The 1969 Portugal earthquake struck western Portugal and Morocco on 28 February at 02:40

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. Originating west of the

Strait of Gibraltar The Strait of Gibraltar is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Europe from Africa. The two continents are separated by 7.7 nautical miles (14.2 kilometers, 8.9 miles) at its narrowest point. Fe ...

, the earthquake registered a

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of 7.8 and the maximum felt intensity was VII (''Very strong'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...

. In total, 13 people died and 80 sustained minor injuries. It is the largest earthquake to hit Portugal since the

1755 Lisbon earthquake The 1755 Lisbon earthquake, also known as the Great Lisbon earthquake, impacted Portugal, the Iberian Peninsula, and Northwest Africa on the morning of Saturday, 1 November, All Saints' Day, Feast of All Saints, at around 09:40 local time. In ...

Tectonic setting

The

epicenter The epicenter (), epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Determination The primary purpose of a ...

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

transpression In geology, transpression is a type of Strike-slip fault, strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. ...

al in style, with dextral (right lateral)

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

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 In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

. The earthquake was located within the Horseshoe Abyssal Plain, where active reverse faulting has been imaged on

seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...

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 The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

. 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

External links

* {{Earthquakes in Africa Earthquakes in Portugal Portugal Earthquake, 1969 Earthquakes in Morocco

Earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

1969 in Morocco 1960s tsunamis

Tectonic setting

Damage and casualties

Characteristics

See also

References

External links