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We present an interpretation of newly acquired audio-magnetotelluric data to reveal the subsurface geometry of Cimandiri Fault Zone, one of the major active faults in the western part of Java. The line section is 25 km long in a nearly north–south direction across the axes of CFZ with 24 stations of 750–1200 m spacing intervals. The 2D AMT inversion model shows two conductive zones in the southern part that may be associated to the Miocene rocks of the Southern Mountains, and a conductive zone in the northern part that is likely to be associated with Gunung Walat fold-belt. The subsurface structures of the Southern Mountains are dominated by south-dipping thrusts that may uplift the shallow marine sediments. Shallow seismicity occurred around CFZ indicating the activity of these blind thrusts.

Continuously increasing population and progressive infrastructural development in the region of Kertajati International Airport, Indonesia, emphasize the need to develop a sustainable water supply network. Airport facilities require sufficient water resources, which can be obtained from surface water and groundwater. Groundwater exploration can provide necessary information for assessing water resources. The purpose of this study is to analyze the configuration of aquifers in the studied area. A Schlumberger array was used to carry out twelve vertical electrical soundings (VES) with AB/2 electrode spacing ranging from 1.5 m to 150 m. IPI2win software was used to qualitatively interpret the VES results and it suggested the presence of three distinct lithological units interpreted as clay, alluvial sand, and a Lower Quaternary formation. In general, resistivity values in the studied area can be divided into five resistivity categories: very low resistivity with values ranging from 1 Ωm to 10 Ωm, low resistivity with values ranging from 10 Ωm to 50 Ωm, medium resistivity with values ranging from 50 Ωm to 100 Ωm, high resistivity with values ranging from 100 Ωm to 200 Ωm, and very high resistivity with values > 200 Ωm. The geo-electric interpretation revealed three geo-electric layers: topsoil (1 - 144 Ω m), sand (1 - 298 Ω m), and clay (1 - 82 Ω m). Aquifers in the studied area are lithologically composed of sand. Clay is the dominant lithology in the studied area, so the presence of aquifers in this area is very limited, and thus the supply of groundwater is also limited. The exploitation of groundwater must be limited and controlled to maintain the sustainability of groundwater in the studied area.

Lineament is one of the most important features showing subsurface elements or structural weakness such as faults. This study aims to identify subsurface lineament patterns using automatic lineament in Citarum watershed with gravity data. Satellite gravity data were used to generate a sub-surface lineament. Satellite gravity data corrected using Bouguer and terrain correction to obtain a complete Bouguer anomaly value. Butterworth filters were used to separate regional and residual anomaly from the complete Bouguer anomaly value. Residual anomaly gravity data used to analyze sub-surface lineament. Lineament generated using Line module in PCI Geomatica to obtain sub-surface lineament from gravity residual value. The orientations of lineaments and fault lines were created by using rose diagrams. The main trends observed in the lineament map could be recognized in these diagrams, showing a strongly major trend in NW-SE, and the subdominant directions were in N-S. Area with a high density of lineament located at the Southern part of the study area. High-density lineament might be correlated with fractured volcanic rock upstream of the Citarum watershed, meanwhile, low-density lineament is associated with low-density sediment. The high-density fracture might be associated with intensive tectonics and volcanism.

The Cenozoic Bogor Basin situated on the SE edge part of Sunda shelf is presumed to have hydrocarbon potential on its turbidite deep-water play. The deep subsurface geometry of this basin may play an important role, yet unexplored, to the hydrocarbon exploration. The gravity method is advantageous to illuminate the subsurface structure on the arbitrary depth and various sources. Eight-hundred-thirty-eight points of ground-based gravity survey were collected on roughly one kilometer spacing in the North of Bandung to Pamanukan region covering the Cenozoic Bogor Basin on the Subang-Purwakarta segment to generate complete Bouguer anomaly (CBA) map. This study examines the two robust methods of gravity anomaly decomposition (i.e., polynomial trend surface and upward continuation) by using multiple parameters to match the geological background. Radially averaged power spectrum was used to estimate the depth of anomalous source corresponds to the top of basement layer and resulting 4 km basement depth in North West Java basin and below 1 km on Tangkuban Parahu volcanic zone. The resulting estimated depth of Cenozoic Bogor Basin was evaluated by the matched a priori published data on those two areas and revealed the depth of depocenter (deepest sediment) on the Cenozoic Bogor Basin is up to 9 km, quite a deeper extent than previously assumed.

