Abstract
Revised estimates of the return periods of Modified Mercalli (MM) intensity for Auckland and Northland, arising from a revision of the attenuation of intensity in New Zealand, and latest data and views on the local seismicity and geology, represent considerable reductions in the hazard given in Smith and Berryman's seismic hazard model of New Zealand. The revised levels are MM6 and MM7 for 150 and 1200 year return periods. This implies that most structures and plant in Auckland and Northland could have much simpler and less onerous earthquake resistant design and construction than required by current codes. This simpler approach would be significantly cheaper for older so-called "earthquake risk buildings" as well as new construction.
Highlights
The seismicity of Auckland and Nmthland was first discussed in 1955 by Biby [12], while more recently the seismic hazard of this area has been evaluated as part of New Zealand-wide studies, those of Smith [23] and Smith and Berryman [24] who described the seismicity and geology and estimated hazards in terms of frequency of occurrence of various levels of Modified Mercalli (MM) intensity
In the above examination of the seismic hazard model it was estimated that the 150 year return period mean intensity is c.MM6 for Downtown Auckland (Fig.4), and the estimated intensity will reduce with distance further north
Assuming 3000 years for the purposes of this discussion, we find from this study's results in Figure 4, that a return period of 3000 years corresponds to a mean intensity of MM71⁄2 in Downtown Auckland. (This means the old normal design level of MM71⁄2 appears to be appropriate for the extreme design level.)
Summary
The seismicity of Auckland and Nmthland was first discussed in 1955 by Biby [12], while more recently the seismic hazard of this area has been evaluated as part of New Zealand-wide studies, those of Smith [23] and Smith and Berryman [24] who described the seismicity and geology and estimated hazards in terms of frequency of occurrence of various levels of Modified Mercalli (MM) intensity. This would mean reducing ai to say 0.25 times the present values Applying this factor and equation (5) to the results, we obtain the intensity versus return period data as in Table 3 and Figure 4. As the effect on Auckland of large distant events is of interest, given in Table 4 is an estimate of the mean intensity in Auckland from the largest events likely to occur in Region D (Fig.2), namely subduction zone events of magnitude 8-8.5 This intensity is only MM6 and as such is probably an overestimate (by about one MM step) as it does not take into account the probable higher inelastic attenuation rate as seismic waves cross the Central Volcanic Region (Region C) noted elsewhere [10, 19].
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