By the EERI Reconnaissance Team.
March 19, 2011. Earthquake Engineering Research Institute.
Aftershocks in Canterbury have persisted since the 4 September 2010 M 7.1 event with 200 aftershocks greater than 4.0 and 15 greater than 5.0.
The period prior to 22/2/2011 contained fourteen events of magnitude 5.0 to 5.6 and forty events of magnitude 4.5 to 5.0. Since 22/2/2011 there have been five aftershocks between 5.0 and 6.3 and eighteen between 4.5 and 5.0.
Figure 1. Christchurch area aftershock sequence – Sep 2010 – Mar 2011.
Christchurch is built on the fine alluvial soils of previously-braided rivers and mashes. Many of the larger aftershocks liquefied new soil and re-liquefied previously-liquefied ground. This activity has slowed the consolidation of large liquefied areas.
This aftershock sequence is active as compared to parametric stochastic aftershock models for both New Zealand and California (Reasenberg and Jones 1989, Reasenberg and Jones 1994, Eberhart-Phillips 1998, Tormann et al 2008). Further, center-city ground motions during the 22/2/2011 aftershock were 50% to 100% stronger than those of the 4/9/2010 primary shock, and exceeded the 10%/50yr uniform hazard spectrum by 60% to 80% at periods from 0.5 s to 2.0 s (Bradley 2011). While the strong motion responses prior to February 2011 could have contributed to progressive weakening of collapse-sensitive structures, the ground motion intensity of the 22/2/2011 aftershock is certainly the dominant factor contributing to these collapses. This event underscores the importance of modeling earthquakes as an event process.
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Bradley, B., (2011) “Comparing the ground motion of the Feb 2011 quake to the September 2010 quake” http://db.nzsee.org.nz:8080/en/web/chch_2011/home , retrieved 24 February 2011.
Eberhart-Phillips, D., (1998) “Aftershock sequence parameters in New Zealand,” Bulletin of the Seismological Society of America, v. 88; n. 4; p. 1095-1097.
Reasenberg, P.A. and Jones, L.M., (1989) “Earthquake Hazard After a Mainshock in California”, Science, v. 243, p. 1173-1176.
Reasenberg, P.A. and Jones, L.M., (1994) “Earthquake Aftershocks: Update”,
Science, v. 265, p. 1251-1252.
Tormann, T., Savage, M.K., Smith, E.G.C., and Stirling, M.W., and Wiemer, S., (2008) “Time-, Distance-, and Magnitude-Dependent Foreshock Probability Model for New Zealand,” Bulletin of the Seismological Society of America, v. 98; n. 5; p. 2149-2160.
Learning from Earthquakes: First person reports
