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Bartolome R.,Center Mediterrani dInvestigacions Marines i Ambientals | Danobeitia J.,Center Mediterrani dInvestigacions Marines i Ambientals | Michaud F.,University Pierre and Marie Curie | Cordoba D.,Complutense University of Madrid | Delgado-Argote L.A.,CICESE
Pure and Applied Geophysics | Year: 2011

Three thousand kilometres of multichannel (MCS) and wide-angle seismic profiles, gravity and magnetic, multibeam bathymetry and backscatter data were recorded in the offshore area of the west coast of Mexico and the Gulf of California during the spring 1996 (CORTES survey). The seismic images obtained off Puerto Vallarta, Mexico, in the Jalisco subduction zone extend from the oceanic domain up to the continental shelf, and significantly improve the knowledge of the internal crustal structure of the subduction zone between the Rivera and North American (NA) Plates. Analyzing the crustal images, we differentiate: (1) An oceanic domain with an important variation in sediment thickness ranging from 2.5 to 1 km southwards; (2) an accretionary prism comprised of highly deformed sediments, extending for a maximum width of 15 km; (3) a deformed forearc basin domain which is 25 km wide in the northern section, and is not seen towards the south where the continental slope connects directly with the accretionary prism and trench, thus suggesting a different deformational process; and (4) a continental domain consisting of a continental slope and a mid slope terrace, with a bottom simulating reflector (BSR) identified in the first second of the MCS profiles. The existence of a developed accretionary prism suggests a subduction-accretion type tectonic regime. Detailed analysis of the seismic reflection data in the oceanic domain reveals high amplitude reflections at around 6 s [two way travel time (twtt)] that clearly define the subduction plane. At 2 s (twtt) depth we identify a strong reflection which we interpret as the Moho discontinuity. We have measured a mean dip angle of 7° ± 1° at the subduction zone where the Rivera Plate begins to subduct, with the dip angle gently increasing towards the south. The oceanic crust has a mean crustal thickness of 6.0-6.5 km. We also find evidence indicating that the Rivera Plate possibly subducts at very low angles beneath the Tres Marias Islands. © 2010 Springer Basel AG.

Geissler W.H.,Alfred Wegener Institute for Polar and Marine Research | Matias L.,University of Lisbon | Stich D.,University of Granada | Carrilho F.,Polytechnic Institute of Lisbon | And 7 more authors.
Geophysical Research Letters | Year: 2010

An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low-magnitude local earthquakes in the complex transpressive plate boundary setting of the Gulf of Cadiz, known for the 1755 Lisbon earthquake and tsunami. 36 relocated earthquakes (ML 2.2 to 4.8) concentrate at 40-60 km depth, near the base of the seismogenic layer in ∼140 Ma old oceanic mantle lithosphere, and roughly align along two perpendicular, NNE-SSW and WNW-ESE striking structures. First motion focal mechanisms indicate compressive stress for the cluster close to the northern Horseshoe fault termination which trends perpendicular to plate convergence. Focal mechanisms for the second cluster near the southern termination of the Horseshoe fault indicate a strike-slip regime, providing evidence for present-day activity of a dextral shear zone proposed to represent the Eurasia-Africa plate contact. We hypothesize that regional tectonics is characterized by slip partitioning. © 2010 by the American Geophysical Union.

Gracia E.,Center Mediterrani dInvestigacions Marines i Ambientals | Vizcaino A.,Center Mediterrani dInvestigacions Marines i Ambientals | Escutia C.,University of Granada | Asioli A.,University of Padua | And 5 more authors.
Quaternary Science Reviews | Year: 2010

