Back-arc Thrust Research Articles

Early Results of Eastern Indonesia P-wave Tomography Study Using Regional Events

The tectonic system of Eastern Indonesia is controlled by several major and minor plates, such as Indo-Australian, Australian plate, and Pacific plates. This area is known for its complexity, and high seismic activity. This study tries to image the complex structures beneath this region by employing regional events data and seismic tomography methods. We used five years of regional events catalog provided by the Indonesian Agency of Meteorology, Climatology, and Geophysics. We have sorted 7336 events recorded between 120° – 136° longitude and 0° – 13°(-) latitude consisting of 46446 P and 15467 S wave arrival data. Relocated hypocenter map shows a better constrain location on seismicity along outer Bandar Arc. A dipping pattern of seismicity is seen that is going deeper to the Banda Sea. The seismicity map also images a steep angle pattern of seismicity that could be related to the subduction slab roll-back model at North of Wetar island. Interestingly, we spotted a seismicity gap in West Seram that could be linked with slab tear zone. The checker-board test suggests a proper resolution is still reliable to a depth of 200 km with a less interpretable model at a depth of 300 km. P-wave tomographic models image the high velocity dipping down going slab. The Banda slab is seen to subduct from south Timor Island to the north, from east Tanimbar and Aru Island to west part, and from north Seram Island to south. We observed the down-going slab meet from all directions at about 300 km beneath the Banda sea. P wave tomogram also shows the Timor Island slab has a steeper dip that agrees with the seismicity pattern. Near the Seram island, we identify a low-velocity anomaly zone infiltrate the Banda slab beneath the shallow part of West Seram, which was previously interpreted as slab tear zone. This study also noticed a higher velocity tomogram model at North of Wetar island that might indicate a back-arc thrust. Lastly, a low-velocity band is also exposed at a shallow depth close to the volcano chain along that Banda volcanic arc.

Early Results of P Wave Regional Tomography Study at Sunda-Banda Arc using BMKG Seismic Network

The plate movement, geological structure, magmatism, and seismic activity in the area of Bali to East Nusa Tenggara are mainly related with the subducting of Indo-Australian Plate underneath the Eurasian plate. The complexity is added with the recent collision of Australian continent lithosphere with the western Banda arc, along the islands of Flores, Sumba and Timor island. Our study area is known as the Sunda-Banda arc transition. With the aim of imaging subsurface structure, we perform seismic tomography inversion using regional events. We collected 5 years of earthquake data (January 2015 – December 2019) from the Indonesian Agency of Meteorology, Climatology, and Geophysics (BMKG). The output of our data processing is not limited to only P wave velocity model, but also relocated seismicity pattern in the region. In general, seismicity pattern shows dominant shallow events in the south that progressively shift into deeper events in the north down to a few 500 km, marking a dipping subduction zone in this region. A group of shallow events down to a depth of 50 km is also seen at the norther region that may relate to back-arc thrust activity. P wave tomogram model show a lower velocity perturbation at a depth of 30 km that could be associated with magmatic activity along the volcanic front line. Higher P wave perturbation model are spotted at two different zones, the first one is marking a dipping Indo-Australian plate down to depth of 400 km. We noticed that the angle of dipping is steeper in the Eastern part compared to the Western part. The second a relatively flat at shallow depth at the northern region from the island of Lombok to Nusa Tenggara Timur that may mark the back-arc thrust region

Initial Result of P Wave Tomography Model in Sunda-Banda Arc Transition using FMTOMO

