1969 Portugal earthquake explained

1969 Portugal earthquake
Timestamp:1969-02-28 02:40:32
Isc-Event:812637
Anss-Url:iscgem812637
Local-Time:02:40:32
Magnitude:7.8 [1]
Depth:22km (14miles)
Location:36.017°N -10.95°W
Countries Affected:Portugal, Morocco
Tsunami:1.14m (03.74feet)
3 runups
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.[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,[3] 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).[4] Linear bathymetric features within this zone, such as the SW–NE trending Gorringe Bank, are thought to be a result of reverse faulting.[3] 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.[5]

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.[6]

Characteristics

The earthquake is interpreted to have resulted from movement on a southeast-dipping reverse fault.[3] 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.[7]

See also

Notes and References

  1. Web site: Comments for the Significant Earthquake. NGDC. August 27, 2010.
  2. Web site: Sismo 1969. O mar borbulhou e o país saiu à rua em pijama. Diario de Noticias. Henriques. Graça. February 28, 2019. March 10, 2022. pt.
  3. Fukao. Y.. 1973. Thrust faulting at a lithospheric plate boundary the Portugal earthquake of 1969. Earth and Planetary Science Letters. 18. 2. 205–216. 10.1016/0012-821X(73)90058-7. 1973E&PSL..18..205F .
  4. Grandin. R.. Borges, J.F.. Bezzeghoud, M.. Caldeira, B.. Carrilho, F.. amp. 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. 171. 2. 807–822. February 13, 2011. 2007GeoJI.171..807G. 10.1111/j.1365-246X.2007.03571.x. free.
  5. 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.. amp. 2009. The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar. Earth and Planetary Science Letters. 280. 1–4. 13–50. 10.1016/j.epsl.2008.12.005. 2009E&PSL.280...13Z .
  6. Web site: Earthquake History for February 28th. United States Geological Survey. December 18, 2009. February 28, 2010.
  7. Grimison . Nina L. . Chen . Wang-Ping . Source mechanisms of four recent earthquakes along the Azores–Gibraltar plate boundary . Geophysical Journal International . 1988 . 92 . 3 . 391–401 . 10.1111/j.1365-246X.1988.tb00003.x. January 31, 2024 . free .