated a creep rate of 3 cm/yr [48].The Chishang and Kendeng are both thrust fault and has a few similarities.InSAR modeling was employed to visualize the fault displacement, revealing an approximate displacement rate of 30 mm/yr along the Chishang Thrust fault.Additionally, creepmeter measurements yielded varied results ranging from 3 to 16 mm/yr.It is worth noting that there are also mud volcanoes in close proximity to the Chishang area.
The laboratory analysis confirmed that the fluid from the mud volcanoes had a high salinity content, indicating its highly salty nature.The gas content of the samples was primarily dominated by CO 2 , followed by Nitrogen (N 2 ) and Methane (CH 4 ).The interaction between CO 2 and the brine fluid with the reservoir rock can cause deformation, potentially contributing to the movement of nearby fault planes [49,50].Experimental studies on the CO 2 -brine interaction with Knox sandstone rocks from Indiana in the United States demonstrated that after 6 months, it resulted in the development of wider cracks in the samples (refer to Figure 8A and 8B).This interaction also led to a 50% decrease in permeability in the sandstone rock samples due to dissolution, migration, and secondary mineral precipitation [49].Similarly, the brine-CO 2 interaction with Eau Claire shale rocks from Wisconsin in the United States resulted in the formation of additional secondary clay layers on the surface of quartz (refer to Figure 8C and 8D).The alteration process in the shale rock sample led to the generation of flaky clay minerals like illite [50].The fluid samples collected from the mud volcanoes contained a high salt content.The salinity and CO 2 content played a role in creating more cracks and additional clay minerals, as clearly shown in the SEM (Scanning Electron Microscope) analysis results (refer to Figure 9).

Conclusions
The geological characteristics of the Kendeng Thrust fault in the Grobogan area closely resemble those of other active creeping faults discussed earlier in this paper.This indicates that the Kendeng Thrust fault has the potential to undergo creep, which is supported by laboratory findings.It is likely that the overpressure conditions in the subsurface are influencing the behavior of the creeping activity along the Kendeng thrust fault.Therefore, it is highly recommended to conduct a more comprehensive field mapping initiative to identify the specific segment where creep is occurring.

Figure 1 .
Figure 1.Old documented major earthquakes in Java Island and location of research.

Figure 3 .
Figure 3. Displacement of surface deformation due to tectonic on the point of P by a ∆R along the direction of LOS [38].

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The flaky clay minerals (illite-smectite) are also present in the mud sample from each mud volcanoes (refer to Figure9).The combination of brine and CO2 from the mud volcanoes material motivate the generation of more flaky clay minerals in subsurface.Additional clay minerals will enhance the possibility of the faulted area to creep.The increased clay content around the fault zone facilitated fault creeping.The significant presence of CO 2 and brine fluid from the subsurface of the mud volcanoes in the Grobogan region may potentially influence the behavior of the surrounding Kendeng thrust fault.This factor strengthens the relationship between the overpressure manifestation in the study area and the activity of the Kendeng Thrust fault.Similarities can also be observed in the gas and fluid content of mud volcanoes in close proximity to the Chishang Thrust fault in Taiwan[51].The Chishang Thrust fault is known as the fastest creeping thrust fault in the world.It is recommended to conduct further field mapping to determine suitable locations for installing creep meters.Installing creep meters will enable continuous monitoring of the creeping behavior.

Figure 9 .
Figure 9. SEM analysis from the mud sample of Crewek mud volcano.The flaky clay minerals of smectite-illite dominated the sample.

Table 1 .
Fluid analysis from each main mud volcanoes in Grobogan.

Table 2 .
Gas analysis from each main mud volcanoes in Grobogan.