Our study area is located near island Sumbawa, Sumba, Flores, West Timor, Indonesia and East Timor, popularly known as Sunda-Banda arc transition zone. The tectonic setting is mainly controlled by the movement of the oceanic lithosphere Indo-Australian plate subducting the Eurasian plate and Northward migration of Australian continental lithosphere into western Banda-arc in the region of Flores, Sumba and Timor island. We tried to image velocity structure beneath these regions using regional events and tomography inversion model. We collected 5 years of regional events from the Indonesian Agency of Meteorology, Climatology and Geophysics. In total, we reserved 3186 events recorded on 29 stations. For data processing, we used fast marching method as ray tracing between sources and receiver. We then employed subspace inversion as the tomography procedure to estimate the best velocity model representing the tectonic model in the region. Hypocenter data distribution is concentrated on shallow parts of the region and along the Benioff zone down to a maximum depth of 400 km. One of challenge of this study is that although events are abundance, the stations used are mostly located onshore and does not extend in the south-north direction that leads us to under determined problem in the inversion process. However, checker-board models show most our target area can be retrieved to its initial model with sign of smearing effects shown start from a depth of 50 km. After six iteration and optimized selection of damping and smoothing parameters, we observed low velocity anomaly under Bali, Lombok, Sumba, East Nusa Tenggara at shallow depth that may be related with volcanic activity. Deeper low anomaly can also be seen that may be related with partial melting process. A band of fast velocity is clearly seen that goes deepen to the north depicting subducting slabs own to a depth of 300 km. We also observed a possible of fast velocity in the northern part of our stations at shallow depth that we believe may represent the back arc thrust.

Preliminary Results of Double Difference Tomography at Sunda-Banda Arc

The Sunda-Arc transition to the Banda Arc is located on the south of the Flores Island, Indonesia, where the Australian lithosphere is moving to the north direction. On-going subduction process dictates the tectonic setting though some studies also suggest a collision and obduction may occur in the past due to of plate buoyancy. This area has active seismicity with frequent large magnitude events. To better understand the tectonic system in this region, we performed double-difference tomography inversion using regional events. We obtained the data catalog from the Indonesian Agency of Meteorology, Climatology, and Geophysics ranging from 116° to 125° east longitude and -6.5° to 12.5° latitude. We collected 4312 events data, detected from 15 stations from January 2015 to December 2019. Final relocated hypocenters showed a reduced fixed-depth problem and a more clustered event, although some deep events disappear. Most events are related to the subducting Benioff zone with some clustered events in the northern area may be related to back-arc thrust. We also observed clustered events near active volcano region and reduced shallow seismicity region to the west of the Timor Island. Resolution test using the checkerboard and Derivative weigh Sum (DWS) shows that fair P wave resolution can be achieved until 300 km, although a smearing start to show at a deeper depth. However, due to lack of arrival S wave data, the resolution test suggest good resolution can only be seen until a depth of 100 km. Tomogram P and S wave models show a clear dipping subducting slab from south to North down to a 250 km. We also spot a fast velocity band near the Timor Island area that similar to the previous tomography study, interpreted as sliver forearm. We spotted a band of lower Vp, lower Vs and higher Vp/Vs at shallow depth close to the volcanic line and we interpreted this as a zone of higher temperature, that may relate to magmatic activity in this region. We also noticed a zone of low velocity and higher Vp/Vs that may relate with dehydration and partial melting. However, we feel this still uncertain due to low Vs resolution.

Subsurface Structure Models Of Sumbawa Island And Flores Back Arc Thrust Based On Gravity Data

Based on the BMKG report on Wednesday, July 29, 2018, at 05.47 WIB, an earthquake with a magnitude of 6.4 SR occurred at the epicenter at a depth of 24 km and was 47 km northeast of the island of Lombok. They are allegedly originating from the Back Arc Thrust Flores, the interaction between the Indo-Australian Plate and the Eurasian plate. This study aims to analyze the subsurface structure of the island of Sumbawa and Flores sea based on the distribution of density values. The analysis was carried out by modeling the subsurface structure based on regional gravity field anomaly data. The data used is secondary data downloaded from the topex.ucsd.edu page 18,400 measuring points. Data processing consists of data correction, separation of anomalies, determination of the Second Vertical Derivative (SVD) value, and 3D and 2D inversion modeling. The data corrections performed are Bouguer correction and terrain correction to produce the total gravity anomaly value. Upward continuation is used to separate regional anomalies and residual anomalies. Analysis of the Second Vertical Derivative (SVD) value was carried out to identify rock contact positions and fault structures. The 3D inversion modeling is done by making a mesh model, and to get a 2D cross-section, six slices are used in the 3D model. The results of the 3D model and 2D cross-section, namely the island of Sumbawa to the Back Arc Thrust zone of Flores, consists of 3 main layers comprised of the upper crust with a density of 2.29 gr/cc - 2.63 gr/cc, the middle crust with a density of 2.64 gr/cc - 2.90 gr/cc, and the lower crust with a density of 2.91 gr/cc - 3.14 gr/cc, and based on the model and SVD value, it shows that the Flores back fault, the reverse fault type, starts at a depth of ± 26 km

