Tuesday, April 11, 2017

Zimbabwe: Earthquakes - How Safe Are We?

The Meteorological Services Department 9pm TV weather forecast last Monday said nothing about the earth tremors that had shaken large parts of Zimbabwe barely an hour earlier, stoking apocalyptic fear among thousands of people.
Not surprising -- the forecast is pre-packed, and thus excludes events that develop later in the day.
But importantly science did not see the Botswana-centred earthquake coming.
Despite the pioneering advances in technology, scientists still cannot predict quakes, a sudden and violent shaking of the ground caused by waves passing through it.
They have a pretty good idea of where an earthquake is most likely to hit, but they still can't tell exactly when it will happen, says University of Zimbabwe geophysicist Lura Gilberta Thwala.
That's just one problem. The biggest is what the 6,5 magnitude Botswana quake has brought to the fore - emerging geological anomalies in places that hitherto have been considered safe zones, such like Zimbabwe and much of southern Africa.
The intensity of an earthquake is measured on a logarithmic scale; the lower the figure the weaker the strength, and vice-versa.
Now, a sub-region known more for its high frequency of drought faces potential future threats from increased seismic activity that could be more damaging, experts warn. Those rattled by tremors last Monday have a fair idea of what to expect in the event of a much stronger earthquake -- buildings can be shaken off their foundations with crashing speed.
How safe are we, now that a 'demon' we have known only from watching television is now standing at our doorstep? We try to answer some of the emerging concerns below.
What are the odds a real powerful quake will strike Zimbabwe in the future?
Seismologists appear divided on this. They do not know really. But studies show large earthquakes can and will occur in areas previously seen as stable.
The country lies to the southern tip of the East Africa Rift System, the most seismic active zone in Africa.
However, this zone is not well defined, according to a geophysicist with the Council for Geoscience in South Africa, who preferred anonymity for professional reasons.
The most active seismic belt passes through Kenya, Uganda and Malawi, and into the Indian Ocean. The other Tanzania, Zambia, northern Zimbabwe and follows the Zambezi river into the Deka fault and terminates in Botswana.
Southwards it stretches through Mozambique, eastern Zimbabwe and into South Africa.
Essentially, Zimbabwe is sandwiched to the north-east and to the east by active seismic belts.
"We cannot completely rule out a large event occurring in Zimbabwean borders," the Council for Geoscience seismologist warned, adding, however, that "the probability of a large earthquake occurring in central Zimbabwe is very low."
Thwala was more blunt. She told The Herald Business: "The odds are high that we might be hit because Zimbabwe lies in the western and eastern extension of the East Africa Rift system which is increasingly becoming active with regard to earthquake activity." The specific day and hour, we will never know.
Which areas face the greatest earthquake risk in Zimbabwe?
Quakes could strike anywhere, including the "stable" regions. But Kariba, the Victoria Falls, Hwange, Bulawayo, Mutare and Chipinge face the biggest risk.
Seismic activity around Lake Kariba is human-induced, correlating with the infilling of the dam in 1963, according to the UK-based International Seismological Centre.
Since 2000, earthquake activity has escalated in Zimbabwe, with no less than half a dozen quakes of moderate to strong intensity striking. In 2004, Nyamandhlovu, a rural area in the west, trembled in the wake of two successive quakes of magnitude 4,0 and 4,3, in February and March, respectively. The two events were felt across Bulawayo, 50km away.
Regions considered safe quaked in 2006, with more violent activity reported in Manicaland, lying closer to the centre of the 7,2 magnitude earthquake that hit Mozambique in February of the same year.
Last year, a moderate magnitude 5,2 shook Mozambique, and also areas along the border with Zimbabwe.
Kariba has reported one of the strongest quakes in recorded history here, at magnitude 6,3, according to Miss Thwala.
The International Seismological Centre says the strongest seismic ever occurred on May 28, 1910 along the Zambezi, measuring 6,0.
How much influence is climate change and global warming having on the increased frequency of earthquakes here and abroad?
The tendency is to link climate change to some of the most pressing problems in the world today -- with good reason -- from depleted fresh water resources to new disease strains in the health sector.

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