The SW margin of the Iberian Peninsula hosts the present-day boundary between the Eurasian and African Plates. Convergence (4-5 mm/yr) is accommodated through a wide deformation zone characterized by moderate magnitude seismic activity. This zone has also been the source of the most important seismic events in Western Europe, such as the 1755 Lisbon Earthquake and Tsunami and 1969 Horseshoe Earthquake. Despite efforts to identify active seismogenic structures in the Gulf of Cadiz in the last ten years, little is known about its paleoseismic history. The turbidite paleoseismology approach was applied for the first time in a low-rate convergent margin to determine the recurrence interval of large earthquake events that occurred in SW Iberia during the Holocene. Four sediment cores collected at strategically located sites offshore Portugal (i.e. Tagus Abyssal Plain, Infante Don Henrique Basin and Horseshoe Abyssal Plain) reveal that these deep-sea basins preserve a record of episodic deposition of turbidites. In the SW Iberian Margin excluding special climatic events, earthquakes are the most likely triggering mechanism for synchronous, widely-spaced distributed turbidites during the Holocene, when the sea level was relatively stable. Age correlation together with textural, mineralogical, physical properties and geochemical signatures of the new cores complemented by pre-existing multicores and gravity cores reveals a total of 7 widespread turbidite events for the Holocene. Precise dating of the most recent turbidite event (E1) based on 210Pb and 137Cs geochronology provides an age of AD 1971 ± 3. This age corresponds to a high-magnitude instrumental earthquake in the region: the 1969 Horseshoe Earthquake (Mw 8.0). Calibrated 14C ages of subsequent widespread turbidite events (E3 and E5) correlate with the dates of important historical earthquakes and paleotsunami deposits in the Gulf of Cadiz area, such as AD 1755 and 218 BC, respectively. If older synchronous events (E6, E8, and E10) with ages ranging from 4960-5510 yr BP to 8715-9015 yr BP are also taken into account, a great earthquake recurrence interval of about 1800 years is obtained for the Holocene. Our correlations suggest that the turbidite record may be considered as a proxy for paleoseismic activity in low-convergence rate margins, and a valuable complementary tool in earthquake and tsunami hazard assessment along the coasts of the Iberian Peninsula and North Africa. © 2010 Elsevier Ltd. All rights reserved.

Patton J.R.,Oregon State University | Goldfinger C.,Oregon State University | Morey A.E.,Oregon State University | Ikehara K.,Japan National Institute of Advanced Industrial Science and Technology | And 7 more authors.
Geosphere | Year: 2015

In order to investigate the possibility of a long-term paleoseismic history from offshore sedimentary records in Sumatra, we collected 144 deep-sea sediment cores in the trench and in lower slope piggyback basins of the Sumatra accretionary prism. We used multibeam bathymetry and seismic reflection data to develop an understanding of catchment basins, turbidity current pathways, and depositional styles, as well as to precisely locate our gravity cores, piston cores, Kasten cores, and multicores. We use detailed physical property data, including computed tomographic X-ray, gamma density, magnetic susceptibility, grain-size analysis, faunal analysis, and smear slides, to evaluate the turbidite stratigraphy and sedimentology at each site. We use radiocarbon age control for piggyback basin sites above the carbonate compensation depth, and use 210Pb and 137Cs to evaluate the timing of the most recent sedimentary deposits. Using well-log correlation methods and radiometric age control, we test for potential correlations between isolated sites in piggyback basins and the trench. We find evidence for very young surface turbidites along the northern Sumatra margin, most likely emplaced within the past few decades at the seafloor in both the 2004 and 2005 earthquake rupture zones, with no overlying hemipelagic sediment. Based on the young soupy deposits, lack of oxidation, and 210Pb and 14C age determinations, we interpret the uppermost turbidite in 21 cores within the 2004 rupture area to have been deposited within a few years of collection in 2007, and most likely as a result of the 2004 moment magnitude (Mw) ~9.2 earthquake. The likely 2004 turbidite has a distinctive stacked structure of three major fining-upward sequences observed at several basin and trench sites, similar to the pattern of moment release in the 2004 earthquake. We observe rapid die out of the 2004 and 2005 deposits with distance from the slip zones, from local sources of sediment supply, and in the segment boundary between the slip zones. Many individual turbidites show strong similarities between isolated sites, as well as having similar emplacement times. Based upon radiocarbon age control and lithostratigraphic correlations between isolated basin and trench core sites, we interpret that 43 turbidites can be linked spatially over a distance of ~230 km within the southern portion of the 2004 rupture zone. Sampling at deep-water sites isolated from terrestrial and shallow-water sediment sources, as well as potential storm or tsunami wave triggers, limits potential mechanisms for initiating turbidity currents to plate boundary, crustal, or slab earthquakes. Other potential triggers, such as tectonic oversteepening, random self-failures, gas hydrate destabilization, are unlikely to be correlative between any two isolated sites. The most probable explanation for the similarity of timing, turbidite sequences, and individual turbidite structure in isolated basin and trench stratigraphic sequences is a seismogenic origin. The mean emplacement time for turbidites (likely triggered by Great earthquakes, magnitude > ~8) in the 2004 rupture region for the past 6.6 ± 0.14 k.y. is 160 yr for 43 turbidites. The ages of 8 of the 10 uppermost turbidite deposits, spanning the past ~1500 yr, are largely consistent with the terrestrial paleoseismic and/or tsunami records in Thailand, Sumatra, India, and the Andaman Islands, suggesting either coincidence or a common origin. The mean interseismic time from the turbidite record for this same period is 170 yr, comparable to the ~210 yr recurrence for regional tsunami. The turbidite record, at 180 yr (6 events), compares reasonably well to the average for all events on northern Simeulue of 220 yr, and is identical to the tsunami interval of 180 yr for the same time period (6 events). Of the 43 correlated turbidites in the 2004 earthquake region, 13 are well correlated in our cores along strike lengths of 150 km or greater, and satisfy criteria for robustness; 24 turbidites correlated along a shorter strike distance may represent other plate boundary earthquakes of shorter spatial extent and may include turbidite beds sourced from crustal and slab earthquakes. © 2016 Geological Society of America.

