Sumatran Fault Zone Research Articles

Study on earthquake and tsunami hazard: evaluating probabilistic seismic hazard function (PSHF) and potential tsunami height simulation in the coastal cities of Sumatra Island

This study uses integrated geological, geodesy, and seismology data to assess the potential tsunami and Probabilistic Seismic Hazard Function (PSHF) near Sumatra’s coastal cities. It focuses on estimating the possible level of ground shaking due to the seismic activity within the Sumatran Fault Zone (SFZ) and subduction zone. It uses the Peak Ground Acceleration (PGA) as a measure. An amplification factor that is based on the previous study is used. It is calculated through the Horizontal-Vertical Spectral Ratio (HVSR), which measures possible surface ground shaking. The Seismic Hazard Function (SHF) is calculated considering magnitudes 6.5 to 9.0 for subduction sources and 6.5 to 7.8 for SFZ sources. Also, the PGA based on the Maximum Possible Earthquake (MPE) magnitude is estimated, and tsunami heights are simulated to assess the possible hazard risk. The tsunami source model in this study is characterized by considering the possibility of the long-term perspectives on giant earthquakes and tsunamis that might occur in subduction zones around the off-coast of southern Sumatra Island. The potentiality source zone is characterized based on the utilization of the cross-correlation of correlation dimension (DC) based on the shallow earthquake catalog of 2010 to 2022 and the SHmax-rate of surface strain rate. Based on the MPE, the relatively high estimated PGA at the base rock was found around Mentawai and Pagai Utara islands at about 0.224 g and 0.328 g, with the largest estimated PGA based on the MPE at the surface with values of about 0.5 g and 0.6 g. The possible maximum tsunami height (Hmax) estimated based on source scenarios position around the west coast of Sumatera Island, such as for Kota Padang and Kota Bungus, reaches up to 12.0 m and 22.0 m, respectively. The findings provide valuable insight into seismic and tsunami hazards, benefiting future mitigation strategies.

Segmentation of the Sumatran Fault Zone based on spatial variation of b-values

Segmentation of the Sumatran Fault Zone based on spatial variation of b-values

Identifikasi Daerah Rawan Longsor secara Mikrozonasi di Jalan Alternatif Provinsi menggunakan Metode Simple Additive Weighting (SAW)

<p class="AbstractText">The alternative provincial in Bengkulu Tengah Regency is located in a location bordering the Musi Fault segment that is part of the Great Sumatran Fault Zone. This condition makes cross-provincial roads in Central Bengkulu Regency vulnerable to natural disasters such as landslides. The purpose of the study was to determine areas that have the potential for landslides in earthquake-prone areas of Central Bengkulu Regency, Bengkulu Province, Indonesia. We performed the HVSR (horizontal to vertical spectral ratio) analysis and the simple additive weighting (SAW) method to achieve the study purpose. Field data acquisition using the PASI Gemini2 broadband seismometer (triaxial geophone). Next, we analyzed the tremor data recorded in the field using a wave spectrum. From the results of this study, we knew that the possibility of landslides in the survey area classified into three categories that is low, medium and high. Landslide risk should be paid more attention in areas with high potential, especially in densely populated areas. The main threat in the study area is an earthquake along the active Sumatran fault. Earthquakes in areas that are very steep and prone to landslides can increase the risk and cause landslides.</p>

Tsunami scenario triggered by a submarine landslide offshore of northern Sumatra Island and its hazard assessment

Abstract. Near the northern border of Sumatra, the right-lateral strike–slip Sumatran fault zone splits into two branches and extends into the offshore, as revealed by seismic sounding surveys. However, due to its strike–slip faulting characteristics, the Sumatran fault zone's activity is rarely believed to cause tsunami hazards in this region. According to two reprocessed reflection seismic profiles, the extended Sumatran fault zone is strongly associated with chaotic facies, indicating that large submarine landslides have been triggered. Coastal steep slopes and new subsurface characteristics of submarine landslide deposits were mapped using recently acquired high-resolution shallow bathymetry data. Slope stability analysis revealed some targets with steep morphology to be close to failure. In an extreme hypothetical case, an earthquake of Mw 7 or more occurred, and the strong ground shaking triggered a submarine landslide off the northern shore of Sumatra. Based on a simulation of tsunami wave propagation in shallow water, the results of this study indicate that a potential tsunami hazard from several submarine landslide sources triggered by the strike–slip fault system can generate a tsunami as high as 4–8 m at several locations along the northern coast of Aceh. The landslide tsunami hazard assessment and early warning systems in this study area can be improved on the basis of this proposed scenario.

