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- Characterization of an Intraplate Seismogenic Zone Using Geophysical and Borehole Data: The Vila Franca de Xira Fault, PortugalPublication . Carvalho, João; Alves, Daniela; Cabral, João; Ghose, Ranajit; Borges, José Fernando; Dias, Ruben Pereira; Ramalho, Elsa; Caldeira, Bento; Casacão, J.; Leote, JaimeABSTRACT: The Vila Franca de Xira (VFX) fault is a regional fault zone located about 25 km northeast of Lisbon, affecting Neogene sediments. Recent shear-wave seismic studies show that this complex fault zone is buried beneath Holocene sediments and is deforming the alluvial cover, in agreement with a previous work that proposes the fault as the source of the 1531 Lower Tagus Valley earthquake. In this work, we corroborate these results using S-wave, P-wave, geoelectric, ground-penetrating radar and borehole data, confirming that the sediments deformed by several fault branches are of Upper Pleistocene to Holocene. Accumulated fault vertical offsets of about 3 m are estimated from the integrated interpretation of geophysical and borehole data, including 2D elastic seismic modeling, with an estimated resolution of about 0.5 m. The deformations affecting the Tagus alluvial sediments probably resulted from surface or near-surface rupture of the VFX fault during M similar to 7 earthquakes, reinforcing the fault as the seismogenic source of regional historical events, as in 1531, and highlighting the need for preparedness for the next event.
- Improving the knowledge on seismogenic sources in Lower Tagus Valley for seismic hazard purposesPublication . Carvalho, João; Pinto, Carlos C.; Costa, M.; Duarte, H.; Terrinha, Pedro; Rabeh, Taha; Borges, José Fernando; Dias, Ruben Pereira; Caldeira, Bento
- Validation of the Lower Tagus Valley velocity and structural model using ambient noise broadband measurementsPublication . Torres, Ricardo Jorge Gomes; Furtado, José Augusto; Silva, H. G.; Borges, José Fernando; Caldeira, Bento; Bezzeghoud, Mourad; Pinto, Carlos C.; Carvalho, João
- Seismogenic sources in the Lower Tagus Area using geophysical dataPublication . Pinto, Carlos C.; Carvalho, João P. F.; Costa, M.; Catrapona, Afonso; Rabeh, Taha; Terrinha, Pedro; Duarte, Henrique; Borges, José Fernando; Caldeira, Bento; Cabral, João
- Depth estimation of pre-Kalahari basement in Southern Angola using seismic noise measurements and drill-hole dataPublication . Carvalho, João; Alves, Daniela; Borges, José Fernando; Caldeira, Bento; Cordeiro, Domingos; Machadinho, Ana; Oliveira, Álvaro; Ramalho, Elsa; Rodrigues, José Feliciano; Llorente, J.; Ditutala, M.; Lobón, Jose Luis Garcia; Máximo, J.; Carvalho, Cristina Isabel Paulo; Labaredas, José; Ibarra, P.; Manuel, JoséABSTRACT: The remote Southern region of Angola is covered by siliciclastic Kalahari Cenozoic formations that host underground aquifers of great importance to local populations affected by water scarcity problems. These aquifers are well developed where Kalahari sands reach appropriate thicknesses. On the other hand, at the eastern end of this area, regional aeromagnetic data recently acquired suggested the possibility of the continuity of the geological structures of the Lufilian Arc, sited in the nearby Zambia and Congo, southwestwards into Angola under the Kalahari formations. Once the Lufilian Arc is associated with the presence of the so-called Central African Copperbelt, this possibility increased the interest in determining the depth to Pan-African rocks under the Kalahari basin. To estimate the thickness of Kalahari formations in this area of difficult access and poor logistics, an expedited and non-invasive geophysical method was needed. Seismic noise and the single-station Nakamura technique were chosen, but due to the large distance of the study area from the ocean, one of the major sources of seismic noise, a test survey was acquired in the Cuvelai region to assess the signal quality, where the data was calibrated using available drill-holes. >170 points of seismic ambient noise were later acquired and the horizontal/vertical (HVSR) amplitude versus frequency curves were 1D inverted for the best velocity/density model for each station. The results were compared with 1D inverted legacy vertical electrical soundings reprocessed and validated in this work, showing similar depth-to-basement, while interpreted velocities/densities of geological formations were sampled and confirmed with measurements. A depth-to-basement map was produced using seismic information, mechanical soundings, and geological information. Despite the relatively reduced geographical area covered, the map presents valuable information for hydrogeology and mineral exploration purposes and agrees with a previously available coarser map of Kalahari thickness and with observations from geological surveys simultaneously conducted at the time of the seismic surveys.
- Ground-motion simulation in the Lower Tagus Valley BasinPublication . Borges, José Fernando; Bezzeghoud, Mourad; Caldeira, Bento; Carvalho, JoãoThroughout history, the Lower Tagus Valley (LTV) region has been shaken by several earthquakes, including some with moderate to large magnitudes and with sources located inside the basin, for example the 1344 (M6.0) and 1909 (M6.0) earthquakes. Previous simulations (Bezzeghoud et al. Natural Hazard 69: 1229–1245, 2011) have revealed strong amplification of the amplitude waves in the region, because of the effect of the low-velocity sediments that fill the basin. The model used in those simulations was updated in this work by including new high-resolution geophysical and geotechnical data available for the area (seismic reflection, aeromagnetic, gravimetric, deep wells, standard penetration tests, and geological data). To contribute to improved assessment of seismic hazard in the LTV, we simulated propagation of seismic waves produced by moderate earthquakes in a 3D heterogeneous medium by using elastic finite-difference wave propagation code. The method, successfully used by Grandin et al. (Geophys J Int 171: 1144–1161, 2007), involves evaluation of the seismic potential of known faults in the area studied and three-dimensional seismic ground motion modelling by use of finite difference methods. On the basis of this methodology, we calculated the ground motion for the April 23, 1909, Benavente (Portugal) earthquake (Mw = 6.0) in dense grid points, then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity (MMI) and peak ground velocity (PGV). The synthetic results, in contrast with available macroseismic and instrumental data, enable validation of the source models proposed for the area, identification of the sources of historical earthquakes, and could also indicate which areas are more exposed to seismic ground motion.
- Velocity and structural model of Lower Tagus Basin (Portugal) derived from ambient seismic noise measurementsPublication . Borges, José Fernando; Torres, Ricardo Jorge Gomes; Silva, Hugo Gonçalves; Caldeira, Bento; Bezzeghoud, Mourad; Carvalho, João; Furtado, Augusto
- Velocity and structural model of the Lower Tagus Basin according to the study of environmental seismic noisePublication . Torres, Ricardo Jorge Gomes; Furtado, José Augusto; Silva, Hugo Gonçalves; Borges, José Fernando; Caldeira, Bento; Bezzeghoud, Mourad; Carvalho, João
- Prospecção geofísica aplicada à detecção de falhas activas - a falha de Vila Franca de XiraPublication . Casacão, J.; Oliveira, R. J.; Caldeira, Bento; Borges, José Fernando; Carvalho, João
- Prospeção geofísica aplicada à deteção de falhas ativas - a falha de Vila Franca de XiraPublication . Casacão, J.; Oliveira, R. J.; Caldeira, Bento; Borges, José Fernando; Carvalho, João