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observed on 12 J u l y 1973 (U.T.) f r om Tafalii Island, northern Tonga, was
associated with an e a r t h q u a k e swarm in t h e same area. The swarm began
about 03'1 on I I J u l y and died away gradually about 07'' on 13 J u l y , ft
comprised 504 events of magnitude Mi, „2= 3.4, the largest being of magnitude
Ml = 5.1. The swarm area for these comparatively low magnitude earthquakes
was abnormally large
The v a r i a t i o n in r a t e of e a r t h q u a k e occurrence during t h e s w a rm shows
two distinct stages, one similar to t h a t in a foresliock sequence, and t h e other
like t h a t in a n a f t e r s h o c k sequence, with t h e largest events occurring between
t h e two. The average value of b, defining t h e f r e q u e n c y - m a g n i t u d e relationship,
was high: 1.77 ± 0.15. Furthermore, this p a r a m e t e r varied during the
course of t h e swarm, a decrease in t h e b value f r om 1.8 to 1.1 being followed
by a sharp increase to 2.5 a f t e r t h e largest earthquakes and principal volcanic
Values of seismic moment were obtained from A B for 118 e a r t h q u a k es
which were well recorded by long-period seismographs. Other source charact
e r i s t i c s were determined from the difference between t h e l o g a r i t hm of seismic
moment and the local magnitude. The source sizes were found to he
u n u s u a l l y large. There was a t i m e variation of source properties during the
swarm. The v a r i a t i o n of t h e displacement shows an inverse correlation with
t h e variation of the coefficient b.
The swarm produced very d i s t i n c t i v e T phases, well recorded at a hydrophone
near Wake Island and at seismograph stations s i t u a t e d on t h e oceanic
side of the Tonga Trench: these were not recorded at seismograph stations
within the island arcs.
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