Statistical analysis of the ULF magnetic field data during earthquake swarm

In this paper, we intend to extract the statistical properties of seismo-ULF signatures and to address their physical mechanism. The statistical properties (i.e. cumulative probability distribution function) are calculated by using each half-hour period in a number of frequency bands ranging from 1mHz to 2Hz for the time period of about three years around Izu Island earthquake swarm. The statistical analysis has been performed during the year of the swarm, one year before and after the swarm. As a result, the remarkable change in the shape of cumulative probability distribution starts about one month before the swarm particularly at the frequency band of 6.7mHz to 0.01Hz. After the swarm, the statistical distribution returns to the original distribution. For other two years (before and after the swarm), the cumulative probability distribution does not show systematic dependence as is seen for the year of the swarm, which indicates that the observed changes of the distribution is due to the swarm.