A 3-D tomographic inversion of first arrival times of shot profiles recorded by a dense 2-D OBS network provides an unprecedented constraint on the P-wave velocities heterogeneity of the ...upper-crustal part of the North Marmara Trough (NMT), over a region of 180 km long by 50 km wide. One of the specific aims of this controlled source tomography is to provide a 3-D initial model for the local earthquake tomography (LET). Hence, in an original way, the controlled source inversion has been performed by using a code dedicated to LET. After several tests to check the results trade-off with the inversion parameters, we build up a 3-D a priori velocity model, in which the sea-bottom topography, the acoustic and the crystalline basements and the Moho interfaces have been considered. The reliability of the obtained features has been checked by checkerboard tests and also by their comparison with the deep-penetration multichannel seismic profiles, and with the wide-angle reflection and refraction modelled profiles. This study provides the first 3-D view of the basement topography along the active North Anatolian fault beneath the Marmara Sea, even beneath the deepest part of three sedimentary basins of NMT. Clear basement depressions reaching down 6 km depth below the sea level (bsl) have been found beneath these basins. The North Imralı Basin located on the southern continental shelf is observed with a similar sedimentary thickness as its northern neighbours. Between Central and Çınarcık basins, the Central High rises up to 3 km depth below (bsl). Its crest position is offset by 10 km northwestward relatively to the bathymetric crest. On the contrary, Tekirda and Central basins appear linked, forming a 60-km-long basement depression. Beneath the bathymetric relief of Western High low velocities are observed down to 6 km depth (bsl) and no basement high have been found. The obtained 3-D Vp heterogeneity model allows the consideration of the 3-D supracrustal heterogeneity into the future earthquake relocations in this region. The topographic map of the pre-kinematic basement offers the possibility to take into account the locking depth variations in future geohazard estimations by geomechanical modelling in this region.
Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of ...North Anatolian Fault within the Sea of Marmara (comprising the "Istanbul seismic gap") has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25
2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5-5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.
Three unburied ocean bottom seismometers (OBS) equipped with Trillium 240 s broad‐band seismometers recorded spheroidal free oscillations of the Earth out to periods over 1000 s period, for the ...M = 8.1, April 1, 2007 Solomon Islands earthquake. In contrast to broadband observatories of the global network that operate in quiet continental locations, these instruments were dropped on the several‐km thick layer of sediments of the forearc and accretionary wedge of the Lesser Antilles subduction zone. Furthermore, a high ambient noise level due to the ocean surface infragravity waves is expected to cover the frequency band of Earth's normal modes band when recorded at these sites. In spite of these hostile environmental conditions, the frequency of clearly defined peaks of the Earth's normal modes were measured after the earthquake. This suggests that the recording of normal modes and long period waves can be extended to parts of the hitherto inaccessible ocean with currently available OBS technology.
Key Points
Normal modes are recorded by free‐fall broadband OBS for a M8.1 earthquake
First time normal modes are measured on the ocean floor down to 1mHz
It opens new horizons toward improving the long period seismic coverage of ocean
The NE dipping slab of the Hellenic subduction is imaged in unprecedented detail using teleseismic receiver function analysis on a dense 2‐D seismic array. Mapping of slab geometry for over 300 km ...along strike and down to 100 km depth reveals a segmentation into dipping panels by along‐dip faults. Resolved intermediate‐depth seismicity commonly attributed to dehydration embrittlement is shown to be clustered along these faults. Large earthquakes occurrence within the upper and lower plate and at the interplate megathrust boundary show a striking correlation with the slab faults suggesting high mechanical coupling between the two plates. Our results imply that the general slab rollback occurs here in a differential piecewise manner imposing its specific stress and deformation pattern onto the overriding Aegean plate.
Key Points
Dense Receiver Functions 3‐D seismic imaging of the SW Hellenic subduction
Slab top segmentation by along‐dip faults
Piecewise roll back induces the Aegean mobility and deformation
In 2007 the Sismantilles II experiment was conducted to constrain structure and seismicity in the central Lesser Antilles subduction zone. The seismic refraction data recorded by a network of 27 OBSs ...over an area of 65km×95km provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15km.
Basement velocity gradients depict that the forearc is made up of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. Whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6km thick sedimentary pile, in relation with the subduction of the Tiburon Ridge and associated seafloor reliefs.
North, offshore Dominica Island, the outer forearc is 70km wide. It extends as far as 180km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop.
The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera Spur.
The outer forearc could be either the eastern prolongation of the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc.
► We present a 3D tomographic model of the central Lesser Antilles subduction forearc. ► The forearc is divided in an inner and an outer forearc regions. ► The subducting Tiburon Ridge is deforming the outer forearc. ► The inner forearc that emerges at La Désirade Island is tilted southward.
