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Possible control of subduction zone slow-earthquake periodicity by silica enrichment

Nature volume 510, pages 389–392 (2014)Cite this article

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Abstract

Seismic and geodetic observations in subduction zone forearcs indicate that slow earthquakes, including episodic tremor and slip, recur at intervals of less than six months to more than two years1,2. In Cascadia, slow slip is segmented along strike3 and tremor data show a gradation from large, infrequent slip episodes to small, frequent slip events with increasing depth of the plate interface4. Observations5,6,7 and models8,9 of slow slip and tremor require the presence of near-lithostatic pore-fluid pressures in slow-earthquake source regions; however, direct evidence of factors controlling the variability in recurrence times is elusive. Here we compile seismic data from subduction zone forearcs exhibiting recurring slow earthquakes and show that the average ratio of compressional (P)-wave velocity to shear (S)-wave velocity (vP/vS) of the overlying forearc crust ranges between 1.6 and 2.0 and is linearly related to the average recurrence time of slow earthquakes. In northern Cascadia, forearc vP/vS values decrease with increasing depth of the plate interface and with decreasing tremor-episode recurrence intervals. Low vP/vS values require a large addition of quartz in a mostly mafic forearc environment10,11. We propose that silica enrichment varying from 5 per cent to 15 per cent by volume from slab-derived fluids and upward mineralization in quartz veins12 can explain the range of observed vP/vS values as well as the downdip decrease in vP/vS. The solubility of silica depends on temperature13, and deposition prevails near the base of the forearc crust11. We further propose that the strong temperature dependence of healing and permeability reduction in silica-rich fault gouge via dissolution–precipitation creep14 can explain the reduction in tremor recurrence time with progressive silica enrichment. Lower gouge permeability at higher temperatures leads to faster fluid overpressure development and low effective fault-normal stress, and therefore shorter recurrence times. Our results also agree with numerical models of slip stabilization under fault zone dilatancy strengthening15 caused by decreasing fluid pressure as pore space increases. This implies that temperature-dependent silica deposition, permeability reduction and fluid overpressure development control dilatancy and slow-earthquake behaviour.

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Figure 1: Subduction zone velocity structure in slow-earthquake source regions.
Figure 2: Downdip vP/vS variations of overlying forearc crust in northern Cascadia.
Figure 3: Conceptual model of silica enrichment controlling slow-earthquake behaviour in northern Cascadia.

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Acknowledgements

Data used in this study come from the Japan Meteorological Agency, New Zealand National Seismograph Network, and the Tectonic Observatory (Caltech). Funding for this work comes from the Natural Science and Engineering Research Council (Canada) and the Miller Institute for Basic Research in Science (University of California, Berkeley). We thank N. Beeler, A. Rubin and P. Ampuero for discussions and comments.

Author information

Authors and Affiliations

  1. Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada,

    Pascal Audet

  2. Department of Earth and Planetary Science and Berkeley Seismological Laboratory, University of California, Berkeley, 94720-4767, California, USA

    Roland Bürgmann

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  1. Pascal Audet
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Contributions

P.A. performed data processing and inversion. Both P.A. and R.B. contributed to the interpretations and preparation of the final manuscript.

Corresponding author

Correspondence to Pascal Audet.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Examples of receiver functions and inversion results for each subduction zone.

In a–c the slab contours from refs 30 and 41 are in yellow, contours of slow-slip patches from ref. 2 are in light green, contours and epicentres of tremors from ref. 2 are in purple, and station locations used in this study are shown as inverted red triangles. For a subset of stations (PLAY, PXZ and IGK, identified by the blue squares on the maps) we show the observed (top, A) and modelled (bottom, B) radial receiver functions ordered by back-azimuth and, for each back-azimuth, by slowness of the incoming P wave. c, A slice through model misfits, with warm colours indicating low values, showing the vP/vS of forearc crust versus the vP/vS of the low-velocity zone. The star shows the minimum value of the misfit plot (best-fitting value).

Extended Data Table 1 vP/vS results from the inversion of receiver function data
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Extended Data Table 2 Data sources for ETS recurrence times and seismic velocity models
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Audet, P., Bürgmann, R. Possible control of subduction zone slow-earthquake periodicity by silica enrichment. Nature 510, 389–392 (2014). https://doi.org/10.1038/nature13391

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