Figure 5 - uploaded by Bernard Pelletier
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Compilation of line drawings from old seismic lines obtained along the BATB during GEOVAN Leg 1 and 2 (1980), SEAPSO2 (1985), and EVA14 (1987) cruises. Vertical scale is similar for all sections.
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This paper presents a combined analysis of data collected both onshore and offshore in the epicentral region of the Mw 7.5, 26 November 1999, Ambrym earthquake. This offshore event occurred at the southern end of the back-arc thrust belt of the Vanuatu subduction zone, east of the island of Ambrym and resulted in permanent uplift of the eastern tip...
Contexts in source publication
Context 1
... In order to confirm the tectonic origin of ridges 1 and 2, we have interpreted a series of unpublished seismic lines perpendicular to the N170° trending steep slopes of Maewo and Pentecost ( Figure 5). The profiles have been collected earlier during various cruises: GEOVAN Legs 1 and 2 (1980), SEAPSO2 (1985), and EVA14 (1987). ...
Context 2
... are stored in archives at the IRD Centre of Nouméa in New Caledonia. The longest lines traverse the western North Fiji basin along distances of more than 100 km eastward of Pentecost and Maewo shorelines (lines 115, 117, and 2 -3, Figure 5), over undisturbed, flat areas lying at 3000-3200 m depth. These western areas are characterized by relatively thick, well-defined sedimentary sequences, with local evi- dence of postdepositional volcanism. ...
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... foot of the deepest ridge (ridge 1) is considered to correspond to the location of the main active fault accommodating present-day shortening of the sedimentary sequence. It is referred to as the main frontal thrust (MFT) in Figure 5. Faults located farther west (labeled thrusts t1 and t2) may be older. ...
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... is indeed in agreement with the fact that Pentecost represents the uplifted border of the Aoba basin, with high relief, whereas Ambrym results partly from recent volcanic con- structions. The scarp east of Ambrym is imaged by profile 133 ( Figure 5). It is locally 900 m high and shows a composite slope including two steps. ...
Context 5
... proposed geom- etries are constrained by data reported above. These are (1) the thickness of the oceanic crust of the North Fiji basin of 8 km, [Pontoise et al., 1994], (2) the thickness of the oceanic crust of the Aoba basin of 11 -13 km [Pontoise et al., 1994], (3) the 1 -2 km thickness of the sedimentary cover of the North Fiji basin (this study), (4) the 4 km maximum thickness of the sedimentary cover of the North Aoba basin [Pontoise et al., 1994], (5) the presence of an asymmetrical crustal root of 18 km below Maewo [Collot and Fisher, 1988], (6) the downbending of the North Fiji basin crust when approaching Pentecost and Maewo ( Figure 5), and (7) the occurrence of sedimentary ridges indicating an accretionary-prism-like structure east of Pen- tecost, including three thrust faults that merge into one fault to the south, eastward of Ambrym (Figures 3 and 5). ...
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... January 2002, the white stains between the two marks had com- pletely disappeared. The evolution of the biological markers between January 2000 and October 2000 can be summa- rized in Figure 15 from observations at site 2 of Ulei Point where four marks were set. The three upper marks (marks 1, 2, and 3) were set in January 2000. ...
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... Erakor the water rose at the entrance of the bungalows and stayed at high level for 10-15 minutes. 250 On 26 November 1999 a Mw 7.5 thrust-type earthquake occurred East of Ambrym Island on the back-arc compressive zone of central Vanuatu (Pelletier et al., 2000;Lagabrielle et al., 2003;Régnier et al., 2003). It is the largest known event on the Vanuatu 260 back-arc. ...
The archipelagos of Vanuatu and Eastern Solomon Islands, scattered over 1500 km along the Vanuatu Arc, include dozens of inhabited volcanic islands exposed to many natural hazards which impact their populations more or less severely. Due to the location of these islands upon a subduction interface, known as the Vanuatu Subduction Zone, tsunamis triggered by earthquakes, volcanic eruptions, and landslides locally, regionally and in the far field, represent a permanent threat. If catalogues already listed tsunamis having occurred in the Vanuatu Arc, they were not exclusively focusing on this region. This study goes further in the listing of tsunamis reported and/or recorded in the Vanuatu Arc, analysing existing catalogues, historical documents, and sea-level data from the 5 coastal tide gauges located in Vanuatu at Port Vila (Efate), Luganville (Santo), Litzlitz (Malekula) and Lenakel (Tanna) and in the Eastern Solomon Islands Province at Lata (Ndende). It allows to identify 100 tsunamis since 1863, 15 of them showing wave amplitude and/or run-up height of more than one meter and 8 between 0.3 and 1 m. If it is known that some tsunamis occurred by the past, information about the wave amplitude or potential run-up is sometime lost (15 events). Also, tsunamis reported in neighbouring islands like New Caledonia, but not reported or recorded in the Vanuatu Arc are discussed, as well as debated events or events with no known origin(s).