One of the most powerful eruptions of the Holocene, known as the Samalas eruption, had a VEI of 7 and took place on Lombok, eastern Indonesia in 1257 CE. Thick tephra fall covered the entire island, and pyroclastic density currents (PDCs) up to 50 m thick buried almost half of Lombok. Until now, there has been no detailed study of the geomorphological evolution of Lombok related to this eruption, especially for the eastern part of Lombok. The main goal of this paper is therefore to reconstruct the paleo-topography on the eastern part of Lombok before the eruption of the Samalas volcano in 1257 CE, and to analyse the subsequent landscape evolution following this eruption. Data were collected from more than 1300 points in order to gather geomorphological, geological, and geochemical information for the eastern part of Lombok over the last millennium. Data were obtained from wells (which are widespread in the studied area), natural outcrops (mainly cliffs along the shoreline and the rivers), and human-made outcrops in quarries, as well as from the two-dimensional resistivity profiling (Dipole-dipole array) carried out on eastern Lombok in 2016 and 2017. We estimate that 4435 ± 5.5 × 106 m3 of pumice-rich PDC, over a surface of 171 km2, buried the eastern part of Lombok in the aftermath of the 1257 CE eruption, and as a result of this rapid pumice-rich PDC deposition there has been some aggradation of the shoreline at some points, resulting in most of the pre-1257 coral reef being buried. After several intense rainfall events, the deposited volcanic material was probably eroded during the first decades following the 1257 CE eruption by mountain streams, producing lahars. We estimate that the remaining volume of pumice-rich PDC deposits following this erosion is 625 ± 5.5 × 106 m3 over a surface of 89 km2, which is equivalent to 14% of the initial volume.

We analysed receiver functions from teleseismic events recorded at 11 broadband seismometers in the western part of Java Island, Indonesia. The stations are mostly located at three main geological environment including Northwest Java Basin, Bogor Zone, and Southern Mountains Arc. A total of about 341 receiver functions were computed using iterative time domain deconvolution. We derived shear-wave velocity structure and crustal Vp/Vs ratio by inverting stacked radial receiver functions using non-linear neighbourhood algorithm. Inversion results show sediment thickness varies between 1 and 2 km thick in Western Java. Our inversion shows that crustal thickness in this region varies between 25 and 32 km. Average crustal Vp/Vs ratio is estimated to be about 1.69–1.78. We hope the study may provide useful information for velocity model and crustal thickness for Indonesia region.

The Sumatran offshore region is known as an active seismogenic zone. Consequently, there were many records of great events that caused a big catastrophe, such as the 2004 Sumatra-Andaman earthquake. In order to investigate the geometry and nature of the earthquake ruptures and structures, we utilize the bathymetry and Multi-Channel Seismic (MCS) profiles from the SO-198 research project in 2008 (R/V Sonne) and KH-10-5 research project in 2010 (R/V Hakuho-Maru). We find that the area along the Sunda Trench to the fore-arc high in the northwestern Sumatra are characterized by parallel anticlinal ridges with distinctive vergence regimes. These vergence regimes classified as seaward, mix and landward vergence system. Our data interpretation recognized structures such as Main thrust, Lower thrust, Middle thrust, Upper thrust, piggy-back basin and West Andaman Fault within this region. Our conclusion suggest that the anticlinal seaward and landward vergence are considered as the product of recent deformation rupture. The co-seismic slip from the past earthquake ruptures tends that it was occurred from the deeper part in the entire northwestern of Sumatra fore-arc.

Most earthquake studies focus mainly on offshore subduction zones that often produce high-magnitude earthquakes. However, onshore active fault earthquakes also common to cause significant devastation, due to their proximity to human activity and their relatively shallow depth. Most of active faults in Java show a west-east (W-E) trending direction. However, the devastating Opak Fault ruptured in a NE-SW trend. Recent studies indicate that some shallow seismic clusters in Java cannot be associated with any known active fault traces. These seismic activities reported occurring in southern Garut, Cipamingkis (Sukabumi), Halimun-Salak Mountain (Bogor), and Grindulu in Pacitan. These seismic clusters show similar direction to the Opak Fault that ruptured an Mw6.4 earthquake in 2006 with thousands of fatalities. Here, we describe a brief review of the NE-SW trending active fault in the southern Garut area. Earthquake is a natural phenomenon with a repetitive mechanism. Therefore, we must be aware of the future hazard that has been demonstrated by recent seismic activities in this area.

