One Ring to rule them all? Maybe not!

Introducing the Southeast Asian Ring of Fire

Citation: Bradley, K., Hubbard, J., 2023. One Ring to rule them all? Maybe not!. Earthquake Insights, https://doi.org/10.62481/dac74c77

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We previously worked as Principle Investigators at the Earth Observatory of Singapore, studying the natural hazards of Southeast Asia - its earthquakes, volcanoes, and tsunamis. This experience led us to develop the concept of the Southeast Asian Ring of Fire. This term is intended to highlight the fact that the active tectonic systems of Southeast Asia - and the populations exposed to the resulting geohazards - differ in important ways from those encircling the Pacific Ocean.

Earlier this month, we gave lectures as part of a training course on earthquake and tsunami hazards jointly organized by the USGS (US Geological Survey) and BMKG (Indonesian governmental agency monitoring meteorology, climatology, and geophysics). As we were preparing for the talks, we were reminded of how the framing of hazards around the Pacific Ring of Fire is inadequate for describing the tectonic systems that impact so much of the world’s population. Thus, this post is a first stab at writing down some of these ideas.

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The Pacific Ring of Fire is possibly the most well-known tectonic feature on Earth - a vast region of seismic and volcanic activity encircling the Pacific, reportedly hosting 90% of all earthquakes and 75% of the world’s active volcanoes. The existence of such a chain - and in particular its volcanoes - was described as early as the 19th century (Fig. 1), but the close association of earthquakes and volcanoes was only adequately explained with the discovery of plate tectonics and the understanding of the role that subduction plays in creating both of these hazardous Earth systems.

Figure 1: Clip from an article in Scientific American, July 13, 1878, describing the “Ring of Fire” surrounding the Pacific. Source: https://books.google.com/books?id=p4o9AQAAIAAJ, p. 26.

In modern times, we usually encounter the term “Ring of Fire” when an earthquake or volcanic eruption occurs. For instance, when Mount Marapi erupted on December 3 to tragic effect, The Guardian reported that the Indonesian volcano lies along the Pacific Ring of Fire. This particular volcano lies in central Sumatra, bordering the Indian Ocean; the Pacific Ocean is located more than 3,000 km to the east - the same as the width of the European Union! The Indian Ocean might feel justifiably slighted about this, as it laps the shores of Sumatra only 50 kilometers from the peak of Marapi. In this post, we take the side of the Indian Ocean - and argue that Southeast Asia has its own Ring of Fire.

Many maps of the Pacific Ring of Fire include Southeast Asia - but you can’t connect Southeast Asia to the Pacific Ring of Fire without jumping through some serious, and scientifically incorrect, hoops. Because no real definition for the Ring of Fire exists, news producers usually have to make their own calls about where it actually lies. The image below is taken from a video made by CNN in response to an earthquake in Chile - it shows the Ring of Fire diverting around the Philippines and running willy-nilly through the tectonically dead South China Sea (Fig. 2).

Figure 2: Map of the Ring of Fire shown on CNN. Screenshot at 0:49 from a CNN video.

The Ring of Fire is a tremendously useful term, as it describes real tectonic phenomena with a simple and intuitive pattern that most people can understand. We propose that Earth actually has two distinct Rings of Fire: the well known Pacific Ring of Fire and the newly defined Southeast Asian Ring of Fire (Fig. 3). (Google the term, and you will find a handful of references, all derived from our time in Singapore.) Here, we will we walk through our thoughts about the tectonic and societal justifications for this two-ring definition.

Figure 3: Global map of volcanoes and earthquakes; volcanoes are colored to represent the proposed Pacific Ring of Fire (bright blue) and Southeast Asian Ring of Fire (purple). Note that these two rings represent only a portion of Earth’s active tectonic regions.

What is a Ring of Fire?

A Ring of Fire forms when large areas of oceanic lithosphere that were created at spreading ridges are finally subducted into the Earth’s mantle. The phenomenal bending of the lithospheric plate and the frictional sticking between the upper and lower tectonic plates together cause an immense number of earthquakes, including huge megathrust ruptures. The subducted lithosphere also carries water to great depth, bound up within the structure of unstable minerals. Metamorphic reactions caused by the increasing heat and pressure free this water, which then rises from the subducting slab and allows the overlying mantle to partially melt. The newly formed magma is less dense than its surrounding rock, so it rises upward through the overlying mantle and crust, erupting along chains of mostly andesitic volcanoes in the upper tectonic plate, known as volcanic arcs (Fig. 4).

