Bento Caldeira

Seismicity along the Azores-Gibraltar region and global plate kinematics

Seismicity along the western part of the Eurasia–Nubia plate boundary displays very complex patterns. The average motion is transtensional in the Azores, dextral along the Gloria transform zone and convergent between the SW Portuguese Atlantic margin and the Ibero-Maghrebian zone. To constrain the factors controlling the seismicity, we provide a new seismotectonic synthesis using several earthquakes. We show that the study area can be divided into six different regions, with each characterised by a coherent seismicity pattern. The total seismic moment tensor and the average slip velocities are provided for each region. To determine the spatial distribution of the seismicity, we computed the slip vector for each earthquake based on its focal mechanism and compared it to the relative velocity between the Eurasian and Nubian plates, deduced from global kinematic models. Despite local departures in the Alboran Sea and near the Mid-Atlantic Ridge, we found a good correlation between these two independent vector sets. Quantitatively, the slip velocities display a linear, non-affine correlation with the norms of the relative kinematic velocities. The norms of the slip velocities also seem to depend on the tectonic regime and on the morphology of the plate boundary.

The Seismicity of Portugal and Its Adjacent Atlantic Region from 1300 to 2014: Maximum Observed Intensity (MOI) Map

In this study, we considered earthquakes with a maximum observed intensity (MOI) Io≥V that occurred throughout the Portuguese mainland and its adjacent Atlantic region to produce the map of maximum intensities of Portugal. This map is based on a wide variety of historical and recent sources, including 175 earthquakes, over the period of 1300–2014. This MOI map allows the regions of high, medium, and low levels of seismic hazard to be highlighted. The entirety of Portugal has been affected by major seismic events, some of which have caused serious damage. In addition, offshore earthquakes have had significant impacts on the coastal cities located in the central and southern regions of the country. Because the southern region of mainland Portugal is more active than the northern region, the largest concentrations of events with Io≥VI are in the southern region, especially on the mainland near the city of Evora, in the Lisbon region, in the Lower Tagus Valley region, and along the Algarve coast, especially in the southwest region in Cape of Sao Vicente and the Gorringe Bank.

Integration of Georradar Methods and Electromagnetic Induction for Caves Mapping: Application to Sono Cave-Arrábida

Neste trabalho apresentam-se os resultados da aplicacao da geofisica ao mapeamento de uma seccao da Lapa do Sono - Serra da Arrabida (Sesimbra), que se sabe ter tido ocupacao funeraria na epoca do Neolitico antigo e atualmente se apresenta extremamente obstruida por sedimentos. Foram escolhidas duas tecnicas frequentemente utilizadas na prospeccao da subsuperficie em ambiente arqueologico, o Georradar (GPR) e a Inducao Eletromagnetica de baixa frequencia (EM). O dificil estado do terreno e os condicionalismos colocados em defesa da protecao da natureza impuseram um programa de aquisicao de dados segundo um paradigma um pouco diferente do habitual. A metodologia seguida para a obtencao de resultados teve que se basear numa analise exaustiva sobre um limitado numero de perfis modelados, utilizando distintas sistematicas de processamento 2D, implementadas para despistar os principais fatores de erro. Os resultados permitem delinear a gruta principal inserida num meio carsico, extensamente fragmentado, principalmente nas areas de cobertura da cavidade. Tambem se recolheram indicios de outras cavidades menores contiguas a gruta principal que podem estar a ela ligadas atraves de galerias menores preenchidas por sedimentos. A partir da interpretacao dos resultados foi feita a avaliacao da espessura de sedimentos acumuladas na periferia da gruta, sob os perfis amostrados.

Grandes terremotos en Azores

The history of the Azores archipelago, from its discovery and settlement in the first half of the 15th century through the present, is marked by the social and economic impacts produced by earthquakes, mainly the high-intensity ones. Information that has been compiled leads to the conclusion that in this period, 33 earthquakes with intensity equal to or greater than VII have affected the Azores, which caused approximately 6,300 deaths and widespread destruction on some islands of the archipelago, principally S. Miguel, Terceira, Graciosa, Faial, S. Jorge and Pico Islands. The accommodation of strain resulting from the dynamics of the Azores triple junction (ATJ) plate boundary and volcanism, which also occurs in the region, are the main factors responsible for the intense seismic activity in this archipelago. This work reviews the scientific issues of the known earthquakes that have severely interfered with the lives of the Azorean people throughout their history, which we call large earthquakes.

Los mayores sismos en Argelia en la época moderna: las fallas de El Asnam y Zemmouri-Boumerdès

Algeria has experienced many destructive earthquakes during the last few centuries (e.g., Ayadi and Bezzeghoud, 2015). The city of El Asnam (formerly Orleanville, today Chlef) was severely damaged in 1954 and 1980 by magnitude 6.7 and 7.3 earthquakes, respectively. On October 10, 1989, a magnitude 5.9 earthquake struck the Mont Chenoua-Tipasa coastal area approximately 150 km west of Zemmouri, which is where the May 21, 2003 earthquake occurred (Mw6.8). Many other large historical and instrumental earthquakes have severely damaged the coastal cities of Algeria over the last few centuries (i.e., around Algiers, Oran, Mascara, Djidjelli, Constantine and Bejaia). These earthquakes suggest active deformation of the margin in conjunction with the clear offshore extent of active coastal faults. According to several studies, the main active geological structures around El Asnam, Algiers, Zemmouri and Boumerdes have experienced several disastrous earthquakes. Many other earthquakes have occurred in and around the Chlef and Mitidja Basins, underlining the seismic activity in the area. In this chapter, we highlight the main characteristics of the two largest earthquakes that have occurred in Algeria in the modern period: the El Asnam earthquakes of September 9 (Mw6.7) and October 10 1980 (Mw7.3), and the Zemmouri-Boumerdes earthquake of May 21, 2003, (Mw6.8).