Earthquake-faulting-related deformation in soil evidenced in S-wave shallow reflection data: Field results from Portugal

Abstract

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, soil-covered areas in the world.