... Earthquakes over the last 75-100 years show that coseismic (during an earthquake) uplift of coasts can be recorded by a range of geomorphic, biological and cultural markers (e.g., Bodin & Klinger, 1986;Carver et al., 1994;Castilla, 1998;Chunga-Llauce & Pacheco, 2021;Clark et al., 2017;Evelpidou et al., 2021;Jaramillo et al., 2017;Lagabrielle et al., 2003;Meghroui et al., 2004;Melnick et al., 2012;Plafker, 1969;Plafker & Savage, 1970;Quezada et al., 2020;Rajendran et al., 2007;Stephenson et al., 2017;Stiros et al., 1994;Taylor et al., 2008, and references cited therein). The geographic distribution of vertical land elevation changes obtained from surveying these markers can be used to infer or model the location, geometry and slip on the fault(s) that ruptured during earthquakes (e.g., Awata et al., 2008;Briggs et al., 2006;Garrett et al., 2019;Hamling et al., 2017;Hayes et al., 2010;Konca et al., 2008;Protti et al., 2014). ...
On rocky tectonic coasts, data from Holocene marine terraces may constrain the timing of coseismic uplift and help identify the causative faults. Challenges in marine terrace investigations include: 1) identifying the uplift datums; 2) obtaining ages that tightly constrain the timing of uplift; 3) distinguishing tsunami deposits from beach deposits on terraces; and 4) identifying missing terraces and hence earthquakes. We address some of these challenges through comparing modern beach sediments and radiocarbon ages with those from a trench excavated across three terraces at Aramoana, central Hikurangi Subduction Margin, New Zealand. Sedimentary analyses identified beach and dune deposits on terraces but could not differentiate specific environments within them. Modern beach shells yielded modern radiocarbon ages, regardless of position or species, showing age inheritance and habitat is likely not an issue when dating shells on these terraces. By integrating terrace mapping, stratigraphy, morphology, and radiocarbon ages we develop a conceptual model of coastal uplift and terrace formation following at least two, possibly three, earthquakes at 5490–5070, 2620–2180, and 950–650 cal. yr BP. A high step and time gap between the upper two terraces raises the possibility that at least one intervening terrace is completely eroded. The trench exposure also showed that terrace stratigraphy may differ from that inferred from surface geomorphology, with apparent beach ridges being of composite origin and draping of younger beach deposits on the outer edge of a previous terrace. Dislocation modelling and comparison of marine terrace and earthquake ages from ~4 km south and ≤73 km north confirms that the most likely earthquake source is the nearshore, landward‐dipping, Kairakau Fault. Alternative sources, such as multi‐fault ruptures of the Kairakau‐Waimārama faults or Hikurangi subduction earthquakes, and/or a combination of the two are also possible and should be examined in future studies.
... Seismicity, neotectonic and deep-sea drilling studies revealed that a backarc thrust belt developed over a length of 200 km since 2-3 Ma (Taylor et al., 1987;Louat and Pelletier, 1989;Pelletier et al., 1994). A Mw 7.5 thrust earthquake occurred in 1999 east of Ambrym Island along the main backthrust which outcrops as a 2 km-deep, 40 km-long, 1 km-high scarp on the seafloor Lagabrielle et al., 2003). The upper bound of the total shortening across this backarc thrust belt has been estimated to be 54 km. ...
... This collision caused the cessation of the south-vergent Vitiaz subduction first in the east and then in the west and the new SZ soon became self-sustained. Recently, the d'Entrecasteaux Ridge carried by the Australian Plate collided with the New Hebrides Arc, triggering an incipient-localized SI stage of subduction polarity reversal at the foot of Ambrym -Pentecost Arc back of Espiritu Santo Island (Collot et al., 1985;Lagabrielle et al., 2003;Régnier et al., 2003). ...
