Research

by Andikagumi & Bradley (2024)

Abstract. The 2018 Lombok earthquake demonstrated the hazard that the Flores Thrust possess and highlighted our lack of knowledge on this crustal-scale back thrust. The Flores Thrust extends from west to east, located at the back of the arc in the subduction zone of the Lesser Sunda Islands, Indonesia, as a result of an extensive crustal deformation of the Sunda upper plate by the northward motion of the subducted Indo-Australian plate. Here we construct a geometry model of the Flores Thrust by fitting the best surface on the relocated earthquake catalogues using simulated annealing. The thrust has a flat and a ramp component, where the flat is generally constant, inclined ~5˚ southward. The ramp of the thrust is dipping southward with 38˚ average dip, where the dip is increasing eastward from Bali (27˚-37˚) to Lombok (28˚-38˚), Sumbawa (30˚-50˚), and western Flores (40˚-58˚), but then shallower in central Flores (29˚-39˚). Based on our analysis of volcano distribution, the Flores Thrust is constrained by arc volcano distribution; bounded by volcanoes at the eastern and the western tips while the deeper part of the thrust is terminated by the along-arc distribution of volcanoes. The clustered distribution of earthquakes and the occurrence of strike-slip focal mechanisms between volcanoes indicate the possibility of the segmented geometry of the thrust, perhaps due to strain localisation by the thermal disturbance from the volcanoes. The Flores Thrust might also affects the arc by migrating the active volcanism northward in Sumbawa Island, leaving extinct edifices in its south while forming the highly potassic rear-arc volcanoes, Tambora and Sangeang Api, in the north. Therefore, the complexity of the Flores Thrust geometry and its interplay with the volcanism should be investigated further, to mitigate the greater effects of any geophysical hazards in the region.

This work was presented at a poster session (T32E-0218) in AGU Fall Meeting 2022, Chicago, USA and at an oral session in EGU General Assembly 2024 in Vienna, Austria.

This article has currently been published on Tectonics. See published article here.

by Andikagumi et al. (2023)

Abstract. The morphology of polygenetic volcanoes is the result of complex processes involving the deposition of volcanic materials that took place over a very long period of time, spanning from hundred thousand to millions of years, making edifice growth observation is impossible to conduct. Here we use analogue modelling approach with controlled-parameters in the laboratory and utilise morphometric parameters to quantify the observation of the volcanic landscape evolution. We investigated the influence of the changes in eruptive volume and magma viscosity on the edifice morphology. We ran the experiment by ejecting vegetable oil repeatedly at a static location in a temperature-controlled room with an adequate interval between ejections to allow this material to solidify. The experiments comprised various scenarios: constant ejection volume and viscosity, decreasing ejection volume with constant viscosity, and increasing viscosity with constant eruptive volume. We described the shapes of the edifices using morphometric parameters, such as height, width, volume, slope, circularity, and regularity. The experiments with decreasing ejection volume produced edifices with taller elevations and steeper slopes, especially near the summit, compared to the edifice produced with constant ejection volume. A similar finding was also observed on the edifice resulting from the experiment with increasing viscosity. The circularity and regularity indexes were insignificantly influenced by ejection volume and viscosity changes, but these parameters vary with the height fraction of the edifice. Moreover, based on the changes of morphometric variables throughout the experiments, we propose three development stages of volcanic edifice growth: basal foundations, flank constructions, and elevation building. Therefore, the development stage in which the edifice currently grows then can be inferred by the relative changes in the eruptive volume and the magma viscosity.

This work will be presented IAVCEI Scientific Assembly 2023, Rotorua, New Zealand.

The article is published on Geomorphology. See published article here.

by Andikagumi et al. (2020)

Abstract. We present a spatial analysis of volcano distribution and morphology in the young, intraoceanic Mariana Arc. Both the quality off it to idealized models and the divergence from those ideals indicate that Mariana Arc volcanoes are arranged into five great circle segments, rather than a single smallcircle or multiple small circles. The alignment of magmatic centers suggests that magma transport is controlled by the stress regime in the deep crust and/or lithospheric mantle of the Philippine Sea Plate, into which the arc is emplaced, and that arc‐normal tension is the dominant process operating in the deep lithosphere along the whole arc. Volcano morphologies indicate that the stress regime in the shallow crust varies between arc‐normal tension and compression, which also implies that the stress field can vary with depth in the arc lithosphere. We show that this horizontal and vertical stress partitioning can be related to the changing dip of the subducting plate and the breadth of the zone where it is coupled with the overriding plate. The variation in stress regime is consistent with both the distribution of seismicity in the PhilippineSea Plate and with the structural fabrics of the non-volcanic part of the plate margin to the south. Our analysis suggests that the upper plate exerts the principal control on the distribution of volcanoes in the Mariana Arc. Where tension in the deeper parts of arc lithosphere is sufficiently concentrated, then a distinct volcanic front is produced.

