Mysterious seismic activity reveals an upside-down section of ocean crust

Mysterious seismic waves revealed a completely overturned piece of hidden ocean crust in Spain.

In 2010, seismic waves from an underground 6.3 magnitude earthquake led researchers to a slab in the sea that was upside down, Newsweek reported.

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According to the findings published inThe Seismic Record, stations based in Spain and Morocco picked these seismic waves as a part of the Program to Investigate Convective Alboran Sea System Overturn (PICASSO). An earthquake that occurred deep under Spain caused these seismic waves.

However, a closer look at the coda waves, or the residual vibrations at the seismogram's end, revealed that these waves lasted much longer than usual.

These mysterious seismic waves led to the discovery of an overturned slab in the Western Mediterranean. The study reported that the waves flipped over the rock as it fell into the Earth's mantle.

According to the authors, this is the first study that shows that the slab was entirely toppled rather than standing vertically or dipping.

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Study author Daoyuan Sun of the University of Science and Technology of China said, "Initially, we were not aiming to better understand the deep earthquake mechanisms, as several prior studies have studied the source nicely. Our intent was merely to plot the waveforms out of curiosity, since there is so much to learn from individual waveforms when one takes the time to look at them closely. Upon examination, we observed these strange arrivals, including the long coda and extra phase."

Low-velocity layers absorb seismic waves. Scientists use these layers to find what kind of material the seismic waves have travelled through.

Sun and his fellow author, Meghan S Miller of the Australian National University, concluded that a "low-velocity layer at the base of the subducting Alboran slab" would have caused the unusually long seismic wave.

"Here, through modeling the detailed waveforms, we are able to image the low-velocity layer underneath the slab surface dipping to the northeast, unlike a normal subducted slab with a low-velocity layer on top of the slab surface," Sun said. "This strange occurrence between the slab and low-velocity layer suggests the occurrence of the overturned Alboran slab," he added.

The study noted that this discovery could help researchers learn more about the Rif-Betic-Alboran region, where Africa and Eurasia are converging. The authors wrote that the discovery will help them understand these rare, deep earthquakes.

(With inputs from agencies)