Percorrer por autor "Matias, Luis"
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- Accurate ocean bottom seismometer positioning method inspired by multilateration techniquePublication . Benazzouz, Omar; Pinheiro, Luis; Matias, Luis; Afilhado, Alexandra; Herold, Daniel; Haines, Seth S.The positioning of ocean bottom seismometers (OBS) is a key step in the processing flow of OBS data, especially in the case of self popup types of OBS instruments. The use of first arrivals from airgun shots, rather than relying on the acoustic transponders mounted in the OBS, is becoming a trend and generally leads to more accurate positioning due to the statistics from a large number of shots. In this paper, a linearization of the OBS positioning problem via the multilateration technique is discussed. The discussed linear solution solves jointly for the average water layer velocity and the OBS position using only shot locations and first arrival times as input data.
- Ambient seismic noise tomography of SW Iberia integrating seafloor- and land-based dataPublication . Corela, Carlos; Silveira, Graça; Matias, Luis; Schimmel, Martin; Geissler, WolframWe used ambient seismic noise recorded by 24 Broadband Ocean Bottom Seismometers (OBS) deployed in the Gulf of Cadiz during the EC funded NEAREST project and seven broadband land stations located in the South of Portugal to image the sedimentary and crustal structure beneath the Eastern Atlantic and SW Iberia. We computed ambient noise cross-correlations to obtain empirical Green's functions (EGFs) between all station pairs using land seismometers and both OBS sensors, seismometers and hydrophones. Despite the great difference in the recording conditions and local crustal structure between the OBSs and land stations, we could compute EGFs, by applying a linear cross-correlation with running absolute mean average time normalization, followed by a time-frequency phase weighted stack. Dispersion analysis was then applied to the EGFs, between 4 and 20s period. The obtained dispersion curves allowed mapping the lateral variation of Rayleigh-wave group velocities, as a function of period. Finally, dispersion curves extracted from each cell of the 2D group velocity maps were inverted, as a function of depth, to obtain the 3D distribution of the shear-wave velocities. The 3-D shear wave velocity model, computed from joint inversion of OBSs and land stations data allowed to estimate the thickness of sediments and crust and the Moho depth. Despite the gap that exists between the OBSs and land station locations, our model displays a good correlation with the known geological structure. The derived sedimentary layer and crustal thicknesses and the obtained Moho depth are locally in agreement with the models proposed by other studies using near vertical, refraction and wide-angle seismic profiling. We conclude that ambient noise tomography could be a valuable tool to image oceanic domains, and also that it is possible to integrate seafloor- and land-based stations to derive a structure model in the transition domain between continent and ocean.
- Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observationsPublication . Pinzón, Juan I.; Custódio, Susana; Silveira, Graça; Krüger, Frank; Mata, João; Matias, LuisIn this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located similar to 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4-9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14-44 s).The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s-1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s(-1). The model reaches an unusually high velocity of 4.9 km s(-1) in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at similar to 70 km depth, which we interpret as the maximum depth for the lithosphere-asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5-4.5 km s(-1) highlighting oscillations of the crustal structure with wavelengths of similar to 25-30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.
- Educating for earthquake science and risk in a tectonically slowly deforming regionPublication . Custódio, Susana; Silveira, Graça; Matias, Luis; Mata, Isabel; Matos, Catarina; Palma‐Oliveira, José Manuel; Rocha, Francisco; Lopes, Fernando C.Over the past decade, scientists have been called to participate more actively in public education and outreach (E&O). This is particularly true in fields of significant societal impact, such as earthquake science. Local earthquake risk culture plays a role in the way that the public engages in educational efforts. In this article, we describe an adapted E&O program for earthquake science and risk. The program is tailored for a region of slow tectonic deformation, where large earthquakes are extreme events that occur with long return periods. The adapted program has two main goals: (1) to increase the awareness and preparedness of the population to earthquake and related risks (tsunami, liquefaction, fires, etc.), and (2) to increase the quality of earthquake science education, so as to attract talented students to geosciences. Our integrated program relies on activities tuned for different population groups who have different interests and abilities, namely young children, teenagers, young adults, and professionals.
