Earthquakes

The Pulse of the Planet

Earthquakes are among the most terrifying yet awe-inspiring natural phenomena on our planet. They have captured the public imagination for centuries, and have been the subject of countless stories, legends and myths. From the mythical Titan Atlas holding up the world on his shoulders to the tale of the gigantic Namazu (catfish) of Japan curled up beneath the sea. And it is not just stories and myths – some of our greatest works of art and literature have also been inspired by earthquakes. For instance, the earthquake that struck China in 2008 served as the impetus for Ai Weiwei’s series of “earthquake artworks”. And in Carol Edgarian’s timeless work, Vera, we follow a 15-year-old girl as she navigates the rubble of San Francisco in 1906, going beyond merely seeking shelter. It is therefore safe to say that they have been woven into mankind’s cultural fabric.

While earthquakes have inspired and awed us, they also unleash tremendous power that has the potential to be lethal and disastrous. One key factor in earthquake formation is the inexorable motion of tectonic plates – big chunks of the earth’s crust that float on the uppermost liquid mantle. Major earthquakes that cause the most damage and kill many thousands of people occur along the margins of colliding plates, where – with time – immense pressures build up in the earth’s crust, sometimes also leading to volcanism. Eventually, all this pent-up energy is abruptly released, causing the ground to shake and rupture with disastrous repercussions, as in the case of San Francisco, which was almost completely levelled in 1906. Those who have witnessed earthquakes claim that they begin with a rumbling noise, like thunder at a distance which grows fiercer as the earthquake draws nearer.

Apart from the magnitude and depth of the epicentre (the point within the earth’s crust where the earthquake originates), the extent of damage caused by an earthquake depends on a lot of “outside” factors such as population density in the affected area, time of occurrence, preparedness (you are more likely to be hurt if you are caught napping!) and local geology. Notably, the potential damage is greater in areas underlain by unconsolidated, soft sediments such as that which many coastal cities are built on, rather than hard rock, as the former offer less resistance to seismic waves, and, instead of rupturing, literally turn into mud. In addition, coastal areas may be threatened by tsunamis – giant waves produced by sudden movements of large volumes of water – such as are caused by an undersea quake. These waves can travel for hours across the open ocean at great speed, and only release their destructive power once they make a landfall, thousands of kilometres from where they originated, overrunning everything in their way as they crash ashore, with the eventual backwash of the furious waters causing as much damage again!

However, not all earthquakes are “killers” or happen along plate boundaries. Smaller movements on faults within the earth’s crust occur almost everywhere. It is difficult to imagine that there are hundreds of such minor earthquakes every day, the majority of which are so slight that no one can feel them, and only the most sensitive instruments pick them up.

To better comprehend the dynamics of our planet, seismologists continuously measure and observe movements within the earth’s crust. They are able not only to pinpoint the location, depth and magnitude of an earthquake from the speed, direction and strength of the seismic waves released, but also to some extent forecast a major earthquake by monitoring build-up of stress, increase of small tremors and similar signs. But despite the advances that have been made over the last decades, it is still not possible to predict the exact date and time of its actual occurrence – or to prevent it!

Still, to be forewarned is to be forearmed, and in earthquakeprone areas around the globe, special building standards, emergency response plans and public awareness campaigns help to make communities more resilient and mitigate the damage, at least to human life. Indeed, several countries, such as the USA, Philippines, Israel and Japan hold regular earthquake drills to rehearse procedures before, during and after a quake in order to be prepared and prevent avoidable chaos.

Recent news from Syria and Turkey has also reminded the Namibian public of the threat posed by earthquakes. So, what are the chances of us being hit? Sitting in the centre of a stable continental plate – far away from its hazardous edges – we have relatively little to fear from this natural hazard. However, even here the earth’s crust is riddled with many fault lines and structures – one of the more prominent ones being the Windhoek Graben – along which movements do now and then occur. One such fault line, the Pahl Fault, runs right across Windhoek, and buildings in its vicinity (such as the Bank of Namibia on Robert Mugabe Ave) have in the past been prone to cracked walls or crumbling foundations, necessitating special reinforcements. Hot springs in the Rehoboth/Windhoek/Okahandja area, as well as other parts of the country, where heated groundwater from great depths is brought to the surface – and often used therapeutically – are further evidence of the existence of such crustal faults.

On average, one or two earthquakes have occurred in Namibia per year over the past century, for a total of about 160. The highest intensity ever reached was magnitude 5.6 on the logarithmic Richter Scale, which can wreak significant damage to poorly constructed structures over small regions, but Windhoekers would likely only feel a slight vibration of the ground. To put it into perspective: the most severe earthquake ever recorded – a magnitude of 9.5 measured in Chile in 1960 – released about 10 000 times more energy than the strongest ever to occur in Namibia!

Natural disasters like earthquakes and volcanism stimulate our imagination and terrify it at the same time by the overwhelming force with which they can make short shrift of man’s works, and life! But by exploring their geophysical properties and stripping them of their mythical associations we deepen, rather than diminish, our appreciation – and respect – for the awesome inherent power of Planet Earth!

Victoria N Nakafingo