Figure 4: Cross section through the Sunda Trench in Central Sumatra, Indonesia, illustrating how earthquakes occur along and around the subduction zone, and volcanoes form above the subducting plate. SG = Singapore.

The famous Pacific Ring of Fire borders the eastern and northern edges of the Pacific Ocean (Fig. 5). From there, it runs around along western border of the Pacific Plate - in turn visiting northern Japan, the Marianas Islands, Papua New Guinea, the Solomon Islands, Tonga, and New Zealand. We suggest that the ring can then be traced southward to Antarctica, following a series of enigmatic volcanoes along the coast (some buried beneath kilometers of ice) eastward back toward the southern tip of South America. Although there is no active subduction and little seismicity along this southernmost edge of the Pacific Ring of Fire, it doesn’t seem unreasonable to include this stretch.

The Pacific Ring of Fire has produced some of the largest ever recorded earthquakes - 1960 M9.5 Chile, 1964 M9.2 Alaska, and M9.0 2011 Tōhoku, each of which triggered a major trans-oceanic tsunami. The ring doesn’t slack in the volcano realm either - having hosted the famous 1980 Mt. St. Helens eruption, as well as giant events like the Taupo mega-caldera eruption in New Zealand and the recent Hunga-Tonga-Hunga Ha’apai eruption in Tonga.

Figure 5: Map of the Pacific Ring of Fire. Magnitude 8+ earthquakes are shown as yellow dots; red dots are all other earthquakes with magnitude > 6. Earthquakes are from the USGS and ISC global catalogs. Plate boundaries are shown as thin white lines, and plate motions are shown with black arrows (plate data are from MORVEL56-NNR). Volcanoes are shown as black triangles and are Pleistocene and Holocene volcanoes from the Smithsonian global database. Cities from Openstreetmap. Graticule is a 10° grid.

The newly proposed Southeast Asian Ring of Fire is smaller, forming a U shape surrounding Southeast Asia: starting in Bangladesh and Myanmar in the northwest, crossing Indonesia along Sumatra, Java, and the Lesser Sunda Islands, then looping north through the Philippines and proceeding to Taiwan and southern Japan (Fig. 6). This ring, too, has caused its fair share of devastation, including the 2004 M9.1 Indian Ocean earthquake and tsunami. Some of the largest historical volcanic eruptions have occurred along this ring - the 1257 Samalas, 1815 Tambora, 1883 Krakatau, and 1991 Pinatubo eruptions being among the most well-known. This ring also hosted the great caldera-forming eruption at Toba about 74,000 years ago.

Figure 6: Map of the Southeast Asian Ring of Fire. See figure above for description of the symbology and data sources.

Together, these two rings incorporate most, but not all, of the active subduction zones in the world, and therefore most, but not all, of the world’s dangerous earthquakes and potentially eruptive volcanoes.

Tectonic comparison

Despite arising from the same tectonic processes, the two Rings of Fire are different in some fundamental ways. It is these differences which we find particularly interesting, in terms of both science and society.

One critical difference between the two Rings of Fire is their size. The Pacific Ring of Fire approximately follows a small circle with an angular radius of 75° - taking up almost an entire hemisphere (small circle is a technical term describing geometry, not a comment on scale!). To sail across the Pacific Ring of Fire, you might have to travel 16,500 km. While this journey might take several months by boat, a tsunami wave takes only 24 hours to make the crossing. These huge distances significantly mitigate the hazard posed to one side of the Pacific Ring of Fire by the opposite side, by diluting energy of physical processes and increasing the time window to prepare for any impacts. The countries along the Pacific Ring of Fire also tend to be pretty long - the USA alone claims about 6,000 kilometers of Ring circumference; Mexico has about 3,000 km, and Chile has over 4,000 km.

In contrast, the Southeast Asian Ring of Fire is compact, with only about 3,500 km separating north Luzon in the Philippines from south Java. Because everything is so closely packed together, the impacts of hazards from across the ring are generally felt more intensely and more quickly - although, as we discuss later, a tsunami on one side of the ring cannot impact the other side due to the land masses that block the path of the wave.

