Temporal variations of the earthquake data in the catalogue of seismicity of New Zealand
DOI:
https://doi.org/10.5459/bnzsee.38.2.87-105Abstract
The authors have analyzed the main temporal characteristics of the earthquake data in the catalogue of seismicity of New Zealand with the objective of providing a general overview of its content and limitations. To this end we have employed different statistical tools which allow for the objective estimate of times of changes in the seismicity rates as well as providing the main reasons for those changes. We have found that the seismicity record of the largest events (M > 4.0) is bracketed by significant changes which occurred during 1940, 1965-1968, and 1987 with other less significant changes taking place during 1960, 1983 and 1992.
By comparing the rates obtained for intervals bounded by the aforementioned dates we were able to determine that a linear correction to the magnitudes in the period 1968 to 1987 for the whole depth range of events may be useful in order to match the frequency-magnitude distribution which is obtained using the current data in the interval 1987 - 2004. A different pattern emerged when separating shallow from intermediate and deep events and we found that most of the temporal variations observed affected mainly the deep events (Z > 40 km), and that the reported rate of shallow seismicity for M > 4.0 has been remarkably homogeneous since 1940. This observation is supported not only by the constancy of seismicity rates but by the similarity of frequency-magnitude distributions during the time intervals analyzed. A systematic evaluation of minimum magnitude of completeness for shallow seismicity yielded values of Mc = 4.4 for the interval 1940 to 1968, Mc = 3.9 for the interval 1968 to 1987 and Mc = 2.6 for the current stage from 1987 to 2004. A simple magnitude shift of 0.2 units applied to data in the interval 1968 to 1987 for intermediate and deep events was found to provide a good match to the observed rate for events from 1987 to 2004. The evaluation of Mc for events in the 40 ≤ z ≤ 600 km range yielded values of 5.5 for data in the interval 1940 to 1968, 4.0 for 1968 to 1987, and 3.6 for the current operative practice starting in I 987. A detailed investigation of the variation of the magnitude of completeness with time was carried out resulting in different trends which seem to correspond to the time when major changes were carried out in the network, in particular with the installation and removal of local microcarthquake networks.
References
Anderson, H.J. and. Webb, T.H. (1994). "New Zealand seismicity; patterns revealed by the upgraded National Seismic Network", N.Z J. Geol. Gephys., 37, 477-493. DOI: https://doi.org/10.1080/00288306.1994.9514633
Dowrick, D., and Rhoades, D. (1998). "Magnitudes of New Zealand earthquakes, 1901-1993". Bull. New Zealand Nat. Soc. Earthq. Eng, 31, 260-280. DOI: https://doi.org/10.5459/bnzsee.31.4.260-280
Gomberg, J. (1991). "Seismicity and detection/location threshold in the southern Great Basin seismic network", J. Geophys. Res., 96 (B10), 16,401-16,414. DOI: https://doi.org/10.1029/91JB01593
Habermann, R.E. (1982). "Consistency of teleseismic reporting since 1963". Bull. Seism. Soc. Am., 72, 93-112.
Habermann, R.E. (1983). "Teleseismic detection in the Aleutian Island Arc", J. Geophys. Res., 88, 5056-5064. DOI: https://doi.org/10.1029/JB088iB06p05056
Habermann, R.E. and Wyss M. (1984). "Background seismicity rates and precursory seismic quiescence: Imperial Valley, California", Bull. Seism. Soc. Am., 74, 1743-1755.
Haines, A.J. (1981). "A local magnitude scale for New Zealand earthquakes", Bull. Seism. Soc. Am., 71, 275-294.
McGinty, P, (2001). "Preparation of the New Zealand earthquake catalogue for probabilistic seismic hazard analysis", Bull. New Zealand Nat. Soc. Earth. Eng., 34, 60-67. DOI: https://doi.org/10.5459/bnzsee.34.1.60-67
Reasenberg, P. A., (1985). "Second-order moment of central California seismicity", 1969 1982, J. Geophys. Res, 90, 5479-5495. DOI: https://doi.org/10.1029/JB090iB07p05479
Smith, W., (2000). "Precursory changes in mean magnitudes: a systematic examination of the New Zealand catalogue", Institute of Geological and Nuclear Sciences Ltd., File Report 2000/2, 36pp.
Wiemer, S., (2001). "A software package to analyse seismicity: ZMAP". Seism. Res. Let., 72, 3, 373-382. DOI: https://doi.org/10.1785/gssrl.72.3.373
Wiemer, S. and Wyss M. (2000), "Minimum magnitude of completeness in earthquake catalogs: examples from Alaska, the western United States and Japan". Bull. Seism. Soc., Amer., 90, 859-869. DOI: https://doi.org/10.1785/0119990114
Wiemer, S., and Zuniga F.R. (1994), "ZMAP - a software package to analyze seismicity", EOS, Transactions, Am. Geophys. U., 75, 456.
Wyss, M. (1991), "Reporting history of the central Aleutians seismograph network and the quiescence preceding the 1986 Andreanof Island earthquake". Bull. Seism. Soc., Amer., 81, 1231-1254.
Wyss, M. and Burford R.O. (1985). "Current episodes of seismic quiescence along the San Andreas fault between San Juan Bautista and Stone Canyon, California: Possible Precursors to local moderate mainshocks?", U.S. Geol. Survey, open .file report 85-745, 367-426.
Zuniga, F.R (1989). "A study of the homogeneity of the NOAA earthquake data file in the Mid-America region by the magnitude signature technique". Geoffsica Internacional, 28, 103-119.
Zuniga, F.R. and Wyss M. (1995). "Inadvertent changes in magnitude reported in earthquake catalogs: Influence on b-value estimates", Bulletin of the Seismological Society of America, 85, 1858-1866.
Zuniga, F.R, Valdes C.M. and Reyes M. (2000). "A general overview of the catalog of recent seismicity compiled by the Mexican Seismological Survey". Geoffsica Internacional, 39, 161-170.
