A Revised Local Magnitude (ML) Scale for New Zealand Earthquakes

In this study, a new local magnitude (M-L) scale is developed for New Zealand and adjacent offshore regions. SeisComP3 (SC3) has been in use for earthquake analysis in New Zealand since September 2012 with the original Richter (1935) log A(0) attenuation relationship for calculating M-L. The attenuation characteristics of New Zealand differ significantly from southern California, where M-L was originally defined, and therefore result in M-L that is consistently high when compared with moment magnitude (M-w). Using M-w from 528 regional moment tensor solutions along with peak observed amplitudes, a new log A(0) curve is derived, along with station correction factors that define a revised M-L scale for New Zealand earthquakes that is more consistent with M-w. The new log A(0) curve is similar to the original Richter (1935) definition at hypocentral distances of similar to 100-200 km but differs significantly at closer and farther distances. The new M-L is more consistent with M-w across New Zealand, including crustal earthquakes and earthquakes below the crust. The California Institute of Technology (Caltech)-U.S. Geological Survey seismic processer (CUSP) system was used for earthquake analysis prior to SC3, and previous studies have derived regression relationships relating CUSP M-L with M-w. After applying the regression relationships to CUSP M-L, we found very good agreement between CUSP M-L and the new SC3 M-L, which is important for developing a consistent M-L between different catalogs.