João P. G. Carvalho

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.

Geophysical study of the Ota-VF Xira-Lisbon-Sesimbra fault zone and the lower Tagus Cenozoic basin

This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota–Vila Franca de Xira–Lisbon–Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr –1 ). A more accurate estimation of the OVLS throw in the Quaternary

Ore prospecting in the Iberian Pyrite Belt using seismic and potential-field data

Ore prospecting using gravimetric and magnetic data has become one of the traditional approaches used in past decades, often complemented with electric and electromagnetic methods. However, due to the problem of non-uniqueness inherent to potential-field modelling, constraints provided by structural methods such as seismic reflection are often used. During the exploration of polymetallic massive sulfide minerals in the Iberian Pyrite Belt, Figueira de Cavaleiros sector, located in the Sado Tertiary Basin, several gravimetric and magnetic anomalies were considered to be interesting targets. In order to reduce any ambiguity in the gravimetric modelling and to confirm the geological model of the area, two seismic reflection profiles were acquired. The interpretation of these profiles was assisted by three mechanical boreholes, two of which were located in the research area to make a seismostratigraphic interpretation. Unfortunately, the gravimetric modelling suggests that the anomaly has a lithological and structural origin and is not related to massive sulfides. Nevertheless, a good agreement between the seismic and potential-field data was achieved and new insights into the geological model for the region were obtained from this work, with accurate data about the Tertiary cover and Palaeozoic basement.

Subsurface Structures and Hydrogeologic Aquifers at the Western Side of Lake Nasser, Southwestern Desert, Egypt

This work deals with the detection of water aquifers and the delineation of subsurface structures predominant in the basement rocks, and their relationship with these aquifers, at west Lake Nasser, southwestern desert, Egypt. An integrated study using geoelectric, geomagnetic, and well logging methods was conducted. Geoelectric surveys were initially performed to obtain vertical electrical sounding (VES) profiles to identify the subsurface geoelectric layers predominant in the area. This was followed by land magnetic surveys using a proton precession magnetometer in a mesh-like network covering the area of investigation. These surveys allowed us to determine the depth to basement rock, and establish the subsurface structures and their relation with the detected aquifers. To confirm the results, more than 20 boreholes were drilled and logged within the study area to determine the distribution of the subsurface water reservoirs and geological sequences in the area. The results indicate that the depth to the...

Introduction and Geological Setting of the Iberian Pyrite Belt

The 250 × 20–70 km Iberian Pyrite Belt (IPB) is a Variscan metallogenic province in SW Portugal and Spain hosting the largest concentration of massive sulphide deposits worldwide. The lowermost stratigraphic unit is the early Givetian to late Famennian-Strunian (base unknown) Phyllite-Quartzite Group (PQG), with shales, quartz-sandstones, quartzwacke siltstones, minor conglomerate and limestones at the top. The PQG is overlain by the Volcanic Sedimentary Complex (VSC), of late Famennian to mid-late Visean age, with a lower part of mafic volcanic rocks, rhyolites, dacites and dark shales, hosting VHMS deposits on top (many times capped by a jasper/chert layer), and an upper part, with dark, purple and other shales and volcanogenic/volcaniclastic rocks, carrying Mn oxide deposits. The VSC is covered by the thousands of meters thick Baixo Alentejo Flysch Group of late Visean to Moscovian age. The VSC comprises a bimodal submarine volcanic succession, with VHMS deposits spatially associated to dacites and rhyolites corresponding to effusive/explosive lava-cryptodome-pumice cone volcanoes. The lava/domes consist of coherent lithofacies surrounded by clast-rotated hyaloclastite breccia and minor autobreccia, with massive VHMS ore at the top of the felsic effusive units and stockworks in the autoclastic and pyroclastic breccias. The eastern IPB rocks are intruded by the voluminous Sierra Norte Batholith (tonalite-trondhjemite-granodiorite, TTG series). Felsic volcanic rocks (dacite to high-silica rhyolite) predominating over basalts and dolerites, belong to the calc-alkaline series and plot mostly in the within-plate field in tectonic discriminative diagrams. Several periods of volcanism, from 384 to 359 Ma are recognized. Dacites and rhyolites exhibit Nd and Sr enrichment, typical of a crustal signature, and their overall geochemistry suggests generation by fractionation/partial melting of amphibolites at low pressure. Trace elemental modelling of the basic rocks, involving tholeiitic lavas and alkaline basaltic lavas and dolerites, points to mixing between E- and N-MORB and assimilation of crustal material. Variscan NW-SE/W-E-trending and SW- or S-verging folds (with NE- or N-dipping planar cleavage) and thrusts, occur in west-central and eastern IPB, respectively. In late to post-Variscan time strike-slip oblique faults formed, either N-S to NNW-SSE or NE-SW to ENE-WSW, dextral or sinistral (both extensional), respectively. The first set hosts late Variscan Cu-Pb-Ba veins and Mesozoic(?) dolerite dykes. IPB contains over 90 VHMS deposits, estimated before erosion at >1700 Million tonnes (Mt), with 14.6 Mt Cu, 34.9 Mt Zn, 13.0 Mt Pb, 46,100 t Ag, 880 t Au and many other metals, particularly Sn. Eight of these are giant (≥100 Mt) VHMS deposits, namely Rio Tinto, Tharsis, Aznalcollar-Los Frailes, Masa Valverde, Sotiel-Migollas and La Zarza (Spain) and Neves Corvo and Aljustrel (Portugal). The VHMS deposits are of the felsic-siliclastic type and mostly of the Zn–Pb–Cu and Zn–Cu–Pb metal content types. The deposits range in thickness from 1 m to tens of meters (plus increase from tectonic stacking) and up to a few kilometers in extension, and many are underlain by large stockwork zones. Their age is either Strunian (palynological age) in the southern IPB or mostly Tournaisian in the northern IPB. The major massive ore minerals are pyrite, sphalerite, chalcopyrite, galena (and cassiterite at Neves Corvo), also present with dominant quartz-chlorite-sericite-carbonate in the stockwork ore. Sericite and chlorite were also formed from additional alteration in the hanging wall rocks. Metal zonation in most VHMS deposits consists of a Cu-rich stockwork and base of the massive ore, with Zn–Pb massive ore above and extending laterally. S-, O-, H- and C-isotope data indicate that ore-forming fluids contain predominant or exclusive modified seawater. A magmatic fluid contribution to the dominant seawater has been proposed for some deposits. The deposits are exhalative or formed by shallow subsurface replacement of either muds/shales or coherent felsic volcanic rocks.

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.