Back-arc Thrust Research Articles

Analysis of Deformation Along the Flores Back-Arc Thrust Using GPS Observation Data

Abstract Indonesia is located on the World Plate and composed of several microblocks. One of the microblocks in Indonesia is along the Back-Arc Thrust of Flores, which is suspected to cause many earthquakes throughout 2022 in Bali and Nusa Tenggara. The high activity of earthquakes encourages the need for deformation monitoring as a mitigation effort. This study aims to analyse the movement and strain patterns that occur in the Flores Back-arc as an effort to see the deformation that occurs. This research uses BIG GPS CORS observation data from 2019-2023 and processed using GAMIT/GLOBK software. The results then will be estimated to obtain velocities, strains, and compare them with the earthquake occurrence. This study shows that the horizontal displacement is dominantly towards the northeast within 20-57 mm/year and dominantly subsidence for vertical displacement with the largest value of −62 mm/year. The high velocity is related to several large earthquakes, one of which is the Flores earthquake in 2021 which caused a offset of up to 19 mm at CBJW, CRTG, CREO, and CLBJ stations. In addition, the strain pattern shows that compression is dominant in the study area with a value range of to 29.3 to 55.7 nstrain/year. While the extension value has a range of 2.7 to 16.9 nstrain/year. The results show that the Southeast region has a high level of energy accumulation as seen from the high values of compression and extension that occur. While the Northwest region shows high energy accumulation indicated by the dominance of compression values due to Flores back-arc thrust in the north.

3-D P-wave velocity structure of the upper mantle beneath eastern Indonesia from body wave tomography

Eastern Indonesia's tectonic setting is well known for its complexity and intense seismic activity. Controlled by several major and minor plates, including the Eurasian, Australian, and Pacific plates, this region is famous for its U-shaped subduction system beneath the Banda Arc. To better understand the architecture of the underlying structure in this region, we performed body-wave travel time tomography using ten years of catalog data provided by the Indonesian Agency for Meteorology, Climatology, and Geophysics. We utilize 9729 events in total, from which 46,446 P-wave arrival times were extracted. We used a double difference method to relocate the initial event catalog, which produced a pattern of seismicity consistent with a curved subduction system. Our tomographic model reveals a high velocity band between 90 and 240 km depth in the upper mantle, which is interpreted to be a concave dipping lithospheric slab that is parallel to the present-day Banda arc. Our results also show that lithosphere subducting from the north and south starts to collide at a depth of 300–350 km and becomes shallower further east. Apparent discontinuities in the high velocity band and a corresponding lack of seismicity supports the presence of a slab tear to the west of Seram. A dipping high velocity structure that is present from south to north beneath the island of Timor represents a subducting slab that dips more steeply beyond a depth of 150–200 km, which appears consistent with slab roll-back. Our tomographic model also shows evidence of back arc thrusting to the north of Sumbawa and Flores Islands in the form of a south-dipping higher velocity band at shallow depth. Furthermore, our tomographic models also reveal the possible presence of underthrust continental forearc in the form of a thin higher velocity anomaly that connects the backarc thrust and northward dipping lithosphere slab in the Timor area. Finally, a zone of low velocity above the higher velocity slab is clearly seen beneath Seram Island at a depth of ∼100 km and may represent a partial melting zone.

Numerical Modelling of the Potential Hazard Due to Future Flores Back Arc Thrust Earthquake Generated Tsunami

Abstract Indonesia is situated at the conjunction of four major plates: Eurasia, India-Australia, the Pacific, and the Philippine Sea. Therefore, Indonesia is exposed to a considerable threat from natural hazards every day. Tectonic-related marine geohazards, particularly earthquakes-generated tsunamis periodically affect Indonesia, causing great loss of life and extensive damage to property and infrastructure in coastal areas. Many studies on tsunami modeling have been addressed in western Indonesia. In eastern Indonesia tsunamis, however, are even more frequent in occurrence and may be more hazardous, there is a lack of study and less attention. The Flores Back Arc Thrust (FBAT) is an active fault that lies on the sea floor of the Flores Sea in the north of Flores Island, extending from north of Bali to the east that may be connected to the Wetar thrust at the north of Wetar Island in the western Banda Sea. The FBAT is a source of seismicity in this area. From 1900 to 2022, there were 79 earthquakes with a magnitude of more than 6.0 Mw, and 11 of them generated tsunamis, and the most devastating one was the 1992 Flores earthquake tsunami. This study presents an estimated FBAT-induced future earthquake-generated tsunami runup based on numerical modeling.

