Mw6.2 offshore of southern Chile

An apparent megathrust earthquake in an unusual location

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On November 8 at 6:38 AM local time, a magnitude 6.2 earthquake was reported just offshore the coast of southern Chile. Due to the incredibly rugged terrain of this remote coast, it is extremely unlikely that many people felt even moderate shaking, although the USGS received a handful of reports of weak shaking from further away.

This isn’t a particularly large earthquake for Chile, but the exact location of the earthquake came as a bit of a surprise to us!

Figure 1. Seismotectonic context of the November 8, 2024 Mw6.2 earthquake offshore of southern Chile.

The focal mechanism and depth are consistent with a shallow megathrust earthquake, resulting from subduction of oceanic lithosphere beneath the continental lithosphere of South America. In plainer language, the crust beneath the Pacific Ocean is sinking eastward beneath the crust of South America. In this part of the world, the subducting ocean crust is young and hot, so the trench is shallow: only about 3 kilometers below sea level.

Offshore of southern Chile, the trench is divided into two sections, separated by an active oceanic spreading ridge that enters the trench just north of 46°S latitude. This plate boundary is revealed by the numerous aligned earthquakes beneath the Pacific Ocean. The ridge divides the Nazca Plate in the north from the Antarctic Plate in the south. Both of those plates are moving east, towards South America, and sinking beneath it, which means that the spreading ridge itself is also being subducted, along with its associated transform faults.

This type of ridge-trench-trench triple junction is rare on Earth. Oceanic lithosphere is created at spreading ridges, and recycled at subduction zones; most spreading ridges safely reside in the center of the large ocean basins that they themselves formed. However, the Pacific Ocean is ringed by subduction zones; here, especially along the eastern side, even fast-spreading ridges aren’t safe! Other examples of similar ridge-trench-trench triple junctions be found near Costa Rica, and in the Woodlark Basin west of the Solomon Islands.

Ridge-trench-trench triple junctions tend to migrate, and the Chile Triple Junction is no exception. The intersection point has moved northward over geological time, as the spreading ridge has subducted. At present, the spreading rate along the ridge system is pretty fast — about 50 mm/yr. That means that the plate to the north of the intersection (the Nazca Plate) is moving eastward relative to South America, subducting at about 70 mm/yr. The truly ultra-great Mw9.5 Valdivia earthquake of 1960, the largest earthquake ever recorded, ruptured about 1000 kilometers of this Nazca-South America plate boundary.

To the south of the triple junction, the Antarctic Plate is moving eastward much more slowly, subducting at only about 20 mm/yr (according to the MORVEL plate motion model). The subducting crust here is still pretty young, but the ridges that created it have already disappeared downward into the mantle.

Although we can guess from the geology that there should be a small subduction zone here, the earthquake record is pretty sparse. While a few deeper earthquakes have been recorded around 48°S latitude, it is unclear if any of those occurred on the megathrust; their scatter and locations seem more consistent with deformation of the subducted slab. A few small events located just to the southeast of today’s earthquake seem more likely to have resulted from megathrust slip. Overall, the November 8 earthquake appears to be one of very few shallow megathrust earthquakes ever recorded which clearly happened on the Antarctica-South America subduction boundary, and is among the largest events associated with this plate boundary.

There’s one more piece to this puzzle. If you re-examine the map above (Figure 1), you will see a number of strike-slip earthquakes in the high Andes mountain range, between 74-72°W latitude. These right-lateral events occur along the Liquiñe-Ofqui Fault Zone, a strike-slip fault system that is more than 1,200 km long. The Chilean forearc (the area between the volcanic arc and the trench) is actually sliding northward relative to South America along this fault. At subduction zones, the volcanic arc creates a line of weakness within the overriding plate. When there is a sideways component to the overall plate convergence, the overriding plate sometimes breaks along that zone of weakness. When a forearc roams like this, it is called a sliver plate: a long, narrow plate that is being dragged along by the sideways component of the overall plate convergence.

In 2021, Luis Astudillo‐Sotomayor and colleagues estimated a Holocene (meaning geologically quite recent) slip rate of 18.8±2 mm/yr for the southern Liquiñe-Ofqui Fault Zone. Here is a map from their study, which shows how the fault winds its way along the volcanic arc (red triangles), and how the southern end of the fault must somehow connect across the forearc to the subduction trench. We have added a red star to the map at the location of the November 8 M6.2 earthquake.

Figure 2: Map modified from Astudillo‐Sotomayor et al. (2021; their Figure 1), showing the main tectonic elements of Southern Chile: the Chile Trench, the Liquiñe-Ofqui Fault System (LOFS), and the Chile Rise.

Thus, it seems like the tectonic setting of today’s earthquake is even more complicated. The ruptured plate boundary is apparently not between Antarctica and South America. Instead, it is between Antarctica and the Chiloé Block, a sliver plate!

References

Astudillo-Sotomayor, L., Jara-Muñoz, J., Melnick, D. et al. Fast Holocene slip and localized strain along the Liquiñe-Ofqui strike-slip fault system, Chile. Sci Rep11, 5970 (2021). https://doi.org/10.1038/s41598-021-85036-5