Tectoplot

[Renita Lukins]

OnAug. 12, 2021, the U.S. Geological Survey reported that a magnitude-7.5 earthquake had occurred in the South Sandwich Islands in the south Atlantic Ocean, more than 2,500 kilometers east of the southern tip of South America (Figure 1). This event led to a small flurry of interest from earthquake scientists, but was generally considered to be low risk with respect to both ground shaking and tsunami hazards because of both its distance from population centers and depth of 60 kilometers.

Figure 1: (Top) Regional setting of the Sandwich Plate, where the August 12, 2021 magnitude-7.5 and magnitude-8.1 earthquakes occurred. Arrows show relative plate velocities at block boundaries. White box indicates extents of maps in Figures 2 and 3. Lower inset shows map location on a lower hemisphere projection of the globe. Map produced using tectoplot (Bradley, 2021) with assistance from Bryan Low Kai Sheng.

Although this magnitude-8.1 earthquake still did not cause damage, a tsunami should have been a much greater concern. However, by the time this earthquake was discovered, the tsunami had already propagated through both the Atlantic and Indian Oceans, a process that took nearly a day. The tsunami was detected at tide gauges as far away as the Azores Islands in the North Atlantic (20 centimeters, 14 hours after the rupture) and eastern Madagascar (6 centimeters, 14 hours after the rupture) (personal communication with Amir Salaree). The tsunami was impressive for its wide yet nondestructive range.

The overlapping seismic waves have made it difficult for seismologists to accurately assess the earthquake. Most critically, the reported depth and mechanism of the event(s) vary widely according to different organizations that monitor earthquakes. Understanding this variability and why it matters requires an examination of the tectonic and geographic setting.

The earthquake occurred on the 700-kilometer-long bow-shaped South Sandwich Subduction Zone (Figures 1 and 2). Here, the South American Plate is subducting westward at a rate of about seven centimeters per year below the Sandwich Plate. The Sandwich Plate is a small piece of young oceanic crust, forming only 300 kilometers to the west at the East Scotia Ridge. On its southern edge, the Sandwich Plate borders the Antarctic Plate across a short transform fault. The plate is marked by a series of regular islands, formed along the volcanic arc of the subduction zone; these volcanic islands (the South Sandwich Islands) give the region its name. They are remote and inhospitable, with no native population, and parts of the islands are permanently covered with ice. This setting helps to explain the lack of nearby seismic or geodetic observations: There are no nearby seismic or GNSS stations and very limited potential for satellite data due to the lack of exposed land.

Figure 2: (Top) Map of the tectonic setting with variously reported focal mechanisms (beachballs), which illustrate either the interpreted initiation point of the earthquake (origin method) or the interpreted center of moment release (centroid method). Colored contours represent depth of slab2.0 (Hayes, 2018). (Bottom) The five profiles are aligned along the 40-km depth contour of the slab, and show that the reported locations broadly vary from above to below the slab, and from deep to shallow. Map produced using tectoplot (Bradley, 2021).

A global map of subduction zones has been generated on the basis of both earthquakes and tomographic images of the mantle, which map the temperature of the mantle (subducting slabs are colder than the surrounding mantle rock). This map, called slab2.0, provides a baseline for understanding the relationship of new earthquakes to the overall setting (Hayes, 2018).

More careful investigation of the seismic records reveals that the already complex story is even more complicated. In addition to reporting the location where the earthquake started (the hypocenter), the USGS also reports depths for the event based on alternate methods that utilize the direction of movement of the rocks on either side of the fault. USGS reported highly variable depths for this event, ranging from 26.5 kilometers based on one method (centroid) to 70.5 kilometers based on another method (W-phase), which is preferred by USGS. As a further complication, other automatic earthquake catalogs report depths of 20 kilometers (reported by GCMT) and 10 kilometers (reported by GEOFON) (Figure 2). As of this writing, several weeks after the event, USGS notes on its event page that the location, depth, mechanism, and magnitude are all preliminary pending further investigation.

It therefore remains unclear if the mainshock was a megathrust event, accommodating slip between the South American and Sandwich plates, or an intraslab event, associated with break-up of the subducting slab.

The orientation of the fault as reported by seismic methods might provide a clue. For a megathrust event, the fault orientation should match the megathrust, whereas for an intraslab earthquake, a steeper rupture surface would be more common. However, reported fault orientations are also variable: GCMT and GFZ report that the fault is tilted either 14 degrees or 11 degrees from horizontal, respectively, generally matching the megathrust, whereas USGS reports variable dips, either 17 degrees in the opposite direction, or 43-54 degrees, depending on the method.

The South Sandwich Islands are inaccessible by ship part of the year due to sea ice, as shown by this satellite image. The inhospitable islands are a challenging place to make seismic measurements. Credit: LANCE/EOSDIS Rapid Response, NASA

Figure 3: (Top) Map of the tectonic setting with the earthquake sequence. The magnitude-7.5 foreshock, magnitude-8.1 mainshock, and aftershocks are shown, with color indicating reported depth. (Bottom) Profiles A and B illustrate the depth distribution of seismicity; background highlights subduction of oceanic lithosphere. Seismicity in the outer rise may be a clue to the slip pattern in this otherwise enigmatic event. Colored contours represent depth of slab2.0 (Hayes, 2018). Map produced using tectoplot (Bradley, 2021).

Notably, intraslab earthquakes tend to produce relatively few aftershocks (e.g., Gomberg and Bodin, 2021). Given the productivity of the aftershock sequence in the South Sandwich Islands, it seems unlikely to have been an intraslab event, despite the reported depths by USGS. The aftershocks suggest that the magnitude-8.1 event ruptured the megathrust, and therefore that the hypocenter depth reported by the USGS is wrong.

We can extend this fingerprinting further. Megathrust earthquakes can either occur on a confined patch of the plate boundary at depth or can rupture the shallow region all the way to the trench. Trench rupture carries a characteristic signal of seismic activation in the outer rise (Sladen and Trevisan, 2018). Notably, this signal seems to be largely absent in deeper ruptures. The South Sandwich Islands earthquake caused significant aftershock activity in the outer rise, which spread across 250 kilometers (Figure 3). This seismicity suggests not only that the earthquake ruptured all the way to the trench, but also that this shallow slip region was at least 250 kilometers long. The density of seismicity has been shown to correlate with the amount of slip at the trench (Sladen and Trevisan, 2018), indicating that it may even be possible to map out the relative amount of slip at the trench using this dataset.

This approach is qualitative, but the seismic, tsunami and gravity data can be used to test the hypothesis of trench rupture, looking for higher tsunami generation near the trench, slow rupture propagation or other signals associated with this type of earthquake.

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