ABSTRACT

This article presents the implementation of an expert‐based method to calculate community intensities (community modified Mercalli intensity [CMMI]) for New Zealand earthquakes, using GeoNet’s “Felt Classic” (2004–2016) online felt reports. The method has been implemented and tested for about 43,000 felt reports from nine recent moderate‐to‐large (Mw 5.7–7.8) New Zealand earthquakes, including the damaging 4 September 2010 Mw 7.1 Darfield, 22 February 2011 Mw 6.2 Christchurch, and 14 November 2016 Mw 7.8 Kaikoura earthquakes. Results from this method (matrix method) have been compared for the 14 February 2016 Mw 5.7 Christchurch earthquake using three independent approaches: (1) postal traditional method, (2) community intensities from New Zealand’s latest ground‐motion intensity conversion equation (GMICE), and (3) community intensities from a recent GMICE using California data. Results show a good consistency in community intensities between the matrix, postal traditional, and California GMICE methods. However, CMMI derived using the New Zealand GMICE shows a tendency to overestimate the intensities by about 2–3 intensity levels in comparison with the CMMI and traditional MMI, showing the need to review the current New Zealand GMICE.

The first database of CMMI for New Zealand felt reports has been generated, comprising 914,000 felt reports from 27,688 different earthquakes during the 2004–2016 period. The database has generated more than 57,000 community intensity values for communities with five or more felt reports. Further quality testing is currently being carried out to improve the quality and robustness of the CMMI method. This method provides a useful way to obtain geographical damage distribution following an earthquake, being the first of its kind using New Zealand felt report data. Its future use in ShakeMapNZ and the GeoNet website will provide local authorities, emergency planners, decision makers, and the public with a fast and easy tool for understanding the damage distribution following an earthquake.

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