An interesting tectonic difference is the direction of subduction (Fig. 7). In the Pacific Ring of Fire, large oceanic plates form along spreading ridges within the Pacific basin. This lithosphere is then subducted outward beneath a surrounding rim of continental lithosphere along broad, undulatory subduction trenches. The tectonic systems along the Pacific Ring of Fire are therefore fairly isolated from each other.

In contrast, the Southeast Asian Ring of Fire lies above a zone of plate convergence, where oceanic plates move inward toward each other and then plunge beneath a central continental mass along tightly curved subduction trenches. A huge amount of complexity is packed into this small area, and there are many structures and interactions in this tectonic nexus that we haven’t even begun to appreciate.

Figure 7: Plate velocities relative to a fixed Sunda plate (Southeast Asia). Red colors indicate fast relative motions (up to 12 cm/yr), while blue colors indicate slower relative motions. All of the oceanic plates around Sunda subduct beneath it. In contrast, the plates beneath the Pacific Ocean - the Pacific, Cocos, Nazca, and Juan de Fuca - subduct outward beneath the surrounding continents. The southern part of the Pacific is also underlain by the Antarctic plate.

By the numbers

Both Rings of Fire include many volcanoes and produce many earthquakes. But how do the numbers actually compare? Let’s take a high-level look at the data (Fig. 8).

It is difficult to directly compare the numbers of volcanoes in the two rings, because of disparities in how well the two areas have been studied, and what counts as a potentially active volcano. The Smithsonian Global Volcanism Program database includes ~1489 Holocene and Pleistocene volcanoes along the Pacific Ring of Fire, and ~545 along the Southeast Asian Ring of Fire. (A 2015 study revised the number for the Southeast Asian Ring of Fire to 744 active and potentially active volcanoes in the region. However, we will stick with the Smithsonian database for our comparisons.) This comes out to an average distance between volcanoes of ~30 km for the Pacific Ring of Fire, and 24 km for the Southeast Asian Ring of Fire.

What about earthquakes? Over the last ten years, the Pacific Ring of Fire has hosted ~10,200 M4+ earthquakes per year (about 0.25 per kilometer), of which ~91 were above M6+. The Southeast Asian Ring of Fire over the same time period has produced ~4,500 M4+ earthquakes (about 0.35 per kilometer), and ~28 M6+ earthquakes per year - slightly more productive on average.

Figure 8: Comparison of the number of earthquakes and volcanoes in the two Rings of Fire.

Overall, the Pacific Ring of Fire is a larger tectonic feature and generates more tectonic and volcanic events. But the Southeast Asian Ring of Fire is slightly more hazard-dense on average. Of course, the impacts of earthquakes and volcanic eruptions on society depend on other factors, which we touch on next.

Hazard and risk

What happens when there is an earthquake or a volcanic eruption? The impact depends on location: the proximity of people and infrastructure, the vulnerability of that infrastructure, and the capability of the event to trigger further hazards, like a tsunami, landsliding, liquefaction, or widespread ash clouds.

Although the Pacific Ring of Fire is much larger geographically, large sections of the ring are relatively unpopulated (e.g., the Aleutians, Marianas, and Antarctica). The total population within the Pacific Ring of Fire is ~422 million people - fewer than the ~584 million people who live within the Southeast Asian Ring of Fire (the areas are as defined by the blue and purple areas in the figure below, and the population is estimated from Landscan data) (Fig. 9). The Pacific Ring of Fire hosts ~84 cities with more than 500k people; the Southeast Asian Ring of Fire includes ~102. In other words, although the Southeast Asian Ring of Fire is smaller, there are more people living within it.

The high population density of the Southeast Asian Ring of Fire means that many people can be directly impacted by a single event. If a great earthquake occurs in this region, such as the 2004 M9.1 earthquake in Sumatra which ruptured a fault 1,300 kilometers long, it is guaranteed that people living nearby will be dramatically affected. In contrast, there are many areas along the Pacific Ring of Fire where a similar sized earthquake could occur, but very few people live in the immediate vicinity. Thus, the 1964 M9.2 megathrust earthquake in Alaska was a tremendous event, similar in many ways to the 2004 earthquake in Sumatra, but was significantly less catastrophic.

