Anatomie einer Subduktionszone vom Ozeanischen Mantel bis zur Kruste: Seismizität und Seismische Strukturen in Norchile

• Antragsteller: Dr. Jörn Kummerow; Dr. Bernd Schurr; Professor Dr. Serge A. Shapiro
• Fachliche Zuordnung: Physik des Erdkörpers
• Förderung: Förderung von 2015 bis 2021
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 275837588

• Erstellungsjahr: 2020

Zusammenfassung der Projektergebnisse

We achieved the principal aims of our project, i.e. to obtain improved images of seismic structures in a subduction regime and to assess if major seismic signatures are fluid controlled. Our most important new findings are: • An extensive, high-quality earthquake catalog of the northern Chile subduction zone forearc containing 101, 601 double-difference relocated earthquake hypocenters was produced; the catalogue results from the analysis of 8 years of continuous seismic data (2007–2014) from the IPOC network. We identify a clear separation of seismicity into three distinct planes. The uppermost plane corresponds to the plate interface, which is observed to terminate downdip at a depth of 50 − 55 km. The other two planes, located about 7 and 26 km below the slab surface, dip at a constant angle of about 20◦ until they are absorbed by a 25 km thick highly active cluster of intermediate-depth seismicity at depths of 80−120 km. Downdip of this cluster, the slab steepens and lower plate seismicity is generally sparser and absent in the northern part of the study area. The catalogue is publicly available. • The detailed vp /vs image of the subduction zone at around 21◦ S features an exceptionally high value of vp /vs > 2.0 within the oceanic mantle. This anomaly motivated us to perform thermodynamic and poro-elastic modeling, which facilitates- compared to most previous studies- a profound analysis of the fluid distribution based on observed seismological signatures. The modeling implicates that the Nazca plate is partially hydrated and that the medium is close to the percolation threshold. This means that the pores are interconnected and provide an effective drainage system. For a postulated vein-like structure, which is consistent with laboratory experiments and rock outcrops, a relatively small porosity value in the order of 10^−3 is sufficient. • The study of source mechanisms reveals a clear segmentation of the stress field in the subducting Nazca slab. We find a sharp termination of the coupling zone, which is characterized by compressional stresses. Beyond and below this zone slab pull is the dominant source of observed stress. Near the slab surface, we also find signatures of the activation of inherited structures. Deeper in the slab, fault orientations are more likely controlled by the stress field alone. Along the subduction pathway, we find indication for an increase of the absolute slab pull component of the stress field that correlates with an increase in event rate and the occurrence possibility of M > 7 events. • The intermediate-depth seismicity cluster exhibits consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab. This can be explained with strong slab pull, which is evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster. Moreover, events in the cluster show a very weak aftershock productivity and a high background event rate, which leads to a temporal distribution of seismicity that is close to a purely random process. Wasja Bloch was one of three award winners for the best young author oral presentation at the DGG meetings in 2015 and 2016.Our work is already of relevance for the seismological and geoscience community, and in particular the new earthquake catalog provides the basis for subsequent studies, e.g. on statistical properties of subduction-related seismicity. In the frame of this project, we further developed our processing algorithms for event detection and event association. We developed a Python wrapper for the HASH code and Python scripts for stress drop estimation. These tools can be easily adapted and applied in comparable studies of different seismological data sets.

Projektbezogene Publikationen (Auswahl)

• (2016): Seismologische Messung und petrophysikalische Modellierung des P-zu-S-Wellen-Geschwindigkeitsverhältnisses im Mantel einer subduzierenden Platte, Mitteilungen / Deutsche Geophysikalische Gesellschaft, 3, 5-8
  W. Bloch, Kummerow, J., Wigger, P., Salazar, P., Kruger, O. S., Shapiro, S.
  
• (FU Berlin, 2017): In-situ Properties of the Subducting Nazca Slab: Constraints on the Deep Water Cycle and the Dynamics of Subduction from Seismological Observations (In-situ Eigenschaften der subduzierenden Nazca Platte: Rahmenbedingungen fur den tiefen Wasserkreislauf und die Dynamik der Subduktion aus seismologischen Beobachtungen)
  Wasja Bloch
  
• (2018): Catalogue of Earthquake Hypocenters for Northern Chile Compiled from IPOC (plus auxiliary) seismic stations
  C. Sippl, Schurr, B., Asch, G., Kummerow, J.
  
• (2018): From Slab Coupling to Slab Pull: Stress Segmentation in the Subducting Nazca Plate, Geophysical Research Letters, 45, 5407– 5416
  W. Bloch, B. Schurr, J. Kummerow, P. Salazar, and S. A. Shapiro
  
• (2018): Seismicity structure of the Northern Chile forearc from > 100, 000 double-difference relocated hypocenters, Journal of Geophysical Research, 123,
  C. Sippl, B. Schurr, G. Asch, and J. Kummerow
  
• (2018): Watching Dehydration: Seismic Indication for Transient Fluid Pathways in the Oceanic Mantle of the Subducting Nazca Slab, Geochemistry, Geophysics, Geosystems, 19, 3189– 3207
  W. Bloch, T. John, J. Kummerow, P. Salazar, O. S. Kruger, and S. A. Shapiro
  
• (2019): Filling the gap in a double seismic zone: Intraslab seismicity in Northern Chile, Lithos, 346–347, 105155
  C. Sippl, B. Schurr, T. John, and S. Hainzl
  
• (2019): Linear Relationship Between Aftershock Productivity and Seismic Coupling in the Northern Chile Subduction Zone, Journal of Geophysical Research: Solid Earth 124, 8726-8738
  S. Hainzl, C. Sippl, B. Schurr
  
• (2019): Probing the northern Chile megathrust with seismicity: the 2014 M8.1 Iquique earthquake sequence, Journal of Geophysical Research: Solid Earth, 124, 12935– 12954
  H. Soto, C. Sippl, B. Schurr, J. Kummerow, G. Asch, F. Tilmann, et al
  
• (2020): Stress Drop Mapping in the Northern Chilean Subduction Zone, Geophysical Research Abstracts, Vol. 22, EGU2020-5019
  J. Folesky, Kummerow, J. and Shapiro, S. A.