- Imaging exhumed lower continental crust in the distal Jequitinhonha basin, BrazilPublication . Loureiro, Afonso; Schnürle, P.; Klingelhoefer, Frauke; Afilhado, Alexandra; Pinheiro, J.; Evain, Mikael; Gallais, F.; Dias, Nuno; Rabineau, Marina; Baltzer, A.; Benabdellouahed, M.; Soares, J.; Fuck, R.; Cupertino, J. A.; Viana, Adriano Roessler; Matias, Luis; Moulin, Maryline; Aslanian, D.; Morvan, L.; Mazé, J. P.; Pierre, D.; Roudaut-Pitel, M.; Rio, I.; Alves, D.; Júnior, P. Barros; Biari, Youssef; Corela, C.; Crozon, J.; Duarte, J. L.; Ducatel, C.; Falcão, C.; Fernagu, P.; Lima, M. Vinicius Aparecido Gomes de; Piver, D. Le; Mokeddem, Z.; Pelleau, P.; Rigoti, C.; Roest, W.; Roudaut, M.Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust.
- Insight on the crustal stress state in Faial and Pico Islands (Azores), from analysis of aftershocks of the 1998 earthquakePublication . Dias, Nuno; Tellez, Julia; Matias, LuisThe seismicity in the Azores plateau can be of volcanic or tectonic origin, the latter usually more destructive, especially in the central group of islands. In this study we aimed to obtain crustal stress indicators from seismic data collected in July 1998, following the July 9th M5.8 earthquake that struck near Faial and Pico islands. The maximum horizontal stress (SHmax) azimuth was obtained from two methods. For the seismogenic volume, the SHmax was obtained from single and composite focal mechanisms, whereas the state of stress beneath the seismic network was derived from shear-wave splitting analysis of local earthquakes. The composite focal mechanisms were obtained from a waveform clustering method applied to identify "identical" earthquakes. The seismic anisotropy analysis could be applied to only six stations, the polarization of the first S arrival being stable, and with a secondary perpendicular pulse arriving 0.6-0.24 s later. The focal mechanisms provided a NW-SE SHmax with azimuth range of 96 degrees-114 degrees, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110 degrees-120 degrees, and coherent with movement along three fault systems. Inland the SHmax azimuths present greater variability. In Faial, the stations located north of the Caldeira present a stable azimuth of 151 degrees-153 degrees and are consistent with the effect of a NNW-SSE fault system present in the islands, rotating to 50 degrees-77 degrees in the eastern sector of the Pedro Miguel Graben, probably due to a complex interaction of the WSW-ENE dominant fault system with old eroded volcanic complexes that segmented the island into several blocks. In Pico island, the SHmax direction azimuth ranges from 93 degrees to 135 degrees, in a radial pattern around the island's main stratovolcano, overlapping the regional stress field.
- Monte Carlo approach to assess the uncertainty of wide-angle layered models: Application to the Santos Basin, BrazilPublication . Loureiro, Afonso; Afilhado, Alexandra; Matias, Luis; Moulin, Maryline; Aslanian, DanielIn the Santos Basin (Brazil), two parallel wide-angle refraction profiles show different crustal structures. One shows moderate crustal velocity gradient, and a clear Moho with topography. The other has an anomalous velocity zone, and no clear Moho reflections. This has large implications on the geological and geodynamical interpretation of the basin. Model uncertainties must be excluded as a source of t hese differences. We developed VMONTECARLO, a tool to assess model uncertainty of layered velocity models using a Monte Carlo approach and simultaneous parameter perturbation using all picked refracted and reflected arrivals. It gives insights into the acceptable geological interpretations allowed by data and model uncertainty through velocitydepth plots that provide: a) the velocity-depth profile range that is consistent with the travel times; b) the random model that provides the best fit, keeping most of the observations covered by ray-tracing; c) insight into valid models dispersion; d) main model features unequivocally required by the travel times, e.g., first-order versus second-order discontinuities, and velocity gradient magnitudes; e) parameter value probability distribution histograms. VMONTECARLO is seamlessly integrated into a RAYINVR-based modelling work-flow, and can be used to assess final models or sound the solution space for alternate models, and is also capable of evaluating forward models without the need for inversion, thus avoiding local minima that may trap the inversion algorithms and providing information for models still not well-parametrised. Results for the Brazilian models show that the imaged structures are indeed geologically different and are not due to different interpretations of the same features within the model uncertainty bounds. These differences highlight the strong heterogeneity of the crust in the middle of the Santos Basin, where the rift is supposed to have failed.