Oceanic island arcs are sites of high magma production and contribute to the formation of continental crust. Geophysical studies may provide information on the configuration and composition of island ...arc crust, however, to date only few seismic profiles exist across active island arcs, limiting our knowledge on the deep structure and processes related to the production of arc crust. We acquired active-source wide-angle seismic data crossing the central Lesser Antilles island arc north of Dominica where the oceanic Tiburon Ridge subducts obliquely beneath the forearc. A combined analysis of wide-angle seismics and pre-stack depth migrated reflection data images the complex structure of the backstop and its segmentation into two individual ridges, suggesting an intricate relation between subducted basement relief and forearc deformation. Tomographic imaging reveals three distinct layers composing the island arc crust. A three kilometer thick upper crust of volcanogenic sedimentary rocks and volcaniclastics is underlain by intermediate to felsic middle crust and plutonic lower crust. The island arc crust may comprise inherited elements of oceanic plateau material contributing to the observed crustal thickness. A high density ultramafic cumulates layer is not detected, which is an important observation for models of continental crust formation. The upper plate Moho is found at a depth of 24
km below the sea floor. Upper mantle velocities are close to the global average. Our study provides important information on the composition of the island arc crust and its deep structure, ranging from intermediate to felsic and mafic conditions.
► In this study we model the deep structure of the Lesser Antilles Island Arc. ► We use a hybrid analysis of refraction and reflection seismic data. ► We image the complex structure of two ridges forming the backstop. ► Island arc crust composition ranges from intermediate to felsic to mafic conditions. ► We discuss the formation of island arc and continental crust.
The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and ...activity by a cluster of active and passive offshore–onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10–20° under the forearc domain, and then to 60° from 70km depth.
At 145km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: “supraslab” earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and “deep flat-thrust” earthquakes at 45km depth on the interplate fault under the Caribbean plate forearc mantle.
None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations are not readily available either for the large crustal thickness of this shallow water marine upper plate, except when remarking its likeness to oceanic plateaus formed above hotspots.
The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose regions of stick-slip behavior on the interplate among stable-gliding areas.
•We resolved seismicity activity pattern beneath the forearc Moho as in Tohoku.•Thicker crust for the Guadeloupe and Martinique islands confirms its LIP origin.•Pyroxenite dykes are present in mantle corner of plume-modified Caribbean lithosphere.•Pyroxenite brittle behavior in serpentinized mantle corner explains downdip ruptures.•Locked interplate in the Antilles down to 45km depth is the same as in Tohoku.
We present the results from a new grid of deep penetration multichannel seismic (MCS) profiles over the 280-km-long north-central segment of the Lesser Antilles subduction zone. The 14 dip-lines and ...7 strike-lines image the topographical variations of (i) the subduction interplate décollement, (ii) the top of the arcward subducting Atlantic oceanic crust (TOC) under the huge accretionary wedge up to 7km thick, and (iii) the trenchward dipping basement of the deeply buried forearc backstop of the Caribbean upper plate.
The four northernmost long dip-lines of this new MCS grid reveal several-kilometre-high topographic variations of the TOC beneath the accretionary wedge offshore Guadeloupe and Antigua islands. They are located in the prolongation of those mapped on the Atlantic seafloor entering subduction, such as the Barracuda Ridge. This MCS grid also provides evidences on unexpected huge along-strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments. Their mapping clearly indicates two principal areas of active deformation in the prolongation of the major Barracuda and Tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the Atlantic seafloor. Although different in detail, the two main deforming forearc domains share similarities in style.
The imaged deformation of the sedimentary stratification reveals a time- and space-dependent faulting by successive warping and unwarping, which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely-oriented elongated and localized topographical ridges. The induced faulting producing vertical scarps in this transport does not require a regional arc-parallel extensional regime as proposed for the inner forearc domain, and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver.
A contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone. Its mapping documents along-arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench. With the exception of the southernmost part, the newly mapped updip limit reaches 25km closer to the trench, thus indicating a possible wider seismogenic zone over almost the whole length of the study area.
•We image the deep structure of the Lesser Antilles Subduction Zone by MCS profiles.•The complex deformation of the outer forearc crust is induced by subducting ridges.•We discuss also the effect of the subducting compressive NAM–SAM Plate-boundary.•Along-strike variations of the seaward edge of the outer forearc crust are discovered.•The updip limit proxy of the seismogenic part reaches 20km trenchwards than believed.
Detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer (OBS) records requires the analysis and identification of noise due to ...instrumental problems, deployment sites or sea state conditions. The temporary OBS deployment at the Lesser Antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor (NVT), long-period (LP) or ultra-long period (ULP) events. Analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise. Episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data. In order to reliably detect NVT (1–10Hz) originating from inside the solid Earth, one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea-bottom as well as on land. The semidiurnal modulation of noise amplitude can be shown here not to be due to that of the NVT from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs. Here, the semidiurnal modulation is rather due to the effect of the tides themselves, such as tidal currents, since they do not affect all types and all components of the unique multi-station array of OBS that could be deployed on this submerged forearc. The short period cut-off of the strong noise due to ocean surface infragravity waves increases to longer periods with OBS depth, thereby increasing the observational window with low noise to lower frequencies, and deep OBS sites may be advantageous for detecting LP events.
► New massive multiple sites and instruments offshore–onshore approach to seismic noise ► Validation of recording in the band of NVT and LP on submerged subduction forearcs ► A different angle on seismogenic zone vs. atmosphere–hydrosphere and coastal origin
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