M6.2 earthquake shakes Papua New Guinea
We look at the regional tectonic setting of this moderate earthquake
Citation: Bradley, K., Hubbard, J., 2024. M6.2 earthquake shakes Papua New Guinea. Earthquake Insights, https://doi.org/10.62481/e3c57007
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On September 5, 2024 at 11:03 AM local time (UTC 01:03:17), a magnitude 6.2 earthquake struck along the northeastern coast of Papua New Guinea. This earthquake made the international news — not because of any real damage (thankfully) — but because Pope Francis will soon visit the island, bringing a lot of extra attention to the region.
The earthquake arose from strike-slip rupture of a fault located just offshore, between the mainland and several volcanic islands. This earthquake joins a long list of moderate to large events that have affected this extremely active region.
The focal mechanism shows almost pure strike-slip motion on either an east-west oriented fault (blue plane in the figure below), or a north-south oriented fault. The pattern of recorded earthquakes in the map above clearly shows that the east-west oriented fault is correct. That makes this a left-lateral strike-slip fault.
It is often pretty challenging to write about earthquakes in Papua New Guinea, because it is one of the most tectonically complicated areas on Earth. Often, the earthquakes at depth do not connect to obvious faults at the surface. We have previously written about these kinds of events.
However, today’s strike-slip earthquake is easier to address because it happened at the very western end of a huge, fast-moving and seismically productive strike-slip fault system. This fault system is quite shallow and isn’t nearly as shy as some of the other Papuan faults. Heading due east from today’s epicenter, the system is marked by a band of earthquakes almost 1000 km long, crossing the entire Bismarck Sea (in the figure below, we only plot earthquakes with centroids shallower than 30 km).
Notice that the focal mechanisms within this band all look similar to today’s earthquake — they are virtually all strike-slip events. These earthquakes record the activity of a major strike-slip fault system. It is truly remarkable that such a large area can be so completely dominated by one type of faulting mechanism.
Here’s our quick take on a tectonic overview map for the Bismarck Sea (today’s earthquake location is shown as a labeled yellow star). We have drawn in the structures as we see them, using shallow focal mechanisms as our primary guide. We have also plotted estimated plate motions from a global plate motion model. Various feature names and fault locations have also been gleaned from the interesting papers listed at the end of this post - don’t forget to browse those if you are interested in the actual science of this area!
As you can probably tell, this whole region is wickedly complex. There are several deep ocean trenches, several different volcanic arcs, a few ocean basins with different average water depths, some extremely mountainous areas, and a huge number of earthquakes.
The collection of strike-slip faults that crosses the Bismarck Sea is actually a plate boundary that separates the South Bismarck and North Bismarck Plates. This fault system moves at a whopping 13 centimeters per year - that’s about six times as fast as the San Andreas Fault! This system ranks among the fastest strike-slip faults in the world.
This plate boundary also has several large step-overs, where the strike-slip earthquakes don’t fall onto a straight line.
The westernmost strike-slip fault in the Bismarck Sea is called the Bismarck Sea Seismic Lineation (BSSL). This is presumably the fault that hosted today’s rupture. We’ll get back to this fault in a moment, after we look at the rest of the system.
The easternmost strike-slip fault of the Bismarck Sea system is the Weitin Fault, which has actually hosted two very large (M7.7 and M8.0) strike-slip earthquakes over the last quarter-century (in 2019 and 2000, respectively). This fault comes onto land in New Ireland, and is therefore the only fault in this system that can actually be seen directly.
Beneath the eastern Bismarck Sea, the large step-overs between the Weitin, Djual, and Willaumez Faults are actually the locations of several mid-ocean ridge segments, where new oceanic crust is being formed as the North and South Bismarck plates pull apart from each other — also at about 13 centimeters per year. Because the ridges are opening so quickly, the hot spreading system is dominated by volcanism and not by tectonic deformation. While slower spreading ridges usually have lots of normal-type earthquakes that betray the ridge location, here we instead see gaps in the earthquake map — like along the Manus spreading center (below), which lies between the Djual and Willaumez Faults.
