# Triggers of Chile's mega-earthquakes

Mohamed Chlieh

► To cite this version:

Mohamed Chlieh. Triggers of Chile’s mega-earthquakes. Nature Geoscience, 2024, 17, pp.5-6.
10.1038/s41561-023-01340-x. insu-04604403

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# Triggers of Chile's mega-earthquakes

Mohamed Chlieh

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Megathrust earthquakes along subduction zones present significant hazards. Evidence from the South Chile subduction zone suggests that the structure and fluid distribution of the megathrust interface governs the size and timing of large earthquakes.

Where and when earthquakes and tsunamis occur remains a major societal issue with fundamental implications for sustainable development of modern societies. The largest earthquakes recorded have occurred at the plate boundaries, along gigantic thrust faults – or megathrusts – within subduction zones. These megathrusts can extend for several thousands of kilometers and are characterized by a wide seismogenic zone of 100–200 kilometers from the trench along the dip direction. Writing in *Nature Geoscience*, Julve and colleagues¹ numerically simulate the seismic behavior of the South Chile megathrust interface constrained by geological and geophysical data. Their findings, based on thousands of seismic cycles rupturing different portions of the shallow megathrust interface, provide insights into the mechanical controls responsible for the observed seismicity.

The Circum-Pacific Belt, which encompasses a continuous series of subduction zones, is frequently affected by large (moment magnitude *M*<sub>w</sub> > 7.0) and great (*M*<sub>w</sub> > 8.5) megathrust earthquakes that can trigger devastating tsunamis. The 1960 Valdivia megathrust earthquake in South Chile, with *M*<sub>w</sub> = 9.5, is the greatest earthquake ever recorded in human history. Its rupture broke a surface roughly 1,000 kilometers

long by 130 kilometers wide². The seismic slip reached more than 30 metres in many places and peaked up to 40 metres locally. It triggered a huge tsunami that was recorded as far as Japan and New Zealand.

After more than a half-century of relative seismic quiescence, the 2017 large Melinka earthquake (*M*<sub>w</sub> = 7.6) ruptured a relatively small and deep portion of the 1960 mega-rupture. What caused the variations in the sizes of these events and what controlled the time gap between them remain open questions.

There have been a wide range of attempts to model the size and frequency of earthquakes using instrumental and historical seismic catalogues as well as deterministic, probabilistic or Artificial Intelligence (AI) assisted methods, with various degrees of success. Little is known about the recurrence time of the giant (M<sub>w</sub> > 9.0) earthquakes as their return period is generally much longer than the historical seismic catalogue. Studies on paleo-earthquakes and tsunamis are slowly advancing our knowledge of major prehistorical seismic events; however, such records remain too sparse and incomplete to build a reliable prehistorical catalogue for most seismically active regions of the world. Based on exceptionally long records of seismicity collected in South Chile³, the analysis of Julve and colleagues provides valuable insights into the physical mechanisms that control the frequency of large and great megathrust earthquakes.

By compiling various geophysical datasets and using laboratory-derived rock friction parameters, Julve and colleagues constrain the frictional parameters of the Chilean megathrust interface. They then conduct numerical modelling of the seismic cycle that includes depth-variable pore fluid pressure along with viscoelastic rheologies for the mantle and are able to reproduce the observed seismic cycles of large and great events. They therefore define great earthquakes as

<figure><img src="image_3.png" /><figcaption>**Fig. 1** | 3D view of a subduction megathrust interface with depth varying rock-fluid interaction processes (from Agard et al.⁴). Julve et al.¹ suggest that the frictional and rheological properties of the forearc, primarily controlled by</figcaption></figure>

the geological structure and distribution of fluids at the megathrust, govern size and recurrence patterns of earthquakes for the Chilean subduction zone. Figure inspired by ref. 5 and adapted with permission from ref. 4, Elsevier.
trench-breaking events that rupture the accretionary prism and the upper and lower seismogenic zones and describe large earthquakes as ruptures that are principally confined in the lower seismogenic zone. Their preferred model matches the estimates from paleo-turbidites and tsunami deposits and predicts a return period of 82 ± 43 years for large (M<sub>w</sub> > 7.0) earthquakes and 271 ± 70 years for great (M<sub>w</sub> > 8.5) earthquakes.

Fluid-saturated sediments and rocks that enter the subduc-
tion channel introduce massive amounts of water at the contact
boundary between the megathrust interface and the upper plate
(Fig. 1). Hydrothermal metamorphisms and dehydration of the
megathrust, facilitated with temperature and pressure rise as depth
increases, expel this water into the forearc and backarc regions.
Julve and colleagues suggest that variations in fluid-rock interac-
tions along the megathrust dip cause differences in the mechanical
stability of the megathrust interface that control the resulting
seismicity.

Even though deployment of dense seismic networks and develop-
ment of space-based technologies have significantly improved our
knowledge of the seismic cycle and fault mechanics in recent decades,
there is still a lot to learn about the stick-slip behaviours of active faults
and subduction megathrusts. In addition, the recurrence time and size
of earthquakes is far from simple to predict. Seismic cycles are often

aperiodic with clustering of large earthquakes within a relatively short time period that alternates with an abnormally long period of seismic quiescence. Although Julve and colleagues have identified a controlling role for the mechanical properties of the Chilean megathrust, many questions remain to understand how, where and when such events occur.

Mohamed Chlieh

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Institut des Sciences de la Terre, Université Grenoble-Alpes,
Grenoble, France.

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e-mail: mohamed.chlieh@univ-grenoble-alpes.fr

Published online: 15 December 2023

References

1. Julve, J. et al. Recurrence time and size of Chilean earthquakes influenced by geological structure. Nat. Geosci. https://doi.org/10.1038/s41561-023-01327-8 (2023).

2. Plafker, G. & Savage, J. C. Geol. Soc. Am. Bull. 81, 1001–1030 (1970).

3. Cisternas, M. et al. Nature 437, 404–407 (2005).

4. Agard, P., Plunder, A., Angiboust, S., Bonnet, G. & Ruh, J. Lithos 320, 537–566 (2018).

5. Angiboust, S., Wolf, S., Burov, E., Agard, P. & Yamato, P. *Earth Planet. Sci. Lett.* **357**, 238–248 (2012).

**Competing interests**

The author declares no competing interests.