Deadly Mw7.3 earthquake shakes Vanuatu

An intra-plate event along a noisy subduction zone

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A powerful magnitude 7.3 earthquake struck the island nation of Vanuatu on December 17, 2024. The quake, which occurred at 12:47 PM local time, initiated just west of the island of Efate, Vanuatu’s largest island, at 57 km depth. The capital city Port-Vila (population ~50k) suffered very strong to violent shaking (intensity VII-IX).

Figure 1: Map of the December 17 M7.3 earthquake and its aftershocks, with contours of shaking intensity estimated by the USGS. Thin colored lines are depth contours of the subduction megathrust from Slab2.0.

Significant damage has been reported in Port-Vila. Footage and reports show a number of collapsed buildings, including damage to the hospital and the building that hosts international embassies. In some cases, cars have been crushed by falling debris. Landslides have blocked roads, including the road connecting the city to the international shipping terminal. The UN Office for the Coordination of Humanitarian Affairs reports at least six unconfirmed deaths. Two main water reservoirs have also been damaged, raising concerns about water supplies. While a tsunami warning was initially issued, the wave was relatively small (about half a meter from trough to peak), and the warning has since been lifted.

Figure 2: Sea level recording at Port Vila, showing the arrival of the moderate tsunami. Source: https://bsky.app/profile/earthjay.bsky.social/post/3ldhvacnzpc2e

While Vanuatu lies above a very active subduction zone and commonly experiences earthquakes, damage is usually minor due to the low population density. The close proximity of this large event to a major city has made it unusually destructive.

Looking at the shaking intensity estimates over a wider region, it is clear that much of Vanuatu felt today’s earthquake. So far, the aftershocks are aligned along a northeast-southwest trend.

Figure 3: The December 17 earthquake and its aftershocks, with contours of estimated shaking intensity modeled by the USGS. Black arrows show movement of the Australia plate relative to the New Hebrides (Vanuatu) plate.

Tectonic setting

Why did this earthquake happen? To answer that, we need to look at the bigger picture.

This earthquake occurred along a fast-moving subduction zone, where oceanic crust of the northeastern Australia Plate is subducting beneath oceanic crust of another small tectonic plate at a rate of about 10 centimeters per year. The overriding plate is known to geologists as the New Hebrides Plate, and it is on that plate that the volcanic islands of Vanuatu have developed.

A quick aside: the New Hebrides and New Caledonia islands were originally named in English by Captain James Cook during his second voyage in 1774. The reef-fringed, coconutty tropical islands somehow reminded him of his Scottish homeland (it was a long voyage). In 1980, the English colonial name New Hebrides was discarded as the new nation of Vanuatu achieved full independence from Britain and France. Our question - should the New Hebrides Plate and the New Hebrides Trench be renamed as the Vanuatu Plate and Vanuatu Trench?

Let’s look at a map of recorded seismicity in this seismically prolific area. The vast majority of earthquakes on this map are related to the active subduction zone. The cross section below shows a slice across the subduction zone.

Figure 4: Recorded earthquakes around the December 17 2024M7.3. event, colored by depth. Cross-section shows the position of the earthquake relative to the subduction interface from Slab2.0.

We know that the rupture that produced the mainshock occurred within the subducting slab, and not on the subduction megathrust (indicated by the black curve in the cross-section above). While the reported depth of the origin (57 kilometers down) is fairly close to the estimated location of the megathrust fault, the focal mechanism allows us to confidently say that the megathrust did not slip. If the plate interface had ruptured, we would expect a thrust-type mechanism with one nodal plane dipping about 50° toward the east. Instead, we have a focal mechanism with one nearly vertical plane, and one nearly horizontal plane. This does not match slip on the megathrust fault.

With intraslab earthquakes, it can be difficult to decide which fault was responsible for the earthquake, because there is no reason to prefer one or the other of the two fault planes indicated by the focal mechanism. The USGS currently provides two possible models of slip, one for each fault orientation. In each case, the earthquake must have been caused by up to ~3 m of slip on a section of fault ~30-40 km across, with all of the slip happening within about 10 seconds. Both possible faults extend beneath the island of Efate. Please note that these are preliminary models, and are frequently updated as better data becomes available.

