Expressions of ductile, soft-sediment deformations induced by ground movements due to past earthquakes are difficult to recognize in near-surface soils. We have carried out shallow S-wave reflection studies in a seismically active area located northeast of metropolitan Lisbon, Portugal. Identifying shallow disturbed zones and hidden fault segments in this area is important but quite difficult because of small vertical slips due to earthquakes, the Holocene alluvial cover hiding the fault segments, and a high rate of surficial sedimentation. We have performed S-wave reflection profiling at two sites -Vila Franca Xira and Castanheira de Ribatejo. We detected different but interrelated evidence of soft-sediment deformation in the seismic data. This evidence includes sharp lateral changes in the S-wave velocity field; changes in the reflection horizons in stacked sections; aligned diffractions in unmigrated sections; discontinuities in common-offset gathers; and discontinuities, backscattered, and diffracted arrivals in common-source gathers. Though not equally clear everywhere, this evidence is recognizable at many locations where earthquake-motion-induced disturbed zones are interpreted. To confirm these interpretations, we have performed synthetic modeling of a seismic wavefield using the same acquisition geometry as in the field experiments, and with multiple disturbed zones present as vertical emplacements through horizontally lying soil layers. The modeling results resemble the observations in field data. It is possible to confirm the signatures of soft-sediment deformation in the shallow S-wave reflection data. The approach that we used will be useful in many seismically active, soilcovered areas in the world.
The Vila Franca de Xira (VFX) fault is a regional fault zone located about 25 km northeast of Lisbon, affecting Neogene sediments. Recent shear-wave seismic studies show that this complex fault zone is buried beneath Holocene sediments and is deforming the alluvial cover, in agreement with a previous work that proposes the fault as the source of the 1531 Lower Tagus Valley earthquake. In this work, we corroborate these results using S-wave, P-wave, geoelectric, ground-penetrating radar and borehole data, confirming that the sediments deformed by several fault branches are of Upper Pleistocene to Holocene. Accumulated fault vertical offsets of about 3 m are estimated from the integrated interpretation of geophysical and borehole data, including 2D elastic seismic modeling, with an estimated resolution of about 0.5 m. The deformations affecting the Tagus alluvial sediments probably resulted from surface or near-surface rupture of the VFX fault during M∼7 earthquakes, reinforcing the fault as the seismogenic source of regional historical events, as in 1531, and highlighting the need for preparedness for the next event.
Main objectives1) Improve seismic hazard evaluation of the Lisbon, Portugal and the Lower Tagus Valley area; 2) To locate and image a fault under the Holocene alluvium that is known to affect Pliocene sediments and deform Pleistocene units; 3) confirm or not if the fault affects 14 000 years aged sediments;
New aspects coveredFirst image of the active fault in the Holocene cover
SummaryThe Azambuja fault is a NNE oriented fault zone located 50 Km north of Lisbon, the capital of Portugal and has been considered as a possible source of historical earthquakes. Therefore, its study is a priority in seismic hazard evaluation. It has a clear morphological signature and Miocene and Pliocene sediments are tilted eastwards and cut by steeply dipping meso-scale faults presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate Quaternary slip on the fault in the range 0.05 to 0.06 milimetres/year. However, the fault has not been observed to affect Pleistocene or Holocene sediments. Here, we present a P-wave seismic reflection study carried out to image the fault below the Holocene alluvium cover southwards of the fault outcrop. We show that the fault is present below the Holocene cover as suspected and, with the aid of nearby well data, that fault segments are affecting the 14 000 year aged alluvium cover, suggesting a larger slip rate then previously admitted.
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