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Strange movement of oceanic plate in Spain detected

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An unusual earthquake measuring 6.3 on the Richter scale occurred beneath Granada (Spain) in April 2010, emitting strange seismic waves picked up by monitoring stations in Spain and Morocco.

Researchers now believe that the unusual seismic signals were caused by a large piece of oceanic crust sinking into the Earth's mantle and flipping over completely in the process.

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Study captures unusual movement of oceanic plate

  • The study was led by Daoyuan Sun of the University of Science and Technology of China and Meghan S. Miller of the Australian National University;
  • He revealed that seismic data from the earthquake indicate that a subducted oceanic plate rapidly descended into the Earth's mantle and flipped over;
  • This means that the water it carried on its surface was now beneath it;
  • Although these findings seem strange, it is hoped that they will actually help researchers better understand the complicated tectonic structure of the point in the western Mediterranean basin where Africa and Eurasia converge.

Initially, we did not intend to better understand the mechanisms of deep earthquakes, as several previous studies have studied the source well. Our intention was to just plot the waveforms out of curiosity, since there is a lot to learn from individual waveforms when it comes time to observe them up close. Upon examination, we note these strange arrivals, including the long coda and the extra phase.

Daoyuan Sun of the University of Science and Technology of China in a statement

Understanding deep earthquakes

According to the IFLSciencein particular, the research is expected to shed new light on the region called Rif-Betic-Alboran, which contains an arc formed by the Betic mountain chain in Spain and the Rif mountain chain in Morocco, in addition to the basin of Alboran Sea, east of the Strait of Gibraltar.

Furthermore, the study could contribute to our understanding of the mechanisms behind rare deep earthquakes that sometimes occur more than 600 kilometers below the region. The researchers observed that earthquake coda waves – the signature of residual vibrations at the end of a seismogram – lasted an unusually long time after the 2010 earthquake. These vibrations were picked up by monitoring stations in Morocco.

There were also signs of what they refer to as an extra late phase of primary P waves – pressure waves that travel faster than other waves through the earth and are therefore detected first – that followed the normal P waves picked up by the stations. in Spain.

Overturned oceanic plate

The team concluded that the long duration of the coda waves and the extra phase of the P waves can be explained by a low-velocity layer at the base of the Alboran plate.

Low-velocity layers, which slow and absorb seismic waves, often suggest that the waves passed through molten or liquid material. Subducted plates may contain a low-velocity layer on their surface, originating from the water they take to the mantle.

The low-velocity layer may also explain the deep earthquakes around Spain. This is because it indicates the presence of hydrated magnesium silicates, which contain water at depths of 600 kilometers. Once dehydrated, silicates become brittle to such an extent that they can lead to deep earthquakes.

Here, through modeling the detailed waveforms, we are able to visualize the low-velocity layer below the slab surface dipping northeast, as opposed to a normal subducted slab with a low-velocity layer on top of the slab surface. This strange occurrence between the slab and the low-velocity layer suggests the occurrence of the overturned Alboran slab.

Daoyuan Sun of the University of Science and Technology of China in a statement

Conclusions and next steps

Hydrated silicates can also provide seismologists with information about the speed of plate subduction in the region. The hydrated magnesium silicates mean that “a significant amount of water was carried into the mantle transition zone, indicating a relatively cool slab,” Sun noted. It is believed that this study may provide a reasonable lower limit for the speed of subduction in this region. region.

Considering the relatively young age of the seafloor in the Western Mediterranean, for the slab to remain cool, the subduction speed must be quite fast, such as a moderate speed of about 70 millimeters per year. In other words, we think that our study could provide a reasonable lower limit on the subduction velocity in this region.

Daoyuan Sun of the University of Science and Technology of China in a statement

The team believes it would be useful to investigate the shapes of seismic waves produced by deep earthquakes in other locations, such as northeastern China, South America, the Sunda-Banda, and places like the Fiji-Tonga region. This would help identify whether they experience similar mechanisms.

The study was published in Seismic Record.



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