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- 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.
- A new tsunami runup predictorPublication . Wronna, Martin; Baptista, Maria Ana; Kanoglu, UtkuWe introduce a new parameter, tsunami runup predictor (TRP), relating the accelerating phase of the wave to the length of the beach slope over which the wave is travelling. We show the existence of a relationship between the TRP and the runup for different initial waveforms, i.e. leading elevation N-waves (LENs) and leading depression N-waves (LDNs). Then, we use the TRP to estimate tsunami runup for past tsunami events. The comparison of the runup estimates against field data gives promising results. Thus, the TRP provides first-order estimates of tsunami runup once the offshore waveform is known or estimated and, therefore, it could be beneficial to be implemented in tsunami early warning systems.
- On the construction and use of a Paleo-DEM to reproduce tsunami inundation in a historical urban environment: the case of the 1755 Lisbon tsunami in CascaisPublication . Wronna, Martin; Baptista, Maria Ana Carvalho Viana; Götz, JoachimIn this study, we present a methodology to reconstruct a Paleo Digital Elevation Model (PDEM) to simulate the propagation of a tsunami similar to the one that occurred on the 1st November 1755 in Cascais, Portugal. The method combines historical data, GPS-measurements, and presentday topographic data to build the PDEM. Antique maps were geo-referenced and altitudinal information was reconstructed using historic descriptions. We used old documents to estimate the original landscape of several sites. Analyses and interpretation of these sources of information served to attribute and approximate elevations of both geomorphologic landforms and building features. We used antique maps to rebuild the boundaries of old river mouths and water courses. Finally, we use GPS-RTK to implement obtained elevation data along creek mouths to interpolate in channelled areas to get their slope. Using this methodology and a numerical tsunami simulation code, we reproduced a 1st November 1755-like flooding in Cascais, Portugal. Our results show that using the PDEM, we can reproduce the inundation described in all of the historical accounts.
- 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.