The alignment of the aftershocks, along a NE-SW trend, seems like it might be more consistent with the steep fault. However, this is only a suggestion at this point.

Figure 5: Slip models for the two possible fault orientations from the USGS. Top: Map view. Bottom: View along the fault surface. Note that the depth. Source: USGS, https://earthquake.usgs.gov/earthquakes/eventpage/us7000nzf3/finite-fault

Intraslab earthquakes are very common globally, because subducting plates have to bend and unbend as they are forced down into the mantle by their own weight. Like the cracking of your knuckles when you bend your fingers tightly, the imposed deformation causes intense stresses within the subducting slabs, producing earthquakes. While intraslab earthquakes never reach the same magnitude as the largest megathrust earthquakes, they can still be quite destructive if they occur close to populated land, as in this case.

This recent earthquake occurred very close to another large event — a M7.3 on August 10, 2010. Unlike the recent earthquake, the slip in that earthquake likely occurred on the megathrust. Notably, although the earthquake definitely caused shaking in nearby Port Vila, news reports indicate limited damage — despite the fact that the earthquake was significantly shallower. One possibility for this difference is the location: the 2010 earthquake was a little to the north, further from the city. It is also possible that the shaking produced by the two earthquakes was fundamentally different: intraslab earthquakes tend to cause higher frequency shaking that megathrust events, producing different damage patterns.

Figure 6: Recorded earthquakes above M7; events above M7.3 are labelled. Black line with teeth shows the subduction trench. Thin colored lines are depth contours to the subduction interface from Slab2.0. Earthquakes and depth contours are colored by depth.

A number of other M7+ earthquakes have been recorded across the region. Some of those have had significant impacts, including the 1999 M7.4 Ambrym earthquake to the north, which triggered a large tsunami and killed between five and ten people. Based on its depth, the Ambrym earthquake must have occurred in the overriding New Hebrides (Vanuatu?) plate, and not the subducting plate — reinforcing how many different types of earthquakes a subduction zone can generate.

What to expect going forward

Large earthquakes are almost always followed by aftershocks, and indeed a number of aftershocks have already been reported for this event, including four earthquakes above M5. While smaller earthquakes are unlikely to cause the same level of groundshaking as the mainshock, they can destabilize already-damaged buildings, so it is important to be cautious. Aftershocks can continue for a very long time — weeks, months, even years — becoming less frequent with time.

There is also the possibility that this earthquake could trigger another earthquake of the same size or larger. This is not a likely scenario, but is important to keep in mind. In general, about 5% of earthquakes end up in this category. Possible candidate faults for triggering could include other fractures within the slab, or even the megathrust fault that forms the subduction plate boundary. Intraslab ruptures have been known in the past to trigger slip on the megathrust in a number of large earthquakes (Lay et al., 2017). In a post in May 2023, we highlighted examples of earthquake sequences where different faults within the New Hebrides subduction system interacted.

References:

Hubbard, J. and Bradley, K., 2023. M7.7 quake at the New Hebrides subduction zone highlights seismic activity in the outer rise. Earthquake Insights, https://doi.org/10.62481/61c31ac3

Lay, T., Ye, L., Ammon, C.J. and Kanamori, H., 2017. Intraslab rupture triggering megathrust rupture coseismically in the 17 December 2016 Solomon Islands Mw 7.9 earthquake. Geophysical Research Letters, 44(3), pp.1286-1292. https://doi.org/10.1002/2017GL072539

Van Daele, M., Araya-Cornejo, C., Pille, T., Vanneste, K., Moernaut, J., Schmidt, S., Kempf, P., Meyer, I. and Cisternas, M., 2019. Distinguishing intraplate from megathrust earthquakes using lacustrine turbidites. Geology, 47(2), pp.127-130. https://doi.org/10.1130/G45662.1