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Yesterday a powerful 7.9 magnitude earthquake hit the west coast of central Peru resulting in hundreds of deaths and injuries. Magnitude 7.9 earthquakes are relatively rare with 1-18 per year in the world, as opposed to almost 120 a year for earthquakes between of a magnitude between 6 and 6.9, the 1886 Summerville earthquake was between a 6.9 and a 7.1. Earthquakes of a magnitude around 7.9 will result in almost all structures either falling or being badly damaged, wide cracks in the ground, and bridges being destroyed. They are also often followed by aftershocks whose magnitudes, in the 5-6.5 range, would be considered extremely hazardous on their own.
Unlike South Carolina, Peru is located on a major tectonic plate boundary. At this boundary one plate, the oceanic Nazca plate is being overridden by the continental South American plate. As the Nazca plate slides under South America it gets stuck, because the rest of the plate is moving relatively quickly; hose places that get stuck build up lots of energy very quickly. This means that even if they release the energy fairly often in the form of earthquakes, the energy buildup is still extremely high and creates very large earthquakes. South Carolina is very different than Peru because it is not on a plate boundary so the motion here is relatively slow and energy builds up slowly. Thus we have far fewer and less catastrophic earthquakes. While the magnitude 6.9-7.0 1886 earthquake would be devastating if it happened today, it would be far less damaging than a 7.9. Geologic history also tells us that while we could have a large earthquake at any time, the big ones seem to come at relatively long intervals of 400 years. One way to visualize this is to look at the earthquake hazard maps from the USGS, note that the one for central Peru indicates a high frequency of devastating earthquakes while the one for South Carolina even has a different scale because the risk is much less. Another indicator of our lower chances for an earthquake can be seen in the historical seismicity maps, each map shows earthquakes larger than about a magnitude 4 for the time period between 1990 and 2000. Note that Peru is covered in earthquakes and South Carolina has a very few.
Curator: hallcr@cofc.edu