Rosas F.M.,Instituto Dom Luiz | Rosas F.M.,University of Lisbon | Duarte J.C.,Instituto Dom Luiz | Duarte J.C.,University of Lisbon | And 12 more authors.
Tectonophysics | Year: 2012

Analog and numerical modeling experiments were carried out to investigate the tectonic interference between intersecting major active strike-slip and thrust faults in the Gulf of Cadiz (Africa-Eurasia plate boundary, offshore SW Iberia). The obtained results show that newly mapped tectonic features located in the fault intersection area (corner zone) consist mostly in oblique (dextral-reverse) faults that accommodate significant strain partitioning. Modeling of this corner-zone faults show that they have endured some degree of rotation, displaying successive evolving geometries and kinematics. Numerical modeling results further show that an interbedded shallow soft layer, accounting for a regional (Late Miocene) gravitational "Chaotic" unit, could explain the mild bathymetric expression of the fault pattern in the corner-zone. Moreover, a recognized depth discrepancy, between the (upper crust) interference fault-pattern and the (lithospheric mantle) seismicity, is interpreted as a manifestation of similar thrust-wrench tectonic interference at different lithospheric depths. Accordingly, an intermediate lower crust-upper mantle aseismic (i.e. softened) depth-domain could be explained by pervasive alteration/serpentinization, prompted by fluid percolation through fault-related fractures associated with the newly revealed corner zone fault-network. Overall obtained results reinforce the relevance of a thrust-wrench multi-rupture seismic scenario as the main cause for the moderate seismicity (Mw < 6.0) in the study area. © 2012 Elsevier B.V.

Alfaro P.,University of Alicante | Bartolome R.,Center Mediterrani dInvestigacions Marines i Ambientals | Borque M.J.,University of Jaén | Estevez A.,University of Alicante | And 6 more authors.
Journal of Iberian Geology | Year: 2012

The Bajo Segura Fault Zone, located at the NE end of the Eastern Betic Shear Zone, has been the site of some of the most intense seismic activity on the Iberian Peninsula in the historical and instrumental time periods. This structure is an active blind fault that does not show any surface rupture. It is characterised by a set of ENE-WSW trending blind thrust faults that offset the Triassic basement and cause active folding of the Upper Miocene-Quaternary sedimentary cover. The main active structures of this fault zone are two ENE-WSW striking reverse blind faults, the Torremendo and the Bajo Segura Faults, and several secondary NW-SE striking dextral faults (San Miguel de Salinas, Torrevieja and Guardamar Faults). These structures continue offshore to the east. From geological, geomorphological and geodetic data, we obtain fault slip rates between 0.2 and 0.4 mm/yr, whereas other authors have proposed higher values ranging between 0.75 and 1 mm/yr. The fault zone can generate earthquakes with maximum estimated magnitudes (Mw) from 6.6 to 7.1 and has approximate recurrence intervals between 4.500 and 21.500 years.

