Atlas of Isoseismal Maps of New Zealand Earthquakes

Technical Abstract

This Atlas of 133 isoseismal maps of New Zealand earthquakes contains the felt intensity distributions of 123 earthquakes, 122 of which occurred within or close to the New Zealand mainland. A magnitude 6.7 event which originated south of the Kermadec Islands but was felt widely on the New Zealand mainland is also included. In five cases detailed maps of the epicentral area are given. Three other earthquakes are represented by two maps, these being either an alternative interpretation of the same data set or an interpretation based on a different data set.

The Atlas is arranged with the isoseismal maps presented in chronological order. Each map is accompanied by the most recently calculated hypocentral parameters and an indication of the method of their determination and placed opposite a brief description of the earthquake, the source of the map and the data on which it has been based. References are listed by earthquake at the end of the Atlas after the last isoseismal map. Sufficient references are given for the reader to be able to obtain a fuller description of the felt effects of the earthquake and an insight into what other aspects have been studied. The Atlas includes all known maps that are considered to have been sufficiently well-determined. Two maps have the isoseismal lines plotted in Rossi-Forel scale, no conversion being possible as the individual felt intensities have not been given and the original felt reports have not been located. The data, however, appear to be more extensive than that which is held in our files and for this reason the maps have been included. All other maps refer to the Modified Mercalli scale adapted for New Zealand conditions by Eiby (1966) or Study Group (1992).

The maps in the Atlas span the years 1848 to 1990, the earliest map being from the 1848 Marlborough earthquake and the most recent, the Weber earthquakes in February and May 1990. Since organised European colonisation began about 1840, New Zealand has experienced many large earthquakes, more than are represented in this collection. Few large earthquakes between 1855 and 1930 have been studied in detail and their inclusion in a collection similar to this awaits further research.

The largest earthquake to be included in the Atlas and the largest known in the recorded history of New Zealand is the Wairarapa earthquake on January 23 1855, estimated magnitude 8.0-8.2. The Wellington and the Wairarapa districts suffered severely, up to intensity MM10. The earthquake was also destructive as far north as Wanganui and along the northeast coast of the South Island and caused extensive landsliding and liquefaction. Regional uplift of the southern part of the North Island, including Wellington, occurred, accompanied possibly by subsidence in eastern Wairarapa and Marlborough as well as vertical and probably horizontal movement on the West Wairarapa Fault. Evaluation of the historic data on this event is continuing.

The Mangonui earthquake on Nov 16 1963, of magnitude 3.5 ML, is the smallest earthquake included in the Atlas. It was one of several earthquakes which occurred in this area of Northland in November and December 1963, the largest event having a magnitude 4.9ML, for which there is also an isoseismal map. They were unusual as few earthquakes of damaging intensity have occurred north of Auckland.

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