Low sulfidation epithermal gold mineralization at Tambang Sawah, Lebong, Bengkulu Province, Indonesia

The Tambang Sawah area, Lebong, Bengkulu Province, is geologically located in the Barisan mountain range and the Sumatran fault zone and is composed of igneous rocks of the Sunda – Banda magmatic arc. This study is aimed to elucidate geological framework and characteristics of the low sulfidation epithermal gold at Tambang Sawah. Gold mineralization is hosted by the lithology of granite and andesitic breccia rocks, and controlled by faults, especially the Ketahun fault which forms a fracture and extensional zone. Hydrothermal fluid filled the structure found to form gold-bearing quartz veins. Hydrothermal alteration recognized at the study area is a typical type of alteration found in epithermal gold deposits, namely, argillic, silicified and propylitic. The characteristic sulfide minerals are low sulfidation epithermal gold deposits in the Sawah Mine area which are found, pyrite, chalcopyrite, covelite, sphalerite and galena with vein textures of colloform banded, cockade-crustiform, moss, and brecciated texture. Gold is interpreted to be coevally deposited with sulfides within quartz veins. Gold grade in rocks and veins varies from 0.07 ppm to 21.3 ppm.

A Comparison of Geologic Structure Detection of Sumatera Island Using Goce Satellite Gravity Data and Sgg-Ugm-2 Data

GOCE gravity satellite data can be used for regional fault detection because the observation area is wide and not limited by area. In this study, GOCE satellite data is used to detect geological structures on the island of Sumatra, the results of which are used as the basis for disaster mitigation. GOCE data and SGG-UGM-2 were processed using the GOCE User Toolbox (GUT) software to produce a gravity disturbance map and a complete bouguer anomaly map. The GOCE obtained results were validated using the SGG-UGM-2 high-resolution gravity model data. The calculation results obtained that the gravity disturbance value from the GOCE data was around -140 to 200 mGal, while the value of the gravity disturbance from the SGG-UGM-2 data was around -180-300 mGal. The GOCE gravity disturbance map and the SGG-UGM-2 can detect the Subduction Trench, Mentawai Fault, and West Andaman Fault on Sumatra Island with negative values, while the Sumatran Fault Zone (SFZ) along Sumatra Island with positive values ​​in line with the presence of mountain ranges. The results of the SGG-UGM-2 data processing for the gravity disturbance are more detailed than GOCE because the SGG-UGM-2 data degree is higher than that of GOCE. GOCE complete bouguer anomaly value is around 40-560 mGal, while the value of complete bouguer anomaly SGG-UGM-2 is around 60-560 mGal. The complete bouguer anomaly maps from GOCE and SGG-UGM-2 can detect patterns from the Subduction Trench, Mentawai Fault, and West Andaman Fault but cannot clearly detect SFZ. The complete bouguer anomaly can also detect differences between oceanic and continental crust. The GOCE and the SGG-UGM-2 complete bouguer anomaly map show almost similar patterns and the ability to detect geological structures for sub and regional Sumatra Island. In addition, GOCE data detect geological structures more clearly than GRACE data.

Analysis of b-Value and Peak Ground Acceleration (PGA) in West Sumatra Province using Maximum Likelihood Method and Empirical Formula (Earthquake data Period 2007-2020)

