Seismic slip on an upper-plate normal fault during a large subduction megathrust rupture

Journal name:
Nature Geoscience
Volume:
8,
Pages:
955–960
Year published:
DOI:
doi:10.1038/ngeo2585
Received
Accepted
Published online

Quantification of stress accumulation and release during subduction zone seismic cycles requires an understanding of the distribution of fault slip during earthquakes. Reconstructions of slip are typically constrained to a single, known fault plane. Yet, slip has been shown to occur on multiple faults within the subducting plate1 owing to stress triggering2, resulting in phenomena such as earthquake doublets3. However, rapid stress triggering from the plate interface to faults in the overriding plate has not been documented. Here we analyse seismic data from the magnitude 7.1 Araucania earthquake that occurred in the Chilean subduction zone in 2011. We find that the earthquake, which was reported as a single event in global moment tensor solutions4, 5, was instead composed of two ruptures on two separate faults. Within 12s a thrust earthquake on the plate interface triggered a second large rupture on a normal fault 30km away in the overriding plate. This configuration of partitioned rupture is consistent with normal-faulting mechanisms in the ensuing aftershock sequence. We conclude that plate interface rupture can trigger almost instantaneous slip in the overriding plate of a subduction zone. This shallow upper-plate rupture may be masked from teleseismic data, posing a challenge for real-time tsunami warning systems.

At a glance

Figures

  1. Location and single source solution.
    Figure 1: Location and single source solution.

    a, Location map. Stations used for CMT inversion are labelled with station codes. Other stations are only for hypocentre relocation (Supplementary Note 1). Shading indicates rupture areas of great earthquakes in 1960 (ref. 12) and 2010 (ref. 6). Inset: Regional tectonic setting. b, Double-couple percentage (%DC) and variance reduction (VR) of the single point-source versus frequency. A transition occurs at 0.057Hz, where VR suddenly decreases because the waveforms cannot be explained by a single source alone. This change is illustrated by representative waveforms at low and high frequencies (see Supplementary Figs 2 and 3 for details).

  2. Two-point-source solution.
    Figure 2: Two-point-source solution.

    a, Observed (black) and synthetic (red) waveforms for the optimum high-frequency (0.02–0.08Hz) solution. Station names are labelled. Numbers alongside each waveform component denote VR. Blue and green shading denotes the contribution from each event. b, Waveform correlation for each event as a function of trial point-source position (numbered). The optimum time shift of Event I and II is shown. Black beach balls are solutions that lie within 90% of the optimum solutions (red beach ball) VR. The red star denotes the earthquakes epicentre. c, Resulting moment-rate function obtained using the NNLS method.

  3. Aftershock analysis.
    Figure 3: Aftershock analysis.

    a,b, Map (a) and cross-section (b) showing locations and focal mechanisms of aftershocks (Groups A and B) and mainshock events (labelled EV-I and EV-II). Faulting style is classified on principal stress orientations25 and minimum rotation angle with respect to plate interface thrust faulting26, accounting for plate interface geometry (black line)15, 16. We plot the revised location of Event II, based on 3D waveform modelling. Mapped faults are shown10, 27; MVFZ, Mocha–Villarrica fault zone. The cross-section background is from P-wave velocity tomography models15, 16. The star denotes the hypocentre of the Araucania earthquake; the triangle shows the coastline.

  4. Schematic interpretation of the Araucania earthquake rupture.
    Figure 4: Schematic interpretation of the Araucania earthquake rupture.

    Plate interface thrusting (Event I) triggered a rupture along an extensional fault in the overriding plate (Event II). It is likely that two great earthquakes in 1960 and 2010 brought both faults closer to failure. As shown by ancient submarine landslide deposits in the area, a larger-scale rupture in the overriding plate has the potential to act as a tsunamigenic earthquake. Beach balls represent the focal mechanisms of both events from Fig. 3. The inset shows the interpreted structure of conjugate normal faulting with the background colour representing the vp/vs ratio15.

References

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Affiliations

  1. Liverpool Earth Observatory, University of Liverpool, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool L69 3GP, UK

    • Stephen P. Hicks &
    • Andreas Rietbrock

Contributions

S.P.H. carried out the single and multiple point-source inversions, as well as the moment tensor inversion and aftershock relocations. S.P.H. wrote the manuscript, interpreted the results, and generated all figures. A.R. carried out the 3D full waveform simulations, wrote the manuscript, and interpreted the results.

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The authors declare no competing financial interests.

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