Tracking Down Strange Seismic Waves

As a rule, oceanic crust is subducted under the adjoining continent or under other oceanic crust at the major fault zones bordering and traversing the Pacific. Seismic waves generated by earthquakes and passing through the earth’s interior provide information about the underground structure of such regions, and with few exceptions they run according to specific patterns. Accordingly, the waves pass through an anisotropic region by splitting up and propagating at right angles to one another in two different directions and at two different velocities. In subduction zones, the fast component of shear waves alignes normally parallel to the trench. This is usually interpreted as due to flow of the mantle parallel to the trench in the region below the slab. However, numerical models show that the flow is perpendicular to the slab.

Cause identified

In his doctoral dissertation, Manuele Faccenda from the Institute for Geophysics at ETH Zurich has now studied the reason why the faster of the two seismic waves propagates in the known directions. Together with Luigi Burlini and Taras Gerya, Senior Lecturers at ETH Zurich, and David Mainprice of the University of Montpellier, he has now published the results of the study today in the scientific journal “Nature”.

Until now it was assumed that the cause of the splitting of the waves – the anisotropy – is situated in the earth’s mantle, but satisfactory replication of this in models was unsuccessful. Therefore the scientists have now included new aspects in their model. The study was based on seismic wave data already recorded. To this were added various parameters representing the conditions in the fault zone, e.g. the subduction angle and the age of the plates, their thickness and the orientation of the minerals to be expected in this crustal region. The expected water content of the minerals was also taken into account.

Hydrated plates as the decisive factor

In their model the researchers were now able to show that the source of the anisotropy lies in the subducting plate and not in the mantle. The computer simulation they developed reveals exactly what happens when an oceanic crustal plate subducts below a neighbouring plate. It is apparent that cracks form in it, depending on its age, depth, subduction angle and pressure. The minerals of the oceanic plate can be hydrated along these cracks by water penetrating down to a depth of 40 kilometres. This forms highly anisotropic platelet-shaped minerals aligned parallel to the cracks.

The seismic waves attain their highest velocity when they penetrate through these platelets longitudinally. In this case the fast seismic waves propagate parallel to the fault. The overall results of the study show that the orientation patterns of the seismic waves relative to the faults match the patterns observed around the Pacific well. The only case in which the researchers were unable to simulate the known pathway of the seismic waves by their model was for a quite young subduction zone in which the plate is still very flexible and subducts at only a gentle slope, and in which consequently scarcely any fractures and hydrated minerals occur. Faccenda explains that in this case probably the cause of the anisotropy really is to be found in the mantle.