Modified mercalli intensities for the M7.8 Kaikōura (New Zealand) 14 November 2016 earthquake derived from 'felt detailed' and 'felt rapid' online questionnaires

Abstract

This paper describes the shaking intensity levels caused by the M7.8 Kaiköura earthquake of 14/11/2016 according to the information from the two current GeoNet online questionnaires, 'Felt Detailed' and 'Felt RAPID'. A recently developed method to extract intensity levels at a community scale using 'Felt Detailed' data is used. These are compared with individual intensities from 'Felt RAPID' survey, instrumental intensities from two recent ground motion to intensity conversion equations, and traditional intensity assignments. While maximum Modified Mercalli instrumental, traditional, 'Felt RAPID' and individual 'Felt Detailed' intensities go up to 8, community intensities using 'Felt Detailed' mostly only go up to 5, with only four communities with MM 6-7. Reasons for this discrepancy include a) lack of data around the epicentre; b) few reports from this event compared to other smaller recent earthquakes; and c) lack of public awareness of'Felt Detailed" surveys, released shortly after the earthquake. In addition, only 47% of reports were used to calculate community intensities, based on a minimum requirement for robust calculation of 5 reports. Although 'Felt RAPID' provided a much larger number of reports (more than 15, 000) for this earthquake compared to 'Felt Detailed' (3500), the reliability of the former may be compromised by their lack of detail. Results from this paper suggest that, when enough reports are submitted, 'Felt Detailed' can provide good quality data that can be used in tools such as the near-real time shaking intensity maps provided in ShakeMapNZ.