Active faults along Indonesia’s first high-speed rail line

Abstract The cities of Jakarta and Bandung, two vital hubs in the western part of Java, are now connected by a high-speed rail line. However, potential earthquake hazards arise as the line intersects two active faults in West Java. To address this concern, we conducted a detailed geomorphological analysis using high-resolution DEM data (8-meter resolution) combine with 30-m resolution data to locate fault lines along the rail infrastructure. In the north, the line intersects the West Java back-arc thrust, while in the south, it crosses the junction of the Lembang Fault and Cimandiri Fault in Padalarang. However, young deposits and vegetation still could obscure fault traces in specific areas observed from high-resolution DEM data. To overcome this challenge, we employed UAV drone mapping to capture finer geomorphology details. This information then combined with paleoseismology trenching to uncover the active structural features. The aim is to enhance the railway’s resilience; and build monitoring and early warning systems for the high-speed train operation system to seismic events.

A note on the seismicity of Sumatra, western Sunda Arc, Indonesia, in relation to the potential for back-arc thrusting

The existence of back-arc thrust faults along the eastern part of the Sunda Arc, ranging westwards from Flores to the western tip of Java, has been recognised for decades. In contrast, it is still unknown whether such back-arc thrust faults exist in Sumatra, which is located in the western part of the Sunda Arc. To investigate the possible existence of back-arc thrusts in Sumatra, we examine regional earthquake data reported by the Agency for Meteorology, Climatology and Geophysics of Indonesia, as well as global earthquake data reported by the International Seismological Centre and the United States Geological Survey. It appears that back-arc thrusts in the study area are not extensively developed, unlike in the eastern Sunda Arc, which may be caused by oblique subduction beneath the Sumatran forearc. The stress associated with the trench-parallel component of subduction is largely accommodated by the ~ 1650-km-long dextral strike-slip fault zone of the Great Sumatran Fault. The seismicity data from various sources do, however, show that there is a dipping seismogenic zone in several parts of the back-arc region of Sumatra, in the opposite direction to the NNE subduction of the Indo-Australian plate. This new observation may be related to the presence of spatially intermittent back-arc thrust faults in the study area, which may need to be taken into account when improving Indonesia's national earthquake hazard maps.

Determining the slip rate of the Kendeng Fault in East Java through GPS observations

The Kendeng Fault is recognized as an extension of the Flores back-arc thrust, primarily exhibiting thrust fault behavior. However, prior research has indicated the presence of a significant right-lateral shear component along this fault. While no significant seismic events have been documented in recent years, historical records suggest that the region experienced significant seismic events prior to the 20th century, necessitating a careful consideration of the earthquake hazard associated with this fault. The question arises whether the absence of recent seismic activity is a consequence of energy accumulation or gradual energy release. Our investigation utilizes GPS data collected over a four-year period starting in 2016 to determine the slip rate of the Kendeng Fault. Our findings indicate that the slip rate along the Kendeng Fault is approximately 0.45 ± 0.07 cm/year. These results provide valuable insights into the behavior of the Kendeng Fault and contribute to a better understanding of its seismic potential and associated risks.