West Sumatra Province has four active fault segments, namely part of the Sumatran fault zone which causes frequent earthquakes. Therefore, it is necessary to research b-value analysis and PGA in the province of West Sumatra by using the maximum likelihood method and empirical formula for earthquake disaster mitigation efforts. This study aims to determine the b-value associated with rock stress and PGA as the level of earthquake activity and analyze the distribution map. The data used is earthquake data for the period 2007-2020 with a magnitude ≥ 5 SR and a depth of ≤ 100 km. The results of data processing produce a map of the distribution of b-value and PGA in the province of West Sumatra. The b-value in each region ranged from 0.8421-1.4477. Based on the b-value distribution map, the area that has the smallest b-value is in region 6, while the largest value is in region 2. A low b-value correlates with high rock stress conditions. This value illustrates that area 6 has a high chance of a major earthquake occurring. Furthermore, the calculation of the PGA value refers to the general form of Lin and Wu's empirical equation to obtain a new empirical formula model. The value of the coefficient a=−1.20543, b=−0.839093, and c=6.88858. The PGA value of West Sumatra province ranges from 10.87-376.98 gal. The city/regency with the largest PGA value is in the Mentawai Islands Regency, which ranges from 84.11-328.17 gal and the lowest value is in Dharmasraya Regency, which is between 10.87-35.28 gal.

Crustal Structure Study in Lampung Region Using Teleseismic Receiver Function Analysis

Lampung region is seismically and volcanic active because located in subduction zone of Indo-Australian and Eurasian plate. We applied receiver function and stacking H-k analysis to estimate the crustal structure in Lampung region. We used teleseismic earthquake data (epicenter distance 30°-90°) and M>6 recorded at 3 seismic broadband stations owned by Agency for Meteorology Climatology and Geophysics (BMKG). Those stations are PSLI (located on Sebesi Island approximately 20 km from Anak Krakatau) represented volcanic arc zone, KASI (located on Kota Agung, Lampung) represented Sumatran Fault Zone and KLI (located on Kotabumi, Lampung) represented back-arc basin. Crustal thickness estimated at PSLI station 32-36 km, KASI station 36-40 km, and KLI station 30-36 km. Furthermore, in 3 stations P wave velocity estimated 4.1-11 km/s, S wave velocity 2.2-6.2 km/s, while vp/vs value estimated 1.7-2.05. We estimated Anak Krakatau volcano’s magma chamber beneath PSLI station in depth 16-30 km, Great Sumatran Fault structure in depth about 8-14 km beneath KASI station, and thick sediment layer about 4 km near surface beneath KLI station. This study result is expected to explain more detail crustal of Lampung region and can be useful for developing of BMKG’s seismic monitoring systems and other geophysical fields in future.

Identification of Capable Fault Location around Mount Betung Area Based on GPS Strain Data

Sumatran Island is located in a very active tectonic zone due to subduction of Indian-Australian plate beneath Eurasian plate. This causes an increase in stress in the subduction zone thereby increasing the pressure and strain. Mount Betung, which is situated in Sumatra Island, is located in the province of Lampung with a developing economy in recent years. Therefore, understanding the strain related to earthquake hazard is important to be conducted. The strain is a tool to help in identifying capable fault location in the geodetic aspect. The purpose of this study is to identify capable fault location to ensure the safety of development and construction around Mount Betung. GPS velocity data from 2006 to 2019 on seven sites are used as tool to calculate the strain which is used to identify the capable fault. GPS strain is calculated on the every triangles and polygons possible formed by the combination of three sites or more. There are two cases for identification, and triangles or polygons within each case do not intersect each other. The capable fault is identified in the area with the strain value of more than the threshold which is determined in the area of Sumatran Fault Zone. The threshold value resulted from the studies is 0.47 microstrain/year. The velocities of sites are ranging from -2.48 to -0.02 cm/year. The area with capable fault is identified around the Sumatran Fault Zone or the extension of the fault. The area close to the capital of Lampung Province, Bandar Lampung is relatively safe since there is no identified capable fault.

Source of the 2019 Mw6.9 Banten Intraslab earthquake modelled with GPS data inversion

The 2019 MW6.9 Banten Intraslab earthquake occurred at ∼100 km to the northeast of the Sunda Trench with two nodal plane models estimated by the Global Centroid-Moment-Tensor (GCMT) Project with a strike of 200° and a dip of 65°.Continuous GPS data from 11 GPS sites were used to model the source of the earthquake in three-components. The coseismic displacements and its uncertainties are obtained from the coordinates of these GPS sites from 7 days before to 7 days after the earthquake. The coseismic slip is the inversion result of those displacements based on the best fit in an elastic half-space. The maximum displacement is ∼5 cm with a large uncertainty that is comparable to the amplitude of displacement. A seismic moment of the best model (strike of 65° and dip of 54°) is 2.79 × 1019 Nm or equivalent to MW6.89. The fault model of the earthquake is highly presumed as a continuation of Sumatran Fault Zone.

