Today fifteen years ago, on Tuesday 17 January 1995 the Great Hanshin earthquake
, or Kobe earthquake, occurred in the southern part of Hyōgo Prefecture, Japan. It measured 6.8 on the Moment magnitude scale. Approximately 6,434 people lost their lives.
A few days ago, particularly on 13 and 14 January 2010, I had much more traffic on my blog than usually, apparently from people in search of more information on the Haiti earthquake and the tectonics in the Caribbean. Sorry, I was enjoying myself in Norway (where the Haiti earthquake was by the way the only World news I heard about), but now I am back.
I cannot say that this earthquake came as a surprise. A strong earthquake in this area was long overdue. A couple of years ago worrisome signs of growing stresses in the fault were reported
showing that the fault was capable of causing a 7.2-magnitude earthquake. The stresses had had plenty of time to grow, as this was the biggest tremor in Haiti in 200 years.
The magnitude is not the only factor determining the devastating potential of an earthquake. Here all the ingredients for a tragedy were present. A powerful strike-slip earthquake that was shallow (depth about 10 km), occurred in loose coastal sands easily agitated (instead of solid hard rocks), in a densely populated area with poor people in poor, far from earthquake resistant, housing. In fact Haiti is the poorest country in the western hemisphere and thousands live/lived in poor quality shacks, many of which were immediately toppled by the earthquake. In a wealthy country with good seismic building codes that are enforced, you would have some damage, but not very much. Haiti has however been politically unstable, and when it comes to natural disasters, Haiti has had its fair share. It is still trying to recover from 2008, when it was hit four times by tropical storms and hurricanes.
The earthquake itself (and the hurricanes) was a natural hazard, but in a wealthier country with a stable government determinant to set strict building codes and enforce them the disaster would certainly have been smaller. It is immoral that we in the rich countries have done so little to help poor countries cope better with this kind of hazards. It basically comes down to money and knowledge.
Haiti is still counting its dead. Estimates of how many people died following the 7.0 magnitude earthquake on Tuesday have varied. The Pan American Health Organization put the death toll at 50,000-100,000, while Haitian Prime Minister Jean-Max Bellerive said 100,000 would seem a minimum. A UN official has said aid workers are dealing with a disaster "like no other" in UN memory because the country had been "decapitated". Three ministers and several senators are reported to have been killed. Prime Minister Jean-Max Bellerive said his house had been destroyed and he had been sleeping in his car. 27 out of 30 senators died in the earthquake, and half of the national police force has not been located, along with their equipment. Apart from that 3 million people have probably been strongly negatively affected - loosing family and property, and being injured, getting ill, out of food and water etc.
And now to the tectonics (mainly as reported by USGS): The earthquake occurred in the boundary region separating the Caribbean plate and the North America plate. This plate boundary is dominated by left-lateral strike slip motion and compression, and accommodates about 20 mm/y slip, with the Caribbean plate moving eastward with respect to the North America plate.
Haiti occupies the western part of the island of Hispaniola, one of the Greater Antilles islands, situated between Puerto Rico and Cuba. At the longitude of the January 12 earthquake, motion between the Caribbean and North American plates is partitioned between two major east-west trending, strike-slip fault systems -- the Septentrional fault system in northern Haiti and the Enriquillo-Plantain Garden fault system in southern Haiti.
The location and focal mechanism of the earthquake are consistent with the event having occurred as left-lateral strike slip faulting on the Enriquillo-Plantain Garden fault system. This fault system accommodates about 7 mm/y, nearly half the overall motion between the Caribbean plate and North America plate.
The Enriquillo-Plantain Garden fault system has not produced a major earthquake in recent decades. The EPGFZ is the likely source of historical large earthquakes in 1860, 1770, 1761, 1751, 1684, 1673, and 1618, though none of these has been confirmed in the field as associated with this fault.Location: 18.457°N, 72.533°WGeneralised map of the plate tectonics in the Caribbean region.
What could happen next? A section of fault approximately 50 kilometers long moved during the earthquake. The largest amount the fault slipped was 4.5 meters. But that is not necessarily the end. It could be the first of several in the region. Researchers are now busy investigating if there could be other faults nearby or perhaps other portions of the fault to the east or west that could give rise to similar or worse tremors in the very near future. Until now Haiti’s political situation had made it a difficult place to do science. It wasn’t until 2003 that researchers were able to begin quantifying the movement along the Enriquillo–Plantain Garden fault system.
I happened to write this before I had read any other blog posts over the past few days (Sorry dear colleagues!).
Chris Rowan at Highly Allochthonous had an excellent post at http://scienceblogs.com/highlyallochthonous/2010/01/tectonics_of_the_haiti_earthqu.php
Well done. He has been featured in a Nature News Briefing: "The Haiti Earthquake in Depth" along with more information about the faults in question and the known seismic risk of the area. at http://www.nature.com/news/2010/100113/full/news.2010.10.html