This work was presented at an oral session (T22C-07) in AGU Fall Meeting 2017, New Orleans, USA.

The article is published on Journal of Geophysical Research: Solid Earth. See published article here.

by Bradley et al. (2019)

Abstract. The death toll and economic impact of an earthquake can be greatly exacerbated if seismic ground shaking triggers landslides. Earthquake-triggered  landslides  typically  occur  in  two different  contexts: localized failure of steep slopes and resulting land-slides that pose a major threat to life in areas below; and lateral spreading of nearly flat sediment plains due to shaking-induced liquefaction, which can damage large areas of critical infrastructure. Unexpected catastrophic  landsliding triggered by the 28 September 2018 earthquake at Palu, Indonesia did not occur in  either  typical context, but produced both destructive out-comes. Here, we show that alluvial ground failure  in the Palu Valley was a direct consequence of irrigation creating a new liquefaction hazard. Aqueduct-supported cultivation, primarily of wet rice, raised the water table to near ground level, saturating sandy alluvial soils that liquefied in response to strong ground shaking. Large-displacement lateral spreads occurred on slopes of 1°. Slopes steeper than 1.5° sourced long-runout landslides and debris flows that swept through villages occupying the gentler slopes  below.  The  resulting damage and loss of life would probably not have occurred in the absence of a raised  water  table. Earthquake-triggered landsliding of gentle, irrigated alluvial slopes is an under-recognized, but avoidable, anthropogenic hazard.

The article is published on Nature Geoscience. See published article here.

by Andikagumi et al. (in review)

Abstract. Volcanism and seismicity along island arc settings are both controlled by subduction process. However, the correlations between them are often neglected in understanding the tectonic system along the subduction margin. In the Sumatran arc, we observed a systematic variation of potassium content of Quaternary silicic tephra samples, both from the whole rock and glass analyses from north to south. Higher K2O (>4.5 wt%) is found in the northern part of Sumatra (1-3ºN), lower K2O (<4 wt%) in the central region (0-3ºS), and a slight increase of K2O (4-5 wt%) in the southern region (3-5ºS). This systematic variation occurred relative to a comparable constant SiO2 content in all regions (76.5 wt%, ± 2.5 wt%). Similar trends are found in several trace elements, particularly La, Ta, Th, U, and Rb. We compare the trends of K2O with various subduction dynamic variables (e.g. crustal thickness, slab depth, slab age, etc.) and with the seismicity occurring along the Sumatran margin. The seismicity along the Sumatran margin was analysed using declustered datasets around the slab interface and quantified by the counts of earthquake events and the b-value in various radii of calculations. Both the earthquake count and b-value vary in the similar manner with the K2O content: higher seismicity takes place in the northern region of Sumatra, lower in the central region and higher again in the southern region. The analysis of earthquake focal mechanisms infers the increasing seismicity is due to the location of active fracture zones in the subducting slab. We propose that the active fracture zones brought more water into the subduction system and that excessive water would persist at greater depths and generate more earthquakes through dehydration embrittlement mechanisms. Slab dehydration at the greater depth (>160 km) would also contribute to the production of supercritical fluids and accumulate more potassium as it allows the breakdown of potassium-bearing minerals in the slab (e.g., phlogopite, K-richterite, phengite) by the higher pressure and temperature. The location of arc volcanoes, expressed in terms of distance from trench along the arc also reflects the K2O trend. Therefore, we suggest that the location of active fracture zones, as observed from the seismicity, controls the accumulation of potassium in Sumatran volcanoes.

