Update of Regional Moment Tensor Analysis for Earthquakes in New Zealand and Adjacent Offshore Regions

Abstract

Abstract New Zealand is one of the more seismically active countries in the world with over 15,000 earthquakes each year in New Zealand and the adjacent offshore region. Routine regional moment tensor (RMT) analysis was implemented by GeoNet in 2007, and nearly 1300 RMT solutions have been calculated for the New Zealand region from August 2003 through early September 2012. The New Zealand RMT catalog contains events with moment magnitude ( M w ) 3.2–7.3 and centroid depth 2–375 km, and includes RMT solutions from aftershock sequences for two major earthquakes: 15 July 2009 M w 7.8 Dusky Sound and 3 September 2010 M w 7.1 Darfield. The RMT solutions provide important constraints on the active tectonics of New Zealand including stress and strain, and the RMT catalog, along with 155 Global Centroid Moment Tensor Project solutions, is used to examine the relationship between M w and local magnitude ( M L ). For shallow focus (≤33 km depth) the M L / M w relationship is found to be similar across New Zealand except for the Fiordland region. For all of New Zealand except Fiordland, M L =(0.93±0.06) M w +(0.54±0.24), and for Fiordland, M L =(0.83±0.04) M w +(1.09±0.13). For deep‐focus earthquakes (>33 km depth) M L is consistently larger than M w by more than a full magnitude unit for some events. The discrepancy between M L and M w is found to be depth dependent with ( M L − M w )=−0.94exp(− h /75.37)+0.80, where h is focal depth in km.