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... The DER and WTM perturb the Vanuatu arc, resulting in the late Quaternary uplift of the western Santo-Malekula islands 47,48 and the growth of a back-arc thrust belt (BATB), uplifting the eastern islands of Pentecost-Maewo (Fig. 1a) [49][50][51] . Ambrym's east coast is located at the southern tip of the BATB, which was last activated in the thrusting 1999 M w 7.5 earthquake 52,53 . We note that the focal mechanisms during the 2018 diking event have P-axis orientations consistent with this regional compression (Fig. 2b). ...
Eruptive activity shapes volcanic edifices. The formation of broad caldera depressions is often associated with major collapse events, emplacing conspicuous pyroclastic deposits. However, caldera subsidence may also proceed silently by magma withdrawal at depth, more difficult to detect. Ambrym, a basaltic volcanic island, hosts a 12-km wide caldera and several intensely-degassing lava lakes confined to intra-caldera cones. Using satellite remote sensing of deformation, gas emissions and thermal anomalies, combined with seismicity and ground observations, we show that in December 2018 an intra-caldera eruption at Ambrym preceded normal faulting with >2 m of associated uplift along the eastern rift zone and 2.5 m of caldera-wide subsidence. Deformation was caused by lateral migration of >0.4 cubic kilometers of magma into the rift zone, extinguishing the lava lakes, and feeding a submarine eruption in the rift edge. Recurring rifting episodes, favored by stress induced by the D’Entrecasteaux Ridge collision against the New Hebrides arc, lead to progressive subsidence of Ambrym’s caldera and concurrent draining of the lava lakes. Although counterintuitive, convergent margin systems can induce rift zone volcanism and subsequent caldera subsidence.
... If lava lakes act as barometer of magmatic plumbing system, monitoring them can possibly warn local inhabitants of possible phreatomagmatic eruptions resulting from dike migration towards the coast Located in central portion of New Hebrides Subduction Zone, which is perturbed by the collision of D'Entrecastetaux Ridge• Close proximity to back thrust, whose end terminates east of Ambrym IslandAdapted from McCall, 1970 andLagabrielle, et al 2003 • Basaltic volcanic island that hosts a 12 km-wide caldera with two main craters • Both intra-caldera (lava lakes, fissure eruptions) and extra-caldera (rift intrusions, phraetomagmatic eruptions) activity ...
In the past 30 years, magmatic activity at the basaltic volcano of Ambrym (Vanuatu), which hosts a notable 12-km- diameter caldera, has been characterized by strombolian activity restricted to the two (or more) semi-permanent lava lakes. Little is known about the magma plumbing system of this volcano, due to its remote location (lack of instrumentation) and its persistent background activity (which does not provide insights into deeper magmatic processes). Ground deformation measurements during the December 2018 eruption show a migration of magma from within the caldera to more than 20 km into the SE Rift Zone and towards the ocean. A similar event has not occurred at Ambrym since the 1930’s, and this is thus the first opportunity to model the propagation and emplacement of magma in the volcano’s well-defined rift zone.
In addition, a spectacular, island-wide signal from a decompressing source accompanied this intrusive event, as well as an extinction of the five lava lakes associated with multiple crater collapse, ash clouds, and followed by a drastic decrease in degassing. We use InSAR ground deformation measurements to track the decaying subsidence signal during the days and weeks following the eruption. By combining SAR images from multiple satellites (ALOS-2, Sentinel-1, and Cosmo-SkyMed), we are able to model the geometries, locations and pressure changes due to multiple deformation sources, including the initial intra-caldera dike intrusion, the SE rift zone intrusion, caldera faulting, and the deep, decompressing source. Together, ground deformation, seismicity, and field observations allow us to investigate how the local stress state at Ambrym favours recurrent voluminous dike intrusions into the rift zone, as well as episodic caldera subsidence.
... Le dernier plus gros séisme enregistré dans la région est celui d'Ambrym, qui date de 1999 (M=7.5 -Base de données IRIS ; Mw=7. 4 -Régnier et al. [2003]). Lagabrielle et al. [2003] identifient la structure responsable de ce séisme comme étant une faille à pendage 40°qui affleure à 20 km d'Ambrym. Le séisme généra un déplacement maximal de 6.5 m, accompagné d'un tsunami à hauteurs maximales de 6 à 7 m au sud de l'île de Pentecôte [Pelletier et al., 2000]. ...
