Frequency of large crustal earthquakes in Puget Sound–Southern Georgia Strait predicted from geodetic and geological deformation rates

Abstract

Earthquake hazard in the Cascadia subduction zone forearc comes from three sources: great subduction earthquakes, Wadati–Benioff slab events, and earthquakes in the forearc crust. This study deals with a concentration of forearc crustal earthquakes in the Puget–Georgia region of southwestern Canada and northwestern United States. These earthquakes are due to margin‐parallel shortening, rather than compression in the direction of plate convergence. The frequency of large earthquakes has previously been based mainly on extrapolation of the statistics of smaller events from the short instrumental record. In this study, an independent estimate has been obtained through the seismic moment rate required to accommodate current rates of deformation from GPS and geological data. The catalogue statistics to Mx = 7.5 give a moment rate of 4.0 × 1017Nm/yr and a shortening rate of 2.9 mm/yr if all deformation is seismic. GPS data indicate local current N–S shortening of 3 ± 1 mm/yr. For a seismogenic thickness of 12 km, this deformation represents a moment rate of 4.1 × 1017Nm/yr. The predicted occurrences are 0.022/yr (45 years) forM> 6, and 0.0025/yr (400 years) forM> 7, within the uncertainties of the catalogue statistics. Large characteristic earthquakes are not required. Forearc long‐term average motion based upon larger‐scale GPS, paleomagnetic, and geological data ranges from 5 to7 mm/yr. The estimated long‐term occurrence of large events thus is approximately double the current rate, and we infer that the extra earthquakes could follow abrupt N–S shortening in this part of the forearc associated with the oblique rupture motion of great subduction events.