NEFITAG - Strong ground motion and near field effects in the Lower Tagus Valley Region

Publications

Soil Classification Maps for the Lower Tagus Valley Area, Portugal, Using Seismic, Geological, and Remote Sensing Data

Publication . Carvalho, João; Dias, Ruben; Borges, José Fernando; Quental, Lídia; Caldeira, Bento

ABSTRACT: The Lower Tagus Valley (LTV) region has the highest population density in Portugal, with over 3.7 million people living in the region. It has been struck in the past by several historical earthquakes, which caused significant economic and human losses. For a proper seismic hazard evaluation, the area needs detailed V-s30 and soil classification maps. Previously available maps are based on proxies, or an insufficient number of velocity measurements followed by coarse geological generalizations. The focus of this work is to significantly improve the available maps. For this purpose, more than 90 new S-wave seismic velocities measurements obtained from seismic refraction and seismic noise measurements, doubling the number used in previously available maps, are used to update available V-s30 and soil classification maps. The data points are also generalized to the available geological maps using local lithostratigraphic studies and, for the first time, satellite images of this area. The results indicate that lithological and thickness changes within each geological formation prevent a simple generalization of geophysical data interpretation based solely on geological mapping. The maps presented here are the first attempt to produce maps at a scale larger than 1:1,000,000 in Portugal, with direct shear wave velocity measurements. A tentative approach to produce more detailed maps using machine learning was also carried out, presenting promising results. This approach may be used in the future to reduce the number of shear wave measurements necessary to produce detailed maps at a finer scale.

2025-04Journal article

Open access

Reappraisal of active tectonics of the Porto Alto buried fault zone (Portugal) considering new geophysical shallow studies

Publication . Carvalho, João; Cabral, João; Ghose, Ranajit; Borges, José Fernando; Dias, Ruben

ABSTRACT: The Lower Tagus Valley area (LTV), where Lisbon is located, has been affected by several destructive, M 6+ earthquakes whose sources remain to be determined. The identification of expectable surface or near surface ruptures in the area is a challenging task that requires a multidisciplinary analysis that includes geophysical techniques, as the source faults are mainly buried despite likely to deform Upper Pleistocene to Holocene alluvial cover of the Tagus River. This paper focuses on the characterization of the Porto Alto fault zone for seismic hazard mitigation purposes. The Porto Alto fault zone was recognized in oil-industry P-wave 1980’s seismic reflection data as an important, Miocene reactivated, deep structure in the LTV. Highresolution P-wave seismic reflection data were later acquired in the early 2000’s to investigate related Holocene fault activity, leading to the identification of a shallow fault zone near the surface. However, the vertical resolution of the acquired P-wave seismic reflection data was considered insufficient to corroborate any presumably small vertical offset related to fault rupture in the ca. 50 m thick alluvium cover. Trenching for the recognition and characterization of surface faulting was previously tested in the study region but it proved to be a challenging and poorly efficient methodology due to the very shallow water table (at ~1 m) and low cohesion of the sediments. Due to these constraints, we revisited the former fault study site to acquire higher resolution S-wave seismic and ground penetrating radar (GPR) data. The new seismic profiles show interruption of the reflectors in the stacked sections. Diffracted energy, changes in amplitude/shape of the reflection hyperbolae in the shot gathers and spatially coincident low velocity anomalies, also indicate the presence of several shallow fault strands deeper than 10 m. The GPR profile, overlapping and extending the seismic profiles in 30 m reaches a maximum investigation depth of about 15 m and shows the presence of deformation at three locations, one of which matches with one of the fault strands detected in the high resolution S-wave seismic data. In this profile, sediment disruption was detected extending upwards to a depth as shallow as ca. 3.5 m, corresponding to alluvium with a poorly constrained age of ca. 2,300 yrs. Slip rate, maximum earthquake magnitude and recurrence, and other parameters are also estimated for the Porto Alto fault zone. These recently acquired seismic and GPR datasets indicate that there were at most three to five maximum earthquakes generated by the fault in the last 13,100 years, with an average recurrence of approximately 4,400 to 2,600 years respectively. However, the data show a grouping of these earthquakes in time, the first two in the period 13,100-12,300 years, separated by about 800 years, and the third or the last grouped three having occurred in the past 2,300 years with a similar average recurrence time of ca. 800 years. However, the regional historical and instrumental seismicity does not show an obvious link of any known major earthquake with the Porto Alto fault zone.

2025-05Journal article

Open access

SedDARE-IB: an open-access repository of sediment data for the Iberian Peninsula and its continental margins

Publication . Torne, Montserrat; Alves, Tiago M.; Jimenez-Munt, Ivone; Carvalho, João; Ayala, Conxi; Ramalho, Elsa; Gomez-Garcia, Angela Maria; Matias, Hugo; Heida, Hanneke; Balaguera, Abraham; Garcia-Lobon, Jose Luis; Verges, Jaume

ABSTRACT: Sediments provide valuable information for geologists and geophysicists whenever they strive to understand, and reproduce, the geological evolution, lithology, rock properties, seismic response, and geohazards of a region. The analysis of sedimentary sequences is thus useful for the interpretation of depositional environments, sea-level change, climate change, and recognition of the sediments' source areas. By integrating sedimentary data into geophysical modeling, such interpretations are improved in terms of their accuracy and reliability. To further help our understanding of the Iberian Peninsula's geological evolution, geological resources, and geohazards, this work presents to the scientific community the SedDARE-IB data repository. This repository includes available data on the depth to the Base of the Cenozoic and Top of the Paleozoic stratigraphic markers for the Iberian Peninsula, the surrounding West Iberian Atlantic Margin and Western Mediterranean Neogene basins, or the acoustic basement as interpreted for the Valencia Trough and Alboran Mediterranean basins. As an example of the broad applicability of the data included in SedDARE-IB, we investigate how sediment thickness affects the depth to the 150 degrees C isotherm in specific basins, as commonly used in geothermal exploration. The calculated trend suggests that, given constant measured surface heat flow and thermal conductivity, the 150 degrees C isotherm becomes shallower as the sediment thickness increases, until a critical threshold value is reached for the latter. The SedDARE-IB database has been compiled thanks to a Portuguese-Spanish collaboration promoting open data exchange among institutions and research groups. SedDARE-IB is freely available at 10.20350/digitalCSIC/16277 (Torne et al., 2024), bringing opportunities to the scientific, industrial, and educational communities for diverse applications.

2025-03Journal article

Open access