Elucidating the Baribis active fault in the central part of West Java is challenging due to the lack of physiography appearance. A gravity survey was conducted on 838 stations around the North of Bandung to Subang region to identify its buried spatial fault continuity. Regular processing routine reduced the observed gravity acceleration to generate the refined Bouguer anomaly map. Additionally, edge detection techniques (e.g., tilt derivative and analytic signal) were analyzed to delineate the proven fault emergence from the published active fault database and surface geological map. The results depict that the Baribis fault was continued to the Subang region and enhanced clearly by applying analytic signal attributes. Thereby, the Baribis fault continuity is better interpreted on the analytic signal anomaly map, whereas the tilt derivative technique explains the factor of unclear surficial discontinuity of Baribis fault on the zone.

Continuous gravity monitoring as geodetic instruments for tidal variation recording has been proving its availability for more applications. The original purpose was to monitor the changes in gravity at one local, which could be correlated to the hydrological variation and geothermal reservoir changes. Most recently explored was its relation to the events of earthquakes. We have been installing the gPhone 145 since 2016 at LIPI Campus in Bandung as a preliminary observation of the instrument. In four years of installation, the gPhone has recorded the relative gravity intermittently, interrupted only by technical problems. Temperature and pressure records indicate the instrument functions properly. However, uneven recording prevented hydrological study, in which we expected to see some changes of the gravity through the years. For earthquakes’ precursor studies, additional samples of events and time-associated gravity anomalies might give more assuring results.

DOI: 10.17014/ijog.4.1.39-47The characteristic of Cimandiri Fault Zone has not been completely defined despite plenty of studies had already been accomplished. Therefore, an audio-magnetotelluric modeling was carried out. An audiomagnetotelluric survey was conducted at two parallel lines (N166oE) that intersected Cimandiri Fault Zone in Cibeber area, Cianjur. The distance between those two lines was 4.5 km and each line consisted of twenty-one stations with the distance between stations was 500 m. From the acquired forty-two apparent resistivity curves, inversion was executed to obtain two models. The models indicate layers with resistivity value of > 1000 ohm.m at about 500 m depth at both lines, which are associated to the basement layer. Columns of low resistivity zones in about the middle of each model represent fault zones as the weak zones of the area, and both models displayed them slightly dip southward as thrust faults.

Simeulue Island is an outer island arc off west of the Sumatra Island. The Island is located close to the interface of the subduction zone between Indo-Australian and Eurasian Plates. Seismic activities around the Island included devastating megathrust earthquakes, such as 2004 M W 9.2 Sumatra-Andaman and 2005 M W 8.7 Nias earthquakes. We investigate the site response characteristics using Horizontal-to-Vertical (H/V) Spectral Ratio method to the continuous ambient noise records from eight broadband seismometers. From the calculation, we generally observe strong peak of H/V spectral ratio which is caused by the strong impedance contrast at these area. However, sites BATU, LABU and DEHI show relatively flat H/V spectral ratio curves that caused by lack of sharp impedance contrast beneath the site. We also observe multiple peaks at several sites that may indicate the presence of a more highly weathered soil/clay layer on top of a more compact medium. H/V peak frequencies, which shows fundamental frequencies resonance of soil sites, generally are observed at range about 2.0 - 16.4 Hz. From the observed H/V spectral ratio, we suggest that the strong impedance contrasts may occur on the shallow part of the medium at 2 up to15 m depth.

Pandeglang is located on the coast of the Sunda Strait, which is tectonically active as the source of geological hazards. Basement configuration mapping is required for further understanding in geological characteristic of the land. Gravity method is one of the geophysical techniques that can be applied for the regional subsurface characterization. Gravity measurements were completed in the Pandeglang area, from Tanjung Lesung to Labuan coastline and all passable roads in Pandeglang Regency. The Bouguer anomaly obtained has indicated a steady decreasing anomaly from south-southwest to north-northeast area. The result of residual anomalies analysis and basement depth estimation presented a deep basin in the northeast most of the study area (Labuan – Picung), which indicates a thick sediment layer in this area.Keywords: gravity anomaly, residual anomaly, basin, basement, Labuan, Pandeglang.

