Understanding megathrust earthquakes

A man observes the aftermath of the earthquake that struck Pedernales, Ecuador.

On April 16, 2016, a historic magnitude 7.8 earthquake occurred offshore Ecuador causing extensive damage along the north central coast and leaving the city of Pedernales in ruins. Seismologist Anne Meltzer helped lead an international team of scientists immediately following the earthquake to organize a rapid response to monitor the aftershock sequence and the resulting data is providing insights into the structure of subsurface and the physical processes behind these events.

Meltzer joined colleagues from the Instituo Geofisica at Escuela Politécnica Nacional in Quito to deploy 55 seismometers on land and 10 ocean-bottom seismometers above the rupture zone and adjacent areas to record aftershocks. The Pedernales earthquake ruptured a 60-mile by 25-mile segment of the subduction zone along the coast. Subduction zones are regions where tectonic plates collide and one plate moves under the other descending back into the interior of the Earth. When the interface between the plates breaks, these areas release considerable amounts of stored energy, producing the largest earthquakes on Earth. As a result, these megathrust earthquakes can cause significant loss of life and damage to property.

“The data collected in the aftermath of large earthquakes tells you something about earthquake rupture processes and can assist in forecasting large aftershocks that can cause additional damage,” said Meltzer, Francis J. Trembley Chair and professor of earth and environmental sciences. “It also tells you a lot about what’s going on in the subsurface where these large earthquakes happen.”

Ecuador sits above the Nazca Plate which is subducting beneath the South American Plate. The downgoing Nazca plate has extensive topography on the seafloor, large volcanic ridges and seamounts. Meltzer and her colleagues are examining the extent to which the roughness on the downgoing plate prevents an earthquake rupture from propagating or serves as a point where the plates couple and build up stress serving as points that initiate ruptures resulting in an earthquake. Meltzer and her colleagues are currently using a dense array of 300 sensor to image the plate interface of the Pedernales earthquake rupture and the physical properties along the plate interface with a specific interest in role played by fluids. Fluids can reduce the stress at which this interface might rupture. In this region, there is a transition from sections that seem to rupture and produce large earthquakes while other parts slip slowly without generating large earthquakes.

“The difference may be related to plate roughness and fluid release in these environments. We now know these slow slip events are part of that earthquake cycle. The energy released during these events can be on the order of a large earthquake.”

During the Pedernales sequence, the mainshock was a magnitude 7.8 while the energy in these slow slip events was equal to a 7.0 magnitude earthquake. They are releasing energy but slowly over weeks and sometimes months rather than seconds. Meltzer is trying to understand what controls the transition from patches that rupture quickly in earthquakes to those that slip slowly.

“This segment of the Ecuador subduction margin has ruptured repeatedly in large magnitude earthquakes. Understanding why these large earthquakes happen where they do and whether there are factors that limit the extent of the rupture helps us understand the likely possible maximum magnitude earthquake that can occur. The larger the magnitude, the larger the ground shaking and the greater the potential damage. We need to understand why, in these interfaces between two plates, some patches rupture and some patches don’t in these large magnitude earthquakes.”

From Acumen, Spring 2021