ULF Seismo-electromagnetic Studies in Kanto, Japan: Progress and Challenges

Abstract:

In order to clarify the ULF seismo-magnetic phenomena, a sensitive geomagnetic network has been installed in Kanto, Japan. In this study, we have analyzed geomagnetic data observed during the past decade in Izu, Boso, and Kakioka (KAK) stations. To verify the relation between ULF geomagnetic anomalies and local seismicity, statistical studies by superposed epoch analysis (SEA) have been carried out. The results have indicated that before a sizeable earthquake there are clearly higher probabilities of ULF anomalies than after the earthquake: for Seikoshi (SKS) station in Izu, about 20~30 days before, one week and few days before, and one day after the event statistical results of daily counts are significant; for Kiyosumi (KYS) station in Boso around two weeks before, few days before, and one day after the event; For KAK station, 30 days before, about 2 weeks before, few days before, and 2 days after the event.

To find out the detailed waveform of anomalous magnetic signals, waveform analysis has been performed. The results show that there are mainly two kinds of seismo-magnetic signature. (1) Noise-like signals: Compared with the background, the signals exhibit small increases of amplitudes at a wide frequency range. (2) Transient/quasi-rectangular signals: the signals have transient/quasi-rectangular waveforms with amplitudes of several nT. The noise-like signals usually persist for several days or even a few weeks, and are mainly associated with large earthquakes; the transient/quasi-rectangular signals have durations of few seconds to few ten seconds, and are registered mainly during slow slip events.

Finally, we have evaluated the precursory information of ULF geomagnetic signals for local sizeable earthquakes using Molchan’s error diagram. The probability gain is around 2-4 against a Poisson model. The above results have indicated that the ULF seismo-magnetic phenomena clearly contain precursory information and have a possibility of improving the forecasting of large earthquakes.