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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - AspSync (Unravelling mechanisms controlling asperities synchronization and triggering mega-earthquakes: insights from analog experiments and subduction zone earthquake statistics)

Teaser

Summary

AspSync focused on one of the most dangerous geo-hazard: subduction megathrust earthquakes. The short instrumental seismic record and the multi-scale, multi-parameter influence complicate our understanding of the origin and loci where those earthquakes may occur in the future. A better understanding of the origins of subduction megathrust earthquakes is of primary importance for both scientific and societal reasons. AspSync aimed at improving our knowledge of subduction megathrust earthquakes using a multidisciplinary approach that featured laboratory experiments as well as statistical analysis of natural subduction zones seismicity. AspSync investigated the following scientific problems:

A. Asperities Synchronization: According to the â€œAsperity Modelâ€, earthquakes magnitude depends on the size and spatial distribution of asperities. When two or more neighboring asperities fail during the same earthquake, the rupture area becomes bigger and give rise to a larger magnitude event. Understanding the physical conditions that lead to synchronization is debated and critical for seismic hazard assessment.

B. Multi-parameter influence on subduction megathrust earthquakes: Subduction zones are complex systems where multiple, intercorrelated geodynamic parameters (e.g., subduction velocity) affect the seismicity simultaneously. Understanding which and how such parameters tune megathrusts seismicity would be pivotal for hazard assessment, because it will help identifying which subduction zone is prone for hosting mega-earthquakes.

Work performed

AspSync included two work packages: WP1 analog modeling; WP2 statistics of natural subduction zones.

WP1A) 14 experiments, where the along-strike length of the barrier and of the asperities are varied systematically in the 0â€“10 cm range and in the 6â€“17 cm range respectively, have been realized. When scaled to nature, these values are well in the range of geodetically observed asperities. The experiments featured a frictionally segmented megathrust embedding two asperities with equal geometry and friction, separated by a barrier. Each experiment produced >80 analog earthquakes characterized by single asperities ruptures and/or asperities synchronization (i.e., both asperities fail during the same earthquake) depending on the boundary conditions.

WP1B) 12 experiments have been realized where the subduction velocity and seismogenic zone width have been varied systematically. With respect to the experiments of WP1A, here the models had no lateral frictional segmentation. Subduction velocity and width of the seismogenic zone have been proposed to exert a key role on interplate seismicity for their first-order control on deformation rate and coupling area between plates, respectively. AspSync investigated how the two selected parameters tune maximum magnitude, seismicity rate and moment release rate.

WP2) One of the most challenging questions for seismologist is which physical parameter, if any, tunes Mmax. AspSync updated an existing database including physical, geodynamical and seismological parameters of worldwide subduction zones and performed multivariate statistics to identify the key controlling factors tuning Mmax. The need of multivariate statistics represented a step forward in this field as the majority of previous studies were based on bivariate statistics. AspSync used the Pattern Recognition algorithm.

OVERVIEW OF THE RESULTS

Experiments focusing on asperities synchronization highlighted the importance of the barrier-to-asperity length ratio Db/Da in tuning megathrust seismicity. In particular, AspSyncâ€™s results highlighted that Db/Da displays a negative correlation with maximum magnitude and seismicity rate. Db/Da controls also the process of asperities synchronization. When Db/Da is < 0.5, asperities synchronization has been observed in the models and the percentage of events with synchronized asperities rupture is inversely proportional to Db/Da. AspSync also shed light on how the process of asperities synchronization takes place. In particular, AspSync imaged a sequence of cracks activating at different times, allowing the rupture to grow laterally.

AspSyncâ€™s experiments highlighted that Mmax is mainly controlled by the width of the seismogenic zone as the rupture potential increases with this parameter. No significant correlation between Mmax and subduction velocity has been observed in AspSyncâ€™s models as well as in natural subduction zones. Subduction velocity rather controls seismicity rate, in particular a high subduction velocity is associated to high seismicity rate. Moment release rate is controlled by both subduction velocity and width of the seismogenic zone. Consequently, the fastest subduction zones with medium to large seismogenic zone widths are expected to have the largest moment release rates.

The statistical analyses of AspSync highlighted the major role of length of the trench and the thickness of subducting sediments, the parameters concurring to enhance long ruptures in the trench-parallel direction. In particular, AspSync verified by means of Monte Carlo simulations that the occurrence of magnitude >8.5 earthquakes along the longest subduction zones characterized by a relatively high sediment supply does not appear to be random. A smooth or smoothened megathrust combined with a long subduction zone enhance the conditions for large trench-parallel extent of the rupture and, in turn, higher earthquake magnitudes.

EXPLOITATION AND DISSEMINATION

AspSyncsâ€™ results have been su

Final results

AspSync improved the existing apparatus for modeling subduction earthquakes adding to it a frictionally heterogeneous megathrust. Additional effort has been requested for the analysis of the experiments. AspSync developed a new routine that allows converting Particle Image Velocimetry data into geodetically and seismologically equivalent parameters. Regarding the analysis of natural subduction zones seismicity the novelty has been analyzing with multi-parameter statistics the most complete subduction zones database.

IMPACT

The identification of the most important parameters that controlled the last century of subduction megathrust seismicity is one of the most important outcome of AspSync. It may help to highlight subduction zones whose seismic potential is higher with respect to others. At the same time AspSync revealed that asperities size to distance ratio Db/Da exert a primary role for the synchronization process. AspSync suggested that since Db/Da is a relatively easily accessible parameter in nature given our increasing knowledge of the distribution of asperities and barriers from either short- or long-term observations, future studies should take it into account when investigating the seismic potential of subduction zones.