Relocation analysis of the Christchurch City "Boxing Day" earthquakes

Abstract

All earthquakes catalogued by GeoNet that occurred near and beneath Christchurch city between Christmas Day 2010 and 5th February 2011, were relocated using a "doubledifference" location procedure, in order to examine their absolute and relative hypocentre locations. The location analysis of these events may possibly assist interpretation of fault structure beneath the city.

The sequence, termed here the "Boxing Day sequence", began with a local magnitude (ML) 4.9 earthquake on Boxing Day 2010, with ML4.6 and ML4.4 events just a few hours later. More than 30 events followed in close spatial proximity over the subsequent 3-4 weeks. The relocation analysis indicates that the ML.9 event was located at -4.0 km depth with an epicentre 1.8 km NW from Christchurch cathedral. Most of the subsequent events occurred at depths between 3.5 and 7 km, and occurred in a single patch less than 1 km2 in area, with epicentres -1 km NE of Christchurch cathedral. Projections of the events indicate a steeply dipping cluster, which is consistent with one of the nodal planes of the ML.9 event (strike 74°, dip 84°, rake 153°). These relocations suggest that the events represent strike-slip rupture on a steeply dipping fault with strike -74°, with activity on that fault spread over a distance of -2.5 km.