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- Study of the 24 September 2013 Oman Sea tsunami using linear shallow water inversionPublication . Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUEL; Omira, Rachid; El-Hussain, IssaIn this study, we present a linear inverse approach for the computation of the initial water displacement of a tsunami. The method uses empirical Green functions and linear shallow water wave theory. We apply this methodology to study the source of the 24 September 2013 tsunami off the Makran coast. We re-analyze the 2013 tsunami data, particularly the tide records in the coast of Oman and the DART buoy record in the Arabian Sea, to infer its source. We use wavelet and Fourier analyses to characterize the frequency content of the tide records and the possible influence of local harbor effects. We compute empirical Green functions for the possible source area, and then use a linear shallow water inversion technique to assess the water disturbance in the early stage of tsunami propagation. The comparison between forward modeling and observations shows a fair agreement with available data, particularly in what concerns the DART record. It also suggests the existence of local amplifications in a number of Omani harbors. The free surface elevation obtained by inversion of the waveforms is coherent with a landslide located close to 61.5 degrees E, 24 degrees N, sliding to the S or SSE direction. Our results show that the inversion technique used here leads to a better characterization of the tsunami source than the simple backward ray tracing which assumes a simple point source. In addition, these results strengthen the hypothesis that landslides might have played an important role in previous tsunami events in the area, and must be taken into account for tsunami hazard assessment on the Omani coast.
- Reanalysis of the 1761 transatlantic tsunamiPublication . Wronna, Martin; Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUELThe segment of the Africa-Eurasia plate boundary between the Gloria Fault and the Strait of Gibraltar has been the setting of significant tsunamigenic earthquakes. However, their precise location and rupture mechanism remain poorly understood. The investigation of each event contributes to a better understanding of the structure of this diffuse plate boundary and ultimately leads to a better evaluation of the seismic and tsunami hazard. The 31 March 1761 event is one of the few known transatlantic tsunamis. Macroseismic data and tsunami travel times were used in previous studies to assess its source area. However, no one discussed the geological source of this event. In this study, we present a reappraisal of tsunami data to show that the observations data set is compatible with a geological source close to Coral Patch and Ampere seamounts. We constrain the rupture mechanism with plate kinematics and the tectonic setting of the area. This study favours the hypothesis that the 1761 event occurred in the southwest of the likely location of the 1 November 1755 earthquake in a slow deforming compressive regime driven by the dextral transpressive collision between Africa and Eurasia.
- Comparison between MUSCL and MOOD techniques in a finite volume well-balanced code to solve SWE. The Tohoku-Oki, 2011 examplePublication . Reis, C.; Figueiredo, J.; Clain, S.; Omira, R.; Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUELNumerical modelling is a fundamental tool for scenario-based evaluation of hazardous phenomena such as tsunami. Nevertheless, the numerical prediction highly depends on the tool quality and therefore the design of efficient numerical schemes that provide robust and accurate solutions still receives considerable attention. In this paper, we implement two different second-order finite volume numerical schemes deriving from an a priori or an a posteriori limitation procedure and we compare their efficiency in solving the non-conservative shallow-water equations. The numerical schemes assessed here are two variants of the a priori Monotonic Upstream-Centred Scheme for Conservation Laws (MUSCL) and the recent a posteriori multidimensional optimal order detection (MOOD) technique. We benchmark the numerical code, equipped with MUSCL and MOOD techniques, against: (1) a 1-D stationary problem with non-constant bathymetry to assess the second-order convergence of the method when a smooth analytical solution is involved; (2) a 1-D dam-break test to show its capacity to deal with irregular and discontinuous bathymetry in wet zones; and (3) using a simple 1-D analytical tsunami benchmark, single wave on a sloping beach', we show that the classical 1-D shallow-water system can be accurately solved by the second-order finite volume methods. Furthermore, we test the performance of the numerical code for the real-case tsunami of Tohoku-Oki, 2011. Through a set of 2-D numerical simulations, the 2011 tsunami records from both DART and GPS buoys are checked against the simulated results using MUSCL and MOOD. We find that the use of the MOOD technique leads to a better approximation between the numerical solutions and the observations than the MUSCL one. MOOD allows sharper shock capture and generates less numerical diffusion, suggesting it as a promising technique for solving shallow-water problems.
