Results of ultra-low-frequency magnetic field measurements during the Guam earthquake of 8 August 1993

We report the results of measurements of ultra-low-frequency magnetic noise during a large earthquake (Ms=7.1) at Guam of 8 August, 1993 (depth similar to 60 km). The ULF observing system is located in the Guam Island, about 65 km from the epicenter. Several distinct features of this analysis are summarized. (1) We have proposed rather sophisticated statistical analyses (monthly mean, standard deviation) in order to estimate the wave intensity and polarization (i.e. ratio Z/H). (2) A comparison between the ULF wav e activity and Sigma Kp, is useful in distinguishing between the space geomagnetic pulsations and non-space emissions. (3) Then, the use of the ratio (Z/H) is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves. (4) The statistical analysis of the temporal evolution of this ratio, has yielded that it shows a broad maximum only about one month before the earthquake, and this suggests that the emissions during this period are very likely to be magnetic precursors. (5) The temporal variation of Z component is similar to that for the Loma Prieta earthquake such that it shows a broad maximum tell days similar to two weeks before the earthquake and another increase a few days before the earthquake. (6) The emissions presumably associated with the earthquake are of noise-like nature, and their main frequency is 0.02 similar to 0.05 Hz (with maximum intensity similar to 0.1 nT).