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Issue Date: 1-Mar-2012
Authors: Raffaele, Rosario Mario
Title: Seismic strutture of subduction zone of the lesser antille
Abstract: In this work, in order to study the relationship between tectonic structures and seismicity, we show results of the experiment SISMANTILLES I on the central Lesser Antilles Subduction Zone where the North and South American plates converge with the Caribbean in a roughly ENE direction, at a rate of about 2 cm/yr. This motion is responsible for volcanic and seismic activity of the Lesser Antilles region. We analyzed local seismicity by using the data recorded, from November 1999 to January 2002, by a temporary combined on/offshore network. A data-set of 155 well located earthquakes, with a total of 4054 and 2617 pickings for P- S-waves respectively, was used to compute first a minimum 1-D velocity model which served as starting model for the subsequent 3-D inversion. To partly estimate the huge structural heterogeneity, the minimum 1-D model is complemented by station corrections that take into account near surface velocity heterogeneity and the geometry of the crust. In particular, the OBS station-terms are in general positive with respect to land stations indicating late arrivals, consistently with the presence of soft mud and sediments at the sea-bottom. The OBS station-terms vary with the thickness of sediments as can be derived here from multi-channel reflection seismic profiles shot over them. The inversion for 3-D P-velocity crustal structure was performed by iteratively solving the coupled hypocentre-velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well-resolved features for interpretation. The resulting 3-D velocity model of the Lesser Antilles region shows that, in the investigated area, the island arc is divided into three layers (upper, middle and lower crust) characterized by a total average thickness of 23-24 km. P-wave velocities between sedimentary values and 6.0 km/s mark the upper crust that is interpreted to consist mainly of volcaniclastics and igneous rocks. Instead the bulk composition of the middle crust (Vp <6.8 km/s) is interpreted to be felsic to intermediate. Finally a plutonic lower crust, that shows P-wave velocities increasing from 7.1 km/s to 7.3 km/s, forms a layer extending from the forearc to the backarc domain. With regards earthquake locations, we observe a very low seismic activity beneath OBS stations that, on the contrary, is mainly concentrated east of the island arc. During the 13-month observation period available, hypocentral distribution highlights earthquakes mostly located between the island arc backstop and the subducting oceanic crust. At depth greater than 50 km we detect earthquakes that can be linked to dehydration of sepertinized slab mantle. In the centre of the arc near 15° 30 , beneath Dominique island, there is a clear concentration of deep events. It is in this area that on November 29, 2007 a 7.4 Mw intraplate earthquake occurred at 150 km depth. A first observation from earthquake locations map suggests that certain seismicity coincides with the extrapolated surface trace of the Tiburon ridge that several authors considered aseismic. Another remark is the no significant seismicity detected either in the accretionnary prism or in the outer forearc region. In our study area we may assume that the inner forearc controls the seismicity since the main seismic activity begins at its eastern border. The occurrence of a high seismicity level far from the backstop position and locate in the slab could be explained by slab bending.
Appears in Collections:Area 04 - Scienze della terra

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File Description SizeFormatVisibility
RFFRRM71M15C351X-0b)Abstract.pdfAbstract84,25 kBAdobe PDFView/Open
RFFRRM71M15C351X-7)Acknowledgements.pdfAcknowledgements71,16 kBAdobe PDFView/Open
RFFRRM71M15C351X-0a)Frontespizio-Indice.pdfIndice116,24 kBAdobe PDFView/Open
RFFRRM71M15C351X-5)Chapter5.pdfChapter51,18 MBAdobe PDFView/Open
RFFRRM71M15C351X-4)Chapter4.pdfChapter45,23 MBAdobe PDFView/Open
RFFRRM71M15C351X-3)Chapter3.pdfChapter37,7 MBAdobe PDFView/Open
RFFRRM71M15C351X-2)Chapter2.pdfChapter25,65 MBAdobe PDFView/Open
RFFRRM71M15C351X-1)Chapter1.pdfChapter12,31 MBAdobe PDFView/Open

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