Analysis of the Severe-Damage Online Felt Reports for the Canterbury (New Zealand) 2011 Aftershocks on 22 February Mw 6.2, 13 June Mw 6.0, and 23 December Mw 6.0

Abstract

Online Material: Description and application of methods for assigning damage grades to online felt reports. In 2004, GeoNet (New Zealand’s national geological hazards monitoring service; http://www.geonet.org.nz/, last accessed March 2014) implemented an algorithm (Coppola et al. , 2010) to automatically assign intensity values in New Zealand’s modified Mercalli intensity (NZ‐MMI) scale (New Zealand National Society for Earthquake Engineering [NZNSEE] Study Group, 1992; Dowrick, 1996; Hancox et al. , 2002), based on felt information captured from a web interface questionnaire. The success of the online questionnaire project was apparent following earthquakes such as the magnitude 6.8 Gisborne event in December 2007, when more than 3400 felt reports were received (Coppola et al. , 2010). The 2010–2012 Canterbury earthquakes have challenged the facility, which needed to deal with more than 15,000 felt reports for the four major events. They produced by far the largest number of reports received by GeoNet since the implementation of the online questionnaire. Although GeoNet’s algorithm has been extremely useful for providing rapid intensity assignments in the Canterbury sequence, it has an important constraint: it is limited to assigning MMI values of no more than VIII. Above MMI VIII, buildings start to suffer considerable damage, and the assignment of intensity values turns into a more complicated task that involves an engineering study of the building’s damage level and building class (Coppola et al. , 2010). This limitation in the algorithm has not been a problem in the past, as prior to the Darfield earthquake only eight felt reports corresponding to one earthquake (Gisborne, December 2007, M w 6.8 event) had been assigned MMI≥VIII (Coppola et al. , 2010). On the contrary, the high levels of damage caused by the Canterbury sequence have led to the accumulation of a considerable number of online felt reports that reach or exceed the …