Sala E.,National Geographic Society | Sala E.,CSIC - Center for Advanced Studies of Blanes | Ballesteros E.,CSIC - Center for Advanced Studies of Blanes | Dendrinos P.,MOm Hellenic Society for the Study and Protection of the Monk Seal | And 26 more authors.
PLoS ONE | Year: 2012

Historical exploitation of the Mediterranean Sea and the absence of rigorous baselines makes it difficult to evaluate the current health of the marine ecosystems and the efficacy of conservation actions at the ecosystem level. Here we establish the first current baseline and gradient of ecosystem structure of nearshore rocky reefs at the Mediterranean scale. We conducted underwater surveys in 14 marine protected areas and 18 open access sites across the Mediterranean, and across a 31-fold range of fish biomass (from 3.8 to 118 g m -2). Our data showed remarkable variation in the structure of rocky reef ecosystems. Multivariate analysis showed three alternative community states: (1) large fish biomass and reefs dominated by non-canopy algae, (2) lower fish biomass but abundant native algal canopies and suspension feeders, and (3) low fish biomass and extensive barrens, with areas covered by turf algae. Our results suggest that the healthiest shallow rocky reef ecosystems in the Mediterranean have both large fish and algal biomass. Protection level and primary production were the only variables significantly correlated to community biomass structure. Fish biomass was significantly larger in well-enforced no-take marine reserves, but there were no significant differences between multi-use marine protected areas (which allow some fishing) and open access areas at the regional scale. The gradients reported here represent a trajectory of degradation that can be used to assess the health of any similar habitat in the Mediterranean, and to evaluate the efficacy of marine protected areas.

Torrecilla E.,Center Mediterrani dInvestigacions Marines i Ambientals | Stramski D.,University of California at San Diego | Reynolds R.A.,University of California at San Diego | Millan-Nunez E.,University of California at San Diego | And 2 more authors.
Remote Sensing of Environment | Year: 2011

Optical measurements including remote sensing provide a potential tool for the identification of dominant phytoplankton groups and for monitoring spatial and temporal changes in biodiversity in the upper ocean. We examine the application of an unsupervised hierarchical cluster analysis to phytoplankton pigment data and spectra of the absorption coefficient and remote-sensing reflectance with the aim of discriminating different phytoplankton assemblages in open ocean environments under non-bloom conditions. This technique is applied to an optical and phytoplankton pigment data set collected at several stations within the eastern Atlantic Ocean, where the surface total chlorophyll-a concentration (TChla) ranged from 0.11 to 0.62mgmΛ3. Stations were selected on the basis of significant differences in the ratios of the two most dominant accessory pigments relative to TChla, as derived from High Performance Liquid Chromatography (HPLC) analysis. The performance of cluster analysis applied to absorption and remote-sensing spectra is evaluated by comparisons with the cluster partitioning of the corresponding HPLC pigment data, in which the pigment-based clusters serve as a reference for identifying different phytoplankton assemblages. Two indices, cophenetic and Rand, are utilized in these comparisons to quantify the degree of similarity between pigment-based and optical-based clusters. The use of spectral derivative analysis for the optical data was also evaluated, and sensitivity tests were conducted to determine the influence of parameters used in these calculations (spectral range, smoothing filter size, and band separation). The results of our analyses indicate that the second derivative calculated from hyperspectral (1nm resolution) data of the phytoplankton absorption coefficient, aph(Λ), and remote-sensing reflectance, Rrs(Λ), provide better discrimination of phytoplankton pigment assemblages than traditional multispectral band-ratios or ordinary (non-differentiated) hyperspectral data of absorption and remote-sensing reflectance. The most useful spectral region for this discrimination extends generally from wavelengths of about 425-435nm to wavelengths within the 495-540nm range, although in the case of phytoplankton absorption data a broader spectral region can also provide satisfactory results. © 2011 Elsevier Inc.

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