As tectonic plates interact with one another, they can move in sudden spurts that release similarly sudden bursts of energy. These are earthquakes, familiar to most—especially those who live in earthquake country. However, plates can also move “aseismically”, which means that they move slowly, almost imperceptibly. Understanding the dominant mode of movement—sudden breaks vs incremental motion—along faults can be especially important for seismic hazard assessment.
Chile is known for several great earthquakes, like the 1960 magnitude 9 Valdivia Earthquake that spawned Pacific-wide tsunamis, and the 2010 magnitude 8.8 Maule Earthquake. These events occurred along the megathrust, where the oceanic Nazca Plate subducts beneath South America. In north-central Chile, few earthquakes greater than magnitude 7 have been observed in recent history in the Atacama seismic gap, which extends from just south of the city of Antofagasta to La Serena. Looking further back, an approximately magnitude 8.5 earthquake struck in 1922, with a similar event rupturing in 1819. However, a section within this gap appears to behave differently, deforming slowly and aseismically.
An international team of scientists led by Marcos Moreno of Pontificia Universidad Católica de Chile and Frederik Tillmann of GFZ Potsdam, Germany deployed a dense geodetic and seismic network in this region. The team used this dual network to generate a microseismicity catalog that they harnessed to develop a high-resolution model of what might be happening in this special section of Chile’s subduction margin. This work, by Diego González-Vidal et al., was published in Geophysical Research Letters.
Moving sans seismicity
Tectonic plates are constantly moving. In Chile, the South American plate and Nazca plate push together, accumulating a slip deficit at the plate boundary, which is a megathrust. That deficit—the movement that needs to happen at the boundary because the rest of each plate has moved—can be released seismically during large earthquakes. However, in cross-section, shallow and deeper parts of the plate boundary that are outside the seismogenic zone (the zone that produces earthquakes) tend to exhibit aseismic behavior that also releases some slip deficit. These slow motions can take days, or even months, to occur. According to an Eos editor’s highlight by Moreno, this aseismic creep can happen in the seismogenic zone itself.
How much slip deficit is accommodated by earthquakes versus aseismic creep varies along the length of most subduction zones. In some cases, aseismic signals can precede large earthquakes with a mixture of slow movement punctuated by foreshocks. Aseismic slip unrelated to large earthquakes has also been observed where the plates are weakly locked—where they’re less stuck.
Along the Chilean megathrust, slow slip events seem to be rare except in north-central Chile—the study area. For instance, one slow-slip event lasted 18 months with a maximum slip of 50 centimeters (about 20 inches). This event, which spanned 2014 and 2015, was observed starting around at the deeper end of the seismogenic zone, extending somewhat farther down from there, said coauthor Christian Sippl, a researcher at the Czech Academy of Sciences. A similar event occurred again in 2020.
Of course this region isn’t entirely devoid of seismicity. In addition to the above-mentioned earthquakes over a century ago, increased rates of swarm-like earthquake sequences, as well as repeating earthquakes that rupture the same fault, have also been documented. Such seismic events may be triggered by aseismic movement.
“When the magnitude of the earthquake is less than 4 and is imperceptible to humans but detectable through a network of seismometers, it is called microseismicity,” said first author Diego González-Vidal of Universidad de Concepción, Chile. Sparse instrumentation in this region means that how microseismicity and aseismic movement are related has been hard to understand, as has the relationship between small, slow movements and the recurrence of great earthquakes—those of magnitude 7 or higher.
Measuring Microseismicity
The team deployed a dense network of 60 seismic stations between 2020 and 2022. In their analysis, they also used data from additional stations, including data from a temporary network that is managed by the Institut des Sciences de la Terre, and data from permanent stations available via the GEOFON data centre and via the SAGE Facility operated by EarthScope. Additionally, the team deployed 28 continuous GNSS stations to densify the existing network of 42 GNSS sites.
With the seismic deployment, the team created a high-resolution microseismicity catalog that comprises more than 30,000 events with magnitudes as low as 0.3 that occurred over about 15 months, from November of 2020 to July 2022. They relocated these events and noted that the apparent decay of seismicity both north and south of the study region is likely due to lower detection capability. (Less dense station coverage means some earthquakes, especially smaller ones, are easier to miss.) But within the study area, they found that the most prominent feature of this earthquake catalog is a continuous band of high seismicity beneath the coastline, at the interface between the overriding South American plate and the downgoing Nazca plate, with events located between 30 and 100 kilometers from the trench. In other words, on a map, this band plots onto the coastline, but it is situated at depths of about 30 to 35 kilometers, Sippl said.
The team compared that band of seismicity to intraplate locking, which they constrained using 70 time series derived from the GNSS data. Armed with these observations, the team explored the interplay between seismic and aseismic processes in this region: what’s locked, what’s not, and what does that mean?
Earthquakes unlocked
The time series information helped the team generate a model of just how locked, or stuck, the plates are. They found high values of intraplate locking offshore along the shallow parts of the megathrust. At greater depths, locking is lower.
The thousands of small earthquakes in the microseismicity catalog appear at the depth where locking along the plate interface starts to decrease. Interestingly, small earthquakes weren’t found at shallower depths where locking is high, implying that the plates are mechanically coupled and accumulating stress that must eventually be released.
Moreover, the team found that the Atacama seismic gap has two highly locked regions of different sizes—two asperities. These locked asperities are separated by a narrow, creeping section that has higher background seismicity levels. Locking is also low in the southernmost part of the study region, possibly because of post-seismic effects from the 2015 magnitude 8.3 Illapel earthquake.
Ridges apart
The curious geometry of the two locked regions separated by a creeping section may result from subduction of the bathymetrically bumpy, possibly hydrated Copiapó Ridge at this latitude. The Copiapó Ridge coincides with a high number of repeating earthquakes, higher seismicity, and low locking. The roughness associated with the ridge may reduce intraplate coupling. This weakly coupled creeping segment could act as a barrier to large earthquakes because stress isn’t accumulating here.
However, the same confluence of observations isn’t present where the Taltal Ridge, located north of the Copiapó Ridge, subducts. This may imply that the Taltal Ridge only recently started to subduct, or it has other properties that are different from the Copiapó Ridge. Alternatively, this observation may imply that the barrier effect of the Copiapó Ridge may be temporary as the two last major earthquakes, in 1922 and 1819, likely ruptured across this region.
Whatever the explanation may be, “our study highlights the need for continued monitoring via dense seismic and geodetic instrumentation,” González-Vidal said.