Termination of a Trench-Linked Strike-Slip Fault Zone in the Sumatra–Java Forearc Basin and Accretionary Wedge Complex

This paper presents a review of several published seismic reflection and seismicity data and analyzes of high-resolution bathymetry data to revise the exact location and reveal detail characteristics of a strike-slip fault zone that formed the southernmost segment of the Sumatran Fault (SF). Previous works interpreted this fault segment as a horst structure to the south of a pull-apart basin. We observe a clear linear trace of dissected seafloor parallels to SF in the high-resolution bathymetric map. This structure extends from the south of a pull-apart basin in the northwest to the Sunda accretionary wedge farther southeast. This lineament exhibits a narrow valley and a linear ridge that in the subsurface are interpreted as negative and positive flower structures, respectively. The structure exhibits a vertical fault plane and appears to have deformed the accretionary wedge sediments and basement at depth. A cluster of shallow seismicity is observed along this NW-trending fault zone, indicating the activity of this zone. Here, we proposed this strike-slip fault as the Ujung Kulon Fault that marks the southeasternmost segment of the SF zone. This segment deformed the area of the Sumatra-Java forearc basin and terminated in accretionary wedge near the trench. The accumulated strain within UKF may trigger large earthquake in the future, close to the highly populated areas in the coast of Sumatra and Java.Keywords: Strike-slip fault, Sumatra Fault, Ujung Kulon Fault, segmentation, earthquake.

Segmentation of Sumatran Fault Zone in Tanggamus District, Lampung based on GPS Displacement and SRTM Data

One of sources of earthquakes in Lampung Province is southern segment of Sumatran fault. The fault is used as consideration in generating hazard map which source comes from stress accumulation of crust which can be obtained by measurement of points in surface. Determining fault location is important for sustainable development in Lampung Province. Southern segment of Sumatran fault stretched on Kota Agung – capital of Tanggamus District, one of the cities with the oldest civilization in Lampung Province. The sedimentation of Kota Agung causes the delineation of Sumatran Fault Zone by using Digital Terrain Model conducted on previous research had flaw. GPS Displacement Data, Shuttle Radar Topography Mission (SRTM) Data and seismic activity are used as tools to determine the location of fault. SRTM Data has 30 meter resolution and measured in 2018. Seismic activity used in this research is obtained from USGS from last 100 years and is used as support data to determine whether the fault delineated with SRTM and GPS displacement is active. GPS displacement is derived from GPS coordinates measured periodically from 2006 to 2019. Displacement used in this research is horizontal displacement since Sumatran Fault is known as strike-slip fault that move on horizontal plane. The result resembles previous research. Location that has the highest possibility of Sumatran Fault Zone segmentation is identified as Sumatran Fault Zone segmentation on previous research named as West Semangko Fault, and East Semangko Fault, respectively. The only fault earthquakes occurred on location which is not identified Sumatran Fault Zone segmentation on previous research.

Geodetic slip rate and locking depth of east Semangko Fault derived from GPS measurement

65 km Semangko Fault is part of southern segments of Sumatran Fault Zone (SFZ) which is complex corresponds to the transition from the strike-slip regime of the SFZ to the normal faulting tectonics of the Sunda Strait. The recent publication showed branches of Semangko Fault: West Semangko Fault (WSF) and East Semangko Fault (ESF). This study estimated geodetic slip rate and locking depth of ESF using Global Positioning System (GPS) time series data from 2007 to 2019 from all available GPS sites. GPS velocities refer to Sundaland Plate were used to estimate the fault parameters of the WSF and ESF simultaneously. Non-uniformity of velocity direction shows the complexities of Semangko Fault possibly caused by the contribution of all faults around ESF. An ESF geodetic slip rate, which is 12.5 ± 2 mm/yr was lower than WSF, which is 16.5 ± 2 mm/yr. It is consistent with the rigid block nature of the SFZ system as northern segment slip rates have similar value. Small slip rates are possibly leading to lower generated seismic moment than the major segment of SFZ.

