Chie Yoshino

Statistical analysis of ULF seismomagnetic phenomena at Kakioka, Japan, during 2001–2010

To clarify and verify the ultralow frequency (ULF) seismomagnetic phenomena, we have performed statistical studies on the geomagnetic data observed at the Kakioka (KAK) station, Japan, during 2001–2010. We investigated the energy of ULF geomagnetic signals of the frequency around 0.01 Hz using wavelet transform analysis. To minimize the influences of artificial noises and global geomagnetic perturbations, we used only the geomagnetic data observed at nighttime (LT 2:30 A.M. to 4:00 A.M.) and utilized observations from a remote station, Kanoya, as a reference. Statistical results of superposed epoch analysis have indicated that ULF magnetic anomalies are more likely to appear before sizable earthquake events (Es > 108) rather than after them, especially 6–15 days before the events. Further statistical investigations show clearly that the ULF geomagnetic anomalies at KAK station are more sensitive to larger and closer events. Finally, we have evaluated the precursory information of ULF geomagnetic signals for local sizable earthquakes using Molchans error diagram. The probability gain is around 1.6 against a Poisson model. The above results have indicated that the ULF seismomagnetic phenomena at KAK clearly contain precursory information and have a possibility of improving the forecasting of large earthquakes.

Statistical analysis of ULF geomagnetic anomaly preceding earthquake at Kakioka, Japan, during 2001–2010

In order to verify and clarify the ULF seismo-magnetic phenomena, a statistical investigation of the relation between magnetic anomalies and local seismicity has been performed. Geomagnetic data observed during 2001-2010 at the Kakioka station has been analyzed. Energy of ULF geomagnetic signals at the frequency around 0.01Hz in nighttime has been investigated using wavelet analysis. To remove anomalous changes due to Ionospheric disturbances, the Kanoya station has been chosen as a reference station. Statistical studies based on the superposed epoch analysis have indicated that before a sizeable earthquake there are clear higher probabilities of ULF anomalies than that after the earthquake: statistical results of daily counts were found significant at about 30 days before, 2 weeks before, 5-7 days before and 2 days after the event. These results are somehow consistent with previous studies in Izu and Boso Peninsulas, which suggests a correlation between ULF geomagnetic anomalies and local seismicity. Further investigations on the Molchan error diagram indicate that local ULF geomagnetic anomaly do contain precursory information of sizeable earthquakes, with a probability gain around 1.5 against a Poisson model. These results show that the ULF seismo-magnetic phenomena at Kakioka contain precursory information and have a possibility of forecasting of large earthquake. The details will be shown in the presentation.

Reduction of ULF external source origin signals (periods t<1,000 s) using of interstation method and wavelet transform

Summary form only given. Ground-based observation of ULF geomagnetic field changes is one of the most promising methods to monitor seismic and volcanic activity due to the skin depth. The observed ULF magnetic and electric potential fields are considered to be the superposition of several possible signals. Intense external source fields in periods of 1-1000 sec include those associated with solar-terrestrial interactions, noise from DC motors, and local sources such as movement of magnetized objects at a site. Therefore, signal separation and effective methods for discrimination of ULF geomagnetic signals are significant. We present an effective time series analysis for reducing the field variations originating from external sources. Our approach, which uses reference geomagnetic data and ULF data collected at different sites, is called the interstation transfer function (ISTF) method. We have adopted a wavelet transform using the Morlet wavelet for the ISTF approach. We have applied the proposed ISTF method to actual data. Results indicate that the proposed ISTF method is effective in reducing signals originating from external source field variations in time periods less than 1000 sec and to enhance the other signals. Both model computation of SES detection and application to the 2002 Boso slip show the capacity of the proposed method.

ULF electromagnetic phenomena induced by underground activities in Izu and Boso Peninsula, Japan during 2000–2010

Recently electromagnetic phenomena have been considered as a promising candidate for short-term earthquake prediction. In this study, we have investigated ULF (Ultra Low Frequency) electromagnetic phenomena possibly associated with underground activities in Izu and Boso Peninsula, Japan during 2000–2010. Daily average energy of geomagnetic signals at the frequency around 100Hz has been examined, and geomagnetic diurnal variation at each station has been investigated. It is found that there are highly confirmed anomalous electromagnetic signals during the 2002 and the 2007 slow slip events in Boso Peninsula. And geomagnetic diurnal variations observed at Boso Peninsula also exhibit unusual behaviors before the 2005 M6.1 and M6.0 earthquakes.

Geoelectric potential differences and geomagnetic data observed at Southern Kanto area, Japan

Summary form only given. In order to investigate the relationship between electromagnetic phenomena and crustal activity, sophisticated geoelectric potential difference and geomagnetic field measurements have been realized since February, 2000. The frequency range is ULF (f<1 Hz). The ULF geomagnetic anomalous changes associated with the crustal activity have been reported based on polarization analysis, which is the spectral density ratio between horizontal and vertical components, using principle component analysis. We observe both 3 magnetic and 2 electric components on the ground simultaneously. In this talk, the electromagnetic environment near the station is investigated over a long period of time. The relationship between polarization and regional seismicity shows that there seems to be a threshold of seismic energy at the site and it is concerned with enhancement of polarization. We remark on the behavior of the orientation on various frequency bands. Further, we calculated the variation of transfer functions, and the variation of MT impedance.