José Fernando Borges

Seismotectonics of Portugal and its adjacent Atlantic area

Abstract A study of focal mechanisms of earthquakes and stress indicators are presented for Portugal and its adjacent Atlantic margins. Recently Riberio et al. (1996) published a work on this subject. In this study, we introduce new focal mechanisms of nine earthquakes. We use these results together with 12 other focal mechanisms and the main tectonic features in the region to define more accurately the type of mechanism and the stress pattern of the region. All the focal mechanisms used in this study differ partially but are compatible with the regional stress field. The stress indicators resulting from focal mechanisms and other geological and geophysical data show that the Portugal continental and its Atlantic margins are under horizontal pressure in NNW–SSE direction, with a greater proportion of strike-slip and reverse-oblique mechanisms for the whole area.

Source Parameters of Old Earthquakes: Semi-Automatic Digitization of Analog Records and Seismic Moment Assessment

Recovering seismic information contained in old analog records could increase our knowledge of seismic source characteristics and the seismicity of a region. This is particularly important in zones with low to moderate seismicity.To extract the available information, it is necessary to digitize the seismic records. This is not an easy task especially owing to the generally poor quality of the original records, with illegible or missing parts. However some exceptions were found in the records of a few seismological stations in Germany, Sweden and France.This paper presents an example of the recovering of source parameters and can be divided in two parts: the first one presents a simple semi-automatic technique for digitization of old analog seismic records, developed using commercial software on a PC; the second part describes the methodology of assessing the seismic moment, using empirical and theoretical relations, as well as the seismic source dimensions.The earthquake selected to illustrate this procedure is the 23 April 1909 Benavente (Portugal) earthquake. This earthquake occurred in the Lower Tagus Valley region and caused great destruction in the meizoseismal area. It is the biggest earthquake that occurred during this century in the central part of the country and its magnitude has been estimated between 6.6 and 7.6.The digitization procedure allowed the recovery of seismic information contained in old analog records, in particular, the seismic moment estimation. The results obtained indicated that 7.6 was a very high value for the magnitude of the 23 April 1909 earthquake, suggesting that the magnitude reported in the Portuguese catalogue is overestimated. The estimated moment magnitude is 6.0.

Incorporating Descriptive Metadata into Seismic Source Zone Models for Seismic-Hazard Assessment: A Case Study of the Azores-West Iberian Region

In probabilistic seismic-hazard analysis (PSHA), seismic source zone (SSZ) models are widely used to account for the contribution to the hazard from earth- quakes not directly correlated with geological structures. Notwithstanding the impact of SSZ models in PSHA, the theoretical framework underlying SSZ models and the criteria used to delineate the SSZs are seldom explicitly stated and suitably docu- mented. In this paper, we propose a methodological framework to develop and docu- ment SSZ models, which includes (1) an assessment of the appropriate scale and degree of stationarity, (2) an assessment of seismicity catalog completeness-related issues, and (3) an evaluation and credibility ranking of physical criteria used to delin- eate the boundaries of the SSZs. We also emphasize the need for SSZ models to be supported by a comprehensive set of metadata documenting both the unique character- istics of each SSZ and the criteria used to delineate its boundaries. This procedure ensures that the uncertainties in the model can be properly addressed in the PSHA and that the model can be easily updated whenever new data are available. The pro- posed methodology is illustrated using the SSZ model developed for the Azores-West Iberian region in the context of the Seismic Hazard Harmonization in Europe project (project SHARE) and some of the most relevant SSZs are discussed in detail. Online Material: Tables describing characteristics and boundaries of the seismic source zones.

Characterization of a Concealed Fault Zone Using P and S-wave Seismic Reflection Data

The Vila Franca de Xira fault zone is the central sector of the OVLS fault zone, one of the major geological structures of the Lower Tagus Valley area in Portugal. Evidences previously gathered from geological and geophysical data suggest that it is an active fault zone and is responsible for the 1531 earthquake that caused extensive damages in Lisbon and the surrounding areas. However, no clear evidence of Quaternary seismic activity has been proven until now. The characterization of the fault zone in Holocenic terrains is therefore important for seismic hazard assessment. A P-wave and an S-wave seismic reflection profiles were acquired over an existing oil-industry and high-resolution P-wave seismic lines. The processing and preliminary interpretation of our data shows that S-wave data provides a superior resolution compared to the P-wave data and is the preferred geophysical method to characterize shallow faults zones in the study area. We have confirmed that faulting affects the Holocene alluvium and a vertical offset of 1-2 m was found in the S-wave data. This finding will allow a better definition of the return periods and the maximum expected earthquake magnitude of the OVLS fault zone.

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.

Is the Vila Franca de Xira fault still active? A shallow seismic reflection shear-wave study in an intraplate environment

The V. F. Xira fault (VFXF) is considered to be the most probable source of several destructive earthquakes that caused significant damage and loss of lives in Lisbon, Portugal and surrounding cities. The fault outcrops in Miocene sediments but is not seen to affect Quaternary terrains. The combination of low slip-rates in the area with erosion/sedimentation rates prevents the identification of active faults in the region. Our goal is to confirm if this fault has been active and to improve seismic hazard assessment in the LTV area. Previous P-wave seismic reflection data indicated the possibility of two shallow fault segments affecting the Holocene alluvium close to the VFXF outcrop. Due to the lack of resolution of this data to detect a small throw (< 2m) fault, the site was revisited with a high-resolution shallow S-wave reflection profile to confirm the fault segments but results were not completely conclusive. Here, we present reprocessed S-wave data and results of full-elastic seismic modelling to confirm the presence of the suspected faults. Clear indications of faulting were found: change of the shape and the amplitude of the reflection hyperbolae, discontinuities in the reflectors in the stacked section, shear-wave velocity dropping in anticipated the damage zones, and weak fault plane reflections. The results are analyzed further through seismic modeling. We conclude that the shallow fault segments affect the entire thickness of the Holocene alluvium and that the VFXF is active.

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.