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Comparison of magnitude estimates for New Zealand earthquakes; moment magnitude, local magnitude, and teleseismic body-wave magnitude



Comparison of magnitude estimates for New Zealand earthquakes; moment magnitude, local magnitude, and teleseismic body-wave magnitude



Bulletin of the Seismological Society of America 99.3



New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M(w) > approximately 3.5-4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude (M(L) ), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M(w) with M(L) for New Zealand earthquakes. In addition to M(w) and M(L) , there are more than 330 teleseismic body-wave magnitude (m(b) ) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M(w) . These are used to examine the relationship between M(w) , M(L) , and m(b) for New Zealand earthquakes. There is a clear distinction in the relationship of M(L) to M(w) for shallow focus ( 33 km depth) earthquakes. Shallow focus earthquakes show M(L) to be fairly consistent with M(w) , particularly for events with M(w) > approximately 4.5 and with M(L) =(0.88+ or -0.03)M(w) +(0.73+ or -0.20). Deep focus earthquakes have M(L) consistently larger than M(w) (more than a full magnitude unit for some events) with M(L) =(1.09+ or -0.10)M(w) +(0.05+ or -0.06). M(w) and m(b) are in fairly good agreement regardless of the depth, whereas m(b) estimates are consistently smaller than M(L) for deep events. This suggests that M(L) is overestimated for deep focus events and that the large M(L) s are not the result of physical properties of the source.

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Accession: 037020470

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DOI: 10.1785/0120080237


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