Seismic and aseismic slip on the Central Peru megathrust

Journal name:
Nature
Volume:
465,
Pages:
78–81
Date published:
DOI:
doi:10.1038/nature09062
Received
Accepted

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40km, heterogeneous surface strain1, 2, 3, 4, 5, 6, 7, 8 suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods9, 10, 11. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude Mw = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50–70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with Mw = 8.0 in the Pisco area is estimated to be 250years.

At a glance

Figures

  1. Seismotectonic setting of the South Peru megathrust.
    Figure 1: Seismotectonic setting of the South Peru megathrust.

    Shown are co-seismic slip, aftershocks and postseismic displacements from the Mw = 8.0 Pisco earthquake in 2007. The focal mechanism shows the Global Centroid Moment Tensor solution (http://www.globalcmt.org/). The 2-m slip contour lines of the 2007 earthquake shown in cyan were derived from the joint analysis of InSAR, teleseismic and tsunami data13. Aftershocks (red dots) were located from the Instituto Geofísico del Perú (IGP) local seismic network. The rupture area of the Mw8.1, 1974 Lima earthquake was estimated from teleseismic data28, and that of the Mw7.7, 1996 Nazca earthquake was derived from the joint inversion of InSAR and teleseismic waveforms29. Grey vectors show the Nazca plate motion relative to South America14. Black vectors show the horizontal postseismic displacements between days 20 and 408 after the mainshock, and the blue vectors show the modelled displacements. The time series of displacements recorded at LAGU, with 2σ uncertainties, is shown in the upper inset. The continuous curve shows the theoretical displacements predicted from the afterslip model shown in Fig. 2. The box in the lower inset represents the area of the main figure panel.

  2. Fault slip derived from modelling of geodetic displacements between days 20 and 408 after the mainshock.
    Figure 2: Fault slip derived from modelling of geodetic displacements between days 20 and 408 after the mainshock.

    The model shown here assumes variable rake. Details regarding the weight put on smoothing and the three principal components selected in this model are given in the Supplementary Information. The 2-m slip contour lines of the 2007 earthquake are shown in cyan13. Pink contour lines show the density of aftershocks in the first month following the mainshock derived from the IGP catalogue. Insets show the slip at the centres of patches A and B, deduced from the inversion of geodetic measurements (blue circles). Red continuous lines show theoretical displacements predicted from rate-strengthening frictional sliding, assuming that frictional stress τ increases linearly with the logarithm of the sliding velocity , as observed in laboratory experiments30. According to this model, postseismic slip U(t) evolves as20 . For patch A, the best fitting parameters are tr2.1years and , assuming a value for the long-term velocity of the order of the convergence rate (that is, V0 = 62mmyr-1; ref. 14). For patch B, we found tr1.2years and , making the same assumptions as for patch A.

  3. Comparison of interseismic coupling with the rupture areas of recent large earthquakes.
    Figure 3: Comparison of interseismic coupling with the rupture areas of recent large earthquakes.

    Rupture areas of the large interplate earthquakes as in Fig. 1. Also shown is the pattern of interseismic coupling, defined as (1-Vi/Vpl), where Vi is the interseismic slip rate, derived from modelling of geodetic data collected between January 1993 and March 2001, all referenced to stable South America14, 15. Data (white vectors) were corrected for 5mmyr-1 of shortening across the Andes by least-squares adjustment of the Euler pole describing the long-term motion of the fore-arc with respect to South America. The rectangular fault model has a strike of 321° and dips 18° to the east. The inversion procedure is described in the Supplementary Information, and the modelled velocities are shown as light blue vectors. The small coupling near the trench may reflect the lack of resolution there, except in the north, where sea-bottom measurements are available15. This model shows that, on average over the study area, aseismic slip in the interseismic period accounts for about 38% to 59% of interplate slip at depths shallower than 40km (the average interseismic coupling is between 0.41 and 0.62).

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Author information

Affiliations

  1. Institut de Recherche pour le Développement, 44 Boulevard de Dunkerque, 13572 Marseille cedex 02, France

    • Hugo Perfettini,
    • Francis Bondoux,
    • Mohamed Chlieh,
    • Laurence Audin &
    • Pierre Soler
  2. Instituto Geofisico del Perú, Calle Badajos 169, Urb. Mayorazgo, Ate, Lima, Peru

    • Hugo Perfettini,
    • Hernando Tavera,
    • Francis Bondoux &
    • Laurence Audin
  3. Tectonics Observatory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • Jean-Philippe Avouac,
    • Andrew Kositsky &
    • Anthony Sladen
  4. GéoAzur, 250 Rue Albert Einstein, 06560 Valbonne, France

    • Jean-Mathieu Nocquet &
    • Mohamed Chlieh
  5. Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California 95064, USA

    • Daniel L. Farber
  6. Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph Fourier/CNRS/IRD/LCPC, Observatoire des Sciences de l'Univers de Grenoble, BP 53, 38041 Grenoble cedex 9, France

    • Hugo Perfettini &
    • Francis Bondoux
  7. Laboratoire des Mécanismes de Transfert en Géologie, Université Paul Sabatier/CNRS/IRD, Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France

    • Hugo Perfettini &
    • Laurence Audin
  8. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA

    • Daniel L. Farber
  9. Ashima Research, Pasadena, 600 S. Lake Ave., Pasadena, California 91106, USA

    • Andrew Kositsky

Contributions

H.P. edited the paper and did modelling and field work; J.-P.A. edited the paper and did modelling; H.T. handled the IGP aftershocks data; A.K. did modelling of postseismic deformation; J.-M.N. did the GPS processing; F.B. was in charge of the GPS network; M.C. did modelling of interseismic deformation; A.S. did modelling of the co-seismic deformation; L.A. did field work; D.L.F. did field work, and helped with editing the paper; P.S. helped with logistics.

Competing financial interests

The authors declare no competing financial interests.

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  1. Supplementary Information (1.2M)

    This file contains Supplementary Information and Data, Supplementary Tables 1-3, Supplementary Figures 1-5 with legends and References.

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