An analysis on the Chilean Earthquake and Tsunami

On September 16, 2015 a magnitude 8.3 earthquake occurred about 29 miles off the coast of the city of Illapel, Chile. The earthquake resulted in 11 deaths in Chile and the evacuation of about one million people from the coastline in anticipation of a possible tsunami.
In fact a tsunami of did occur, with a sea level rise of as much as 14 feet observed in Chile. Although there was extensive waterfront damage, no reported deaths occurred from the tsunami. Part of the reason is improved warning systems put in place after the recent devastating tsunamis, including an automated system that notified many of the local residents by cell phone.
Tsunamis occur when an earthquake or underwater landslide displaces a large amount of water. Typically the earthquakes that generate dangerous tsunamis are “dip-slip” earthquakes, in which one piece of seafloor is displaced vertically with respect to another, such as when one tectonic plate is being subducted under another. In this case the Nazca plate is being subducted beneath the South American Plate.
Tsunami waves can travel long distances across the ocean with very little loss of energy, so even very distant shorelines can be at risk for tsunamis from large earthquakes. In the U.S., a tsunami advisory was issued for the Hawaiian coastline and parts of the coastline of Southern California. In Hawaii a tsunami of up to 3 feet was observed, as can be seen in this plot of tides in Hilo.

tsunami-1image

Here the blue line shows the tides that were predicted, the red line shows what was actually observed, and the purple line shows the difference. The appearance of the tsunami waves is obvious in this plot.
In Southern California, the tsunami was much smaller, but could still be seen on the tide gauges. The following plot shows the tide at the Scripps Institution of Oceanography pier in La Jolla, California.

tsunami-image2

While the amplitude of the tsunami is much smaller here—only a few inches—the arrival is still obvious by the “jittery” nature of the plot after about 5am local time. Another interesting feature on this plot is the relatively large difference (about 9 inches) between the observed tide and what was predicted. This has nothing to do with the tsunami, since it can be seen before the tsunami arrival, but is instead is associated with the current strong El Nino, which has raised sea level in many places.