This work was presented at an oral session (V11B-05) in AGU Fall Meeting 2021 (online).

by Andikagumi et al. (in review)

Abstract. The relative importance of structural control on caldera-forming eruptions, versus eruptive or magmatic processes, remains unclear. In arc settings, it is widely accepted that a magma source in the mantle wedge is required to generate the thermal and mechanical conditions to develop caldera-forming events. Toba, in Sumatra, is one example where the formation of a large-scale caldera has been linked to a voluminous magma supply. However, recent investigation shows that the feeder system accumulated magma not only beneath Toba but also hundreds of kilometres south of the caldera; yet, the large-scale caldera only formed in a specific region. We used spatial analysis to explore the location of Toba in the context of an observed alignment of Sumatran vents into great circle segments with stepover geometry. The alignments of arc volcanoes reflect focussing of magma pathways controlled by arc-normal tension at the base of the deforming lithosphere. We propose that Toba, located at an arc-segment stepover, formed in a zone of weak, extended arc lithosphere with enhanced extension and thinning of the arc lithosphere. The extended weakness zone developed due to northeastward migration of the more northerly arc-segment. This occurred because of substantial changes in the age, for about 50 Ma, and the buoyancy of the subducting oceanic plate. The thinning in arc lithosphere may also increase the amount of upwelling, and therefore the amount of crustal melting. We conclude that the latest caldera-forming event at Toba was driven by both a voluminous magma supply and by the arc lithosphere structure.

This work was presented at an oral session (V11B-05) in AGU Fall Meeting 2019, San Francisco, USA.

The article is currently under review with Geology.

by Andikagumi et al. (in prep)

Abstract. We employ the distribution of arc volcanoes in the Java and Lesser Sunda Islands (Java LS) as a proxy for the stress regime of the upper plate. Three great circle segments in the Java LS form an en-echelon pattern suggesting an oblique subduction system, in contrast to the common view that this is an orthogonal subduction system in Java. We hypothesise that the lineaments represent the strike-slip stepover structures in the deep crust and that their overlaps, in West and East Java, are pull-apart structures. Surface structures, mostly folds and formation boundaries, also show trench-parallel strain manifest as the 'S'-shape deformation patterns observed along the islands from lineament mapping. Trench geometry, great circle volcano segments, and subduction directions are compared to constrain obliquity variations along Java LS. Most of the Java LS southern block, bounded by the trench in the south and the volcanoes to the north, experienced eastern lateral movement with a low trench-parallel component (<0.45 ratio over the trench-normal component). This is in contrast to strongly trench-parallel movement (0.8 to 1.5 average ratio) of Sumatra's southern block movement towards the west, most conspicuously shown by the dextral strike-slip Great Sumatra Fault. Lateral movements of the Java LS and Sumatra blocks are influenced by trench geometry which has been modified at each end of the arc due to the subduction of Australia and India. The northward deflection of the arc by Australian plate caused a rotation of Java LS about a pole of rotation, about which the lateral movement changes from eastern directed to western directed, located at about 108oE which coincides with the great circle lineament overlap in West Java. Sinistral strike-slip deep crustal movement occurs to the east of the pole while dextral movement occurs to the west of the pole and continues to Sumatra and propagates to the surface as the trench-parallel component gets significantly higher. Therefore, we suggest the rotation of Java LS developed block's lateral movement and generated strike-slip stepover structures in the deep crust.

This work was presented at a poster session (T21G-0313) in AGU Fall Meeting 2018, Washington DC, USA.

by Andikagumi et al. (in prep)

Abstract. The locations of arc volcanoes are usually viewed as being situated above the locus of melting in the mantle which itself is interpreted as a function of slab dehydration at some critical depth. This critical depth was inferred as the depth from arc to slab while assuming that arc volcanoes are distributed as small circles on the Earth’s surface. However, this approach does not account for how the upper plate may influence arc volcano locations. Instead, we fit great circles on arc volcano distribution and suggest the distance from trench to arc-segment as an important variable to address arc volcano location. Using statistical quantification by Hough Transform analysis of 19 arcs we show that volcanoes in most arcs are distributed as segments of great circles, rather than single or multiple small circles. We compare subduction dynamic variables with properties of these segments to determine which factors most probably control arc-segment location. The arc-segment distance from trench is correlated with the crustal thickness (R=0.57) and the shallow slab dip (R=-0.74), while the latter two also correlated (R=-0.76) with each other. The great circle alignments reflect the arc-normal tension at the base of upper plate as the result of stress regime partitioning by lithosphere flexure. At subduction margin, plate flexure occurs at the end of down-pulled zone where the breadth of such zone is defined by the coupling between upper and lower plates. Thicker crust and shallower slab dip both lead to wider plate coupling zones that locate lithosphere flexure and arcs farther from the trench. Therefore, we propose that the thickness of the upper plate and slab dip in shallow depth control the location of arc volcano segments relatively to the trench.