Strong earthquake shakes northern Chile: Concern about a seismic swarm?

On the night of Thursday, July 18, a strong earthquake shook a large part of northern Chile. The magnitude 7.3 tremor had its epicenter 20 kilometers south of San Pedro de Atacama, in the Antofagasta region, and a depth of 166 kilometers, as reported by the National Seismological Center (CSN). The seismic activity, recorded at 9:50 pm local time, was felt in the regions of Arica and Parinacota, Tarapacá, Antofagasta, Atacama, and Coquimbo, according to the National Service for Disaster Prevention and Response (Senapred). Since June 30, 2024, the Northern Chile region - comprising the regions of Arica and Parinacota, Tarapacá, Antofagasta, and the northern half of Atacama - has experienced a series of low-magnitude earthquakes. These events raised concerns among both scientists and local residents. Many local media outlets reported that the event was due to a 'seismic swarm.' Luis Donoso, a seismologist and academic in Applied Geophysics at the University of Desarrollo, explained to France 24 that "a seismic swarm is a series of earthquakes in a specific location, over a finite time, it can be two days, three days, five months, but it ends and there is no earthquake of greater magnitude than another." The seismic movements recorded mainly occurred in the Tarapacá Region and areas near the Collahuasi Mine, with aftershocks felt in locations such as Antofagasta, Socaire, and Pica. According to the National Seismological Center, over 50 tremors were recorded with magnitudes ranging from 2.7 to 3.8 on the Richter scale. But, do these movements qualify as a seismic swarm? Chile, located in the Pacific Ring of Fire, is one of the most seismically active regions in the world due to the subduction of the Nazca Plate under the South American Plate, a phenomenon that generates significant stress accumulation in the Earth's crust. Donoso explained that "when there is an active subduction zone, such as the one found on the Pacific margin, large earthquakes of magnitude 8 or higher will always be associated with the plate boundary." The expert mentioned that in Latin America, "we have been shaking for 40 million years and we will continue for another 40 million." In that sense, Donoso claims that what is happening in northern Chile actually refers to a 'seismic nest.' In this context, he emphasized the importance of not confusing the concept of a seismic nest with other phenomena. "A seismic nest is a volume of rock where earthquakes occur constantly; seismic events are constantly generated. But once an event occurs, for example, of magnitude 5 or 6, there is no relaxation of stresses, but rather it is maintained and persists over time." Examples of seismic nests include the Bucaramanga nest in Colombia, and others in Romania, Afghanistan, Fiji, and Ecuador. The Tarapacá region is known for its high seismic activity due to the subduction of the Nazca Plate under the South American Plate. The concept of subduction refers to the sliding of one plate's edge of the Earth's crust beneath another plate's edge. The likelihood of the recent earthquakes in northern Chile triggering a larger event in the near future is a complex issue that depends on various geological and statistical factors. Orietta Nicolis, a doctor in Applied Statistics for Economic and Social Sciences from the University of Padua, Italy, and an academic at the Andrés Bello University, explained to France 24 that so far, "the indicators we have investigated have not detected clear anomalies that could predict a major earthquake." The northern region of Chile, where the subduction of the Nazca Plate under the South American Plate occurs, is known for its high seismic activity and its ability to generate large earthquakes. Nicolis warned that "the risk remains very high, given that some earthquakes occur without precursory signals." For the academic, the analysis of the current seismic sequence suggests that, although numerous tremors have been recorded, the energy released has not been sufficient to fully relieve the accumulated stress in the subduction zone. This implies a high risk of a larger earthquake occurring in the near future, in the coming months or years. On the other hand, Donoso emphasized the importance of distinguishing these earthquakes from others that could pose greater problems, such as the major earthquakes that have affected Mendoza and San Juan, Argentine provinces bordering Chile. "Our historical memory in seismological terms is quite weak; we forget about the earthquake within five or ten years as a vague memory." Donoso explained that all other seismic activity, outside the subduction plane, is related to mountain systems. "That rock deforms plastically, folds are generated, and when it can no longer withstand the stress, it fractures. When the rock fractures, an earthquake occurs." Residents of the affected areas, such as Antofagasta, Socaire, and Pica, have also felt the impact of these earthquakes. Although the recorded tremors have been of low magnitude, the frequency of these events has created a sense of uncertainty and anxiety among the population. Local authorities have issued recommendations for citizens to prepare adequately, including creating family emergency plans and reviewing the structures of their homes to ensure their safety in the event of stronger earthquakes. Chile has an advanced seismic monitoring system, and events like this underscore the importance of maintaining and improving these capabilities. Donoso highlights three fundamental pillars for facing earthquakes: education, building regulations, and governance. "If people do not know how to react before, during, and after an earthquake, everything becomes more complex," the expert emphasizes. He stresses the importance of having basic elements such as bottled water, as earthquakes can disrupt logistical systems for 72 hours to a week. He adds that "the culture generated by a major earthquake has a memory that does not go beyond 30 to 40 years." Regarding building regulations, Donoso highlights the evolution in Chile over nearly 100 years. "We have had earthquakes since 1927 in Talca, 1935 in Chillán, where the first Building Code was developed, reinforced by the great Valdivia earthquake in 1960, modified again by the earthquake in 1985 in Algarrobo, and again in 2010." This regulation adds value to infrastructure, but ensures that even after an 8.8 magnitude earthquake, "in terms of infrastructure, habitability, and logistics, the country was functioning within a week." The third guideline is governance. Donoso explains that states must define public policies aimed at people's safety, business operational continuity, and minimizing operational losses. "If I have an industry and it is affected, for example, by an earthquake, I do not want to be without production for a month or two." Mitigating damage and reducing downtime is crucial, as "earthquakes generate poverty, but if that damage can be mitigated, that period is shortened." Chile has an advanced seismic monitoring system and a disaster preparedness culture. However, the unpredictable nature of earthquakes demands continuous improvement in mitigation strategies and ongoing education for the population. In this context, collaboration among scientists, authorities, and citizens is crucial to address the challenges posed by seismic activity in one of the most active regions in the world.