The magnitude 7.0 earthquake that struck just outside Port-au-Prince, Haiti could be just the beginning.
According to geologists monitoring the region, the quake could have primed faults crisscrossing the island of Hispaniola to unleash additional destructive temblors.
Situated where the North America and Caribbean plates collide, the island is a jumble of broken up crust. Two large, east-west running faults cut the island — the Septentrional fault zone in the north, and the Enriquillo-Plantain Garden in the south, which broke last night.
It’s too early to say for certain exactly how Tuesday’s event changed the earthquake threat in the region, Uri ten Brink of the United States Geological Survey said. The faults in the area simply aren’t well understood. But there are several possibilities that geologists are watching very closely:
1. The Enriquillo fault could rupture further to the west, out along Haiti’s southern Tiburon peninsula. The fault extends through the more sparsely populated region of Haiti, offshore and runs near to Kingston, Jamaica. Only a small portion of it — ten Brink estimates 50-60 kilometers (31-37 miles) — ruptured yesterday.
The quake could set in motion a string of powerful tremors, each further west than the last, as the rest of the fault unloads its pent up stress. The trouble is knowing when. In 2004, a 1200-kilometer (746-mile-)-long stretch of the Sumatra fault broke, unleashing a magnitude 9.2 earthquake and a horrifying tsunami that pummeled coastlines throughout the Indian Ocean. But only three months later the fault popped again along a previously locked section further south, letting loose a magnitude 8.6 tremor.
However, it may be years, even decades, before the next big quake hits the Enriquillo fault. “We can’t go around saying, ‘oh look out, there’s going to be another one in three months,’” ten Brink said. “Ultimately it will reach Jamaica, but it could be 10 or 20 years. We have no way of knowing.”
2. Within 50 kilometers (31 miles) of a quake, faults are prone to what scientists call “near field” effects — large changes in tectonics stresses that can greatly increase the chance of a fault rupturing. This is different than the long term fault behavior noted above, and there is a small probability that the area immediately west of yesterday’s quake could rupture any day.
3. At the east end of the Enriquillo fault, the seismic picture becomes even fuzzier; a jumble of lesser, but still dangerous faults slice through the Dominican Republic. Yesterday’s quake could have pushed any of these fractures closer to their breaking point.
4. The Septentrional fault could break. This is a worst-case scenario, because this major fault can host earthquakes approaching magnitude 8.0 (32 times more energetic than yesterday’s 7.0). Ten Brink was again very cautious to say that no sophisticated modeling work has yet been carried out to assess the likelihood that the Septentrional was pushed closer to failure. But he added that his initial impression was that it’s far enough away that it shouldn’t have been greatly affected.
At this early stage, the only thing we know is that the combination of a strong quake almost directly beneath a huge population center like Port-au-Prince is guaranteed to bring a heavy cost in human life.
“We’ll be lucky if only 30,000 people have died,” ten Brink said.