High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal (2023)

Ghose, R., Carvalho, J., Alves, D. V., Santos, L. A., Ressurreição, R., Alves, P. H. B., & Leote, J. (2023). High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal. Tectonophysics, 859, [229851]. https://doi.org/10.1016/j.tecto.2023.229851

Ghose, Ranajit ; Carvalho, João ; Alves, Daniela V. ; Santos, Luiz Alberto ; Ressurreição, Ricardo ; Alves, Paulo Henrique Bastos ; Leote, Jaime. / High-resolution P- and S-wave reflection studies of an intraplate structure : The Azambuja fault, Portugal. In: Tectonophysics. 2023 ; Vol. 859.

@article{bc66c39fa32e4b6fa443a490826aebbe,

title = "High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal",

abstract = "The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Understanding this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault segments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05–0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P- and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.",

keywords = "2D viscoelastic modeling, Azambuja fault, Holocene fault activity, Seismic reflection, Shear-wave",

author = "Ranajit Ghose and Jo{\~a}o Carvalho and Alves, {Daniela V.} and Santos, {Luiz Alberto} and Ricardo Ressurrei{\c c}{\~a}o and Alves, {Paulo Henrique Bastos} and Jaime Leote",

year = "2023",

doi = "10.1016/j.tecto.2023.229851",

language = "English",

volume = "859",

journal = "Tectonophysics",

issn = "0040-1951",

publisher = "Elsevier",

}

Ghose, R, Carvalho, J, Alves, DV, Santos, LA, Ressurreição, R, Alves, PHB & Leote, J 2023, 'High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal', Tectonophysics, vol. 859, 229851. https://doi.org/10.1016/j.tecto.2023.229851

High-resolution P- and S-wave reflection studies of an intraplate structure : The Azambuja fault, Portugal. / Ghose, Ranajit; Carvalho, João; Alves, Daniela V.; Santos, Luiz Alberto; Ressurreição, Ricardo; Alves, Paulo Henrique Bastos; Leote, Jaime.

In: Tectonophysics, Vol. 859, 229851, 2023.

Research output: Contribution to journal › Article › Scientific › peer-review

TY - JOUR

T1 - High-resolution P- and S-wave reflection studies of an intraplate structure

T2 - The Azambuja fault, Portugal

AU - Ghose, Ranajit

AU - Carvalho, João

AU - Alves, Daniela V.

AU - Santos, Luiz Alberto

AU - Ressurreição, Ricardo

AU - Alves, Paulo Henrique Bastos

AU - Leote, Jaime

PY - 2023

Y1 - 2023

N2 - The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Understanding this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault segments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05–0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P- and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.

AB - The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Understanding this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault segments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05–0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P- and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.

KW - 2D viscoelastic modeling

KW - Azambuja fault

KW - Holocene fault activity

KW - Seismic reflection

KW - Shear-wave

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U2 - 10.1016/j.tecto.2023.229851

DO - 10.1016/j.tecto.2023.229851

M3 - Article

AN - SCOPUS:85158050739

VL - 859

JO - Tectonophysics

JF - Tectonophysics

SN - 0040-1951

M1 - 229851

ER -

Ghose R, Carvalho J, Alves DV, Santos LA, Ressurreição R, Alves PHB et al. High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, Portugal. Tectonophysics. 2023;859. 229851. https://doi.org/10.1016/j.tecto.2023.229851