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

Abstract

This paper examines systematic differences between the coverage of New Zealand earthquakes in the New Zealand local catalogue and the PDE catalogue put out by the United States National Earthquake Information Centre (NEIC). Only events with local magnitudes ML ≥ 5 in the New Zealand catalogue, and with body‐wave magnitude mb ≥ 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°S, and longitudes 165–183°E. The main differences found are: 1. before 1983, the PDE catalogue misses many events (mainly of intermediate depths) with ML ≥ 5 listed in the New Zealand catalogue; after 1983 most of the New Zealand shallow events are recorded, but some intermediate events are still missing; 2. the New Zealand catalogue misses many events with mb ≥ 4.5 listed in the PDE catalogue as occurring to the northeast of New Zealand, and a few listed as occurring to its southwest; otherwise, virtually all events with mb ≥ 4.5 listed in the PDE catalogue are also found in the New Zealand catalogue; 3. the epicentres of events to the northeast of New Zealand are systematically displaced to the east in the New Zealand catalogue, relative to the PDE catalogue; many such events listed as having intermediate depths in the New Zealand catalogue are classified as shallow events in the PDE catalogue; 4. this region aside, for shallow events in the given magnitude ranges, there seems to be no systematic difference between the PDE body‐wave magnitude mb and the New Zealand local magnitude ML; however, magnitudes of individual events may differ by up to one unit in either direction; 5. for intermediate depth events there is a small but systematic tendency for mb to be less than ML for the same event; the effect appears to increase with depth down the descending plate; 6. an extremely large swarm occurred in Bay of Plenty in 1984, some 9 months after the Edgecumbe earthquake, and marked the start of an unusually active period in the northeast of the region covered by the New Zealand catalogue; 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 intermediate depths, in the northern and central regions of New Zealand, in the last few years of the study (1992–95).