The Miami Herald, BY CURTIS MORGAN,Posted on Sun, Oct. 24, 2010
The earthquake that left Haiti in ruins and killed more than 200,000
people may not have been the ``big one'' and almost certainly wasn't the
last one.
New studies published Sunday point to a previously unmapped ``blind''
fault as the likely trigger for the catastrophe nine months ago and found
no evidence it had eased more than two centuries of increasing seismic
strain along the island's major pressure point, which geologists call the
Enriquillo-Plantain Garden fault zone.
If anything, the studies conclude, Haiti now faces a heightened risk of
repeat quakes along the Enriquillo fault -- particularly near the heavily
damaged, densely populated capital of Port-au-Prince.
``Even if this earthquake did not occur along the entire fault, it's
certainly an indication that stress has built up in the area,'' said
Andrew Freed, a Purdue University geophysicist and co-author of one of
several papers published online in Nature Geoscience. ``It's locked and
loaded. My concern is that we are in the beginning of new cycles of
earthquakes.''
What scientists stress they can't pinpoint with certainity is when or how
frequently temblors might again shake the devastated country. Before the
January's 7.0 magnitude quake, Haiti was last rocked by significant
earthquakes in 1751 and 1770.
But University of Miami earthquake expert Timothy Dixon, who co-authored
another study in the journal, said the series of quakes in similar
``strike-slip'' fault zones in places like Sumatra and Turkey strongly
suggest it won't take centuries for the next big quake.
Typically, he said, other large quakes follow within decades and at
either end of the fault zone, where earlier quakes can increase tensions
between massive, slow-moving tectonic plates. The sudden, violent shifts
that finally relieve that strain are what generate the intense shaking of
an earthquake.
``There is another shoe waiting to drop at one or both ends of the
rupture zone,'' said Dixon, a professor of geophysics at UM's Rosenstiel
School of Marine and Atmospheric Science. ```We can't say very much about
when that other shoe will drop. It could be 100 years from now or it
could be next month.''
The studies, published in a special edition of Nature Geoscience focused
on the Jan. 12 quake, are among the first peer-reviewed research on the
quake's origins.
The findings echo concerns many geologists had raised before and after
the quake. They also underline the difficult rebuilding challenges that
face Haiti, where aging, weakly re-enforced and poorly constructed
buildings multipled the death toll and left 1.5 million people homeless
and living in tents.
Hispaniola, the island Haiti shares with the Dominican Republic, sits
atop two major faults -- borders between the North American and Caribbean
tectonic plates, which grind against each other as they move about one
inch per year. The 200-plus years of pent-up strain in the Enriquillo
fault, which runs from Jamaica east through southern Haiti and the
capital city into the Dominican Republic's Enriquillo Valley, has long
been considered a trouble zone.
Dixon and colleagues authored a paper in 1998 warning of serious seismic
instability in Haiti centered along the Enriquillo fault line. In 2008,
Freed and Purdue colleague Eric Calais, a study co-author on leave in
Port-au-Prince to serve as science advisor to the United Nations on
Haiti's recovery plans, produced another study predicting an impending
7.2 quake near Port-au-Prince.
That's why geologists initially made the Enriquillo fault the prime
suspect.
But months of field observations and measurements taken from global
positioning systems, satellite images and radar-and-light ranging
systems, found little change along the main fault line.
For one thing, despite widespread structural damage, there were no major
surface ruptures or sideways landscape shifts that are telltale signs of
quakes in ``strike-slip'' faults. The patterns of deformation instead
pointed to a parallel, previously unknown fracture running along the
coast, now dubbed the Leogane fault line.
Anthony Crone, a research geologist with the U.S. Geological Survey in
Denver who co-wrote one of the studies, said it's not unusual for an
earthquake to reveal a ``blind,'' meaning unknown, fault in boundaries
between tectonic plates that can stretch for a hundred miles or more.
``There is a lot of squeezing and grinding and crunching going on,'' he
said.
``It's not totally surprising that you are going to have much larger
features in these very complex structures.''
Though confined underground, the seismic ruptures were powerful enough to
raise coastal elevations along a 50-to 60-mile stretch west of Carrefour,
Crone said.
Researchers found small, near-shore corals that normally grow no higher
than low tide levels suddenly exposed above the waves -- in some spots
thrust as much as two feet higher.
The consensus view is that Enriquillo fault remains -- as Freed put it --
``locked and loaded'' and a serious seismic hazard. The risk is
particularly pronounced in sections east of the epicenter that are
closest to Port-au-Prince.
Dixon stressed that the studies amount to a preliminary investigation and
it will take more research to definitely determine the origin of the
quake. There is the possibility, for instance, that some strain may have
eased in deep, unobservable sections of the Enriquillo.
If there is a positive message from the work, he said, it's that the
country has time to prepare for the next one.
``You probably have a few decades to enforce your building codes and
build stronger and better.'' But, scientists cautioned, when the
Enriquillo fault inevitably goes, it could generate far more force than
the January quake.
``This was a small event,'' Freed said. ``We are talking much larger. The
historic earthquakes were 7.5 or 8, orders of magnitude more.''
Under the complex algorithm of the scale, an 8.0 quake would produce 30
times the energy of a 7.0 quake. A 9.0 would generate 30 times that --
meaning 900 times stronger than what Haiti endured. Researchers stressed
they were not being alarmist but simply reporting the hard facts of the
country's unstable seismology.
``Unless we've just really got it wrong about what has happened in the
past,'' Freed said, ``it's not going to be two centuries until the next
one. It's going to be a matter of decades.''