Tsunami Hazard Potential Modeling as Tourism Development Considerations in the North of Lombok Strait

Currently, the central government and provincial governments of Bali and West Nusa Tenggara are promoting tourism development in North Bali and North Lombok. In the development of tourism, besides consider the potential of existing tourism objects, the economy, and its supporting facilities, the potential threats of disasters must also be considered. Based on historical records, one of the potential disasters in northern Bali and Lombok is a tsunami. This study aims to determine the height and the arrival time of the tsunami along the northern coasts of Bali and Lombok. This study was carried out by simulation and numerical modeling, using the TUNAMI Model. Modeling was carried out for 2 hypothetical scenarios caused by an earthquake from the Flores Back-arc Thrust. Base on this model, it is known that the height of tsunami on Trawangan Island is ± 6 m with an arrival time of ± 3 minutes, while on the north coast of Bali the height of the tsunami is ± 0.6 m with an arrival time of ± 21 minutes. The arrival times of the tsunami on the north coast of Bali and Lombok are so fast.

Local earthquake tomography of the source region of the 2018 Lombok earthquake sequence, Indonesia

SUMMARY We develop and present a 3-D seismic velocity model of the source region of the 2018 Lombok, Indonesia earthquakes by using local earthquake tomography. The data consist of 28 728 P- and 20 713 S-wave arrival times from 3259 events which were recorded by 20 local seismic stations. The results show that most of the significant earthquakes occur to the edge of high-velocity regions. We interpret these to represent coherent blocks of the Flores Oceanic Crust underthrusting Lombok. At depths shallower than the nucleation area of the largest earthquake, many triggered aftershocks are located within a low-velocity, high-Vp/Vs region which is probably a highly fractured fault zone with a large amount of fluid. This fault zone is parallel to the dip of the Flores Back Arc Thrust and probably ruptured during this earthquake sequence. A prominent low-velocity, high-Vp/Vs region is colocated with the northwest and southern flank of the Rinjani volcanic complex. This large aseismic region is probably related to a wide area of the crust containing fluids due to ongoing magma intrusion beneath the volcano. To the east of Rinjani Volcano a cooled intrusive complex was imaged. It is characterized by high-velocity and low-Vp/Vs, supported by the presence of a high Bouguer anomaly. We confirm the existence of the Sumbawa Strait Strike-Slip Fault and find it is characterized by an elongated low-velocity, high-Vp/Vs zone.

E-Alert Application In Facing Earthquake Disaster

Disaster is a phenomenon that has a destructive effect and arises with or without predictions that always accompany human life. This damaging impact can be in the form of loss of life and / or property loss so that it disrupts the natural and social order. Lombok is one of the islands in the group of islands in West Nusa Tenggara. The island of Lombok is flanked by two large faults, the Flores Back Arc Thrust (FBAT) which extends north of Nusa Tenggara to Bali and Megathrust on the south of Lombok. Seeing from the great potential of earthquake disasters that can occur at any time on the island of Lombok, it is necessary to prepare disaster mitigation well in order to minimize the impact of earthquake disaster risk. From the problems outlined, the researchers plan to create a mobile-based earthquake e-standby application. This study aims to create a mobile-based earthquake e-alert application that can be used to view the latest earthquake information, information for determining the nearest evacuation location and educational media related to earthquake disaster awareness. The stages of the research use the prototype development method with stages, namely the study of literature, collection of needs, data collection, needs analysis, design, prototype implementation, prototype evaluation, implementation and testing and conclusions. The conclusion of this research is the construction of a mobile-based earthquake e-alert application. This application is useful for improving, understanding and public awareness of the risk of earthquake disasters and as an educational media to create disaster resilient communities

