ULF electromagnetic emissions associated with a large earthquake occurred a
t Biak Island, Indonesia at 5h 59m UT on February 17, 1996 (magnitude (Mw)
= 8.2 and depth = 20 km from USGS catalog), have been investigated on the b
asis of ULF magnetic observations at two stations, Biak and Darwin in Austr
alia (about 1,200 km apart). Though a simple analysis of magnetic field int
ensity (horizontal and vertical magnetic field components) at both stations
was found to be closely correlated with geomagnetic Sigma Kp activity, a d
etailed analysis of the difference of H and Z components at the two station
s, the polarization analysis (Z/H) and fractal analysis (frequency spectrum
slope) at these two stations has yielded that the ULF emissions (in the fr
equency range from 5 mHz to 30 mHz) about 1.5-1.0 months before the quake a
re likely to be a precursory signature of the quake with its intensity on t
he order of 0.2-0.3 nT.