The 1969 Tulbagh earthquake occurred at 20:03:33 UTC on 29 September. It had a magnitude of 6.3 and a maximum felt intensity of VIII (''Severe'') on the

Modified Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. It caused widespread damage in the towns of

Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...

Tulbagh Tulbagh, named after Dutch Cape Colony Governor Ryk Tulbagh, is a town located in the "Land van Waveren" mountain basin (also known as the Tulbagh basin), in the Winelands of the Western Cape, South Africa. The basin is fringed on three side ...

and Wolseley and led to 12 deaths. The earthquake was a result of

strike-slip faulting 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 ...

along a NW-SE trending near vertical fault plane, as shown by the

focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...

and the distribution of

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...

Tectonic setting

The Western Cape lies on the

Cape Fold Belt The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mount ...

, which is characterised by many

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s. Some of these thrust faults were reactivated during

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing as

extensional fault An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create a ...

s, such as the Worcester Fault, which comes to the surface close to the epicentral area, but does not appear to be active.

Earthquake

The earthquake was estimated to have a magnitude of 6.3 . The ISC-GEM catalogue records it as 6.3 . The focal mechanism shows that the earthquake was a result of strike-slip faulting, either sinistral movement on a NW-SE trending fault or dextral movement on a NE-SW trending fault. As the zone of aftershocks was elongated in a NW-SE direction, the NW-SE plane is regarded as the fault responsible. There is no evidence of a surface fault trace and it has not been possible to tie the earthquake to movement on a known fault structure. However, faults of similar orientation are known from nearby areas.

Aftershocks

The main-shock was followed by a long series of aftershocks. The largest aftershock occurred nearly six months later on April 14 1970 and had a magnitude of 5.7 .

Damage

Damage was particularly severe in the towns of Ceres, Tulbagh, Wolseley and

Prince Alfred Hamlet Prince Alfred Hamlet is a small town in the Western Cape province of South Africa. It was founded by Johannes Cornelis Goosen, who was born in the Klein Drakenstein and came to the Warm Bokkeveld as a young farmer. In March 1851 Goosen bought the ...

. There was also significant damage in Porterville and

Worcester Worcester may refer to: Places United Kingdom * Worcester, England, a city and the county town of Worcestershire in England ** Worcester (UK Parliament constituency), an area represented by a Member of Parliament * Worcester Park, London, Engla ...

and the villages of Gouda,

Saron SARON stands for Swiss Average Rate Overnight and represents the overnight interest rate of the secured funding market for the Swiss Franc (CHF). (Swiss Average Rate Overnight) is an overnight interest rates average referencing the Swiss Fra ...

and

Hermon Mount Hermon ( ar, جبل الشيخ or جبل حرمون / ALA-LC: ''Jabal al-Shaykh'' ("Mountain of the Sheikh") or ''Jabal Haramun''; he, הַר חֶרְמוֹן, ''Har Hermon'') is a mountain cluster constituting the southern end of th ...

. The earthquake severely affected Church Street in Tulbagh, which was renowned for its 18th to 20th-century buildings in

Cape Dutch Cape Dutch, also commonly known as Cape Afrikaners, were a historic socioeconomic class of Afrikaners who lived in the Western Cape during the eighteenth and nineteenth centuries. The terms have been evoked to describe an affluent, apolitical se ...

, Victorian and

Edwardian The Edwardian era or Edwardian period of British history spanned the reign of King Edward VII, 1901 to 1910 and is sometimes extended to the start of the First World War. The death of Queen Victoria in January 1901 marked the end of the Victori ...

styles.

Aftermath

The buildings in Church Street were restored, initially by the National Committee for the Restoration of Historic Buildings in Tulbagh and its Environment, and later by the Tulbagh Valley Heritage Foundation.

References

{{Earthquakes in Africa Earthquakes in South Africa