The Early Model of Tomography in Eastern Indonesia Using FMTOMO

Eastern Indonesia has a complicated structure due to the collision of the Eurasian Plate, the Pacific Plate, the Indo-Australian Plate, and the Philippine Plate, creating an active seismic zone. The Banda - Seram segment of eastern Indonesia comprises subduction, rollback, detachment, slab tear, back-arc thrust, and the Banda volcanic arc. Sri Widiyantoro did a previous study in Eastern Indonesia utilizing seismic tomography methods to detect the upper mantle and the 180-degree curved transition zone under the Banda arc. In addition, Robert Hall and Spakman’s research of the upper mantle revealed the existence of a spoon-shaped mechanism in the band arc. Seismic tomography modeling, this research seeks to identify the tectonic features in Eastern Indonesia, such as partial melting, subduction, slab rips, back-arc thrust, Banda volcanic-arc, detachment, and faults, and to establish the most suitable mitigation strategies. This study uses earthquake data from the Meteorology Climatology Geophysics Agency database (BMKG) and the form of P waves. In this research from 1 January 2016 to 31 December 2020, with coordinates 1 - 12 LS and 120 - 135 BT, there were 81 recording stations and 11299 earthquake occurrences consisting of 175643 P waves and 27338 S waves. This work applies the FMTOMO method to a seismic tomographic model. This research indicates that high-velocity anomalies characterize subduction to a depth of 650 kilometers. On Buru Island, the high-velocity anomaly is diagnosed as a slab tear at a depth of more than 400 kilometers. This research identifies a 100-kilometer-deep Back-arc thrust on Wetar Island that produces shallow earthquakes. In addition, a low-velocity anomaly at a depth of 70 kilometers - 90 kilometers is characterized as partial melting due to its parallel location with the Banda volcanic arc. This study examines the connection between subduction and intense seismic and magmatic activity in Eastern Indonesia.

Hypocenter relocation of the Mw 5.9 eastern Manggarai earthquake 2022 and its aftershocks based on BMKG seismic network

The moderate earthquake (Mw 5.9) struck eastern Manggarai and its surroundings on February 21, 2022, at 12:36:00 UTC. Although this earthquake was classified as a moderate earthquake, it produced a significant number of aftershocks. The aftershocks could be observed as the post-seismic activity, which was still well recorded until March 31, 2022. The hypocenter location of the aftershocks is updated by applying the double-difference method. The aftershock distribution is striking in west-east orientation and dipping to the south, emphasizing a geometry of faults. The strike and dip directions are consistent with the focal mechanism determined by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Most aftershocks after the relocation are distributed at a depth of < 25 km; the depth of the mainshock is 21.5 km. The aftershock distribution in this study shows that the earthquake sequence propagates toward the up-dip direction. The aftershock relocation also reveals that most aftershocks are situated slightly off the main fault plane in the complex splay faults. Therefore, we conjecture that the co-seismic activity may trigger the aftershocks. In addition, this study also shows that the fault system consists of several fault segments. Our study benefits earthquake disaster mitigation, especially in mapping the segment faults of the Flores backarc thrust.

Deformation feature and tectonic model of the Timor Trough: New interpretation of the evolution and mechanism of Banda arc-continent collision

The deformation of the Timor Trough is controlled by the combined dynamics of Australian Plate subduction and the arc–continent collision between Australian continent and Banda arc, and its development remains controversial. Based on the seismic, earthquake, and well datasets, we focus on these controversial problems and analyzed the deformation features and mechanism of the Timor Trough, and we established a tectonic model to discuss the arc–continent collision evolution and plate coupling. Our results and model suggest that the subsidence of the Timor Trough and uplift of the Timor Island begin at ∼3.3 Ma and marked the deformation of the Australian northwestern continental margin. We propose that the arc–continent collision underwent two phases and transformed the subduction zone into collision zone. The earliest phase uplifted and closed the forearc basin since ∼6 Ma, and the second phase of regional deformations at ∼3.3 Ma is represented by the development of backarc thrust, the cessation of volcanism, the uplift of Banda arc, and the subsidence of Timor Trough. The tearing oceanic crust provides a window for the development of backarc thrust to the southeastward. The outer and inner Banda arcs may have become a cohesive block after the collision; consequently, it is possible for them to move northward as a whole along the backarc thrust.

3-D shallow shear velocity structure of the Jakarta Basin from transdimensional ambient noise tomography.

