Strike-slip fault
Courtesy USGSLast weekend’s massive magnitude 8.8 earthquake off the coast of Chile released 500 times the energy generated by the magnitude 7 earthquake that hit in Haiti the month before. Damage is extensive in both countries but so far Haiti seems to have taken a worse hit than Chile, despite suffering a less-powerful quake. In fact, there's news today that the death toll in Chile has been lowered, which is unusual with earthquake tolls. They're usual revised upward. So why the discrepancy between the two quakes? There are several reasons but a big one is the types of earthquakes involved.
The Haiti tremblor occurred along a strike-slip fault where stress is created as two tectonic plates (in Haiti’s case, the Caribbean plate and North American plate) scrape and grind past each other in opposite directions, like two cars trying to squeeze past each other on a single lane bridge. Tension builds as the plates catch and grind and energy is released in fits and starts in the form of tremors. California’s San Andreas fault is a classic strike-slip boundary. There the Pacific plate is moving in a northwesterly direction beside the North American plate.
Subduction zone: Megathrust earthquakes occur near subductions zones.
Courtesy USGSWith a subduction zone megathrust quake – like that which occurred in Chile - it’s more like a head-on collision, where a lighter oceanic crust slams into a heavier continental crust and pushes (or is pulled) beneath it. This creates tremendous tension which eventually gets released, and when it is does, megathrust earthquakes can sometimes occur. They don’t occur all the time, in fact, megathrust earthquakes are rare – only fourteen have been recorded in history – but they only happen in subduction zones, like the one along the coast of Chile where the oceanic Nazca plate is subducting beneath the continental South American plate. Chile’s Andes mountain range rose up as a result of this subduction.
To give an idea of the incredibly huge amount of energy involved with the Chilean quake, it’s been estimated the jolt shifted Earth’s axis 3 inches, caused the planet’s entire mass to contract, become denser, and it’s rotation to speed up, thereby shortening the length of a day by 1.26 milliseconds! (see story)
The nearness and depth of an earthquake’s epicenter is another factor in the amount of perceived intensity and actual damage (measured using the modified Mercalli Intensity Scale), and this figures in the Haiti-Chile comparison. Haiti’s tremblor occurred six miles below the surface, and within ten miles of the severely over-populated capital of Port-au-Prince. That’s a fairly shallow earthquake, so the intensity level was high. Chile’s earthquake was centered 22 miles underground and five miles offshore, more than 70 miles from the nearest large population center (Concepcion).
Haiti’s capital is also built on loose soil that’s been eroded and carried down from the hillsides, and since earthquakes are rare in the region, the poor island nation is ill-prepared and under-equipped to deal with them. Building construction is flimsy, and collapsed easily when shaken, even by many less intense after-shocks. Chile, however, is on solid bedrock, both geologically and in regards to their central government. The country has a long history of dealing with the many quakes that occur there (the normal-sized ones anyway), and has had building codes in place since the 1920s.
MORE EARTHQUAKE INFO
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