August 20, 2008 Tsunami: Southern Atlantic Ocean, South Africa

Name: August 20, 2008 Tsunami: Southern Atlantic Ocean, South Africa
Start: 2008-08-20
Location: Southern Atlantic Ocean, South Africa

August 20, 2008 · 2.55 m · Probable tsunami

A tsunami of unknown cause began at Southern Atlantic Ocean, South Africa on August 20, 2008 at 23:00:00 UTC. The water rose as high as 2.55 meters. Its effects were recorded at 2 places.

Where the waves reached

The source (white) and every runup with a place, colored by the water height recorded there. A ring is a runup NCEI marks doubtful.

Under 1 m
1 to 3 m
3 to 10 m
10 to 30 m
30 m and more
Height not recorded

Tsunami warnings in force

Travel times

How long the first wave took to reach each part of the ocean, one line for each hour up to 24, estimated from the source and the depth of the sea.

The travel times are computed in your browser, which needs JavaScript.

1 to 4 hours
5 to 6 hours
7 to 14 hours
15 hours and more

Computed in your browser the way NOAA’s own travel time maps are: the wave travels at the square root of gravity times the depth, over NOAA’s ETOPO1 depths averaged to a 10 arc-minute grid. Checked against NOAA NCEI’s maps for 21 tsunamis over the first 24 hours, these lines fall within 10 minutes of NCEI’s at half the points compared and within 52 minutes at 9 in 10. They give when the first wave could arrive, not how high it is; bays, harbors and island chains finer than the grid can see it later. Not for use in a warning: see tsunami.gov.

The tsunami

Date: August 20, 2008, 23:00:00 UTC
Source: Southern Atlantic Ocean, South Africa
Cause: Unknown
Earthquake: The earthquake
Maximum water height: 2.55 m (8.4 ft)
Runups recorded: 2
Warning: No warning issued
Waters: Ocean
How certain: Probable tsunami
Source location: 28.500° S, 13.200° E

The toll

Damage: Limited (under about $1 million)

Where the literature gives no number, NCEI records a description on a scale of 1 to 4, printed here in its words. Damage is in millions of US dollars of the year of the event, not adjusted for inflation.

What the record says

NCEI’s account, from the sources it cites.

Around 06:11 SAST (04:11 GMT) on Thursday, August 21, 2008, the port installations at St. Helena were flooded by a series of waves which inflicted significant damage, notably to the infrastructure of a shipyard, this phenomenon lasting for about 15 min.

On the other hand, Hartnady et al. (2009) documented instrumental observation of the 2008 waves by maregraphs over an 850-km stretch of coastline extending from Luderitz, Namibia, in the north to Granger Bay near Cape Town in the south.

Based on the timing of the largest waves, Hartnady et al. (2009) proposed to locate the source of the main component of the tsunami around 28:5 S; 13:2 E, with an origin time of 23:00 GMT on August 20, 2008. Noting the presence of the Chamais Slump in the proposed source area, and the existence of documented gas hydrate structures in a structurally similar area 260 km to the southeast (Ben-Avraham et al. 2002), Hartnady et al. (2009) suggested to interpret the 2008 waves as due to a prolonged episode of slumping triggered by the destabilization of gas hydrates, and reaching its climax 24 h after it started.

However, Hartnady et al. (2009) noticed that barometric records at Port Nolloth showed signals inversely correlated with sea level residuals, speculating that while the ocean waves may have been driving the atmosphere, the opposite, i.e., a meteorological origin of the 2008 waves, might also be true [C. Hartnady, pers. comm., 2013].

(all above frome reference #11087)

An undersea earthquake is believed to have triggered a mini-tsunami off the Cape west coast last week, causing noticeable fluctuations in tidal patterns, the National Sea Rescue Institute (NSRI) said yesterday. The earthquake caused a mudslide, which in turn caused the tsunami, said NSRI spokesperson Craig Lambinon. The tidal changes caused damage to some factories in St Helena Bay. "You get about five earthquakes around the world every day, so it is not unusual - it's just that it came without warning," said Lambinon, adding that the quake was not severe enough to warrant a tsunami warning. South Africa is part of an early-warning tsunami monitoring system based in Hawaii. (reference #3998)

Every runup

Nearest the source first. Heights are above sea level at the time; a tide gauge reads half the wave’s full height.

Where	Country	Km from source	Water height, m	Inland, m	Measured by	First wave, UTC	Deaths
Saint Helena Bay	South Africa	662	2.5	70	Runup height, post-tsunami survey
Brandhuis	South Africa	664	2.55	20	Runup height, post-tsunami survey

Every tsunami in 2008 The 2000s Tsunamis of South Africa The earthquake This day in weather history Alerts in force

Sources

Tsunami events and runups from NOAA’s National Centers for Environmental Information, Global Historical Hazard Database, cited as NCEI/WDS, doi:10.7289/V5PN93H7. Public domain. Read 2026-09-24. NCEI tsunami event 3588.