Situated on the northern coast of the Indonesian island of Java, Jakarta and its metropolitan area (Greater Jakarta) are subject to significant earthquake hazards from a subduction zone south of Java and nearby active crustal faults. The seismic risk may be even higher because Greater Jakarta resides on a sedimentary basin filled with thick Pliocene-Pleistocene sediments. A comprehensive study of Jakarta Basin's properties and geometry is important for creating robust seismic hazard and risk assessments. The main objective of this study is to develop a 3-D model of Jakarta Basin's shallow shear-wave velocity (VS ) structure and improve on previous models that did not cover the basin edge due to the extent of data coverage. Between April and October 2018, we deployed a new temporary seismic network to extend the spatial coverage beyond that of a previous deployment in 2013, and sampled 143 locations through sequential deployments of 30 broad-band sensors covering Jakarta and its adjacent areas. We conducted a 2-stage transdimensional Bayesian inversion of Rayleigh wave phase velocity dispersion curves derived from seismic noise. To begin, we applied tomography and constructed 2-D phase velocity maps for periods 1-5s. Then, at each point in a regular grid defined on these maps, we invert each dispersion curve into 1-D depth profiles of VS . Finally, these profiles at gridpoints with ∼2km spacing are interpolated to form a pseudo-3-D VS model. Our results reveal the edge of the Pliocene-Pleistocene sediments along the south. Also, we resolve a basement offset across south Jakarta that we suggest may be related to the western extension of the Baribis Fault (alternatively, the West Java Backarc Thrust). We recommend using this 3-D model of the Jakarta Basin for scenario earthquake ground motion simulations. Such simulations would help establish how important it might be to re-assess seismic hazard and risk in Greater Jakarta so that basin resonance and amplification are considered.

Mapping of Destructive Tectonic Earthquakes in the West Nusa Tenggara (NTB) Region Based on the Zhao Attenuation Function

NTB is a group of small islands located between the earthquake zone, namely in the south there is a subduction zone of the Eurasian and Indo-Australian plates and in the north, there is a back-arc fault or Flores Thrust. Earthquakes in this area are very active in the form of shallow tectonic earthquakes of large magnitude, so the risk of disaster is very high. Based on this, our research aims to map the distribution of destructive earthquakes using the attenuation function Zhao, released in 2006. The Zhao attenuation function has been applied to earthquake sources in subduction zones which are inputs for seismic hazards in various parts of the world, because the distance uncertainty factor used has better accuracy. The distribution of earthquake epicenter points in the West Nusa Tenggara region is clustered in four places, namely the Eurasian and Indo-Australian plate subduction zones in the sea south of the West Nusa Tenggara islands, the non-volcanic zone to the south of the West Nusa Tenggara islands, the volcanic zone to the east, and the Back Arc Thrust zone in the waters north of the NTB islands. Meanwhile, the hypocenter distribution is spread majorly on the Eurasian plate. Most of the islands of West Nusa Tenggara are characterized on the MMI VII-VIII intensity scale, and only a small part is characterized on the V-VI MMI intensity scale. The implications of the results of this research are useful in disaster mitigation, so it is hoped that there will be a reduction in disaster risk.

Analisis Kerentanan Bahaya Gempa Bumi Tektonik Merusak Berdasarkan Fungsi Atenuasi Zhao Di Nusa Tenggara Timur

&lt;p class="AbstractText"&gt;The East Nusa Tenggara (NTT) region which is located between 2 earthquakes, namely the tectonic plate subduction zone in the south and the Back Arc Thrust zone in the north, is very interesting to study considering the very active tectonic earthquakes that are significantly destructive in this region which have earthquake characteristics shallow and large magnitude. A study has been carried out that aims to determine the vulnerability of the tectonic earthquake to significant damage when seismic waves pass through these islands. The vulnerability information generated can be useful as a reference for disaster mitigation and factors that need to be considered before carrying out development in the NTT region. The input data studied are shallow earthquake data from 1997-2018 at a depth of 60 km and a magnitude M≥5 SR at coordinates 8&lt;sup&gt;o&lt;/sup&gt; 30’-13&lt;sup&gt;o&lt;/sup&gt; 10’ South Latitude and 118&lt;sup&gt;o&lt;/sup&gt; 30’-125&lt;sup&gt;o&lt;/sup&gt; 30’ East Longitude. The number of earthquake events is 198 data. The attenuation function used refers to the attenuation function Zhao, et. al 2006. The results of the study show that. the vulnerability of the tectonic earthquake to significant damage in the East Nusa Tenggara region clustered on the MMI V-VIII intensity scale. Most of these islands are clustered on the MMI VII-VIII intensity scale and only a small part is clustered on the V-VI MMI intensity scale. The NTT region which has an intensity scale of VII-VIII MMI has a high vulnerability to destructive earthquakes, so it is necessary to disseminate disaster mitigation from an early age to the public about the threat of earthquakes and it is recommended that this information be used as a reference in developing the area.&lt;/p&gt;

