Postseismic Deformation due to the 2012 Mw 7.8 Haida Gwaii and 2013 Mw 7.5 Craig Earthquakes and its Implications for Regional Rheological Structure

Observations of similar to 7 years of postseismic deformation following the 2012 M-w 7.8 Haida Gwaii and 2013 M-w 7.5 Craig earthquakes in western British Columbia and southeastern Alaska provides insight into the regional rheologic structure and seismological hazards. A stress-driven aftersliponly model cannot alone explain rates or spatial patterns of the far field and later stage postseismic deformation measured by Global Positioning System (GPS). We thus develop an integrated model that fits the observations by combining afterslip on the deeper extension of the faults and distributed viscous flow in the asthenosphere. The best-fit integrated model has a 60-km thick lithosphere overlying a Burgers body asthenosphere with a Maxwell viscosity of 2 x 10(19) Pa s (0.8-3 x 10(19) Pa s at 95% confidence), with a Kelvin viscosity assumed equal to 10% of that value. These values are in good agreement with rheological models derived from glacial isostatic adjustment data. The early, near-field postseismic deformation from both events is dominated by their corresponding afterslip, however, throughout the Craig postseismic period, the dominant mechanism in the far field is viscoelastic relaxation. For the intermediate region between the two earthquakes, both events contribute to the observed postseismic displacements. Because there was no obvious afterslip following the Haida Gwaii event or known large historical earthquakes on the southernmost part of the QCF, this region continues to have great potential to suffer a large earthquake in the future.

Automated summary

Observations of approximately 7 years of postseismic deformation following the 2012 M-w 7.8 Haida Gwaii and 2013 M-w 7.5 Craig earthquakes in western British Columbia and southeastern Alaska provide insight into regional rheologic structure and seismic hazards. A stress-driven afterslip-only model is insufficient to explain the rates and spatial patterns of the far-field postseismic deformation, leading to the development of an integrated model combining afterslip and distributed viscous flow. The integrated model suggests a 60-km thick lithosphere overlying a Burgers body asthenosphere with a Maxwell viscosity of 2 x 10(19) Pa s, in good agreement with glacial isostatic adjustment data.