Differences in coverage between the PDE and New Zealand local earthquake catalogues

This paper examines systematic differences between the coverage of New Zeal and earthquakes in the New Zealand local catalogue and the PDE catalogue pu t out by the United States National Earthquake Information Centre (NEIC). O nly events with local magnitudes M-L greater than or equal to 5 in the New Zealand catalogue, and with body-wave magnitude m(b) greater than or equal to 4.5 in the PDE catalogue, are considered in the study, which covers the period 1965-93, and a series of four quadrats covering the landmass of New Zealand and extending between latitudes 33-49 degrees S, and longitudes 165 -183 degrees E. The main differences found are: (1) before 1983, the PDE catalogue misses many events (mainly of intermedia te depths) with M-L greater than or equal to 5 listed in the New Zealand ca talogue; after 1983 most of the New Zealand shallow events are recorded, bu t some intermediate events are still missing; (2) the New Zealand catalogue misses many events with m(b) greater than or equal to 4.5 listed in the PDE catalogue as occurring to the northeast of N ew Zealand, and a few listed as occurring to its southwest; otherwise, virt ually all events with m(b) greater than or equal to 4.5 listed in the PDE c atalogue are also found in the New Zealand catalogue; (3) the epicentres of events to the northeast of New Zealand are systematic ally displaced to the east in the New Zealand catalogue, relative to the PD E catalogue; many such events listed as having intermediate depths in the N ew Zealand catalogue are classified as shallow events in the PDE catalogue; (4) this region aside, for shallow events in the given magnitude ranges, th ere seems to be no systematic difference between the PDE body-wave magnitud e m(b) and the New Zealand local magnitude M-L; however, magnitudes of indi vidual events may differ by up to one unit in either direction; (5) for intermediate depth events there is a small but systematic tendency for m(b) to be less than M-L for the same event; the effect appears to incr ease with depth down the descending plate; (6) an extremely large swarm occurred in Bay of Plenty in 1984, some 9 mont hs after the Edgecumbe earthquake, and marked the start of an unusually act ive period in the northeast of the region covered by the New Zealand catalo gue; the swarm was followed 11 years later by a magnitude 7 event off East Cape, which itself initiated an exceptionally large aftershock sequence; (7) both catalogues indicate a modest increase in activity, mainly at inter mediate depths, in the northern and central regions of New Zealand, in the last few years of the study (1992-95).