A preliminary report on seismicity declustering methods and completeness magnitude in eastern Sunda Arc

Data declustering separates mainshocks from both foreshocks and aftershocks while a reliable estimate of completeness magnitude is a key point in seismic parameter determination. These play a role in seismicity-related work. In this preliminary study, we reported seismicity in two Indonesian provinces, namely NTB and NTT, as part of eastern Sunda Arc using the USGS catalogue during 1970-2021 based on performance of three declustering methods (Gardner and Knopoff, Reasenberg, Uhrhammer). These methods were tested along with three techniques of M c determination (MAXC, EMR, BC) provided by ZMAP to estimate minimum magnitude cut-offs, leading to an accurate completeness magnitude. After careful examination, the Reasenberg and BC techniques were proved to be suitable for characterising seismicity in the regions of interest, where M c was calculated under a linear assumption of the cumulative frequency-magnitude distribution (FMD), widely known as the Gutenberg-Richter law. The results revealed that b and a parameters are influenced by the choice of a specific declustering algorithm and calculation of M c. NTT was found to have a higher level of seismicity than NTB and seismicity rates in the southern part of both provinces were higher than those in the northern part. However, the number of strong ground motion with M w ≥ 6.5 in the northern area was larger than that in the southern, indicating the potency of Flores Back-arc Thrust for generating large earthquakes hence possible tsunamis.

Application of Scenario Earthquakes for Analysis of Seismically Triggered Landslide Hazard: A Case Study in Costa Rica

In this study, we demonstrate the capabilities of hypothetical scenario earthquakes as a new tool for assessment of hazards associated with earthquake-triggered landslides. Costa Rica offers an ideal environment for demonstrating the utility of scenario earthquakes due to its diverse tectonic environments and associated widespread seismic hazard, rugged topography, and high landslide susceptibility. We investigate the relative influence of landslide proxies such as topographic slope, peak ground velocity (PGV), and compound topographic index (CTI), and earthquake source parameters such as magnitude and depth, on predicted landslide probability and fatality. We examine five distinct tectonic environments, including subduction events beneath the (1) Nicoya and (2) Osa peninsulas respectively, (3) intraplate earthquakes beneath the Central Volcanic Range (CVR) and (4) the Central Costa Rica Deformed Belt (CCRDB), and (5) back-arc thrust events on the eastern Caribbean coast. Our results demonstrate that the slope, PGV, and CTI thresholds necessary to produce landslide probabilities greater than 10% vary by tectonic environment. In all cases, we observe magnitude to be the primary control on the predicted maximum landslide probability and overall areal landslide coverage. We validate model predictions with observed landslide inventories from the 2009 Cinchona and 1991 Limon earthquakes, demonstrating a good fit, where over 70% of landslides occurring in zones of greater than 20% probability. We also use a global model of landslide impact to predict exposure and fatality ranges for each scenario earthquake of this study, revealing that moderate-sized earthquakes in the CCRDB and CVR and large subduction megathrust earthquakes each pose a significant hazard to Costa Rica’s population.

The Kalaotoa Fault: A Newly Identified Fault that Generated the Mw 7.3 Flores Sea Earthquake

AbstractWe reveal the existence of a previously unknown fault that generated the Mw 7.3 Flores Sea earthquake, which occurred on 14 December 2021, approximately 100 km to the north of Flores Island, in one of the most complex tectonic settings in Indonesia. We use a double-difference method to relocate the hypocenters of the mainshock and aftershocks, determine focal mechanisms using waveform inversion, and then analyze stress changes to estimate the fault type and stress transfer. Our relocated hypocenters show that this earthquake sequence ruptured on at least three segments: the source mechanism of the mainshock exhibits dextral strike-slip motion (strike N72°W and dip 78° NE) on a west–east-trending fault that we call the Kalaotoa fault, whereas rupture of the other two segments located to the west and east of the mainshock (striking west-northwest and southeast, respectively) may have been triggered by this earthquake. The Coulomb stress change imparted by the rupture of these segments on nearby faults is investigated, with a focus on regions that experience a stress increase with few associated aftershocks. Of particular interest are stress increases on the central back-arc thrust just north of Flores and the north–south-striking Selayar fault in the northwest of our study region, both of which may be at increased risk of failure as a result of this unusual earthquake sequence.

