National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1990

Body-waveforms and source parameters of moderate-sized earthquakes near North Island, New Zealand

Dziak, R.P., and S.G. Wesnousky

Tectonophysics, 180, 273-286, doi: 10.1016/0040-1951(90)90313-W (1990)

P-wave first motion and synthetic seismogram analysis of P- and SH-waveforms recorded at teleseismic distances on the WWSSN are used to estimate source parameters of seven of the largest earthquakes (6.1 ≤ mb ≤ 6.3) that occurred in the vicinity of North Island, New Zealand since 1965. The source parameters of three other (mb ≥ 6.1) events determined outside of this study are included and considered in the final analysis. Four of the earthquakes occurred at shallow depths (< 20 km), of which three were located within and to the north of North Island. Two of the shallow events show strike-slip and normal focal mechanisms with T-axes oriented in a manner consistent with their location in an area of known back-arc extension. One of the shallow events occurred in northern South Island and shows a reverse-type mechanism indicating horizontal contraction of the crust in an easterly azimuth. Six events occurred at intermediate depths (h = 39 to 195 km) of which five exhibit thrust mechanisms with T-axes consistently oriented near vertical. In the light of previously published plate tectonic models, the near vertical orientation of T-axes of the intermediate-depth events may be used to infer that the southern Kermadec plate boundary immediately north of North Island is not strongly coupled, and hence, not likely capable of producing great earthquakes. A similar inference cannot be made for the section of the Hikurangi Margin adjacent to North Island since the intermediate-depth events considered in this study lie to the north of this segment of the plate boundary.

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