Mon, 14 Oct 2019

Fifty years ago I was a teenager growing up in Cape Town. At about 10pm on 29 September 1969, our house started to shake violently. Alarmed, my family and I fled out of doors; our neighbours did the same.

The news soon broke that a deadly earthquake had struck the Ceres-Tulbagh district, some 120 km away. Many homes were destroyed and 12 people died - among them children at the Steinthal children's home. Thousands were left homeless and landslides ignited wildfires. The aftershocks continued and could be felt for several months.

The Ceres-Tulbagh earthquake measured 6.3 on the Richter magnitude scale. It is the strongest earthquake to shake South Africa since measurements began around 1900. Magnitude (M) is a measure of the energy released by the earthquake. Earthquakes are usually divided into the following categories: micro M7 have been recorded since 1900, the largest being a M7.3 event in Tanzania in 1910. African countries exposed to the highest risk are Morocco and Algeria, and countries that straddle the East African Rift.

No time for complacency

But people on the continent can't afford to be complacent. It has been said, "Earthquakes don't kill people; buildings do." A moderately sized earthquake that occurs close to a town can be devastating, especially if buildings are not designed to be earthquake-resistant or if the terrain is steep and prone to landslides.

One such earthquake struck Agadir, a town on the Atlantic coast of Morocco, on 29 February 1960. The M5.7 event killed some 13 000 people - over one-third of the population. It was the deadliest earthquake to strike Africa since 1900 and the most destructive "moderate" quake (magnitude of less than 6) anywhere in the world during the 20th century.

An important consequence of the Ceres-Tulbagh earthquake was the establishment of the South African National Seismograph Network to monitor earthquakes, assess hazards, and guide efforts to mitigate the risk.

As the anniversary of the earthquake draws closer, it's a useful moment to explore how probable future, large quakes are in the region and how the risks might be handled.

There are three important issues to consider when thinking about quakes: first, what causes them; second, how to prepare and plan for them; and third, how to move on after a damaging quake.

What makes the Earth shake?

Natural earthquakes are caused by irresistible forces in the Earth's crust. They're powered by radioactive heat generated in the core that slowly moves continents and build mountains.

But not all earthquakes are natural. Some of South Africa's gold and platinum mines are so deep that the great stresses around excavations exceed the strength of the rock, which may rupture suddenly. Micro-earthquakes (as small as M=1) can cause shaking strong enough to damage nearby excavations.

These rockbursts pose a risk to mine workers and nearby communities. Rockbursts occur anywhere where mining is deep enough that the mining-induced stresses exceed the strength of the rock. Countries where this occurs include Australia, Canada, Chile, China, Poland, Sweden, Russia and the US.

The first seismographs were installed in Johannesburg in 1910 after a commission had investigated tremors related to mining activities. The largest mining-related tremor that has occurred to date is a M5.5 earthquake that struck Orkney in August 2014. It caused widespread damage in nearby Khumo, while the shaking was strong enough to alarm residents of high-rise buildings in Johannesburg, over 150 km away.

The filling of large dams such as Kariba, between Zambia and Zimbabwe; Gariep in South Africa and Katse in Lesotho has also triggered earthquakes. The biggest earthquake of these - at Kariba in 1960 - was a M6.1.

Can earthquakes be predicted?

Despite massive advances in knowledge and technology in the last century, the exact time and place of earthquakes can't be predicted. But scientists can assess the likelihood that an earthquake will occur.

For every large earthquake, hundreds of smaller earthquakes occur, most too small to be felt by people but easily detected by sensitive seismographs. This microseismic activity is an indicator of stress accumulation within the Earth's crust. Paleoseismologists search for ancient fault scarps that provide valuable clues of the size and frequency of prehistoric quakes.

There have been a few instances where it has been claimed that successful prediction led to the saving of lives. The most famous was the M7.3 earthquake that struck Haicheng in China in 1975. But there have been numerous failures, such as the 1976 Tangshan (China) earthquake that killed 250,000 people.

In many ways, earthquakes are similar to lightning. We can forecast the likelihood of thunderstorms, but it is nigh impossible to predict exactly when and where lightning will strike.

The best we can do is mitigate the risks. Disaster managers, emergency first responders, town and regional planners, and architects and engineers have key roles to play. The first is to identify areas with thick soil or sand cover where ground shaking is amplified. Then there's the formulation and enforcement of building regulations. Third involves providing training to scholars and the public about what to do in the case of a quake. Lastly, they can provide emergency response in the case of a disaster.

Ideally, earthquake-risk reduction should not be seen in isolation, but be part of an integrated strategy to mitigate the risks posed by both natural (earthquakes, floods, wind storms, epidemics) and human-induced hazards like chemical spills, terrorist attacks.

Much still to learn

I now hold a research chair in seismology and apply my knowledge to mitigate the risks posed by natural and mining-induced earthquakes in South Africa. I have recently assessed the risk that earthquakes pose to proposed new gas pipelines and extensions to the electricity grid.

South Africa's deep mines are earthquake "laboratories", and I am participating in an international project that has drilled into the fault rupture that hosted the 2014 M5.5 Orkney earthquake. Even Japanese and American scientists come to South Africa to study the physics of earthquakes.

Author: Ray Durrheim - Research chair, University of the Witwatersrand The Conversation

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