Hypocenter and Magnitude Analysis of Aftershocks of the 2018 Lombok, Indonesia, Earthquakes Using Local Seismographic Networks

Abstract The island of Lombok in Indonesia is located between the Indo-Australian and Eurasian subduction trenches and the Flores back-arc thrust, making it vulnerable to earthquakes. On 29 July 2018, a significant earthquake Mw 6.4 shook this region and was followed by series of major earthquakes (Mw>5.8) on 5, 9, and 19 August, which led to severe damage in the northern Lombok area. In this study, we attempt to reveal the possible cause of the sequences of the 2018 Lombok earthquakes based on aftershock monitoring data. Twenty stations were deployed to record earthquake waveform data from 4 August to 9 September 2018. In total, 3259 events were identified using 28,728 P- and 20,713 S-wave arrival times during the monitoring. The aftershock hypocenters were determined using a nonlinear approach and relocated using double-difference method. The moment magnitude (Mw) of each event was determined by fitting the displacement spectrum amplitude using a Brune-type model. The magnitudes of the aftershocks range from Mw 1.7 to 6.7. The seismicity pattern reveals three clusters located in the Flores oceanic crust, which fit well with the occurrences of the four events with Mw>6. We interpret these events as the main rupture area of the 2018 Lombok earthquake sequence. Furthermore, an aseismic zone in the vicinity of Rinjani extending toward the northwestern part of Lombok was observed. We propose that the crust in this area has elevated temperatures and is highly fractured thus inhibiting the generation of large earthquakes. The aseismic nature is therefore an artifact of the detection threshold of our network (Mw 4.6).

Identification of Fault Structure in Lombok region, West Nusa Tenggara using Tomography Lombok Earthquake Data of July-August 2018

Lombok and Nusa Tenggara are one of the regions in Indonesia that have quite complex tectonic arrangements. With this order, not infrequently in the region there are often natural disaster phenomena. One of the most striking things is the occurrence of a series of earthquakes of magnitude Mw > 5.0 that rocked the northern region of the Island of Lombok on 29 July 2018 (Mw = 6.4), 5 August 2018 (Mw = 6.9), 9 August 2018 (Mw = 5.9), 19 August 2018 (Mw = 6.3 and 6.9) and 25 August 2018 (Mw = 5.5). This study aims to identify the condition of tectonic structures under the surface of the earthquake occurring region using the tomography method. This method utilizes earthquake travel time data recorded at 15 BMKG seismic stations. The tomogram results show the contrast of anomalous values in the Vp and Vs models. The contrast of the anomalous is associated with the presence of fault structures that have an allowance angle of about ± 20-30° and a thrust fault type, verified by focal mechanism ball data. This thrust fault is then indicated as the cause of the occurrence of earthquakes with magnitude Mw > 5.0, which shook the northern part of the island of Lombok in July-August 2018. According to the tomography result, the fault is located closer to the north side of Lombok Island than Flores Back arc Thrust.

Relocated aftershocks and background seismicity in eastern Indonesia shed light on the 2018 Lombok and Palu earthquake sequences

