A comparison of ground motions and first-hand experience of the 2011 Mw6.3 Christchurch, New Zealand and 2011 Mw9.0 Tohoku, Japan earthquakes

摘要

This poster provides a comparison between the strong ground motions observed in the 22 February2011 Mw6.3 Christchurch earthquake with those observed in Tokyo during the 11 March 2011 Mw9.0Tohoku earthquake. The destuction resulting from both of these events has been well documented,although tsunami was the principal cause of damage in the latter event, and less attention has beendevoted to the impact of earthquake-induced ground motions. Despite Tokyo being located over100km from the nearest part of the causative rupture, the ground motions observed from the Tohokuearthquake were significant enough to cause structural damage and also significant liquefaction toloose reclaimed soils in Tokyo Bay. The author was fortunate enough (from the perspective of anearthquake engineer) to experience first-hand both of these events. Following the Tohoku event, theathor conducted various ground motion analyses and reconniassance of the Urayasu region in TokyoBay affected by liquefaction in collaboration with Prof. Kenji Ishihara. This conference is therefore afitting opportunity in which to discuss some of authors insights obtained as a result of this first handknowledge.Figure 1 illustrates the ground motions recorded in the Christchurch CBD in the 22 February 2011 and4 September 2010 earthquakes, with that recorded in Tokyo Bay in the 11 March 2011 Tohokuearthquake. It is evident that these three ground motions vary widely in their amplitude and duration.The CBGS ground motion from the 22 February 2011 event has a very large amplitude (nearly 0.6g)and short duration (approx. 10s of intense shaking), as a result of the causal Mw6.3 rupture at shortdistance (Rrup=4km). The CBGS ground motion from the 4 September 2010 earthquake has a longerduration (approx. 30s of intense shaking), but reduced acceleration amplitude, as a result of the causalMw7.1 rupture at a short-to-moderate distance (Rrup=14km). Finally, the Urayasu ground motion inTokyo bay during the 11 March 2011 Tohoku earthquake exhibits an acceleration amplitude similar tothe 4 September 2010 CBGS ground motion, but a significantly larger duration (approx 150s ofintense shaking). Clearly, these three different ground motions will affect structures and soils indifferent ways depending on the vibration characteristics of the structures/soil, and the potential forstrength and stiffness degradation due to cumulative effects.Figure 2 provides a comparison between the arias intensities of the several ground motion recordsfrom the three different events. It can be seen that the arias intensities of the ground motions in theChristchurch CBD from the 22 February 2011 earthquake (which is on average AI=2.5m/s) isapproximately twice that from the 4 September 2010 earthquake (average AI≈1.25). This is consistentwith a factor of approximately 1.6 obtained by Cubrinovski et al. (2011) using the stress-based (i.e.PGA-MSF) approach of liquefaction triggering. It can also be seen that the arias intensity of theground motions recorded in Tokyo during the 2011 Tohoku earthquake are larger than ground motionsin the Christchurch CBD from the 4 September 2011 earthquake, but smaller than those of the 22February 2011 earthquake. Based on the arias intensity liquefaction triggering approach it cantherefore be concluded that the ground motion severity, in terms of liquefaction potential, for theTokyo ground motions is between those ground motions in Christchurch CBD from the 4 September2010 and 22 February 2011 events.