20-Jan-2010
By Paul Tapponnier and Kerry Sieh
Earth Observatory of Singapore (EOS), Nanyang Technological University

Another hammering home that we are not living sustainably on our dynamic Earth!

Large disasters developed a doleful cadence through the first decade of the 21st century.  First it was India, then Iran, Indonesia, Pakistan and China.  And now, in the first month of the century’s second decade, it is Haiti.  The anguishing post-earthquake scenes of monumental devastation have become all too familiar.

In the last half of the last century we built too much, too inadequately, in too many dangerous places, with too little regard for how the Earth works. Winston Churchill’s ominous remark, a few years before the Second World War, is apt: "Owing to past neglect, in the face of the plainest warnings, we have now entered upon a period of [greater] danger... The era of procrastination, of half-measures, of soothing and baffling expedients, of delays, is coming to a close. In its place we are entering a period of consequences..."

Last week’s Haiti earthquake was caused by the sudden rupture of a tectonic fault that traverses the entire length of a long peninsula that juts out to the west from the capital city, Port au Prince. (Fig 1)  The rupture occurred at quite a shallow depth as earthquakes go, and ran for a few tens of kilometers just southwest of the impoverished city of about 2 million.  The proximity of the fault rupture and the lack of infrastructural defenses conspired to magnify the severity of the catastrophe.  The death toll may well exceed the 40,000 each of the 2001 Bhuj and 2003 Bam earthquakes in India and Iran and could even exceed the nearly 100,000 deaths that resulted from both the 2005 Kashmir and 2008 Lungmen Shan earthquakes in Pakistan and China. 

The fault that generated last week’s earthquake is well known to geologists as the Enriquillo-Plantain Garden Fault Zone (EPGFZ). It is part of the northern boundary of the Caribbean Plate, which currently moves about 2 cm/yr toward the northeast relative to North America. (Fig 2) It is the southernmost fault zones that shape the northern and southern edges of Hispaniola, crossing both Haiti and the adjacent Domincan Republic. Its trace on the ground, which skirts the southern suburbs of Port au Prince (Petionville), and then veers eastwards in the direction of Lake Enriquillo, is strikingly exposed in the landscape. It looks equally spectacular on digital topography and satellite images, now available to all through Google Earth: a steep, narrow cut along which large river channels divert toward the left.  (Figure 3) Kinks of these riverbeds by as much as 14.5 km demonstrate the persistence of horizontal motion leftward along the fault.  (Figure 4)  At a couple of meters every 250 years or so, it would have taken several thousand movements like last week’s to create the current landscape!  A pretty predictable neighbor, wouldn’t you say?

Preliminary seismological data suggest that it was a section of the fault west of the city that ruptured last Tuesday. In less than a minute, the mountains south of the fault slipped eastwards by a couple meters, likely over a length of about 70 km, from west of Petionville all the way to Lake Miragoane.

History amply shows that the EPGFZ had the potential to produce large, destructive earthquakes.  Two in the 18th century were particularly well recorded. Moreover, recent measurements of deformation of the region using GPS satellite technology had singled the fault out as a site of dangerously accumulating crustal strain (at a rate of 7 to 9 mm/yr). Warnings of the potential for damaging earthquakes, as well as calls for more detailed scientific study, had been issued in recent years, most recently after small shocks occurred beneath the city in September 2008.

In the second half of the 18th century, it was not just one but two huge earthquakes, only 19 years apart, that razed the colonial town of Port-au-Prince.  As related by French Historian Moreau de Saint-Méry, the first one, at 2PM on October 18th, 1751, devastated the whole region between Léogane and Gonaives.  It even toppled several buildings in the Spanish harbour of Santo Domingo, more than 250 km to the east, suggesting a fault rupture east of Port-au-Prince. It was followed by strong aftershocks for about two months. Within Port au Prince, only one masonry building did not collapse.

The second earthquake, on the evening of June 3rd, 1770, was more violent than the first. It turned the reconstructed town into a vast field of ruins, where only aligned trees marked the tracks of former streets. Rock falls and landslides cascaded down mountain slopes, and watery quicksand fountains erupted on the plains.  Aftershocks continued for 4 months.  Several small shocks that jolted the region in the preceding three years were unrecognized portents of the great earthquake to come.