SUMMARYHigh seismicity rates in eastern Indonesia occur due to the complex interaction of several tectonic plates which resulted in two deadly, destructive earthquake sequences that occurred in Lombok Island and the city of Palu, Sulawesi in 2018. The first sequence began in July with an Mw 6.4 event near Lombok, culminating in an Mw 7.0 event 8 d later. This was then followed by a nearby Mw 6.9 event 12 d later. Approximately 1000 km to the northeast, a separate sequence began several weeks later near Palu where an Mw 7.5 event occurred that triggered a tsunami. In this study, we present hypocentre relocations for both earthquake sequences as well as all other regional earthquakes in eastern Indonesia. The relocations were performed using a teleseismic double-difference relocation method and arrival times for P and S waves from stations at local, regional, and teleseismic distances. The catalogue and phase data were taken from the Agency for Meteorology, Climatology and Geophysics (BMKG) of Indonesia and the International Seismological Centre (ISC) for the period of April 2009 through November 2018. The relocated catalogue provides an improved view of seismicity in eastern Indonesia over the study period, sharpening locations and interpretations of seismogenic features throughout the region. In the Lombok area, the relocated earthquakes clearly show a backarc thrust to the north of the Sunda-Banda Arc transition zone. The relocated aftershocks show that the destructive Mw 7.0 and Mw 6.9 earthquakes of the Lombok sequence ruptured two different regions: The Mw 7.0 earthquake propagated westward, whereas the Mw 6.9 earthquake propagated eastward. The entire sequence of Lombok earthquakes was most likely started by the Mw 6.4 event as the initial event or foreshock, which then triggered backarc thrusts on both sides. Several weeks later and far to the northeast, the Mw 7.5 Palu earthquake occurred along the Palu-Koro Fault, filling a seismic gap that had not ruptured in an Mw 6.0 event or larger since at least 1900. The distribution of aftershocks indicates that the northern part of the Palu-Koro Fault has lower relative seismicity rates than the southern part at shallow depths, and that off fault aftershocks are mostly located to the east of the Palu-Koro Fault.

Did the Flores backarc thrust rupture offshore during the 2018 Lombok earthquake sequence in Indonesia?

SUMMARYThe Flores thrust forms the west segment (∼450 km) of a very active, ∼E–W striking, ∼800-km-long backarc thrust along the east Sunda Arc. In 2018, a deadly earthquake sequence composed of ~110 M4+ events rattled the Indonesian island of Lombok near the Flores thrust and caused tremendous damage on the island, however what is the nature of rupturing during this earthquake sequence remains unknown. Here, using a total of 2120 km of high-resolution seismic profiles covering ∼300 km of the Flores thrust off Bali, Lombok and Sumbawa, in addition to earthquake data and InSAR measurements, we investigated the active thrusting during this earthquake sequence. Our seismic interpretation and structural mapping show that offshore north of Lombok and Bali, the remarkable Flores thrust is essentially blind, deforming the seabed by folds, not faults. The Lombok earthquakes were all shallow thrust events with depth <40 km and occurred within ∼35 km north of the Rinjani volcano beneath the Lombok Island and its northern extremity. The InSAR measurements suggested that the most of the crustal deformation caused by these earthquakes occurred the north the and northeast of the island. The maximum vertical deformation was ∼36 cm near the northwest margin of the island, caused by the 5th August Mw 6.9 event. These observations combined with the presence of blind thrusts off Lombok suggest that the offshore portion of Flores thrust did not rupture during the 2018 Lombok earthquake sequence; the most coseismic slip must have occurred along a deep-rooted, north-verging basal fault and a range of imbricate thrusts beneath the north of the island, not along the buried thrusts offshore. Despite being blind off Lombok and Bali, the Flores thrust can still pose tsunami threats to the adjacent population centres by rupturing the seafloor during future large earthquakes (M > 7) that occur directly on the offshore blind thrusts, not beneath the island like the Lombok sequence. The proximity of the Rinjani volcano and thrust earthquakes suggests a possible role of volcanic activity (e.g. magmatic fluids and gas migration, stress change induced by pressurized magma chamber) in inducing the Lombok earthquakes.

Penentuan Persamaan Empiris Percepatan Tanah Di Wilayah Bali Berdasarkan Data Gempa Bumi Periode 2008-2016

The earthquake was the most common natural disaster in Indonesia, one ofwhich was the Province of Bali. The southern part of Bali is a meeting area of two earth plates (subduction zones) between the Eurasian and Indo-Australian plates and in the northern region there is a back arc thrust fracture which results in earthquake frequencies in the Bali area. One of the parameters that is very instrumental in determining the level of damage to land and buildings that occur on the surface of the earth due to earthquake shocks is the acceleration of the soil. A research has been conducted which aims to find out the empirical equation of land acceleration for Bali in the 2008-2016 period, namely by determining the empirical equation of land acceleration (PGA) conducted using magnitude parameters, hypocenter distance and PGA (observation) values in 2008-2016 with coordinates 7 0 -12 0 LS and 113 0 -118 0 BT. Through the regression analysis approach, to obtain the empirical equation of land acceleration in the Bali region is 𝑙𝑜𝑔10(𝑃ðo𝐴) = (−1,89) 𝑙𝑜𝑔10(𝑅) + (0,71) ð‘s𝑏 + 1,18 . Then several approaches and correlation analysis are performed to see the compatibility of the new empirical equation with the existing one.

