Terra is not so firma

Wednesday 31st December 2014

The surface of the Earth (terra) may be solid (firma) but that doesn’t mean it is stable. Riding on a semi-molten undercarriage, the various tectonic plates that make up the Earth’s more rigid crust continually jostle and jockey for supremacy. Some of these plate interactions occur on land, resulting in great faults and rising mountain ranges. Others occur beneath the Earth’s oceans, and usually go largely unnoticed. But the recent passing of the 10th anniversary of one of the world’s worst natural disasters in recorded history has served as a reminder that these submarine plate interactions on occasion may be no less dramatic than those on land.

Early on December 26th 2004, a powerful undersea earthquake occurred in the Indian Ocean off the west coast of Sumatra, Indonesia (see image below). The earthquake occurred along part of the boundary that marks where the Indian plate is being subducted beneath the overriding Sumatran and Andaman islands. Subduction is far from the smooth process depicted in textbook diagrams, but rather occurs in fits and starts, with the plates initially sticking and then, as the accumulated strain becomes too great, violently slipping past each other. As the plates slip, vast amounts of stored energy are released as an earthquake. The Richer scale assigns a magnitude number to quantify the energy released by an earthquake, ranging from less than 2.0 (‘minor’) to 9.0 or more (‘great’). In the 2004 Indian Ocean event, the magnitude was around 9.1–9.3, making it one of the largest earthquakes ever recorded on a seismograph. More significantly, the violent movement and shaking of the sea floor over several minutes displaced billions of tones of seawater and produced wave trains (tsunamis) that radiated in all directions (see image).

Here is an animation of tsunami showing how it
 radiated from the entire length of the 1 600 km rupture on the sea floor (Source: http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#mediaviewer/File:2004_Indonesia_Tsunami_Complete.gif)

Animation of the tsunamis showing how they radiated from the 1600 km long rupture on the sea floor (Source: http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami#mediaviewer/File:2004_Indonesia_Tsunami_Complete.gif)

The tsunamis took from as little as a few minutes to several hours to reach coastlines bordering the Indian Ocean, typically increasing in height as they entered shallower water. In some locations, tsunami waves with run-up heights (surges) reaching 30 m above normal sea level inundated coastal communities, typically with very little warning. In the unfolding disaster, now known by various names including the Boxing Day tsunami, the Christmas tsunami, the 2004 Indian Ocean tsunami, and the South Asian tsunami, the morphology of vast swathes of coastline were re-modelled. Beaches and dunes were severely eroded, new deposits were laid down, vegetation was stripped, and homes, businesses and infrastructure were decimated. More than 230 000 people were killed and millions were made homeless, with the disaster being most acutely felt in parts of Indonesia, Sri Lanka, India, and Thailand.

Over the past 10 years, much has been written about the natural and human factors contributing to this natural disaster event, the humanitarian response, and the social and economic impacts. In March 2011, the Japanese Tōhoku earthquake and ensuing tsunami provided yet further evidence of the instability of terra firma. In this case, a magnitude 9.0 earthquake occurred in the Pacific Ocean east of Japan. Here, the Pacific plate dives beneath the overriding Eurasian plate, and the violent slippage generated tsunami waves with run-up heights reaching 40 m. The images of coastal re-modelling were even more dramatic, with some of the devastation being captured on film (see photographs below) and beamed around the world’s media outlets in real time. By comparison with the Boxing Day tsumani, the loss of life was not on the same scale (in a recent official count, somewhere between 15 000 and 20 000 people are reported dead or missing) but meltdowns at the Fukushima nuclear power plant and resulting radiation leaks have created more sinister legacies.

Source: http://blog.salvationarmyusa.org/wp-content/uploads/743246-minamisoma-japan-tsunami.jpg (courtesy of www.theaustralian.com.au)

A tsunami wave impacting on part of the Japanese coastline (Source: http://blog.salvationarmyusa.org/wp-content/uploads/743246-minamisoma-japan-tsunami.jpg [cited as courtesy of www.theaustralian.com.au])

But back to the Boxing Day tsunami. The 10th anniversary of this disaster has not gone unnoticed by the world’s media outlets, many of which have published some remarkable photos of the ‘then and now’ variety: the ‘then’ being the severely eroded coastline and damaged buildings in the immediate aftermath of the disaster, and the ‘now’ showing the reconstructed physical and built environment (see example below).

