We performed moment tensor inversion of
waveform data from the temporary seismic network Southwest Pacific Seismic
Experiment (SPASE) from 1994 to 1995 for focal mechanism, focal depth and moment
magnitude of shallow earthquakes in the Tonga-Fiji region. We focused the
shallow earthquakes, since they are more important than deep earthquakes in the
viewpoint of disaster mitigation such as tsunami warning. We classified the
quality of our solutions mainly on the basis of similarity to those in the
Global Centroid Moment Tensor (GCMT) catalog as ‘good’, ‘moderate’ and ‘poor’.
We also used P-wave polarity data and proportion of double-couple component for
the classification. We could obtain ‘good’ solutions for all of earthquakes
with Mw > 6. A proportion of ‘good’ solutions is a little greater than half
for earthquakes with Mw < 5.9. Geographical dependency of the quality shows
that the quality of our solutions is much better in the region covered by the
SPASE network, while worse in the northern end or south of Tonga. Comparing to
Mw by GCMT and depths from the United States Geological Survey (USGS), Mw of
this study is underestimated by about 0.1. Differences of Focal depths of this
study and the USGS depth are within 15 km. The result based on data from the
SPASE network suggests that it is feasible for quick and reliable determination
of earthquake parameters for shallow Tongan earthquakes of Mw > 6 off
Ha’apai, Vava’u, and Tongatapu, using data only from a regional sparse network.
To improve a reliability in the earthquake parameters in the northern Tonga
arc, where seismicity is very high, a few stations should be deployed there.
Keywords: Focal mechanism, Tonga-Fiji region, Moment tensor inversion