Jakarta groundwater basin is one of the most developed basins in Indonesia as Jakarta city is located within the basin, with an elevation ranging from 0 to 1000 m above sea level. The study of the basement characteristics of groundwater basins still needs further study in the Jakarta Groundwater Basin. The objective of this study is to examine the basement characteristics of Jakarta Groundwater Basin by satellite gravimetry. Gravity forward models were constructed using Oasis Montaj and 2-D GM-SYS software for two north-south sections, A and B, and west-east section C-D. Regional anomalies in the study area with a value range of 36.7 to 53.2 mGal, while the residual anomaly values in the study area were in the range of -7.0 - 10.0 mGal. There are four rock formations based on forwarding modeling. Holocene Beach Ridge Deposit formation has a density of 2.0 gr/cc. The thickness of this formation is estimated to be around 10-20 meters. Late Pleistocene Alluvial fan has a density of about 2.2 gr/cc. The depth of this layer ranges from 10-50 meters. Early Pleistocene Formation, has a density of 2.35 gr/cc. The thickness of this formation is at a depth of 50-200 meters in the cross-section. The Tertiary Marine Sediment Formation is a basement of the Jakarta Groundwater Basin. This formation has a rock density of 2.45 gr/cc.

Pemetaan bawah permukaan diperlukan sebagai acuan dasar dalam kajian sumberdaya alam di daerah Serang, yang merupakan salah satu daerah dengan pertumbuhan industri dan populasi yang sangat pesat. Sebagai tahap awal dalam kajian regional, survey gayaberat dilakukan untuk memetakan anomali gayaberat di Kota Serang dan sekitarnya. Gayaberat diukur pada 204 titik pengamatan di Kota dan Kabupaten Serang dengan jarak antara titik kurang lebih sejauh 1 km. Dari hasil pengukuran diperoleh peta anomali gayaberat Bouguer yang menunjukkan setidaknya tiga kelompok anomali. Sisi barat (Kota Serang ke selatan) memiliki anomali gayaberat tinggi, sisi timur (Ciruas) memiliki anomali gayaberat rendah, dan sisi utara (hingga kepantai utara) memiliki anomali gayaberat sedang. Pemodelan bawah permukaan berdasarkan data anomali gayaberat tersebut menunjukkan adanya cekungan pada batuan dasar di sekitar Serang dan Tanara, yang diapit oleh tinggian batuan dasar di sisi barat dan timurnya. Kondisi batuan dasar demikian akan mempengaruhi keberadaan akuifer dan kemungkinan arah aliran airtanah. Subsurface mapping is required as a basic reference in the study of natural resources in Serang area. The Serang City and County are one of the areas with rapid industrial and population growth. In this preliminary study, a gravity survey was executed to map gravity anomaly of the area. The gravity field was measured at 204 stations in Serang City and County, with approximately 1 km distance between two stations. The result is a Bouguer anomaly gravity map that classified the region into 3 (three) units. The western part of study area, which includes Serang City to south, has a high gravity anomaly. The eastern part (Ciruas region) has a low gravity anomaly. And the northern part (to the north coast) has a moderate gravity anomaly. Subsurface modeling indicated a presence of shallow basin at the bedrock beneath the east part of Serang, flanked by ridges on the west and the east sides. Such bedrock topography condition would affect the origin of aquifers and possible flow of groundwaters.

