An upside-down piece of hidden ocean crust has been revealed by mysterious seismic signals.
The seismic waves, which came from an underground 2010 magnitude 6.3 earthquake in Spain, led scientists to a subducted slab in the sea that had been completely overturned. The finding is detailed in a study published in The Seismic Record in February.
These seismic waves were first picked up by stations based in Spain and Morocco as part of the Program to Investigate Convective Alboran Sea System Overturn (PICASSO) project.
They were produced as part of an earthquake that occurred deep under Spain.
Upon closer look, scientists realized that coda waves—residual vibrations at the end of a seismogram—lasted a lot longer than is usual. And these provided clues into a very unexpected subsequent discovery under the Western Mediterranean: a slab that was completely overturned. These waves painted a picture of a slab that flipped over as it descended into the Earth's mantle, the study reported.
This study is the first one to establish that the slab was completely overturned rather than standing vertically or dipping, the authors wrote.
"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," study author Daoyuan Sun of the University of Science and Technology of China said in a summary of the findings. "Upon examination, we observed these strange arrivals, including the long coda and extra phase."
Sun, and fellow study author Meghan S. Miller at Australian National University, concluded that the unusually long seismic wave could have been due to a "low-velocity layer at the base of the subducting Alboran slab."
Seismic waves are absorbed by low-velocity layers. And these layers indicate to scientists what type of material the seismic waves have traveled through.
"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."
These findings could drastically help research into the Rif-Betic-Alboran region, the study reported, which is where Africa and Eurasia are converging. The study will also help shed insight into these deep earthquakes that remain fairly rare, the authors write.
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