Identification of ULF Emissions for Izu Islands Earthquakes in July 2000

摘要

The purpose of this report is to find out any ULF signature for the earthquakes in Izu islands in July 2000 (associated with volcanic activity) by means of a new signal processing method. The ULF data observed at closely-spaced three stations in Izu peninsula, are subjected to the principal component analysis (PCA), and we have found that this PCA is very effective in distinguishing among different sources of ULF emissions (geomagnetic variation, man-made noise, and seismogenic emission). While, the first and second principal components are found to be associated with geomagnetic variation and man-made activity, respectively, the third principal component (residual noise like seismogenic emission if it exists) at night has indicated a significant temporal evolution; an enhancement for about 1.5 months (from early May to middle June) and a conspicuous minimum about ten days before the first earthquake on July 1 (magnitude 6.4), followed by a sharp increase a few days before the quakes. Furthermore, this component is found to be back to the background level a few months later than the quakes. This temporal pattern is in good agreement with those for previous large earthquake (Spitak, Loma Prieta, Guam and Biak earthquakes), so that this temporal evolution for the third principal component found by PCA is likely to be a precursory ULF signature of those Izu islands earthquakes with its intensity of the order of 5% of the first principal component.