Figure 9: Comparison of populations along the two Rings of Fire. Population density is from Landscan. Cities with population over 500,000 are derived from OpenStreetMap. While hazard is relatively uniformly distributed around each Ring of Fire, population exposure is not. Populations are highly variable around the Pacific Ring of Fire (ranging from uninhabited to highly dense), but the Southeast Asian Ring of Fire has high population density everywhere.

Volcanic eruptions can also have large geographic impact: the 1991 eruption of Pinatubo brought a thin layer of volcanic all the way to Singapore, 2,300 km to the southwest. Because even small amounts of volcanic ash can disrupt air travel, an eruption anywhere along the Southeast Asian Ring of Fire could impact the whole region. The fact that large populations in Southeast Asia are encircled by these volcanoes increases this risk significantly, compared to the wider Pacific Ring of Fire.

This density - not just of people, but also of countries - carries a further risk: that of trans-national disasters. An earthquake or volcanic eruption that impacts multiple countries at the same time can be challenging to manage, as many communities require simultaneous access to emergency response, medical supplies, food, and clean water. Because of this issue, the ASEAN Coordinating Centre for Humanitarian Assistance (AHA Centre) was established in 2011 to help coordinate disaster response across Southeast Asian nations.

To be clear, portions of the Pacific Ring of Fire carry this same risk - for instance, in Central America where countries are smaller - but along most of the Ring, countries are larger and trans-national events are less common.

A ring filled by ocean also carries a different risk profile than one surrounded by ocean. A tsunami in Japan, for instance, is a problem for Chile - there is transoceanic tsunami hazard within Pacific Ring of Fire, and the potential for tsunami triggering on all sides. In contrast, the tsunami hazard around the Southeast Asian Ring of Fire is not universal: a tsunami sourced from the subduction zone south of Java will not affect the Philippines. However, such a tsunami can propagate into non-tectonically active regions on the opposite sides of the Indian Ocean, surprising coastal populations in Sri Lanka, Madagascar, and Australia. Similarly, a large earthquake on the Manila Trench could devastate coastlines bordering the South China Sea, including southeastern China and Vietnam. Thus, tectonic events along the Southeast Asian Ring of Fire can impact countries far away from the source.

Finally, portions of the Pacific Ring of Fire are well studied, and some countries along the ring have well developed and regularly enforced building codes (e.g. Japan, USA, Chile) and large budgets for geohazards research. While these policies cannot prevent earthquakes or volcanic eruptions, they can reduce both property damage and loss of life, and accelerate the recovery process. In contrast, most countries along the Southeast Asian Ring of Fire have limited resources for research and have yet to develop and enforce rigorous building codes. Coastal subsidence, typhoons, and endemic tropical diseases like malaria can multiply the impacts of geodisasters along this ring.

The maps below (Fig. 10) show the average annual expected losses due to ground shaking, calculated by the Global Earthquake Model (GEM) Foundation. These maps emphasize that seismic risk is not evenly distributed across the two rings. For instance, while Indonesia and the Philippines (Southeast Asian Ring of Fire) face both high average annual economic and human losses, the west coast of the United States and New Zealand (Pacific Ring of Fire) can expect high economic losses but only low to moderate human losses. This is because these richer countries contain more valuable buildings that can be damaged in earthquakes, but are unlikely to collapse. Meanwhile, the Aleutian Islands (also along the Pacific Ring of Fire) experience regular earthquakes, but are exposed to almost no risk, because they have such low population density.

Figure 10: Maps of average annual economic and human losses due to earthquake ground shaking. Sourece: Global Seismic Risk Map (v2023.1) from the GEM Foundation. Silva et al. (2023). Does not consider the impacts of tsunamis, liquefaction, landslides, fires, or other triggered hazards. Curved lines approximately circle areas bordering the two rings of fire. https://maps.openquake.org/map/grm-2023-1/#3/32.00/-2.00

Other maps of the rings

How do our two proposed rings compare to previous views of a single ring? Because of the lack of a clear definition, maps of the “Ring of Fire” vary widely - some including the entire region from the Marianas through Sumatra in addition to the Caribbean; others choosing a more precise - but still arbitrary - path through the many subduction zones of Southeast Asia.

The following figure shows different maps of the Pacific Ring of Fire (Fig. 11).

