Strong shaking in recent New Zealand earthquakes

Abstract

On 4 September 2010 a surface‐rupturing crustal earthquake (Mw 7.1) struck the Canterbury Plains region of New Zealand's South Island [Gledhill et al., 2011]. The Canterbury Plains is a region of relatively low seismicity in New Zealand, and the structure that ruptured was a previously unmapped fault (Figure 1a). Fortunately, even though parts of the region experienced liquefaction of unconsolidated sediments and sands—including neighborhoods of the city of Christchurch (population 377,000)—no fatalities occurred. Compared to the average New Zealand aftershock decay model, the aftershock sequence for the 2010 earthquake was relatively underproductive for the first 5 months. But on 22 February 2011 anMw 6.2 aftershock (teleseismic and regional estimates range from (Mw 6.1 to (Mw 6.3 with regional inversions favoring higher values) occurred within kilometers of the center of Christchurch (A. E. Kaiser et al., The (Mw 6.2 Christchurch earthquake of February 2011: Preliminary report, submitted to New Zealand Journal of Geology and Geophysics, 2011). The event increased the productivity of other aftershocks (Figure 1b). This particular aftershock was devastating, generating much more destruction than theMw 7.1 event, including more than 180 fatalities. Recorded peak ground acceleration (PGA) in the city was more than double the acceleration of gravity (g). Many of the poorly consolidated, low‐shear‐wave‐velocity soils liquefied during the shaking. Damage estimates reached approximately US$15 billion, making the aftershock New Zealand's costliest natural disaster.