Earth science

Trouble under Tonga?

Earthquakes occur in cool, foundering tectonic plates deep within the Earth. But seismic data from the southwestern Pacific indicate that the minerals that make up the plates at depth don't behave as if they are cool.

Is there something wrong with our understanding of basic seismological features of Earth's mantle, the seismic discontinuities at depths of 410 km and 660 km? A report by Rigobert Tibi and Douglas A. Wiens, just published in the Journal of Geophysical Research1, provides cause for thought about this possibility.

First, however, some context. Earth's surface consists of a mosaic of rigid tectonic plates, many of which have a hard life. They form at rock-melting temperatures at mid-ocean ridges, sally across ocean basins and then sink out of sight in trenches, where they collide with another plate in a subduction zone. At the end of their life cycle, however, they leave a Cheshire-cat-like reminder of their former presence in the form of a sheet of earthquakes that extends, in many cases, from the surface down to about 700 km deep in the Earth. Here the grin of earthquakes fades.

The earthquakes in subducted plates happen because these plates are as much as 1,000 °C colder than their surroundings and are therefore brittle. They are, in essence, cold fingers stuck into the warm mantle and so provide a deep-Earth laboratory to probe phenomena that occur where the mantle is cooler than normal. One such phenomenon is the behaviour of the worldwide velocity jumps in seismic-wave speed at depths of 410 km and 660 km. For 30 years these discontinuities have been attributed to pressure-induced changes in mantle minerals2, and, as such, have predictable responses to changes in temperature in subduction zones3,4,5. Tibi and Wiens, however, conclude that they don't behave entirely as expected.

The problem is sketched in Fig. 1. A cold subducted plate should cause a mineralogical change related to the 410-km discontinuity (that of olivine to wadsleyite) to occur at lower pressure, and so shallower in the mantle. Conversely, the change related to the 660-km discontinuity, that of the higher-pressure form of wadsleyite, ringwoodite, to perovskite and magnesiowüstite, should occur at higher pressure, and so deeper. Careful analysis of subduction-zone earthquakes yields the depths of the ‘410’ and ‘660’ discontinuities from the timing of secondary signals arising from the reflection and conversion of seismic waves at the discontinuity boundaries.

Figure 1: Tibi and Wiens' observations1 of the northern Tonga subduction zone.
figure1

Earthquakes in the subducted slab emit two types of seismic wave, P (solid black lines) and S (dashed lines), which reflect off the 660-km discontinuity below the slab and travel back up to the surface to seismic stations on Fiji and Tonga. Upgoing S waves also interact with the 410-km discontinuity and convert to P waves. The time lags between the direct P waves from the quake (not shown for clarity) allow the depth of the discontinuities to be estimated. The ‘660’ behaves as expected. But the ‘410’ seems to be depressed (purple; dashed where not observed, but inferred), not raised (white), near the slab. A conjectural flow of hot mantle material might explain the depression here, and a possible region of metastable olivine inside the slab (yellow) might bend the 410 into a downwards wedge-shaped form, making it more difficult to observe. (Modified from Fig. 2 in ref. 1.)

Tibi and Wiens1 deployed portable seismographic stations on Fiji and Tonga, two islands that bracket the Tonga subduction zone in the southwestern Pacific, and looked for the signals from the 410 and 660 discontinuities in their data. This was a challenging project — observing conditions on ocean islands are extremely tough because of ocean-wave-generated seismic noise and problematic logistics. What Tibi and Wiens found, however, was that the 660 went down as expected but the 410 didn't go up everywhere, in part contradicting the mineralogical-change explanation for the discontinuities.

What is going on? Possibly two things. One, discussed by the investigators, is that hot mantle affected by the warm, ascending material that causes ‘hotspot’ volcanism in Samoa, nearby to the north, might be drawn southwards by the subduction beneath Tonga, so heating the vicinity of the slab. If it does, in my view there must be terrific thermal gradients in the slab, because parts of it are still cold enough to cause earthquakes. But this hot mantle might narrow the uplifted part of the 410 discontinuity, making it difficult to observe, and depress the 410 outside the slab. The observed 25-km depression corresponds to about 280 °C hotter temperatures5, not impossibly high for hotspots. Yet somehow the 660 behaves appropriately, suggesting that the thermal perturbation does not extend that deep or arose only recently, after the deeper parts of the slab had slipped past the present warm flow.

Alternatively, although Tibi and Wiens don't favour this explanation, the transition from olivine to wadsleyite could be hindered by the low temperatures in the slab, and be pushed downwards in the form of a metastable olivine wedge in the slab6,7. If so, not all subduction zones behave in this way, because studies8 of such a zone near Japan found the 410 raised to 350 km by cold slab temperatures.

Before giving up on the explanation that the 410-km discontinuity is caused by mineralogical change, it would be worth checking regional Tongan topography with underside reflections8,9. These are best investigated with observing stations that are at least 5,000 km distant, leading to reflection points that lie farther into the slab's interior because of the geometry. They are more sensitive to detecting topographic peaks than the methods used by Tibi and Wiens because reflection amplitudes remain strong for vertically travelling waves, whereas they decrease to zero for transmitted waves. This would settle the doubts about the dashed line in Fig. 1. On the other hand, the fading earthquake-limned grin of the hard-luck plate might be one of amusement at the head-scratching these observations are causing among Earth scientists.

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Helffrich, G. Trouble under Tonga?. Nature 436, 637–638 (2005). https://doi.org/10.1038/436637a

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