Structural Controls on Rupture Extent of Recent Sumatran Fault Zone Earthquakes, Indonesia

AbstractWe present geodetically derived coseismic slip models for four Mw ~6 strike‐slip earthquakes that struck the mainland island of Sumatra, Indonesia, between 2007 and 2016. Three of these earthquakes ruptured segments of the Sumatran Fault Zone: the 2007 Mw 6.3 and 6.4 Lake Singkarak doublet, and the 2009 Mw 6.6 Dikit event. The fourth earthquake, the 2016 Mw 6.6 Pidie Jaya event, ruptured an unmapped active fault in a region of historically low seismicity near the northern coast of Aceh. Our results suggest that (1) the estimated rupture extents of the 2007 Lake Singkarak doublet and 2009 Dikit event were limited by geological structures such as step overs that define segment boundaries, (2) the 2016 Pidie Jaya event did not produce surface rupture despite the fact that it produced large shallow slip of more than 2.5 m, and (3) the remaining seismic gaps on the Sumatran Fault Zone are primarily located between the equator and northern Sumatra. We strongly recommend that seismic hazard assessment for densely populated areas in Sumatra include threats not only from the Sumatran Fault Zone but also from additional potentially active faults outside of the main Sumatran Fault Zone.

Characteristic of Geothermal System at Semurup Manifestation, Kerinci: Geological and Geochemistry Investigation-Based

Semurup Village on Kerinci Regency, Jambi is indicated potential for geothermal resources; it is proved by the presence of hot springs (Gao Luluk, Gao Dili, and Gao Mudik), non-active geyser, and silica sinter deposit. This research aimed to investigate reservoir characteristics which the approach was to interpret the Semurup hot spring fluid type and reservoir temperature based on elements fluid contained. This research was carried by the analysis of B, Al, Ca, Mg, Na, K, Li, F, Cl, SO4, HCO3 contained of hot spring for reservoir characterization conducted by Cl-SO4-HCO3, B-Li-Cl, and Na-K-Mg ternary diagram and conductive silica geothermometer for reservoir temperature estimation. Semurup hot spring was classified by volcano-tectonic geothermal system, because it was associated with Quartenary Lumut Volcano and Sumatran Fault Zone. Unit stratigraphy was composed by Eosene pyroclastic rocks of Bandan Formation (kaoliniteclay minerals, calcite, feldspar) and Undifferentiated Volcanic Rock Formation (silica minerals, chlorite, and calcite). Geological setting was possibly vary of hot springs (chloride -bicarbonate) and deposits (silica -bicarbonate). There were three types of geothermal manifestation, which were neutral hot spring, geyser, and silica sinter. The result showed that GaoLuluk fluid was dominated by Cl at 53.06% while that of GaoMudik was high in bicarbonate at 72.81%. Silica conductive geotermometers gave equilibirum reservoar temperatures ranging from 166°C -241°C. Based on the geological and geochemical investigation, the reservoir fluid of Semurup manifestation had occured dilution with calcite-contains on groundwater and the reservoir temperature was classified as moderate -high, which was high potential for geothermal energy resource.

Preliminary Result of Hypocenter Relocation Using Double Difference Method along Sumatran Fault, Indonesia

The Sumatran fault zone has a high seismicity and seismic hazard threat levels due to this fracture through densely populated areas. The fault zone of Sumatera has 19 main segments which each segment has the possibility to generate a strong earthquake. Therefore, in this study we attempt to improve the accuracy of hypocenter locations which is needed as basic analysis in advance seismology and in seismic hazard analysis. The final objective of this study is to analyze the subsurface seismic velocity structure of the Sumatran fault by applying seismic tomography method. For the preliminary result, we have been conducting double-difference method to relocate 967 events recorded by 49 stations compiled by Meteorological, Climatological, and Geophysical Agency of Indonesia (MCGA) from 2009 to 2017 along Sumatran fault. Our preliminary results from the relocation proces indicate prominent features of event cluster around Sumatra fault system. The next step, we will improve the onset of P-and S-wave arrival time by using waveform cross-correlation in order to get more reliable hypocenter location.

