Abstract
The Modified Mercalli intensities of the 1956 Mw 6.3 Bay of Plenty and 1987 Mw 6.5 Edgecumbe earthquakes have recently been reviewed and about one-third of them were found to be erroneous. The resulting revisions to their isoseismal maps are substantial, and both new maps now show the strong influence of the high attenuation in the Taupo Volcanic Zone (TVZ). An analysis of the causes of the errors in the intensities is given. The new maps will help improve the modelling of attenuation in the TVZ, and will contribute to improvements in assessments of seismic hazard and risk in that region. An important implication is that the mean damage ratios estimated from studies of damage costs in the Edgecumbe earthquake by Dowrick and Rhoades are likely to be erroneously low, and need to be reviewed.
Highlights
The 1956 Mw 6.3 Bay of Plenty earthquake and the 1987 Mw 6.5 Edgecumbe earthquake are the largest historical earthquakes to have occurred in the Taupo Volcanic Zone (TVZ)
That of the 1956 event, prepared by the New Zealand Seismological Observatory in 1956 and republished in Downes (1995) (Figure 1), shows no marked effect of the high attenuation that occurs in the TVZ, and the distances between the isoseismals appear to be too small
As a result of the above study of the intensities in the 1956 Mw 6.3 Bay of Plenty and the 1985 Mw 6.5 Edgecumbe earthquakes, the following conclusions have been drawn: (1) Substantial numbers of errors have been found in the intensities assigned for both of these earthquakes, 33 percent being erroneous in the case of the Edgecumbe event
Summary
The Modified Mercalli intensities of the 1956 Mw 6.3 Bay of Plenty and 1987 Mw 6.5 Edgecumbe earthquakes have recently been reviewed and about one-third of them were found to be erroneous. The resulting revisions to their isoseismal maps are substantial, and both new maps show the strong influence of the high attenuation in the Taupo Volcanic Zone (TVZ). An analysis of the causes of the errors in the intensities is given. The new maps will help improve the modelling of attenuation in the TVZ, and will contribute to improvements in assessments of seismic hazard and risk in that region. An important implication is that the mean damage ratios estimated from studies of damage costs in the Edgecumbe earthquake by Dowrick and Rhoades are likely to be erroneously low, and need to be reviewed
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