The 2006 Yogyakarta Earthquake had caused a disaster in Bantul area. Several institutions had reported different results for the epicenter location. However, aftershocks studies indicated that the rupture area was at about 10 km east of Opak Fault. Analysis of gravity anomaly, including several degrees of residual anomalies and tilt derivative, facilitated this regional tectonic study to determine the structural constraints on the main earthquake and its aftershocks. The Yogyakarta area was primarily characterized by several SW-NE faults; one of them is the Opak Fault. Among those faults,, there are a series of WNW-ESE faults. Several groups of these lineations indicated a presence of some pairs of parallel strike-slips faults that formed pull-a-part basins. The obtained structural pattern has signified the dynamic response of the force from the subduction of the Australian Plate toward Sunda (Eurasia) Plate. The subduction force produced the strike-slip fault in a parallel direction of subduction, and subsequently, the faults caused the formation of thrust structures that are perpendicular to them.Gempabumi Yogyakarta pada tahun 2006 telah menyebabkan bencana di daerah Bantul dan sekitarnya. Lokasi episenter yang ditentukan oleh beberapa lembaga menunjukkan hasil yang berbeda. Tetapi analisa gempabumi susulan telah menunjukkan daerah pegerakan hingga 10 km ke sebelah timur dari Sesar Opak. Analisa anomali gayaberat yang terdiri dari perhitungan anomali sisa dan turunan kemiringan (tilt derivative) diharapkan dapat membantu studi tektonik regional dalam menentukan batasan struktur yang menyebabkan kejadian gempabumi di daerah Yogyakarta. Daerah ini dicirikan oleh sesar-sesar berarah BD (Barat daya)-TL (Timur laut), yang salah satunya adalah Sesar Opak. Di antara sesar-sesar tersebut, terdapat pula deretan sesar-sesar berarah BBL (Barat barat laut)-TTG (Timur tenggara). Beberapa kelompok kelurusan-kelurusan membentuk kemungkinan adanya cekungan pull-a-part, yang terbentuk karena adanya deretan sesar-sesar strike-slip. Pola struktur yang diperoleh menunjukkan respon dinamik dari subduksi Lempeng Australia terhadap Lempeng Eurasia (Sunda). Tekanan dari gaya subduksi menyebabkan terbentuknya sesar-sesar strike-slip. Kemudian sesar-sesar tersebut menyebabkan adanya struktur sesar naik yang tegak lurus terhadapnya.

The dimensionality and regional strike analyses of the Cimandiri Fault, West Java, Indonesia have been investigated. The Cimandiri Fault consists of six segments. They are Loji, Cidadap, Nyalindung, Cibeber, Saguling and Padalarang segments. The magnetotelluric (MT) investigation was done in the Cibeber segment. There were 42 observation points of the magnetotelluric data, which were distributed along 2 lines. The magnetotelluric phase tensor has been applied to determine the dimensionality and regional strike of the Cibeber segment, Cimandiri Fault, West Java. The result of the dimensionality analysis shows that the range values of the skew angle value which indicate the dimensionality of the study area are -5 ≤ β ≥ 5. These values indicate if we would like to generate the subsurface model of the Cibeber segment by using the magnetotelluric data, it is safe to assume that the Cibeber segment has the 2-D. While the regional strike analysis presents that the regional strike of the Cibeber segment is about N70-80°E.

We analyzed receiver functions to estimate the crustal thickness and velocity structure beneath two stations of Geofon (GE) network in the Sunda-Banda arc transition zone. The stations are located in two different tectonic regimes: Sumbawa Island (station PLAI) and Timor Island (station SOEI) representing the oceanic and continental characters, respectively. We analyzed teleseismic events of 80 earthquakes to calculate the receiver functions using the time-domain iterative deconvolution technique. We employed 2D grid search (H-κ) algorithm based on the Moho interaction phases to estimate crustal thickness and Vp/Vs ratio. We also derived the S-wave velocity variation with depth beneath both stations by inverting the receiver functions. We obtained that beneath station PLAI the crustal thickness is about 27.8 km with Vp/Vs ratio 2.01. As station SOEI is covered by very thick low-velocity sediment causing unstable solution for the inversion, we modified the initial velocity model by adding the sediment thickness estimated using high frequency content of receiver functions in H-κ stacking process. We obtained the crustal thickness is about 37 km with VP/Vs ratio 2.2 beneath station SOEI. We suggest that the high Vp/Vs in station PLAI may indicate the presence of fluid ascending from the subducted plate to the volcanic arc, whereas the high Vp/Vs in station SOEI could be due to the presence of sediment and rich mafic composition in the upper crust and possibly related to the serpentinization process in the lower crust. We also suggest that the difference in velocity models and crustal thicknesses between stations PLAI and SOEI are consistent with their contrasting tectonic environments.

Java Island has a unique tectonic features and one of it is the pattern of the volcano distribution along the island. The volcano pattern occurrence might be connected to the deeper process of geodynamic. Hence, thermal structure modeling is employed to find any association between the subducting plate and the tectonic of the overriding plate. Thermal structure of a subducting plate depends on the age of the lithosphere and the dip of the subduction. With the lithospheric age increases from west to east of Java, varies from about 90 my to 120 my old, there are dissimilarities in thermal structure models. The comparison with volcanoes positions shows that differences of thermal structure might have associated to variation in the formation of volcanoes in Java Island.

