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Aquaplaning in the geological underground

'Chilean megathrust earthquake recurrence linked to frictional contrast at depth' DOI 10.1038/s41561-018-0089-5 (Credit: M. Moreno; Nature Geoscience / SPRINGER NATURE)
'Chilean megathrust earthquake recurrence linked to frictional contrast at depth', Nature Science, DOI 10.1038/s41561-018-0089-5 (Credit: M. Moreno; Nature Geoscience / SPRINGER NATURE)
Links: Das Erdbeben von 2016 hat die südliche Hälfte der Bruchzone des Ereignisses von 1960 beeinflusst. Weißgraue Linien zeigen die Bruchzone des Erdbebens von 1960 und der rote Stern das Epizentrum des Events 2016. Rechts: Verteilung der Verwerfungszone, Horizontale und vertikale koseismische GPS-Verschiebungen werden durch blaue bzw. grüne Pfeile angezeigt. Die Abbildungen stammen aus dem Paper 'Chilean megathrust earthquake recurrence linked to frictional contrast at depth', welches in Nature Geosciences erschien. DOI 10.1038/s41561-018-0089-5 (Credit: M. Moreno; Nature Geoscience / SPRINGER NATURE) Link: https://www.nature.com/articles/s41561-018-0089-5
The stronger shallower segment strains and stresses the deeper segment of the seismogenic zone. The deeper segment is weaker and fails at lower stress build-up in 2016-class earthquakes. Higher shear strength along the shallow segment allows longer periods (>110 yr) of stress build-up, the release of which may result in great 1960-class earthquakes that can also extend into the deeper segment. Interseismic GPS velocities indicate high crustal shortening above the deeper section of the seismogenic zone (right inset). Pore fluid pressure gradients along the plate interface are related to the geological configuration of the margin, and exert a mechanical control on the strength segmentation and timing of stress release in the seismogenic zone (left inset). 'Chilean megathrust earthquake recurrence linked to frictional contrast at depth', Nature Science, DOI 10.1038/s41561-018-0089-5 (Credit: M. Moreno; Nature Geoscience / SPRINGER NATURE)

The 2016 Mw 7.6 earthquake of Southern Chile was the first large earthquake to occur within the rupture bounds of the great 1960 Mw 9.5 Valdivia earthquake, the largest ever observed in historical times. Using GPS, InSAR, gravity, seismic reflection, and geological data, Marcos Moreno and colleagues from GFZ as well as Chile show that the 2016 earthquake occurred at the deep boundary of a persistent asperity on the interface between the subducting Nazca and overriding South American plates, where both plates are coupled and not sliding past each other in spite of the high convergence velocity of 68 mm/year. This asperity broke during the 1960 Chile earthquake b has since healed and recovered. Their study, published in Nature Geoscience, presents a mechanical model in which the highest stresses gradually accumulate at the deeper edge of such an asperity. The 2016 event released these high stresses. Depending on the frictional parameters of the asperity and the deeper segment of the plate interface, the model predicts the failure times of the deeper and shallower portions of the interface.

According to this model, the shallower failure is representative of a great event (1960-class) and the deeper event represents a large earthquake (2016-class). Given the lag time of 56 years since the 1960 event, the model suggests that the pressure of fluid (i.e. largely water) at the plate interface zone is close to lithostatic at the deeper interface and is slightly lower at the shallower interface. If the water pressure at the plate interface zone becomes as high as the pressure of the overlying rock column, the strength of the rocks at the plate interface becomes practically zero – an effect akin to aquaplaning will initiate eventually triggering an earthquake. It is proposed that the development of this modelling strategy could enable the estimation of critical failure thresholds for other mapped subduction asperities where subducting and overriding plates are currently locked.

Original study: Moreno, M., Melnick, S. Li, D. . Bedford, J. R. , Baez, J. C. ., Motagh, M., Metzger, S., Vajedian, S., Sippl, C., Gutknecht, B.D., Contreras-Reyes, E., Deng, Z., Tassara, A., Oncken, O., 2018. Chilean megathrust earthquake recurrence linked to frictional contrast at depth. Nature Geoscience. DOI: 10.1038/s41561-018-0089-5

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