Large Apparent Stresses from the Canterbury Earthquakes of 2010 and 2011

An earthquake of Mw 6.1–6.31 (Beavan et al. 2011, page 789 of this issue) that struck Christchurch, New Zealand, on 22 February (21 February, UTC) produced recorded ground motion acceleration over 2 g. The event caused widespread damage with dense recordings of non-linear site behavior. Globally, dense near-field recordings of shallow intraplate earthquakes are rare. It is possible that extreme ground motions are common with this type of earthquake and that their rarity is merely a function of inadequate seismic sampling in the near field of such low-probability, high-potency events. To better define the nature of these events, we calculate apparent stress ( τa ) of the three largest earthquakes in the Canterbury sequence and compare them to global and regional data. We then place recorded PGA and spectral accelerations into the context of regional and global ground motion prediction equations and discuss the implications of high-stress events for future seismic hazard estimates for the region. For the February event, we also briefly explore the implications of directivity on measured ground motions in central Christchurch. The earthquakes that occurred in the Canterbury region of the South Island, New Zealand, from September 2010 to the present have disproportionately large energy magnitudes ( Me ) to their moment magnitudes ( Mw ). They have produced the largest ground motions ever measured in New Zealand. The sequence began with the Mw 7.1 earthquake that occurred about 40 km west of the city of Christchurch on 4 September 2010. The maximum recorded ground acceleration recorded during the event was over 1.25 g, which was experienced near the intersection of the triggering thrust on which the rupture began and the strike-slip Greendale fault that carried most of the moment in the earthquake (Gledhill et al. 2010). Peak ground accelerations (PGA) in the central business district of Christchurch averaged …

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