Fault activity studies in the Lower Tagus valley and Lisbon region using geophysical data

he Metropolitan Area of Lisbon and the Lower Tagus Valley (LTV) region are located in central Portugal and inhabited by nearly 4 million people. The region has suffered throughout its history the effect of destructive earthquakes caused by hidden faults, possibly related to the plate boundary, which is sited approximately 400 km south of the region (Figure 1). In spite of low slip-rates and big recurrence times that have been estimated for these local, regional faults, they can produce moderate-to-large earthquakes that cause large damage and loss of life, as in 1344, 1531, or 1909 (e.g. Justo and Salwa, 1998; Cabral et al., 2003; 2013). The shorter occurrence time of the earthquakes might be owing to the existence of multiple active faults and/or time clustering owing to stress drop caused by proximal faults (e.g. Carvalho et al., 2006). Therefore, the seismic hazard and risk evaluation of the region has long been a reason of concern. Geological outcrop and geomorphologic mapping identified several regional faults in the LTV region that could be the source the historical earthquakes, but some of them do not affect. Quaternary sediments and lacked the proofs that they were active faults. On the other side, in the vast quaternary alluvial plains that cover the region, it was difficult to identify active faults, as the sedimentation/erosion rates erase any possible surface rupture caused by the low slip-rate faults (<0,35 mm/y). By the late-20th century, seismic reflection data that had been acquired for the oil-industry till the beginning of the 1980s began to be used to identify the major hidden fault zones (e.g. Cabral et al., 2003; Vilanova and Fonseca, 2004; Carvalho et al., 2006). Potential field data was also used to locate active faults in the areas where no seismic data is available (Carvalho et al., 2008; 2011). Though a few more active faults have been proposed, the vast majority of authors agree that the following active faults threaten the region: Nazaré-Caldas da Rainha, Lower Tagus Valley, Ota, Azambuja, Vila Franca de Xira (VFX), Pinhal Novo and Porto Alto faults (Garcia-Mayordomo et al., 2012; Vilanova et al., 2014). In this work, we discuss the acquisition, processing and interpretation of near surface geophysical works carried out over three of these faults — the VFX, Porto Alto and Azambuja faults — in order to confirm they have had activity during the Holoceneera. Their location is shown in Figure 2. We further estimate some of its fault parameters (vertical displacement, slip-rate, length, etc.) and respective implications in terms of seismic hazard

Managing past landfills for future site development: A review of the contribution of geophysical methods

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Application of the Marchenko method to a land seismic dataset

The Marchenko method is capable of estimating Green's functions between the surface of the Earth and arbitrary locations in the subsurface. This method enables the isolation of the response of a specific layer or package of layers, free from the influence of the overburden and underburden. In this study, we apply the Marchenko-based isolation technique to land S-wave seismic data acquired in the Groningen province, the Netherlands, even though applying the Marchenko method to land data is very challenging. We apply the technique for three scenarios: overburden elimination, underburden elimination, and combined elimination of the overburden and underburden. Our results indicate that this approach partly enhances the resolution of reflections and eliminates internal multiples. These enhanced reflections can be utilized for imaging and monitoring applications, where we expect improved results due to the elimination of the overburden and underburden.Comment: 10 pages, 7 figure

Predicting the Dielectric Response of Saturated Sandstones Using a 2-electrode Measuring System

4-electrode setups are usually used to measure the dielectric response (complex conductivity) of sandstones, as it is known that 2-electrode systems are sensitive to unwanted electrode polarization at low frequency. Moreover, electrode polarization (EP) occurs in the frequency range where the characteristic relaxation associated to the grain size also occurs, which can therefore theoretically be assessed using 4-electrode setups. Nonetheless, we find that other parameters of interest (porosity, salinity) can easily be extracted from the frequency range ~ 1–10 kHz, beyond the one affected by EP using a 2-electrode setup. An additional unwanted effect (“pseudo-inductance”) is observed in the frequency range 10 kHz–1 MHz during our experiments. Even though the origin of this effect remains unknown, it is shown to be correlated with the ionic strength of the system and the electrode separation. The bulk polarization region, i.e., the region of intermediate frequencies devoid of EP and pseudo-inductance polarizations, is the one of interest, as the complex conductivity of the system is there only dependent on material parameters such as the porosity of the sandstone and the conductivity of the electrolyte. We demonstrate that in the bulk region the model predicts the complex conductivity response, when these porosity and ionic strength are known. The model has been validated using laboratory measurements on a Bentheim sandstone saturated with five different NaCl concentrations: 5, 10, 100, 170, and 540 mM

Characterization of an Intraplate Seismogenic Zone Using Geophysical and Borehole Data: The Vila Franca de Xira Fault, Portugal

The Vila Franca de Xira (VFX) fault is a regional fault zone located about 25 km northeastof Lisbon, affecting Neogene sediments. Recent shear-wave seismic studies show thatthis complex fault zone is buried beneath Holocene sediments and is deforming thealluvial cover, in agreement with a previous work that proposes the fault as the sourceof the 1531 Lower Tagus Valley earthquake. In this work, we corroborate these resultsusingS-wave,P-wave, geoelectric, ground-penetrating radar and borehole data, con-firming that the sediments deformed by several fault branches are of Upper Pleistoceneto Holocene. Accumulated fault vertical offsets of about 3 m are estimated from theintegrated interpretation of geophysical and borehole data, including 2D elastic seismicmodeling, with an estimated resolution of about 0.5 m. The deformations affecting theTagus alluvial sediments probably resulted from surface or near-surface rupture of theVFX fault during M∼7 earthquakes, reinforcing the fault as the seismogenic source ofregional historical events, as in 1531, and highlighting the need for preparedness for thenext even