- On the source of the 8 May 1939 Azores earthquake - tsunami observations and numerical modellingPublication . Reis, Cláudia; Omira, Rachid; Matias, Luis; Baptista, Maria Ana Carvalho VianaOn 8 May 1939, an earthquake (Ms7.1) occurred near the Azores archipelago, with an epicentre located close to the western end of the Gloria fault. Previous studies present different epicentre locations spreading over a large area, and two different types of focal mechanisms. Given these uncertainties, the interpretation of the seismological information in a complex tectonic environment between the Gloria Fault and the Terceira Ridge is a matter of debate. The event caused a small tsunami recorded in the Azores Islands. In this study, we use the tsunami observations and tsunami numerical modelling to select the earthquake fault rupture that best fits the tsunami observations. We consider the different focal mechanism solutions, perform tsunami numerical modelling, and compute synthetic tsunami waveforms at the tide gauge locations. We find that an earthquake caused by a low-angle dipping fault with dominant strike-slip movement generates a tsunami that reproduces well the record at Ponta Delgada tide gauge. Finally, in areas where earthquakes are rare, the study of ancient earthquakes must use all information available, namely tsunami observations and mareograph data.
- Reply to comment on "Probabilistic Tsunami Hazard in the Northeast Atlantic From Near- and Far-Field Tectonic Sources'' by Fonseca (Pure and Applied Geophysics, 2016)Publication . Omira, Rachid; Matias, Luis; Baptista, Maria Ana Carvalho VianaThe recent events of the Indian Ocean 2004 and the Tohoku-Oki 2011 have brought to the fore the hazardous nature of the tsunami phenomenon. Consequently, understanding and quantifying the tsunami hazard have gained a significant interest from researchers worldwide. Traditionally, deterministic approach, based on the maximum credible event or the worst-case scenario, has been used to assess the tsunami hazard. However, the absence of a single comprehensive way to define this scenario makes the usefulness of the deterministic method limited (Geist and Lynett 2014). Probabilistic tsunami hazard assessment (PTHA), on the other hand, takes into consideration the contribution of multiple tsunamigenic sources to elaborate tsunami hazard maps. PTHA is now widely used in different tsunami-prone areas of the globe (Annaka et al. 2007; Thio et al. 2007; Sørensen et al. 2012; Yadav et al. 2013; Roshan et al. 2016). In the NE Atlantic, many efforts have been conducted in the aftermath of the Indian Ocean tsunami of 2004 to assess hazard posed by tsunamis. They mainly addressed the tsunami hazard through the deterministic approach with a particular focus on the coastal impact from the 1755-like event (Omira et al. 2010, 2011, 2013; Baptista et al. 2011). Alternatively, Omira et al. (2015) and, more recently, Omira et al. (2016) introduced the probabilistic tsunami hazard assessment in the NE Atlantic from sources of tectonic origin. We welcome the comment by Fonseca (2016) as it gives us the opportunity to better discuss the progress in the PTHA in the NE Atlantic. In his comment, Fonseca (2016) focuses on three main points: (1) the communication of the hazard; (2) the conservatism in the hazard analysis; (3) the uncertainty treatment. The reply to the comments raised by Fonseca (2016) will be addressed in the following sections.
- Synthetic tsunami waveform catalogs with kinematic constraintsPublication . Baptista, Maria Ana Carvalho Viana; Miranda, Jorge Miguel; Matias, Luis; Omira, RachidIn this study we present a comprehensive methodology to produce a synthetic tsunami waveform catalogue in the northeast Atlantic, east of the Azores islands. The method uses a synthetic earthquake catalogue compatible with plate kinematic constraints of the area. We use it to assess the tsunami hazard from the transcurrent boundary located between Iberia and the Azores, whose western part is known as the Gloria Fault. This study focuses only on earthquake-generated tsunamis. Moreover, we assume that the time and space distribution of the seismic events is known. To do this, we compute a synthetic earthquake catalogue including all fault parameters needed to characterize the seafloor deformation covering the time span of 20 000 years, which we consider long enough to ensure the representability of earthquake generation on this segment of the plate boundary. The computed time and space rupture distributions are made compatible with global kinematic plate models. We use the tsunami empirical Green's functions to efficiently compute the synthetic tsunami waveforms for the dataset of coastal locations, thus providing the basis for tsunami impact characterization. We present the results in the form of offshore wave heights for all coastal points in the dataset. Our results focus on the northeast Atlantic basin, showing that earthquake-induced tsunamis in the transcurrent segment of the Azores-Gibraltar plate boundary pose a minor threat to coastal areas north of Portugal and beyond the Strait of Gibraltar. However, in Morocco, the Azores, and the Madeira islands, we can expect wave heights between 0.6 and 0.8 m, leading to precautionary evacuation of coastal areas. The advantages of the method are its easy application to other regions and the low computation effort needed.