The 1,400 km long Vanuatu subduction zone is located in the southeast Pacific Ocean. It is part of the "Ring of Fire" and one of the most seismically and volcanically active subduction zones in the world. The Vanuatu arc delineates the boundary between the Australian plate and the North-Fijian microplate. The former subducts beneath the latter with a convergence rate ranging from 170 mm/yr in the north to 130 mm/yr in the south. This rapid convergence rate is abruptly decreases to 35 mm/yr, at the central segment. This slowing is attributed to the subduction/collision of the d’Entrecasteaux fracture zone. The slowing is accompanied by segmentation of the forearc and uplift of the islands close to the trench by up to 6 mm/year.
The purpose of this thesis is to understand the deformation generated in the forearc of the Vanuatu central segment due to the interaction of the trench with the seafloor reliefs. We use seismological data from the ARC-VANUATU program (2008-2009), which included a temporary (10 month) network of 30 seismic stations (20 on land and 10
at sea). The network recorded more than 40,000 earthquakes. We calculated the first 3-D P and S velocity models of the region, using local passive tomography. We then used these models to locate 11,703 earthquakes beneath the network during the ARCVANUATU
experiment, and 58,303 further earthquakes using a 5-station network left in place after the experiment (from 2009-2014). The 2009-2014 indicates a change in the seismic dynamic from the 2008-2009 data and illuminates areas not seen in the ARC-VANUATU catalogue.
The tomographic inversion highlighted a strong P and S waves heterogeneity in the first 40 km below the surface [Foix et al., 2019] : 1) a trench-parallel alignment of low velocity anomalies is observed west of the large forearc islands (Santo and Malekula) and directly correlated with subducted reliefs ; 2) a trench-parallel alignment of high velocity anomalies further to the east and between 5 and 15 km deep, which could play the role of a backstop ; 3) a relatively thick upper plate crust (29 ± 3 km), compatible with recent indications of a continental origin. 4) a seismogenic zone with an average 15° dip, observed by aligned seismicity in the area not facing seafloor reliefs.
A receiver function study of the central Vanuatu subduction is currently underway. The data is very complex, probably related to both the complexity of the area and noise from the ocean-island setting, and may be difficult to interpret.
... For example, in the northeastern Caribbean, a northverging accretionary prism lies to the north of the eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a Sverging thrust belt lies to the south (Fig. 1). GPS-derived velocities and the occurrence of large earthquakes suggest that significant convergence across the plate boundary can take place in the backarc thrust belt (Java-Timor; Genrich et al., 1996;Vanuatu;Lagabrielle et al., 2003). Therefore, the seismic hazard of backarc thrust belts should be considered (ten Brink et al., 2009). ...
Oblique convergence between the North American and Caribbean plates along the eastern Greater Antilles island arc has yielded the compressive Muertos margin in the backarc region. The Muertos margin is characterized by an asymmetric thrust belt with overall opposite vergence to the subduction system in the forearc region. Offshore south of Puerto Rico, this thrust belt disappears and is replaced by a succession of NE─SW- and E─W-trending deep basins and steep ridges that characterize the western Anegada passage, resulting in a complex deformation pattern. Using new high-resolution multibeam bathymetry and a dense data set of mostly new seismic reflection profiles, we studied the along- and across-strike variations of the geomorphology and shallower structure of the southern Puerto Rico offshore margin. We have identified four morphotectonic provinces: the Puerto Rican sub-basin and Muertos trough, the Muertos margin, the insular shelf and the western Anegada passage. The Muertos margin province shows two distinct slope sub-provinces: the active Muertos thrust belt formed by N─S-compression between the island arc and the Caribbean plate’s interior ─which includes lower and upper thrust belts with distinct deformational styles and lateral continuity─ and the shelf slope covered by a thick package of layered sediments highly-incised by a dense canyon network. This network is disrupted by the Investigator fault zone consisting of a 130 km-long E─W-trending band of active extensional deformation. The Investigator fault zone shows differential surface expression caused by along-strike changes in the magnitude and distribution of the deformation, though this deformation is driven by a N─S-oriented extension, and if there is any amount of strike-slip it is insignificant. In the western Anegada passage province, the Whiting basin and Whiting and Grappler ridges are formed by large dip-slip normal faults driven by a NW─SE-oriented extensional regime. The western St. Croix rise shows a complex structure as attested by the existence of NE─SW- and E─W-oriented normal faults. The NE─SW-trending NW-dipping normal faults observed at the summit of the rise predate the E─W-bounding faults that could accommodate the extensional deformation at the Present. This study provides detailed observations on the active tectonic and sedimentary processes to help future studies assessing the natural resources and the seismic and tsunamigenic hazard in the Puerto Rico region.
