New Zealand Journal of Geology and Geophysics abstracts
Differences in coverage between the PDE and New Zealand local earthquake
catalogues
DAVID HARTE
DAVID VERE-JONES
Institute of Statistics and Operations Research
School of Mathematical and Computing Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
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 33-49deg.S, and longitudes 165-183deg.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).
Keywords PDE catalogue; New Zealand catalogue; location
errors; magnitude scales; earthquake swarm
New Zealand Journal of Geology and Geophysics, 1999, Vol. 42: 237-253
0028-8306/99/4202-0237 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (1652K); (scanned from paper original: notes about this process)
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