The 1969 Tulbagh earthquake occurred at 20:03:33

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

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) 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 ...

. It caused widespread damage in the towns of

Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid and first to be discovered * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion ...

Tulbagh Tulbagh (), previously named Roodezand, then Land van Waveren later in 1804 named after Cape of Good Hope Governor Ryk Tulbagh, is a town located in the "Land van Waveren" mountain basin (also known as the Tulbagh basin), in the Cape Winelands ...

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

strike-slip faulting In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geology ...

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

focal mechanism The focal mechanism of an earthquake describes the Fault (geology)#Slip.2C heave.2C throw, deformation in the Hypocenter, source region that generates the seismic waves. In the case of a Fault (geology), fault-related event, it refers to the ori ...

and the distribution of

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...

Tectonic setting

The Western Cape lies on the

Cape Fold Belt The Cape Fold Belt (CFB) is a long fold-and-thrust mountain belt along the western and southern coastlines of Western Cape, South Africa. The Cape Fold Belt formed during the Permian period (300 to 250million years ago) in the late Paleozoic ...

, 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. I ...

s. Some of these thrust faults were reactivated during

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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-graben ...

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 ...

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, Engl ...

and the villages of Gouda,

Saron SARON stands for Swiss Average Rate Overnight and is a measurement of the overnight interest rate of the secured funding market denominated in Swiss Franc (CHF). It is based on transactions and quotes posted in the Swiss repo market, and is ad ...

and

Hermon Mount Hermon ( / ALA-LC: ('Mountain of the Sheikh', ), , ) is a mountain cluster constituting the southern end of the Anti-Lebanon mountain range. Its summit straddles the border between Syria and Lebanon and, at above sea level, is the ...

. 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, educated sect ...

Victorian Victorian or Victorians may refer to: 19th century * Victorian era, British history during Queen Victoria's 19th-century reign ** Victorian architecture ** Victorian house ** Victorian decorative arts ** Victorian fashion ** Victorian literatur ...

and

Edwardian In the United Kingdom, the Edwardian era was a period in the early 20th century that spanned the reign of King Edward VII from 1901 to 1910. It is commonly extended to the start of the First World War in 1914, during the early reign of King Ge ...

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