Tsunami hazard in Lombok and Bali, Indonesia, due to the Flores back-arc thrust

Abstract. The tsunami hazard posed by the Flores back-arc thrust, which runs along the northern coast of the islands of Bali and Lombok, Indonesia, is poorly studied compared to the Sunda Megathrust, situated ∼250 km to the south of the islands. However, the 2018 Lombok earthquake sequence demonstrated the seismic potential of the western Flores Thrust when a fault ramp beneath the island of Lombok ruptured in two Mw 6.9 earthquakes. Although the uplift in these events mostly occurred below land, the sequence still generated local tsunamis along the northern coast of Lombok. Historical records show that the Flores fault system in the Lombok and Bali region has generated at least six ≥Ms 6.5 tsunamigenic earthquakes since 1800 CE. Hence, it is important to assess the possible tsunami hazard represented by this fault system. Here, we focus on the submarine fault segment located between the islands of Lombok and Bali (below the Lombok Strait). We assess modeled tsunami patterns generated by fault slip in six earthquake scenarios (slip of 1–5 m, representing Mw 7.2–7.9+) using deterministic modeling, with a focus on impacts on the capital cities of Mataram, Lombok, and Denpasar, Bali, which lie on the coasts facing the strait. We use a geologically constrained earthquake model informed by the Lombok earthquake sequence, together with a high-resolution bathymetry dataset developed by combining direct measurements from the General Bathymetric Chart of the Oceans (GEBCO) with sounding measurements from the official nautical charts for Indonesia. Our results show that fault rupture in this region could trigger a tsunami reaching Mataram in &lt;9 min and Denpasar in ∼ 23–27 min, with multiple waves. For an earthquake with 3–5 m of coseismic slip, Mataram and Denpasar experience maximum wave heights of ∼ 1.6–2.7 and ∼ 0.6–1.4 m, respectively. Furthermore, our earthquake models indicate that both cities would experience coseismic subsidence of 20–40 cm, exacerbating their exposure to both the tsunami and other coastal hazards. Overall, Mataram is more exposed than Denpasar to high tsunami waves arriving quickly from the fault source. To understand how a tsunami would affect Mataram, we model the associated inundation using the 5 m slip model and show that Mataram is inundated ∼ 55–140 m inland along the northern coast and ∼230 m along the southern coast, with maximum flow depths of ∼ 2–3 m. Our study highlights that the early tsunami arrival in Mataram, Lombok, gives little time for residents to evacuate. Raising their awareness about the potential for locally generated tsunamis and the need for evacuation plans is important to help them respond immediately after experiencing strong ground shaking.

Focal Mechanism Analysis of the Earthquakes Beneath the Sunda-Banda Arc Transition, Indonesia, Using the BMKG Data

Structural complexity in the Sunda-Banda arc transition is a topic of much debate amongst Earth scientists. We have processed focal mechanism study using moment tensor inversion for 20 events in the region using the Agency for Meteorology, Climatology, and Geophysics (BMKG) data from 2014-2016 for earthquakes of magnitude Mw ≥ 5.0. Our result shows different solutions that depend on the source region of the earthquakes that include subduction zones and collision zones, which host active faults and the back-arc thrusts. Earthquakes that occurred in the subduction zone appear to rupture on thrust faults for shallow and intermediate events, while the deep events have normal fault mechanisms. The shallow events in the collision zone occur on thrust faults with differing strike directions, but generally, there are largely parallel to the Timor trough. We also found normal fault mechanisms at play for deep events below the collision zone. The occurrence of deep earthquakes in this area is consistent with remnant slab activity that persists to the present day. Focal mechanism solutions for shallow events in the north of Sumbawa island indicate a thrust fault with a strike direction that is almost parallel to the back-arc thrust in this area. We also found evidence of strike-slip motion along local-scale active faults in the area.