... Earthquake rupture zone occurs along well-identified seismic gap instead exhibiting different recurrence intervals and different lengths of the main rupture as well. A clear relationship between coastal forearc uplift and earthquake activity was long been recognized (Plafker, 1969;Thatcher, 1984;Berryman, 1987;Muhs et al., 1990;Nelson and Manley, 1992;Chappell et al., 1996;Lagabrielle et al., 2003). In most cases, the earthquake rupture induces the segmentation of the upper plate tectonic deformation (Chappell et al., 1996;Ota and Yamaguchib, 2004;Bourgois et al., 2007;Melnick et al., 2009). ...
... Because it occurred during the last deglacial sea level rise ( Figure 9b and Table 1), these regressive terraces most likely evidence a sudden fall of relative sea level primarily caused by rapid tectonic uplift that, in addition, prevents their destruction by reoccupation ( Figure 10). Indeed, historic reports of sudden uplift of strandlines during large earthquakes suggest that older strandlines may have been raised coseismically during recurrent earthquakes [Plafker, 1969;Thatcher, 1984;Lajoie, 1986;Berryman, 1987;Nelson and Manley, 1992;Ortlieb et al., 1996;Chappell et al., 1996;Peltzer et al., 1996;Reilinger, 2000;Lagabrielle et al., 2003;Ota and Yamaguchib, 2004;Taylor et al., 2005]. We surveyed these features using two TRIMBLE 4600 LS receivers. ...
... Subduction of the d'Entrecasteaux Ridge beneath Vanuatu pushes the central Vanuatu segment eastward relative to adjacent arc segments (Taylor et al., 1995). Indentation is accommodated there by trench-perpendicular strike-slip faults, and by the activation of the Central Vanuatu back-arc thrust belt that is now accommodating more shortening than the subduction zone itself does (Lagabrielle et al., 2003). ...
... The comparison with experiment 11 that does not include any overriding plate, and in which the trench geometry modifies much more quickly, shows that the overriding plate rigidity opposes rapid indentation in this experimental set. Natural examples evidence that the indentation of the overriding plate by a subducting oceanic ridge may be very rapid: the indentation rate is larger than half the convergence rate between the subducting and overriding plates in Central Vanuatu (Lagabrielle et al., 2003). In the Maipo orocline, Arriagada et al. report post-10 Ma clockwise rotations between the locus of ridge subduction and~38°S, which may account for more than 70 km of differential shortening (~10% of the convergence accommodated between the subducting plate and South America). ...
We present analogue models simulating the subduction of a buoyant ridge oriented perpendicularly or obliquely with respect to the trench, beneath an advancing overriding plate. The convergence velocity is imposed by lateral boundary conditions in this experimental set. We analyze the three-dimensional geometry of the slab, the deformation and topography of the overriding plate. Experiments suggest that ridge subduction diminishes the dip of the slab, eventually leading to the appearance of a horizontal slab segment in case boundary conditions impose a rapid convergence. This result contrasts with that obtained in free subduction experiments, in which ridge subduction diminishes the convergence velocity which, in turn, increases the dip of the slab beneath the ridge. The slab dip decrease is accompanied by the indentation of the overriding plate by the ridge, resulting in arc curvature. Experiments suggest that indentation is larger for small convergence velocity and large slab dip. Ridge subduction also uplifts the overriding plate. Uplift first occurs close to the trench (~ fore-arc area) and is accompanied by the flexural subsidence of the overriding plate behind the uplifted area (~ back-arc subsidence). The uplifted area migrates within the overriding plate interiors following the appearance of a horizontal slab segment. These results are compared with natural examples of ridge subduction in the circum-Pacific area. They explain why ridge subduction may have contrasted effects on the overriding plate dynamics depending on the global conditions that constrain the converging system.