Geological constraints for disaster mitigation model in South Sulawesi

South Sulawesi is one of the provinces in Indonesia with the highest economic growth index. This situation gives an impact to intensive infrastructure development including road access, bridge, dam, airport, seaport and other public facilities in the province. However, due to geological complexity of the region, the infrastructures developments are threatened by natural hazard. This paper outlines the characteristic of natural disaster in South Sulawesi using geological consideration of the region for mitigation strategy. The natural hazard in South Sulawesi consists of earthquake, landslide, tsunami and flood. Earthquake occurs in association with some major faults such as Walanae Fault, Palu-Koro Fault and Matano Fault. All areas located near these fault zones are classified as earthquake-prone areas. Landslides are classified as slides, flow and rock fall and takes place in some high topography areas which composed of moderately to highly weathered volcanic product material. Geologically the landslides are closely related with morphology, stratigraphy and structure (seismic-related) condition. Some coastlines in South Sulawesi province are tsunami-prone area, especially the area facing to the seismically-active region such as southern tip of the peninsula where tsunami is generated by the Flores back-arc thrust fault. Flooding shows a strong relationship to geological condition in the area especially low-lying topographic areas in some coastlines and river mouth areas. Three main causes of the flood have been considered namely deforestation due to land conversion, high sedimentation rate and sporadic urbanization coupled with heavy rain fall and tides. The study shows that all natural disasters in South Sulawesi are mainly controlled by geological condition such as fault, morphology, stratigraphy, petrology and seismicity. Therefore, in order to mitigate these natural hazards, geological constraints should be taken into consideration in natural hazard mitigation scheme in the regional level.

Ground Deformation Identification related to 2018 Lombok Earthquake Series based on Sentinel-1 Data

Lombok Island was hit by a series of earthquakes in July and August 2018 with magnitude 6 class. This series of earthquakes resulted in fatalities and material losses that even reached Sumbawa Island to the east of Lombok Island. The earthquake was triggered by Flores back arc thrust resulting in ground deformation. Ground deformation can be identified by satellite-based remote sensing method. The Sentinel-1A and Sentinel-1B satellites are two satellites carrying C-band SAR sensors with a temporal resolution of 12 days each for the same orbit, and the difference in time between the two is 6 days. Therefore, ground deformation related to seismo-tectonic or volcanic activities can be identified by interfering two SAR images (interferometric synthetic aperture radar or InSAR) at least in 6 days. By utilizing Sentinel Application Platform (SNAP) a free open source software (FOSS) and combining with other InSAR software, an interferogram that represents line of sight displacement (LOS) between ground and satellite can be generated. Line of sight displacement can then be interpreted as ground deformation signals. It is shown that the series of Lombok earthquake cause an uplift up to 70cm and subsidence up to 25cm. This deformation affects areas around epicentre. A field survey was conducted to obtain information directly and it was seen that the ground deformation that was identified with the InSAR technique were consistent with the findings in the field. This shows the advantages of remote sensing in terms of ability to cover a wide area in a short time.

The parameter estimation of conditional intensity function temporal point process as renewal process using Bayesian method and its application on the data of earthquake in East Nusa Tenggara

A temporal point process is a one-dimensional point process where events are observed by time. A temporal point process can be viewed as a renewal process with time between events as random variables. The conditional intensity function of the process represents the expected rate dependent on time between events so that we can predict earthquakes activities. This research will be discussed about parameters estimation of conditional intensity function using Bayesian method and be applied to earthquake data in East Nusa Tenggara (ENT) from January 2015 to April 2018. The ENT is categorized as earthquake prone, because it is flanked by two zones of earthquakes. In southern region there are the Eurasian and Indo-Australian plates sub ducted, and in the north there is a back arc thrust. Based on the research results, it was obtained the conditional intensity estimation. The application of conditional intensity estimation represents the expected rate of earthquake occurrence in East Nusa Tenggara at a certain time interval. The time intervals are in units of days. The greatest conditional intensity is at the time interval (day 1096th to 11897th) that is 0.058798617. It means that the probability earthquake occurrence in East Nusa Tenggara at that time period is very low with the total number of occurrences are 28 earthquakes.