The accretionary prism complex has unique morphological characteristic as a result of deformation and sedimentation processes that develop in the area. Simeulue Island is one example of the islands in the accretionary complex. Geological study was arranged to map geological structure and morphology of Simeulue Island by satellite images interpretation and field survey. We use Landsat TM 5, Landsat 8 and Aster-GDEM imageries for morphology and structural geology interpretation. The collision in the deformation front has generally developed reverse and thrust faults in the Simeulue Island. The north-northeast blocks tend to be higher than the south-southwest blocks. Based on imagery interpretation, the dominant thrusts are the trenchward-vergence thrusts. Measurement of structural features and folds reconstruction indicated the existence of the out-of-sequence thrusts, which might activated the arcward-vergence thrust. This arcward-vergence thrust could be interpreted as a backthrust fault of a backstop mechanism in the inner accretionary prism. The thrusts might be correlated to the mud diapirisms that were also found in some locations in the island. The finding of thrusts and mud diapirism traces concurred to the theory that the melange complexes in subduction zones were formed through mud diapirism that facilitated by backthrust faults. ABSTRAK Kompleks prisma akresi memiliki karakteristik morfologi yang unik sebagai hasil dari proses deformasi dan sedimentasi yang berkembang di daerah tersebut. Pulau Simeulue merupakan salah satu contoh pulau yang terbentuk pada prisma akresi. Studi geologi dilakukan untuk menggambarkan keadaan struktur geologi yang terbentuk di Pulau Simeulue yang berhubungan dengan mekanisme pembentukan sesar-sesar anjak pada prisma akresi, pembentukan kompleks batuan bancuh dan mekanisme kubah lumpur. Morfologi dan struktur geologi Pulau Simeulue dipetakan melalui interpretasi citra dan survey lapangan. Citra yang digunakan dalam studi ini adalah citra Landsat TM5, Landsat 8 dan Aster-GDEM. Tumbukan di bagian muka deformasi mengakibatkan berkembangnya struktur berupa sesar-sesar naik dan anjak dimana di bagian utara-timurlaut cenderung lebih naik daripada bagian selatan-baratdaya. Berdasarkan hasil interpretasi citra, sesar-sesar yang berkembang tersebut umumnya merupakan sesar-sesar anjak berarah palung (trenchward-vergence thrusts). Berdasarkan hasil pengukuran fitur struktur di lapangan dan rekonstruksi lipatan di Pulau Simeulue, perkembangan struktur anjak juga terjadi secara out-of-sequence yang mengakibat-kan teraktifkannya sesar anjak berarah busur (arcward-vergence thrusts). Sesar anjak berarah palung dapat diinterpretasikan sebagai backthrust fault dari sebuah mekanisme backstop di bagian belakang prisma akresi. Sesar-sesar anjak ini dapat dikaitkan dengan keberadaan kubah lumpur (mud diapir) yang jejaknya juga ditemukan di beberapa lokasi di Pulau Simeulue. Hal-hal tersebut mendukung teori bahwa kompleks batuan melange di Pulau Simeulue terbentuk melalui proses mud diapirism yang muncul melalui sesar-sesar anjak.

The subduction of the Indian-Australian Plate beneath the Eurasian Plate is oblique (~45o) along Sumatra. A major zone of dextral strike slip displacement along Sumatra Island, called the Sumatra Fault, partially accommodates the oblique convergence off Sumatra. On the contrary, the subduction is normal along Java; therefore, no major fault zone can be found along the Java Island. The transition zone of the Sunda Strait fore arc, which is located between Java and Sumatra fore arc, is subject to northwest-southeast extension related to the motion of the Sumatra Fault and north-south compression because of subduction. Geophysical studies show that continuous extension dominated the Sunda Strait fore arc region. The results are interpreted as showing ongoing separation of the Sunda Strait fore arc region as the Sumatra fore arc plate has moved northwest, bounded by the Sumatra Fault. Therefore, the Sumatra Fault can also be interpreted to extend across the fore arc to the trench in the form of several graben systems. Key words: Sunda Strait, Sumatra Fault Zone, fore arc tectonic