Figure 11: Image sources: Map A: Universal Images Group Editorial, found at https://www.mirror.co.uk/science/what-pacific-ring-fire-facts-12342864. Map B: Time for Kids, map by Joe Lemonnier, found at https://www.timeforkids.com/g2/ring-of-fire/. Map C: United States Geological Survey, https://pubs.usgs.gov/gip/dynamic/fire.html. Map D: Wikipedia, Map by Sting https://commons.wikimedia.org/wiki/File:Tectonic_plates_and_ring_of_fire.png

Map A includes the Caribbean, Southeast Asia, the Marianas Trench, and the southern tip of South America. Map B includes Southeast Asia, all of the Philippine Sea Plate, most of the Caribbean (but not its eastern boundary), and the southern tip of South America. Map C is the USGS map, which does not include the Caribbean, the Marianas Trench, or the southern tip of South America. Map D is a more data-centric view like ours, showing plate boundaries (black lines) and volcanoes (red triangles); the Ring of Fire is marked as a narrow band that excludes Southeast Asia, the Marianas, and the Caribbean, but includes the southern tip of South America. Our proposed Pacific Ring of Fire includes the Marianas, an active subduction zone often neglected due to its distance from land. We do not, however, include the Caribbean - a small tectonic plate bordered by volcanoes and earthquakes; this system is also distinct from the Pacific and should not be lumped in.

There you have it - a first look at the Southeast Asian Ring of Fire. We are very interested in people’s perspectives on this topic. Is it a useful idea, or not? Are there other factors to consider, or better ways to express them? Any feedback is highly appreciated!

THE END… or is it?

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Bonus Figure

While writing this post, we came up with the idea of “unwrapping” the Southeast Asian Ring of Fire using a single long swath profile, but couldn’t decide where to put it in the main text (Fig. 12). So here it is, a bonus for the committed reader!

Figure 12: Map and along-strike profile of the Southeast Asian Ring of Fire.

This profile, running from Bangladesh to south Japan, shows off the lateral variability in the seismicity and volcanoes that arises from the complex geology of this region. Note how the “gaps” in volcanoes and seismicity generally coincide.

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We’re still not done!

Did you make it to the actual end? Guess what, we have news for you! Those two Rings of Fire are not the only source of tectonic hazards around the world!

There is also the Alpine-Himalayan collisional belt. (We would like to point out that Galadriel gave belts to Boromir and two hobbits as gifts - continuing the Ring theme of this post).

This collisional belt is more than 10,000 km long, reaching from north Africa and Spain in the west to India and China in the east. Because most of this collision occurs between continents (India, Arabia, and Africa in the south; Europe and Asia in the north), this belt exhibits relatively few volcanoes - there are only occasional subducting plates to carry water into the mantle. This lack of subduction also means that deformation occurs at shallow depth over a very broad area, in some places more than 2,000 km wide. A huge number of countries are impacted by this system, including parts of southern Europe, north Africa, the Middle East, and South and East Asia.

The complexity of plate deformation here also means that there are many different styles of faulting. For instance, the February 6, 2023 M7.6 and 7.8 earthquakes in Turkey and Syria occurred on strike-slip faults in the central part of the this belt, while the September 8, 2023 M6.8 earthquake in Morocco occurred near its western tip on a thrust fault. The Tibetan Plateau exists because of this collisional system; faults within and along the borders of the plateau regularly slip in earthquakes, including the May 12, 2008 M7.9 earthquake in Sichuan, China and the April 25, 2015 M7.8 earthquake in Nepal. As with the two Rings of Fire, differences in population, geography, and vulnerability are very important for assessing risk along this system.

But is there a simple term for this belt which can rival the tremendously successful “Ring of Fire”? We haven’t come up with one yet - but any suggestions are welcome!

THE END (really, for sure this time)

References:

Silva, V., Calderon, A., Caruso, M., Costa, C., Dabbeek, J., Hoyos, M.C., Karimzadeh, Z., Martins, L., Paul, N., Rao, A., Simionato, M., Yepes-Estrada, C., Crowley, H., Jaiswal K., 2023. Global Earthquake Model (GEM) Seismic Risk Map (version 2023.1), https://doi.org/10.5281/zenodo.8409623

Van Wyk de Vries, M., Bingham, R.G. and Hein, A.S., 2018. A new volcanic province: an inventory of subglacial volcanoes in West Antarctica. Geological Society, London, Special Publications, 461(1), pp.231-248., https://doi.org/10.1144/sp461.7.