1969 Portugal earthquake

Tectonic setting

The epicenter of the earthquake lies within a diffuse zone of seismicity known as the Azores–Gibraltar seismic belt, which marks the boundary between the African plate and the Eurasian plate. The deformation at this plate boundary is transpressional in style, with dextral (right lateral) strike-slip accompanied by slow convergence (4 mm/yr). Linear bathymetric features within this zone, such as the SW–NE trending Gorringe Bank, are thought to be a result of reverse faulting. Investigations using multibeam swathe bathymetry have revealed additional SW–NE trending reverse faults and fold axes and a set of WNW–ESE trending lineaments, interpreted as strike-slip faults. The earthquake was located within the Horseshoe Abyssal Plain, where active reverse faulting has been imaged on seismic reflection data.

Damage and casualties

At magnitude 7.8, the earthquake was considered very powerful. The resulting damage killed 13 people (11 in Morocco and 2 in Portugal). Damage to local buildings was "moderate", according to the United States Geological Survey. Overall, structures were prepared for the earthquake and responded well, sustaining slight, if any, damage.

Characteristics

The earthquake is interpreted to have resulted from movement on a southeast-dipping reverse fault. Analysis of seismigrams of the earthquake revealed two subevents comprised the rupture process. The first subevent, a pure thrust-faulting mechanism, occurred during the first 15 seconds of the rupture. Twenty seconds after the rupture onset, a larger subevent occurred with a strike-slip mechanism at shallower depths, reaching the seafloor.

References

References

1. NGDC. "Comments for the Significant Earthquake".
2. (February 28, 2019). "Sismo 1969. O mar borbulhou e o país saiu à rua em pijama". [[Diario de Noticias]].
3. Fukao, Y.. (1973). "Thrust faulting at a lithospheric plate boundary the Portugal earthquake of 1969". Earth and Planetary Science Letters.
4. Grandin, R.. (2007). "Simulations of strong ground motion in SW Iberia for the 1969 February 28 (Ms = 8.0) and the 1755 November 1 (M ~ 8.5) earthquakes – II. Strong ground motion simulations". [[Geophysical Journal International]].
5. Zitellini, N.. (2009). "The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar". Earth and Planetary Science Letters.
6. (December 18, 2009). "Earthquake History for February 28th". United States Geological Survey.
7. (1988). "Source mechanisms of four recent earthquakes along the Azores–Gibraltar plate boundary". Geophysical Journal International.
8. ISC. (2014). "ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009)". [[International Seismological Centre]].
9. (September 4, 2009). "PAGER-CAT Earthquake Catalog". United States Geological Survey.