Modeling the Potential of Tsunami Hazard in Labuan Bajo Towards A Disaster-Resilient Tourism Area

In 2019, Labuan Bajo was designated a super-priority tourism destination, but this area is prone to earthquakes and tsunamis. The potential threats of disasters need to be considered when developing the tourism sector. Therefore, this study aims to determine the height and arrival time of tsunami waves based on a worst hypothetical scenario on the north coast of Labuan Bajo and its surroundings. The method used was numerical modelling with open source software TUNAMI F1, which calculated the wave propagation based on linear equations in spherical coordinates based on worst scenarios. The results showed that the Flores back-arc thrust earthquake caused a reasonably high tsunami with a fast arrival time at Labuan Bajo. The wave height of the tsunami around Labuan Bajo, Rinca, and Komodo Island is at least 3 m, but it reaches 8-9 m under certain conditions. Furthermore, its arrival time on the coast of Labuan Bajo is less than 2.5 minutes. This is much faster than those in Aceh (2004), Pangandaran (2006), and Palu (2018). Even though the early warning came 5 minutes after the earthquake, a high level of preparation and awareness is required to create a disaster-resilient tourist area in Labuan Bajo.

Coastal vulnerability mapping due to tsunami using Geographic Information System in Buleleng Regency, Bali Province

Bali is one of the areas vulnerable to disasters because of its geographical position, which is flanked by two earthquake sources in the form of a subduction zone and back-arc thrust zone, which can cause a tsunami in Bali region. This research aims to identify and map the level of coastal vulnerability to the tsunami in Buleleng Regency, Bali Province. The mapping was carried out using Geographic Information System (GIS). This study used secondary data and field data. The parameters used in analyzing the level of tsunami vulnerability were land elevation, slope, landuse, distance from the beach, and distance from the river. The level of vulnerability was grouped into five classes, namely very safe (35,466.9 ha), safe (70,485.0 ha), moderately vulnerable (17,645.0 ha), vulnerable (6,903.3 ha), and very vulnerable (438.9 ha) located in the Districts of Gerokgak, Seririt, Buleleng, and Sawan which are close to the river.

Coeval Miocene development of thrust belt-backarc and forearc extension during the subduction of a continental margin (Western-Central Mediterranean Sea)

The position of the middle-upper Miocene volcanic arc, encompassing the Maghrebides, the Sardinia Channel and the Sardinia-Corsica block, implies that the Algerian and Tyrrhenian basins developed, respectively, as backarc and forearc extensional zones in the Western-Central Mediterranean. The opening of the Western-Central Mediterranean Neogene extensional basin has been commonly interpreted as a two-step process: the opening of the Provençal-Algerian basin during the early-middle Miocene, followed, in the late Miocene, by the formation of the Tyrrhenian Basin. This article is an attempt to synthesize knowledge about the hinge zone between Algerian and Tyrrhenian basins by combining the analysis of seismic reflection profiles with dredge and borehole data in order to investigate how the transition between the eastern Algerian backarc and Tyrrhenian forearc geodynamic settings took place. We identified three sectors: the western Tyrrhenian characterized by a Tortonian forearc extension; the Sardinia Channel, which preserves the architecture of the lower Miocene Maghrebian thrust belt formed during the collision between Europe and Africa plates; and the easternmost Algerian basin-Sicily Channel where a backarc–thrust belt system developed during the Tortonian stage. During the extensional events, we hypothesize the re-activation of inherited structures during Tortonian rifting, (that is a negative tectonic inversion of pre-existing Eocene and early Miocene thrust faults). The contemporaneity of two different geodynamic environments, the forearc extension in the northern area and backarc–thrust belt system in the southern area, can be directly related with a lateral variation of the lower plate paleogeography of the Africa continental margin. This evidence contributes to the understanding of how the paleogeography of the lower plate can control, to a certain extent, the tectonic evolution of the upper plate in a subduction setting.