Op-ed contribution: A scientific perspective on earthquake, tsunami hazards and risk in the Caribbean

THE PAST 14 months seem to have produced an unparalleled succession of devastating earthquakes, starting in Haiti on January 12, 2010, followed by the Chilean mega-earthquake and tsunami on February 27, 2010, the two in Christchurch, New Zealand, in November 2010 and February 2011, and, most recently, the earthquake and ensuing tsunami in Japan.

One thing is scientifically very clear: earthquake rates and peak magnitudes have not changed in recent history and, unlike meteorological hazards - which will be affected by man's influence on global climate - we do not find any evidence for man being able to influence, either for better or worse, the rate of seismic activity.

Geological forces are generally slow and steady actors, building pressure along plate boundaries over decades and centuries before releasing that pressure, seemingly on a whim and with devastating effect. Even our most outrageous treatment of Mother Earth has had no discernible effect on these geological forces and, for better or worse, we will need to live with them for ever more.

However, two things have changed: first, population growth and, in particular, the urbanisation of the world's people, has led to dramatic increases in the number of people exposed to earthquake hazards and the vulnerability of those people to impacts; and second, the availability of information instantly and in incredible richness of detail, compared to even a decade ago, has greatly increased our knowledge of far-off events that we might previously have not even heard about.

While the first of these two factors has clear and substantial negative consequences, the second has definite positive aspects, most obviously in the raising of awareness of earthquakes, tsunami, and the risks they pose.

First, we will review the science behind answering the question: Could an earthquake as big as the ones in Japan and Chile affect the Caribbean? The short answer is no.

While there are some parts of the Caribbean region that can generate large quakes of Moment Magnitude 8.0 to 8.5 - Moment Magnitude has superseded Richter Magnitude as the accepted baseline measurement of an earthquake's source energy release; the two are generally comparable - most of the region is at threat from smaller, shallow earthquakes of magnitudes 6.5 to 8.0, such as caused such devastation in Haiti and significant damage and economic impact in Christchurch.

Critical controlling factor

The Japan quake was 1,000 times more powerful than the Haiti quake and to generate that power, a substantial length of the earth's crust must break simultaneously.

The maximum possible so-called rupture length of an earthquake is the most critical controlling factor in estimating an earthquake magnitude limit in any particular region. In the offshore subduction zones of Japan, Chile and many other places in the world, rupture lengths of upwards of 500km are possible.

The Japan quake appears to have ruptured just under 500km of the crust while the Chile quake ruptured about 700km.

Such rupture lengths are simply not possible in the majority of the Caribbean Basin due to the segmented nature of the plate boundary.

The longest stretches of potentially un-segmented plate boundary are along the eastern end of the Greater Antilles - between the Mona Passage and the Anegada Passage north of Puerto Rico - and southwards from the Anegada Passage to offshore east of Dominica, well to the east of the northern Lesser Antilles; neither appears able to produce rupture lengths of more than 200-300km, corresponding to a peak magnitude of around 8.5.

However, there is certainly no cause for complacency. Magnitude 8.0 to 8.5 events are able to generate high levels of shaking far from their source, as well as major tsunami waves which would leave little time for warnings.

For example, the north coast of Puerto Rico, and particularly the major urban area of San Juan, is vulnerable to both high-intensity shaking and rapid and substantial tsunami inundation following a mega-earthquake along the Puerto Rico Trench.

Tsunami risk is heightened by the propensity of the trench to produce massive undersea landslides, which themselves can generate high-energy tsunami wave sets. This having been said, the frequency of such events is dependent on the rate of energy build-up, which, in turn, is dependent on the rate at which the earth's crustal plates are moving towards or past each other. In the northeastern Caribbean, plate motions are about one-quarter of those off the northeast coast of Japan, so making major earthquakes substantially less frequent.

Given the rate of slip to the north of Puerto Rico, it seems unlikely that a magnitude 8.5 event can occur more than once every thousand years or so, with a greater likelihood of pressure being dissipated in more frequent, smaller events.

Tele-tsunamis

Unfortunately for the region, large offshore earthquakes are not the only potential source of tsunami waves.

In recorded history, the Caribbean has been impacted by tele-tsunami - those caused by earthquakes far away, with the great Lisbon earthquake and tsunami in 1755 being the best-recorded; by volcano collapse-generated tsunami, which occurred on a small scale during the ongoing Soufriére Hills eruption on Montserrat; and by tsunami waves generated by explosions at the underwater volcano off Grenada's north coast, Kick-'em-Jenny.

In the geological record, there is strong evidence to suggest that massive volcano collapses have occurred from many of the eastern Caribbean volcanoes at rates comparable to magnitude 8.0 to 8.5 mega-earthquakes, and modelling combined with examples from elsewhere in the world indicate that such collapses produce tsunami waves which dwarf those produced by the Chile and Japan earthquakes.

But what also of the Caribbean's risk to smaller earthquake events? The Haiti earthquake was a minnow compared to the Japan and Chile quakes, but the death toll and impact on infrastructure was catastrophic.

While due in part to a particularly vulnerable built environment, the Christchurch quake - affecting a much better-prepared city - demonstrated the vulnerability of all man-made infrastructure to shallow, moderately-sized earthquakes close to urban centres.

To write Haiti's tragedy off as the consequence of poor planning and substandard building practices is to miss the point; concentrations of people in urban areas built in earthquake hazard zones are inherently at risk.

Kingston, Port-of-Spain and Santiago, the second city of the Dominican Republic, are all examples of such populations at risk. All are subject to the level of shaking felt in both the Haiti and the more recent of the Christchurch earthquakes at frequencies of occurrence of tens to a few hundred years and, while none might fare as badly as Port-au-Prince in such an event, it is reasonable to assume that none would fare as well as Christchurch.

It is also noteworthy that much critical infrastructure across the region, including almost all power-generation facilities, is located on the coast and often both in an area of likely shaking amplification, as well as within the likely inundation zone of a major tsunami event.

So, while our thoughts are with the people of Japan, just as theirs were with our Caribbean neighbours in Haiti a year ago, and indeed before that - it was Japan that provided the funds to complete the preparation for and launch of the Caribbean Catastrophe Risk Insurance Facility, in recognition of the need for the region to build economic resilience to natural disaster impacts, and the Japanese have also been long-standing funding agents for regional disaster management programmes - we must not fail to learn the lessons that both of those events, and the many in between, have taught us.

Earthquakes are a terrible force of nature. They occur much more rarely than the meteorological hazards which also afflict our region, but when they do happen, they can be truly devastating. When they are followed by a tsunami, that devastation is further magnified.

As we look at the heart-wrenching pictures from Sendai, we must remember that Japan is perhaps, in fact probably, the best-prepared country in the world for the natural disasters which so commonly befall her.

That preparedness has surely mitigated what could have been a much greater tragedy, unimaginable though that seems.

The Caribbean region has worked hard and made progress in preparedness and response, but there is much still to do.

We have been perhaps less focused on the role of development planning in creating a built environment which is more resilient to natural hazards, both meteorological and geological, and we must strive to better integrate disaster risk management across all aspects of our regional development process.

And most of all, we must each as individuals, families and communities, be aware of the environment in which we live and the hazards it poses, and seek to foster resilience through improved preparedness and response at local, national and regional levels, and across both public and private sectors.

Dr Simon R. Young is chief executive officer of Caribbean Risk Managers Limited, a member of the CGM Gallagher Group and facility manager of the Caribbean Catastrophe Risk Insurance Facility.

syoung@caribrm.com