I recently thought about the earthquake that hit the Western Cape when I was a child in 1969. We have not had a major earth tremor since then and I got to wondering how often these things happen in our part of the world and whether they have ever been documented. To my delight I found that there is an historical list of quakes and tremors and on examining it found the data very disturbing. Why? Because according to the patterns displayed in the data, we are long overdue for another 'significant event'. Let's take a look.
On the website of the Council for Geosciences there is a list of known events dating as far back as 1620 when a ship which was becalmed off Robben Island reported "two startling sounds like thunderclaps". The next recorded incident of significance was in 1695 when there was a"loud noise like thunder, earth trembled, no damage, no injury". Modern experts estimated that this tremor would have measured approximately 4.5 on the Richter Scale to produce this kind of effect.
In 1739 there was one that registered 4 on the Richter Scale and in 1766 a "Strong tremor, noise like thunder, at Simonstown, hospital beds knocked against wall, people frightened, only damage caused was that old cracks in walls and gables opened again." That one was estimated to be about 4.3 - 4.8 on the Richter Scale.
In 1809 an earthquake almost identical in intensity to the 1969 quake shook Cape Town. "Three strong quakes, second most violent on 4th [December], loud reports, everybody frightened, fled to streets, tremendous noise continuous few minutes, no wave-like motion, all buildings suffered numerous cracks, water spouts etc. in places. Another 4 tremors from 5th to 12th, some strong" It registered 6.1 on the Richter Scale. Another of 5.6 followed in June 1811: "Two loud reports and shocks, second about same violence as first shock of 4 Dec 1809, strange unusual motion felt, last 5-6 seconds, everybody ran outside, walls cracked, some unsafe, urns again tumbled from parapets"
In November 1835 a tremor measuring 4.8 rumbled through the Cape. "Smart shock, rumbling sound lasted 30 seconds, residents awakened and alarmed, furniture moved, water shaken from glass, windows rattled, no damage to buildings" There was a tremor of similar intensity in 1857. Then there is a bit of a gap until 1902. There could be two reasons for this - either it was a 'quiet period' geologically speaking or quite simply no reports of tremors have been found.
Then in 1912 and 1920 two big quakes measuring 6 and 6.2 on the Richter Scale were experienced and were felt all over South Africa. After this quakes measuring 4 and more were recorded every 2 - 5 years, right through the rest of the 1920's up to the fateful day of 29th September 1969 when an earthquake measuring 6.3 on the Richter Scale rocked the Cape. I remember it well. The comparison to loud thunder is justified and I seem to recall that this roar continued for what seemed like ages. Our family tried to get outside but the floor was shaking so much that we slipped and stumbled around until it was over.
It is now almost 40 years since the 1969 quake - if one looks at the full list on the Council for Geosciences website one can't help noticing that this is quite a long stretch of time without a major incident even though the Ceres fault is quite active and minor tremors are recorded often. Mmmm...makes one think doesn't it?
Another great site to visit is The US Geological Survey website which has a 'Latest earthquakes in the World' map and one can zoom in and see what's been happening.