Why is it that Port au Prince was no more prepared for last week’s earthquake than it was for the twin events two and a half centuries ago?  The answer is quite simple:  Just as we humans cannot hear sounds or see colors that are emitted at frequencies beyond what our ears or our eyes have evolved to perceive, we are not naturally attuned to events that happen less frequently than every few decades.  We teach our children to look both ways before crossing the street, because cars come by every few seconds or minutes.   We regulate the financial markets (well, maybe!) because financial debacles occur every few decades.  But a seismic, volcanic or climatic disaster that might occur every few centuries or millennia or even longer!  Should we lose too much sleep over them?

For civilization’s future the answer is most assuredly "yes."  We no longer live as a primitive humanity, where our natural senses are adequate for our survival. Our modern civilization, dependent upon intricate machines and complex economic, political and cultural institutions, will likely not make it without paying attention to the science that has enabled us to extend our "eyes" and "ears" to see and hear the inner working of Earth’s tectonics and climate.   And once we perceive these dangers, we will need to utilize the powerful "hands" that modern planning and engineering have given us to build safely.

This is particularly appropriate for Port-au-Prince and Haiti.  It is quite plausible that last week’s earthquake is the beginning of a sequence of such events, analogous but likely not identical to that of the late 18th century. We now know that large earthquakes on long-quiescent, "locked" fault zones commonly trigger one another, and hence cluster in time. It is thus reasonable to suspect that another large earthquake will strike the city in the coming decades, as happened more than two centuries ago. Given the location of last Tuesday’s fault rupture, this second quake will likely emanate from a section of the EPG Fault Zone a bit farther to the east, now pushed closer to the brink of failure by stresses imparted by last week’s sudden slip on the Miragoane-Petionville segment.  Cities in a similar fix include Istanbul and Padang, where history has shown that clusters of earthquakes are common and where science has shown that another one is likely within the next few decades.

Insofar as this tragic event focuses the world’s attention on Haiti, it could well be a last chance to turn things around there.  Recall the billions of dollars that poured into Aceh for its reconstruction after the great 2004 earthquake and tsunami.  Those who develop the plans for using the money and the good will that will likely continue to pour into Haiti would do well to expect another shock at least as violent within the next few decades.   This means that it would be smart to rebuild the physical and the human infrastructure of Port au Prince to be more resilient to another strong shake. An investment in scientific infrastructure would be an important part of that resilience " to measure the strains as they accumulate and understand possible forewarnings.

The lessons of the Haitian earthquake apply here in Asia, as well.  We, too, have earthquake-generating faults close to some of our important cities.  Istanbul sits just a few kilometers away from a great fault that failed progressively across Turkey throughout the 20th century.  The only section of that great fault that has not yet broken is the underwater section nearest that great city straddling the boundary of Europe and Asia.  Mandalay, the famous ancient city of northern Myanmar, lies less than 10 km from the Sagaing fault, whose northern stretch has not ruptured since the M ≈ 8 Ava earthquake of March 23rd, 1839.  Myanmar’s new capital Naypyidaw straddles a section of the fault that has not broken in recorded history.  In Sumatra, Padang is only 40 km from the Sumatran fault, a fault very similar to Haiti’s EPGFZ;  Bukinttinggi sits virtually atop it and Banda Aceh half way between two of its branches.  Wellington, New Zealand, has a similar fault running right through the city.  So do the suburbs of Manila.  (Figure 5)

At the dawn of the second decade of the 21st century, the Haitian tragedy illustrates in exemplary fashion the dangers that supercities, megacities and their hinterlands face from natural hazards:  We are largely unprepared, overexposed and naturally disinclined to do things differently.  Unless we embrace a new man-Nature paradigm, we are in for an endless litany of what has just happened in Haiti.  

Best if we show that the famous 20th-century writer E.B. White was wrong about us in this century: "I would feel more optimistic about a bright future for man if he spent less time proving that he can outwit Nature and more time tasting her sweetness and respecting her seniority."

Prof Paul Tapponnier is Head, Earthquakes and Tectonics Group, EOS

Prof Kerry Sieh is Director, EOS

More maps on the Haiti Earthquake

An edited version of this article appeared in the Straits Times of 20th January 2010.

See Also
"Lessons from the Haiti earthquake" by Roger Bilham
Reprinted by permission from Macmillan Publishers Ltd: Nature Vol 463/18 February 2010, Issue 7238, pp 878-879, copyright 2010