The strike-slip faults connecting the mid-ocean ridges in the eastern Bismarck Sea are (fairly standard) ridge-ridge transform faults; they allow new oceanic crust to spread out in both directions from each ridge.
The central and western Bismarck Sea presents more of a geological puzzle. It isn't clear that there is active oceanic spreading between the BSSL and the Willaumez Fault (in fact, there are a few thrust-type mechanisms within those gaps!). The westernmost BSSL certainly does not link up with a spreading ridge — instead, it turns into an oblique (sideways-slipping) thrust fault! This marks the area where the North Bismarck Plate is colliding with, and shouldering past, Papua New Guinea. The BSSL is therefore some kind of ridge-trench transform fault — it balances out the extension in the east with compression in the west, at least to some extent.
Why is this whole area so tectonically complicated? Well, it’s probably because there are too many cooks in the kitchen, so to speak.
First, the Pacific Plate is just rocketing westward compared to Papua New Guinea. The Pacific and Caroline Plates (which might be one plate, actually) seem to have jammed within the West Melanesian Trench, effectively adding the North Bismarck Plate to the Pacific Plate and dragging it along westward.
Second, the Australian Plate is moving rapidly northward, colliding with Papua New Guinea and driving up mountains that are tall enough to maintain glaciers near the equator (at least for the next few years, until the last ones finally melt). This geological uppercut from the south prevents Papua New Guinea from retreating away from the Pacific onslaught.
But that’s not all! To the east, the oceanic Solomon Sea Plate is subducting along the New Britain Trench. If that plate is dense enough, it could be causing slab rollback, basically dropping the floor out from underneath the Bismarck Sea and pulling it open from the east.
All of this tectonic judo is happening at great speed (tectonically speaking). And as with judo, it is the ultimate balance of forces that matters, not just the specific forces applied in one area. Because the plates are moving so fast here, most of the geographic features we see along their boundaries are probably only a few million years old. That presents a great challenge, but also gives us the chance to study newly formed tectonic systems.
So — today’s M6.2 earthquake is just the tip of the tectonic iceberg, representing the western edge of a fascinating and active fault system! But to emphasize the real seismic complexity of this region, we also plotted a map featuring ALL of the recorded earthquakes in our catalogs:
As always, we love to get feedback from people who have worked on or lived within the regions we write about. Please feel free to leave a comment below!
Further reading:
Dyriw, N.J., Bryan, S.E., Richards, S.W., Parianos, J.M., Arculus, R.J. and Gust, D.A., 2021. Morphotectonic analysis of the East Manus Basin, Papua New Guinea. Frontiers in Earth Science, 8, p.596727. https://doi.org/10.3389/feart.2020.596727
Holm, R.J., Rosenbaum, G. and Richards, S.W., 2016. Post 8 Ma reconstruction of Papua New Guinea and Solomon Islands: Microplate tectonics in a convergent plate boundary setting. Earth-Science Reviews, 156, pp.66-81. https://doi.org/10.1016/j.earscirev.2016.03.005
Hubbard, J. and Bradley, K., 2024. M6.9 earthquake shakes Papua New Guinea. Earthquake Insights, https://doi.org/10.62481/89b816d8
Llanes, P., Silver, E., Day, S. and Hoffman, G., 2009. Interactions between a transform fault and arc volcanism in the Bismarck Sea, Papua New Guinea. Geochemistry, Geophysics, Geosystems, 10(6). https://doi.org/10.1029/2009GC002430
Martinez, F. and Taylor, B., 1996. Backarc spreading, rifting, and microplate rotation, between transform faults in the Manus Basin. Marine Geophysical Researches, 18, pp.203-224. https://doi.org/10.1007/BF00286078
Wow, this is very interesting and complicated... impressive post. Thanks.