Multi-Data-Type Source Estimation for the 1992 Flores Earthquake and Tsunami

We revisit the source of the 1992 Flores earthquake and tsunami using finite-fault inversion. We simultaneously invert teleseismic body and surface waves together with coseismic uplift/subsidence datasets. We then verify the inverted source against tsunami run-up heights along the northern coast of Flores Island and the only tide gauge recording of the tsunami. Our preferred source model provides a good fit to all the datasets, whereas previous models only explained a subset of the available data. We show that the fault geometry implies segmentation of the back-arc thrust system in the eastern Sunda Arc.

Earthquake Relocation Using Double Difference Method for 2D Modelling of Subducting Slab and Back Arc Thrust in West Nusa Tenggara

West Nusa Tenggara is classified into earthquake prone zone as it is located between subduction and back arc thrust earthquake sources. Accurate hypocenter determination in this area is necessary for strong motion calculation and earthquake source zone modelling. Earthquake relocation in the region is needed to produce a more accurate hypocenter location and 2D modelling of subduction slab and back arc thrust. A double difference method was employed with earthquake data from 2009-2017. The results show better accuracy in the distribution of the travel-time residual. Subduction slab modelling shows a dip value of about 7-13° from a trench to an arc, 49-55° from an arc to a transition zone, and 60-64° dip at a depth of 300 km. Back arc thrust modelling shows a dip value of about 19-28° at a depth of 15-30 km. The results provide a reliable 2D model for subduction slab and back arc thrust in West Nusa Tenggara. Therefore, the developed models can be used as reference for earthquake zones and seismic hazard assessment in West Nusa Tenggara.

Tectonic Model of Bali Island Inferred from GPS Data

DOI: 10.17014/ijog.5.1.81-91Seven periods of GPS campaign have been conducted for three years since March 2013 - October 2015 on fourteen GPS sites across Bali Island. The GAMIT/GLOBK 10.6 version was used to compute data with respect for thirteen reference sites of International Terrestrial Reference Frame (ITRF) 2008 surrounding Bali. The result shows that horizontal displacement varies between 1.93 and 22.53 mm/yr dominantly northeastward. Vertical displacement ranges at -184.34 to 33.79 mm/yr. The result of modeling using Coulomb 3.3 version indicates the deformation in Bali was mostly contributed by subduction at the southern part, West and East Flores Back-Arc Thrust at the north, Lombok Strait Fault and a fault at the eastern coast of Bali with the estimation maximum magnitude of 7.1, 6.6, 6.8, 5.8, and 5.2, respectively.

Determination of the seismicity and peak ground acceleration for Lombok island: an evaluation on tectonic setting

Lombok Island is located in the Nusatenggara region, which is considered as one of the most active seismic parts of Indonesia. The determination of seismicity is crucial to the island, since Lombok Island is a relatively dense populated small island. Earthquakes that occurred around the island were generated by southern subduction mega-thrust and northern back-arc thrust. Mostly, distributions are concentrated on the north-west and south-east parts of Lombok Island. Probabilistic data analysis of 309 earthquake events during the period of 1973 - 2017 results in the Gutenberg-Richter parameters (a and b) of 3.62 and-0.53 indicating medium to high seismic activities. The Joyner-Boore attenuation relation seems to be suitable for seismic characteristics of the island. The reconstruction of tectonic setting indicates that the Benioff zone may develop at the depths of 150 km beneath the island causing horizontal tectonic forces working within the direction of N 171 °E - N 351 °E. This could create a strike-slip fault on the crustal basement within the NE - SW direction.