1969 Portugal earthquake

The 1969 Portugal earthquake struck western Portugal and Morocco on February 28 at 02:40 UTC. Originating west of the Strait of Gibraltar, the earthquake registered a magnitude of 7.8 and the maximum felt intensity was VII (Very strong) on the Mercalli intensity scale. In total, 13 people died and 80 sustained minor injuries. It is the largest earthquake to hit Portugal since the 1755 Lisbon earthquake.[4]

1969 Portugal earthquake
1969 Portugal earthquake is located in Morocco
1969 Portugal earthquake
Location off the coast of Morocco and Portugal
1969 Portugal earthquake is located in Portugal
1969 Portugal earthquake
1969 Portugal earthquake (Portugal)
UTC time1969-02-28 02:40:32
ISC event812637
USGS-ANSSComCat
Local dateFebruary 28, 1969 (1969-02-28)
Local time02:40:32
Magnitude7.8 Mw[1]
Depth22.0 km (13.7 mi)
Epicenter36.017°N 10.950°W / 36.017; -10.950
Areas affectedPortugal, Morocco
Max. intensityVII (Very strong)
Tsunami1.14 m (3 ft 9 in)[2]
3 runups[2]
Aftershocks6.3 Mw Feb 28 at 04:25:35 UTC[3]
Casualties13 killed[2]
80 injured[2]

Tectonic setting
See also: Geology of the Iberian Peninsula

The epicenter of the earthquake lies within a diffuse zone of seismicity known as the Azores–Gibraltar seismic belt,[5] 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).[6] Linear bathymetric features within this zone, such as the SW–NE trending Gorringe Bank, are thought to be a result of reverse faulting.[5] 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.[7]

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.[8]

Characteristics

The earthquake is interpreted to have resulted from movement on a southeast-dipping reverse fault.[5] Analyzing 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.[9]

See also

References
  1. NGDC. "Comments for the Significant Earthquake". Retrieved August 27, 2010.
  2. USGS (September 4, 2009), PAGER-CAT Earthquake Catalog, Version 2008_06.1, United States Geological Survey
  3. ISC (2014), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 1.05, International Seismological Centre
  4. Henriques, Graça (February 28, 2019). "Sismo 1969. O mar borbulhou e o país saiu à rua em pijama" (in Portuguese). Diario de Noticias. Retrieved March 10, 2022.
  5. Fukao, Y. (1973). "Thrust faulting at a lithospheric plate boundary the Portugal earthquake of 1969". Earth and Planetary Science Letters. 18 (2): 205–216. Bibcode:1973E&PSL..18..205F. doi:10.1016/0012-821X(73)90058-7.
  6. Grandin, R.; Borges, J.F.; Bezzeghoud, M.; Caldeira, B. & Carrilho, F. (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" (PDF). Geophysical Journal International. 171 (2): 807–822. Bibcode:2007GeoJI.171..807G. doi:10.1111/j.1365-246X.2007.03571.x. Retrieved February 13, 2011.
  7. Zitellini, N.; Grácia E.; Matias L.; Terrinha P.; Abreu M.A.; DeAlteris G.; Henriet J.P.; Dañobeitia J.J.; Masson D.G.; Mulder T.; Ramella R.; Somoza L. & Diez S. (2009). "The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar" (PDF). Earth and Planetary Science Letters. 280 (1–4): 13–50. Bibcode:2009E&PSL.280...13Z. doi:10.1016/j.epsl.2008.12.005.
  8. "Earthquake History for February 28th". United States Geological Survey. December 18, 2009. Retrieved February 28, 2010.
  9. Grimison, Nina L.; Chen, Wang-Ping (1988). "Source mechanisms of four recent earthquakes along the Azores–Gibraltar plate boundary". Geophysical Journal International. 92 (3): 391–401. doi:10.1111/j.1365-246X.1988.tb00003.x.
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