OPTIMAL BENCH MARK DISTRIBUTION TO MONITOR SOUTHERN SEGMENT OF SUMATRAN FAULT

<p><em>The s</em><em>outhern segment of</em><em> a</em><em> Sumatran fault zone </em><em>was</em><em> one of </em><em>the </em><em>sources of earthquakes in Lampung Province. The source of hazard c</em><em>ame</em><em> from stress accumulation of crust</em><em>,</em><em> which can be derived from movement of </em><em>bench mark</em><em> in surface.</em><em> Lack of research in southern segment </em><em>was</em><em> caused by small numbers of monitoring bench mark. </em><em>This research show</em><em>n</em><em> optimal monitoring bench mark distribution by considering existing bench mark and location which is decent and representative to monitor Sumatran Fault Zone movement by considering on its position relative to Sumatran Fault Zone, earthquake history, Digital Elevation Model and land use.</em> <em>Decent location </em><em>was</em><em> determined by overlaying land use and slope processed from Digital Elevation Model</em><em>.</em> <em>Representative location was determined by taking into account the distance to Sumatran Fault Zone and earthquake history.</em> <em>Very decent location is around 10.5 percent of the total area. Very representative location is around 44.5 percent of the total area.</em> <em>There are total 15 planned bench marks location to make southern Segment of Sumatran Fault Zone monitoring bench mark more optimal.</em><em></em></p>

Moment tensor analysis using regional and temporary deployment 2008 data for Sumatran active fault zone earthquakes

The purpose of this study is to determine the focal mechanisms of the earthquakes through moment tensor inversion of seismograms derived from local events in Sumatran fault zone. We use ISOLA software for data processing which is widely used to estimate the source parameters from local events. We retrieve seismogram data from three selected local events recorded by ZB temporary deployment and BMKG-GEOFON networks. The inversion results reveal that the deformation style around this fault zone is dominated by strike slip motion with shallow focal depths. These results are also consistent with regional active tectonics revealed by other geophysical and geological studies.

Revisiting seismic hazard assessment for Peninsular Malaysia using deterministic and probabilistic approaches

Abstract. Seismic hazard assessments, both deterministic and probabilistic, for Peninsular Malaysia have been carried out using peak ground acceleration (PGA) data recorded between 2004 and 2016 by the Malaysian Meteorological Department using triaxial accelerometers placed at 19 seismic stations on the peninsula. Seismicity source modelling for the deterministic seismic hazard assessment (DSHA) used historical point sources whereas in the probabilistic (PSHA) approach, line and areal sources were used. The earthquake sources comprised the Sumatran subduction zone (SSZ), Sumatran fault zone (SFZ) and local intraplate (LI) faults. Gutenberg–Richter law b value for the various zones identified within the SSZ ranged between 0.56 and 1.06 (mean=0.82) and for the zones within the SFZ, between 0.57 and 1.03 (mean=0.89). Suitable ground motion prediction equations (GMPEs) for Peninsular Malaysia along with other pertinent information were used for constructing a logic tree for PSHA of the region. The DSHA “critical-worst” scenario suggests PGAs of 0.07–0.80 ms−2 (0.7–8.2 percent g), whilst the PSHA suggests mean PGAs of 0.11–0.55 ms−2 (0.5–5.4 percent g) and 0.20–1.02 ms−2 (1.9–10.1 percent g) at 10 % and 2 % probability of exceedance in 50 years, respectively. DSHA and PSHA, despite using different source models and methodologies, both conclude that the central-western cities of Peninsular Malaysia, located between 2 and 4∘ N, are most susceptible to high PGAs, due to neighbouring active Sumatran sources, SFZ and SSZ. Of the two Sumatran sources, surprisingly, the relatively less active SFZ source with low magnitude seismicity appeared as the major contributor due to its proximity. However, potential hazards due to SSZ mega-earthquakes should not be dismissed. Finally, DSHA performed using the limited LI seismic data from the Bukit Tinggi fault at a reasonable moment magnitude (Mw) value of 5.0 predicted a PGA of ∼0.40 ms−2 at Kuala Lumpur.

Structure of the central Sumatran subduction zone revealed by local earthquake travel-time tomography using an amphibious network

Abstract. The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp∕vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw = 8.7) and to the south in 2007 (Bengkulu earthquake, Mw = 8.5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ∼ 180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s−1 at 10 km depth and the vp∕vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp∕vs values of 1.85 at ∼ 150 km distance to the trench in the region of the Mentawai Fault. vp∕vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust.