Abstract
A submarine volcanic eruption near Curagoa Reef, first 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 eruption. 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.
Highlights
Focal mechanisms of shallow earthquakes fromthemainpart of the Tonga arc, confined to the region between t h e trench axis and the volcanoes and defining the common boundary of the Pacific and Australian plates, indicate that the Pacific Ocean floor underthrusts the arc to the west along shallow dipping fault planes
The fault plane solutions for shallow earthquakes from the northern end of the arc are different. They reflect dip-slip motion on nearly vertical, easterly striking faults with the south side moving downward (1(i'18). These earthquakes are associated with the formation of a hinge fault within t h e Pacific plate rather t h a n with movement of one plate past another, and higher stress at the northern end might be expected t h a n elsewhere in the zone (1U)
The special attention paid to T phases in this study arises from the need to use them for the determination of magnitudes for the smaller earthquakes in the swarm
Summary
An earthquake swarm, beginning on 11 July 1973 (*), took place near CuraQoa Reef, at the extreme northern end of the Tonga arc, shortly before an observed volcanic eruption. This is an area in which the tectonics are relatively well understood C6'18'22). Anumber of the larger earthquakes throughout t h e Cura§oa Reef sequence were recorded on both short- and long-period seismographs a t Afiamalu, and this allows the time variation of source properties during the swarm to be considered. The others recorded T waves from the largest events only
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
