Major earthquakes occur when the stress on rocks between two tectonic plates reaches a critical breaking point, allowing the earth to move along the connecting fault. While the slow creep of the tectonic plates makes earthquakes inevitable along major faults, the timing and exact location of the quake epicenter can be influenced by outside forces pushing down on Earth's crust. For example, the sloshing of water into the Eastern Pacific during El Niño events has been linked
to magnitude 4, 5, and 6 earthquakes on the seafloor below, due to the extra weight of water caused by local sea level rise. Sea level rise due to rapid melting of Earth's ice sheets could also potentially trigger earthquakes, though it is unknown at what melting rate such an effect might become significant.Figure 1. Google Earth
image of Haiti taken November 8, 2010, showing the capital of Port-Au-Prince and the mountainous region to its west where the epicenter of the 2010 earthquake was. Note the brown color of the mountains, where all the vegetation has been stripped off, leaving bare slopes subject to extreme erosion. Heavy rains in recent years have washed huge amounts of sediment into the Leogane Delta to the north.Figure 2.
Zoom-in view of the Leogane Delta region of Figure 1, showing the large expansion in the Delta's area between 2002 and 2010. High amounts of sediments have been eroded from Haiti's deforested mountains and deposited in the Delta. Recent expansion of the river channel due to runoff from Hurricane Tomas' rains is apparent in the 2010 image. Image credit: Google Earth
, Digital Globe, GeoEye.
At last week's American Geophysical Union (AGU) meeting last week in San Francisco, Shimon Wdowinsky of the University of Miami proposed a different method whereby unusual strains on the crust might trigger an earthquake. In a talk titled, Triggering of the 2010 Haiti earthquake by hurricanes and possibly deforestation
, Wdowinsky studied the stresses on Earth's crust over the epicenter of the mighty January 12, 2010 Haiti earthquake that killed over 200,000 people. This quake was centered in a mountainous area of southwest Haiti that has undergone severe deforestation—over 98% of the trees have been felled on the mountain in recent decades, allowing extreme erosion to occur during Haiti's frequent heavy rainfall events. Since 1975, the erosion rate in these mountains has been 6 mm/year, compared to the typical erosion rate of less than 1 mm/yr in forested tropical mountains. Satellite imagery (Figure 2) reveals that the eroded material has built up significantly in the Leogane Delta to the north of the earthquake's epicenter. In the 2008 hurricane season, four storms--Fay, Gustav, Hanna, and Ike--dumped heavy rains on the impoverished nation. The bare, rugged hillsides let flood waters rampage into large areas of the country, killing over 1,000, destroying 22,702 homes, and damaging another 84,625. About 800,000 people were affected--8% of Haiti's total population. The flood wiped out 70% of Haiti's crops, resulting in dozens of deaths of children due to malnutrition in the months following the storms. Damage was estimated at over $1 billion, the costliest natural disaster in Haitian history. The damage amounted to over 5% of the country's $17 billion GDP, a staggering blow for a nation so poor. Tragically, the hurricanes of 2008 may have set up Haiti for an ever larger disaster. Wdowinsky computed that the amount of mass eroded away from the mountains over the epicenter of the 2010 earthquake was sufficient to cause crustal strains capable of causing a vertically-oriented slippage along a previously unknown fault. This type of motion is quite unusual in this region, as most quakes in Haiti tend to be of the strike-slip variety, where the tectonic plates slide horizontally past each other. The fact that the 2010 Haiti quake occurred along a vertically moving fault lends support to the idea that the slippage was triggered due to mass stripped off the mountains by erosion over the epicenter, combined with the extra weight of the extra sediment deposited in the Leogane Delta clamping down on the northern portion of the fault. Wdowinsky gave two other examples in Taiwan where earthquakes followed several months after the passage of tropical cyclones that dumped heavy rains over mountainous regions. His theory of tropical cyclone-triggered quakes deserves consideration, and provides another excellent reason to curb excessive deforestation!
Figure 3. Two of 2008's four tropical cyclones that ravaged Haiti: Tropical Storm Hanna (right) and Hurricane Gustav (left). Image taken at 10:40 am EDT September 1, 2008. Image credit: NASA/GSFC.Christmas in HaitiPortlight.org
will brighten the lives of hundreds of kids in Haiti this week, thanks to their successful Christmas in Haiti
fundraiser. Portlight raised $1800 to buy toys, candies, and other assorted goodies. The shipment left Charleston last week, and will arrive in time for Christmas. Thanks to everyone who helped support this worthy effort!