1 May 2017
By Larry O’Hanlon
The most populated central region of Chile could be vulnerable to large tsunamis generated by a deceptively moderate kind of earthquake that might be overdue, say scientists who have sorted out the source of an earthquake and tsunami that struck the area 287 years ago. The region is the same that trembled from a magnitude 6.9 earthquake on April 24.
Historical records and new reconstructions of the 1730 Chile earthquake suggest that part of the megathrust rupture — an unzipping of the Earth’s crust — was relatively shallow. Although the quake struck deep underneath the sea, near the ocean trench off the coast of Valparaiso, the rupture occurred close to the top of the Earth’s crust. Shallow earthquakes like this one that occur deep underneath the ocean are likely to generate tsunamis, even when they are smaller in magnitude, say Chilean researchers who conducted the study.
“Shallow megathrust ruptures occur near oceanic trenches, where ocean depth is usually greater than elsewhere,” said Matías Carvajal of the Escuela de Ciencias del Mar of the Pontificia Universidad Católica de Valparaíso in Valparaíso,
Chile, and lead author of the new study in the Journal of Geophysical Research: Solid Earth, a journal of the American Geophysical Union. “The deeper the water above the source region, the larger the tsunami will be.”
In the new study, Carvajal and his colleagues ran simulations of hypothetical earthquakes of various magnitudes and depths off the coast of central Chile. They then calculated the tsunami flooding that would result from such earthquakes and compared their results to historical observations of the 1730 tsunami from Chile and Japan.
The researchers found that a shallow rupture of 9.1 to 9.3 magnitude offshore central Chile best fit the historical observations. They concluded that the 1730 quake ruptured 600 to 800 kilometers (400 to 500 miles) of crust and caused it to slip 10 to 14 meters (33 to 46 feet). The quake also produced shaking that destroyed buildings up and down the coast for more than 1,000 kilometers (600 miles). The rupture was shallow in the north, near Valparaiso, and deeper to the south. The shallow end of the rupture generated tsunamis that caused damage as far away as Japan.
But the same shallow rupture zone has been quiet ever since, which is worrisome, the researchers say.
“The effects produced by major earthquakes that occurred later in that region suggest that these earthquakes involved little or no slip at shallow depths,” Carvajal said. “This may imply that any future earthquake in the region could release the slip that has been accumulated since 1730. The large, offshore, shallow slip during the 2011 Japan earthquake was the main responsible for the huge tsunami that claimed thousands of lives”
Carvajal and his colleagues estimate that the accumulated slip since 1730 could be as much as 20 meters (66 feet) – more than the slip that occurred in 1730 and enough to produce a devastating tsunami in the region that’s now home to more than a million people.
“I feel confident that the main features that characterized the real 1730 rupture are rather well represented in our proposed 1730 rupture,” Carvajal said. “This is, a very large earthquake rupture, with a magnitude greater than 9, and with significant shallow slip in the north, with a deeper slip in the south.”
The message to the people of Chile is that they need to be aware of the risk of shallow earthquakes, which can generate large tsunamis even with less ground shaking, Carvajal said.
“I have lived in coastal areas of Chile most of my life,” Carvajal said. “People here are very used to earthquakes in all their sizes, and that is why we only tend to evacuate to high ground when we feel very strong shaking.”
The Valparaíso area has not experienced a shallow earthquake similar to the 1730 quake in recent history, he said. “Because shallow ruptures produce only moderate shaking on land, I believe that immediate evacuation is not guaranteed for this type of earthquake. Hopefully I’m wrong,” he added.
— Larry O’Hanlon is a freelance science writer, editor and social media manager in New Mexico. He manages the AGU blogosphere. Follow him on Twitter at @Earth2larryo