Part of Banda Aceh shortly after the tsunami and now (Source: http://www.bbc.co.uk/news/world-30550586)

Part of Banda Aceh in the Province of Aceh, Indonesia, showing photographs taken shortly after the tsunami and recently (Source: http://www.bbc.co.uk/news/world-30550586)

This and other good examples can be found through the following links:




For geomorphologists, study of the landscape changes resulting from these kinds of extreme events is terra firma of the intellectual kind. Extreme events – be it from earthquakes, tsunamis, volcanic eruptions, hurricanes, catastrophic floods, landslides and so on – provide some of the most visible examples of geomorphology in action. Aside from the commonly tragic social and economic disruption, large amounts of geomorphic work (erosion, deposition) are done very rapidly. But while these extreme events may be highly dramatic, they occur relatively infrequently, commonly being spaced decades, hundreds or even thousands of years apart. In between times, more moderate size or smaller events fill the void: say, freeze-thaw cycles, rainfall-runoff events, coastal storms, seasonal floods, and so on. These ‘normal’ events commonly weather rock, and erode, transport and deposit sediment on quasi-continuous or seasonal bases. So, over timescales of, say, ten thousand years, are the extreme events or the cumulative effects of the normal events (‘the dripping tap’) more significant agents of landscape change?

This question has a generated long-standing, ongoing debate in geomorphology, with a recent Annual Meeting of the British Society for Geomorphology (University of Liverpool, June 2011) being devoted to this theme. The jury is still deliberating, but the verdict is unlikely to be unanimous. The question asks whether extreme events or normal events are more significant but rarely is nature so dichotomous. For instance, normal events (e.g. accumulating strain along a plate boundary) help to ‘prepare the ground’ for extreme events (e.g. the dramatic earthquake), while extreme events (e.g. tsunamis) cause dramatic, rapid change but ‘recovery’ to pre-event conditions can occur through the cumulative effects of the normal events (e.g. regular wave and tidal action). The ‘then and now’ photos highlighted above bear witness to the restorative impacts of these normal events on the morphology of tsunami-impacted Indian Ocean coastlines. Admittedly, in many areas human agency may have played a helping hand – perhaps through beach replenishment and vegetation replanting schemes – but likely this has only hastened processes that would have occurred naturally.

Of course, recovery of the physical and built environment is only part of the post-disaster story. The emotional and wider social recovery from this and other similar events – if indeed recovery is even possible – cannot be illustrated so simply. These ‘human interest’ stories tend to capture most of the media’s attention, but for most geomorphologists these are definitely not intellectual terra firma. Nonetheless, across the geographical, earth and social sciences more broadly, focus is now sharpening on closely related concepts, including societal ‘vulnerability’, ‘resilience’ and ‘adaptive capacity’. To what extent are communities susceptible to, and unable to cope with, disasters (show ‘vulnerability’), or can they absorb the impacts of disasters (display ‘resilience’)? Can they even develop an ability to adjust to disasters (‘adaptive capacity’), perhaps by changing land use or evacuation procedures to moderate potential damages? In the case of the Boxing Day tsunami, the lessons learned have increased adaptive capacity, with many countries cooperating to create and operate a tsunami early warning system.  

Given projections of future increases in the exposure of a growing human population to a variety of hazards, particularly those related to a changing global climate, other case studies will continue to accumulate. The experience of the Philippines also shows just what can be achieved in terms of developing adaptive capacity; following the devastation of Typhoon Haiyan in November 2013 (more than 7 000 dead or missing), improved disaster management helped to greatly minimise the loss of life during Typhoon Hagupit in December 2014 (see http://www.bbc.co.uk/news/world-asia-30375007).

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