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PRINCIPAL NEW ZEALAND EARTHQUAKES IN 1985 Warwick D. Smith*
During 1985 the most seismically active part of the N e w Zealand region w a s an offshore area to the north and east of East C a p e , far enough away from the coast that earthquakes there present no
significant h a z a r d s . An earthquake of m a g n i t u d e 7 occurred on September 26 about 4 00 km north east of E a s t C a p e . It was felt at intensity MM V at C a p e Runaway and other locations on the East Cape p e n i n s u l a , and at lower intensities as far south as C h r i s t c h u r c h . Three other e a r t h q u a k e s reached m a g n i t u d e 6 in the same region during the y e a r :
m a g n i t u d e 6.4 on M a y 7, m a g n i t u d e 6 on June 29 a n d ' m a g n i t u d e 6.5 on N o v e m b e r 8.
N o n e of these w a s felt at intensities h i g h e r than M M IV. The only potential hazard., from e a r t h q u a k e s in this area is from t s u n a m i s , but none w a s generated on any of the above o c c a s i o n s .
The h i g h e s t intensity reported during the :ye'ar w a s M M V I -at' B l e n h e i m on March 7 w h e n an earthquake' of m a g n i t u d e 5.5 occurred in the M a r l b o r o u g h S o u n d s . It w a s felt from O p u n a k e (MM V) to Greymouth
(MM V) and Christchurch ("strong").
A n o t h e r shock of m a g n i t u d e 5.5, w h i c h occurred on January 2 4 in the vicinity of the M o t u River in the eastern B a y of P l e n t y , w a s felt less strongly.
T h e m a x i m u m intensity was only MM V, in O p o t i k i , although it w a s felt throughout the Bay of Plenty and in G i s b o r n e . The epicentre of this shock w a s very close to that of the largest w i t h i n the land area of N e w Zealand in recent y e a r s , the m a g n i t u d e 6.4 event which occurred on 1984 March 8 at a depth of 80 km.
A number of other earthquakes d e s e r v e m e n t i o n : the January 31 shock in Fiordland, which was just less than m a g n i t u d e 6 and w a s felt in Dunedin and Stewart Island as well as closer locations; the m a g n i t u d e 5.8 shock to the south of G i s b o r n e on July 2 0 , w h i c h was felt in G i s b o r n e and Whakatane and south to W e l l i n g t o n and had an aftershock of m a g n i t u d e 5.2 the next day, and the September 13 event in F i o r d l a n d of m a g n i t u d e 5.2, which w a s felt in Dusky Sound, I n v e r c a r g i l l , G o r e and Te A n a u .
* Seismological O b s e r v a t o r y , Department of Scientific and Industrial Research, W e l l i n g t o n .
The task of analyzing the y e a r1s earthquake activity has been focussed to a very significant extent on the Bay of Plenty earthquake of m a g n i t u d e 6 on 1984 December 31, for this earthquake was followed by a very large sequence of
aftershocks. M a n y hundreds of small events followed the m a i n shock, and lasted
throughout the first few w e e k s of January 1985. It is the largest such sequence to occur in N e w Zealand since the Inangahua aftershocks of 19 68. A s such it serves to remind us that large earthquakes are never isolated e v e n t s : they are normally followed by long sequences of a f t e r s h o c k s , some of w h i c h can be significant earthquakes in
their own right. A casebook example is the m a g n i t u d e 7 aftershock of the Hawke's Bay earthquake in 1931.
BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol. 19, No. 1, March 1986