- Reconstructing the architectural evolution of volcanic islands from combined K/AR, morphologic, tectonic, and magnetic data. The Faial Island exemple (Azores)Publication . Hildenbrand, A.; Marques, Fernando O.; Costa, A. C. G.; Sibrant, A. L. R.; Silva, Pedro; Henry, B.; MIRANDA, JORGE MIGUEL; Madureira, P.The morpho-structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass-wasting processes. We present here a multi-disciplinary study of the successive stages of development of Faial (Azores) during the last 1 Myr. Using high-resolution digital elevation model (DEM), and new K/Ar, tectonic, and magnetic data, we reconstruct the rapidly evolving topography at successive stages, in response to complex interactions between volcanic construction and mass wasting, including the development of a graben. We show that: (1) sub-aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca. 0.85 Ma, followed by its partial destruction over half a million years; (2) beginning about 360 ka a new small edifice grew on the NE of the island, and was subsequently cut by normal faults responsible for initiation of the graben; (3) after an apparent pause of ca. 250 kyr, the large Central Volcano (CV) developed on the western side of the island at ca 120 ka, accumulating a thick pile of lava flows in less than 20 kyr, which were partly channelized within the graben; (4) the period between 120 ka and 40 ka is marked by widespread deformation at the island scale, including westward propagation of faulting and associated erosion of the graben walls, which produced sedimentary deposits; subsequent growth of the CV at 40 ka was then constrained within the graben, with lava flowing onto the sediments up to the eastern shore; (5) the island evolution during the Holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano-tectonic depression. We conclude that the whole evolution of Faial Island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation. Each pulse has been separated by considerable periods of volcanic inactivity during which the Faial graben gradually developed. We propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement, as observed historically, when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the Capelinhos eruptive crises (1957-58).
- Generation of N-waves in laboratoryPublication . Lima, Vania Veloso; Avilez-Valente, Paulo; Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUELIn this research work, a novel theoretical first-order formulation for the generation of N-waves in laboratory, by means of a piston-type wave-maker, is presented. The plate's trajectory, velocity and acceleration equations for the generation of tsunami N-waves in a wave flume are deduced. A set of laboratory experiments performed for the generation of leading depression N-waves in a wave basin is described. A Tadepalli-Synolakis N-wave profile is assumed and the equations for the paddle's trajectory, velocity, and acceleration and for the piston stroke and stroke period are derived and discussed in detail. Limits for the initial paddle velocity and acceleration are established. A set of experiments, devised and performed in the 28 m long wave basin of the Laboratory of Hydraulics, at the Faculty of Engineering of the University of Porto, is described. Six leading depression N-waves were selected and classified into three categories, according to their Stokes parameter. The results show that the shorter waves undergo a strong transformation, tending to a solitary wave-like pulse, followed by a trough. The longer waves show a distinct behaviour, tending to a bore-like wave exhibiting a leading trough. The generated longer waves presented the best results when compared with the expected Tadepalli-Synolakis N-wave profiles. Further investigation on the formulation of the wave velocity of the N-waves and on the behaviour of waves with higher order Stokes parameter is necessary.
- Reevaluation of the 11 november 1858 earthquake and tsunami in Setubal: a contribution to the seismic and tsunami hazard assessment in Southwest IberiaPublication . Wronna, Martin; Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUELThe southwest Iberian Margin (SWIM) hosted a series of tsunamigenic earthquakes. However, strong magnitude earthquakes M > 7.0 are scarce and geological and geophysical evidence suggest that slow deformation occurs on a large area without a discrete plate boundary. The reanalysis of this event is crucial because of its location in a possible transition zone between the diffuse plate boundary and the stable continent regime. The 11 November 1858 earthquake in Setubal, Portugal, with an estimated M similar to 7, ruined a large part of the city and was felt all over Portugal and large parts of the Iberian Peninsula. Earlier studies suggest an epicentre close to the shore, less than 50 km away, and there exists a description of a tsunami in Setubal. We use macroseismic intensities and present a reevaluation of the event parameters: depth, epicentre, magnitude, and fault orientation. We select the candidate faults according to the epicentral area and plate kinematics and use scaling laws for the magnitude range. We use tsunami numerical modelling to check the tsunamigenic potential of the event. Finally, we compute the tsunami propagation and inundation for Setubal for seven candidate scenarios and analyze waveform data obtained at virtual tide gauges. Our results favour an offshore inverse fault compatible with the tsunami observation corresponding to a reverse fault with a 40 degrees strike angle at 13 km depth and a magnitude range of 6.8 +/- 0.3.