Misplaced Pages

1969 Portugal earthquake

Location off the coast of Morocco and PortugalShow map of Morocco1969 Portugal earthquake (Portugal)Show map of Portugal
UTC time	1969-02-28 02:40:32
ISC event	812637
USGS-ANSS	ComCat
Local date	28 February 1969 (1969-02-28)
Local time	02:40:32
Magnitude	7.8 Mw
Depth	22.0 km (13.7 mi)
Epicenter	36°01′01″N 10°57′00″W / 36.017°N 10.950°W / 36.017; -10.950
Areas affected	Portugal, Morocco
Max. intensity	MMI VII (Very strong)
Tsunami	1.14 m (3 ft 9 in) 3 runups
Aftershocks	6.3 Mw Feb 28 at 04:25:35 UTC
Casualties	13 killed 80 injured

The 1969 Portugal earthquake struck western Portugal and Morocco on 28 February 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.

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.

See also

• List of earthquakes in 1969
• List of earthquakes in Portugal
• List of earthquakes in Morocco
• 1761 Portugal earthquake

References

1. NGDC. "Comments for the Significant Earthquake". Retrieved 27 August 2010.
2. ^ PAGER-CAT Earthquake Catalog, Version 2008_06.1, United States Geological Survey, 4 September 2009
3. ISC (2014), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 1.05, International Seismological Centre
4. Henriques, Graça (28 February 2019). "Sismo 1969. O mar borbulhou e o país saiu à rua em pijama" (in Portuguese). Diario de Noticias. Retrieved 10 March 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 13 February 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. 18 December 2009. Retrieved 28 February 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 (inactive 1 November 2024).{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)

External links

• The International Seismological Centre has a bibliography and/or authoritative data for this event.

• Earthquakes in Portugal
• 1969 earthquakes
• Earthquakes in Morocco
• 1969 in Portugal
• 1969 in Morocco
• 1960s tsunamis