By Gabriel Lotto
The April 16, 2016 Pedernales, Ecuador, earthquake was the latest in a string of large quakes that have repeatedly ruptured the Ecuador-Colombia subduction zone over the last 100+ years.
The earthquakes kicked off in 1906, when a megathrust earthquake – estimated between M=8.5 and M=8.8 – ruptured the entire interface between the down-going Nazca Plate and the North Andean Sliver (NAS), a small segment of the South American Plate.
Since 1906, there have been not-quite-as-huge-but-still-large earthquakes in 1942, 1958, 1979, 1998, and 2016, the last of which was probably very similar to the one in 1942. If you were sitting at the plate interface at the equator (0 degrees latitude), you would have seen the Nazca Plate slip past the NAS three times since 1906.
Why have there been so many large earthquakes on Ecuador’s coast so close together in time?
A team of geophysicists from Ecuador, Colombia, and France recently published a paper in Nature Geosciences that aims to answer that question.
Away from plate boundaries, Earth’s tectonic plates move at a steady speed with respect to one another. But where two plates meet, they often lock together, sticking to each other as stress builds up and then rapidly releases in an earthquake. Off the coast of Ecuador, the Nazca Plate moves at 47 mm/yr with respect to the NAS. The Ecuador-Colombia subduction zone is locked, and it should experience earthquakes that just barely account for that 47 mm/yr of long-term deformation. For example, an earthquake that slips 4.7 meters every 100 years would get the job done.
But in Ecuador, things aren’t so simple. The 2016 earthquake’s peak slip of 6 meters was almost double the 3.5 meter slip deficit accumulated since the 1942 quake, according to the authors of the recent Nature Geosciences paper. And the 1942 earthquake itself had a seismic moment three to five times larger than would be needed to release the stress built up since 1906.
In other words, if you only look at the last 100 years, the earthquake slip budget has been in the black.
How can this be? Earthquakes sometimes dynamically overshoot the slip required to keep up with the long-term plate motion, but not by this much. Instead, the authors suggest that there was probably an extended lull in earthquake activity prior to 1906. Before the 20th century, there are no historical records of major subduction zone earthquakes in Ecuador, even though coastal cities like Manta and Portoviejo have been around since the 16th century.
What appears to have happened is that the great 1906 earthquake kicked off a phase of enhanced seismic release, evening out the earthquake budget after a long period of quiescence. Has the Ecuador-Colombia subduction caught up to the long-term plate convergence rate? We can’t be sure, but there’s no reason to believe the period of greater earthquake activity is over yet.
Credit: Tremblor, http://temblor.net