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Prolonged Canterbury earthquake sequence linked to widespread weakening of strong crust

Nature Geoscience volume 7, pages 34–37 (2014)Cite this article

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Abstract

The Canterbury earthquake sequence in New Zealand was characterized by high-energy earthquakes1, a complex pattern of faulting and an extended series of aftershocks2. The mainshock occurred in September 2010, yet the largest aftershock occurred 172 days later. Beneath the Canterbury region, the Hikurangi Plateau—a large igneous province that was subducted about 100 Myr ago3—remains attached to the crust, welded to greywacke rocks that were fractured during the earthquakes. Here we use three-dimensional seismic tomographic data to investigate the role this crustal structure played in the unusual Canterbury earthquake sequence. We identify a broad, 5-km-wide region centred beneath the surface break and coincident with the depth of maximum fault slip during the mainshock that is characterized by unusually low seismic P- to S-wave velocity ratios of 1.60. Yet before the mainshock this region exhibited ratios4 of 1.71. We interpret the reduced velocity ratios as the signature of greywacke rocks that have been weakened by the fresh rupture front producing widespread cracking around the fault zone. We suggest that recovery of rock strength between the mainshock and largest aftershock could explain the long delay between the two events. In contrast to a common assumption in aftershock forecasts that crustal strength is constant, we conclude that energetic earthquakes can lead to widespread changes in the strength of Earth’s crust.

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Figure 1: The Canterbury earthquake sequence.
Figure 2: A depth section of the P-wave velocity model normal to the plate boundary and through Lyttelton volcano.
Figure 3: Maps of Vp/Vs at 3 km depth.
Figure 4: Cumulative number of aftershocks with magnitude ≥3.0 with time (blue line).

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Acknowledgements

This paper was improved by reviews by B. Fry and S. Ellis. GeoNet (www.geonet.org), financially supported by the New Zealand Earthquake Commission, provided the earthquake data used in the study. Research was supported by the Marsden Fund administerd by the Royal Society of New Zealand and by the Natural Hazards Research Platform of the New Zealand Ministry of Business, Employment and Innovation.

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Authors and Affiliations

  1. GNS Science, Lower Hutt 5040, PO Box 30368, New Zealand

    Martin Reyners & Donna Eberhart-Phillips

  2. Department of Geology, University of California Davis, Davis, California 95616, USA

    Donna Eberhart-Phillips

  3. School of Geography, Environment, and Earth Sciences, Victoria University of Wellington, Wellington 6140, PO Box 600, New Zealand

    Stacey Martin

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Contributions

M.R. helped to co-ordinate the deployment of portable seismographs after the earthquakes and led the analysis of the resulting earthquake data. D.E-P. led the tomographic inversions. S.M. assisted with earthquake analysis. M.R. wrote the manuscript, with contributions from all authors.

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Correspondence to Martin Reyners.

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Reyners, M., Eberhart-Phillips, D. & Martin, S. Prolonged Canterbury earthquake sequence linked to widespread weakening of strong crust. Nature Geosci 7, 34–37 (2014). https://doi.org/10.1038/ngeo2013

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