Figure 1 shows a typical configuration of our ULF-EM observation system. We observe 3 component geomagnetic fields (NS, EW, and vertical) and two horizontal geoelectric components at a site with GPS synchronization. The both electric and geomagnetic sensors are buried about 1-2 m depth. Collected data are digitized at 24 bit AD and fed to the data acquisition PC and transferred to our Laboratory. Observation in Japan is cooperated by EPRC, UEC, and so on.

Fig1. Typical configuration of ULF-EM observation system.

Figure 2 indicates the station map in Japan. We installed 5 stations in Kanto - Tokai area in taking account of the seismically active regions and practical importance in population and social infrastructure in Japan. The detailed location is provided in Table 1. At Matsushiro station, we installed ULF induction magnetometer, acoustic emission sensor, electrodes for electrical potential measurement in the cave cooperated with JMA, who operates the several seismometers, tiltmeter, and strainmeter in the cave. Nishi-Izu and Minami-Boso stations are faced to the one of the most seismically active region and small array observation with inter-sensor distance of 5 km have been performed for accurate observation.
Iyogatake station is also unique station with integrated observation. At Iyogatake station, ULF-EM, atmospheric electricity, GPS, seismometer, radon, ion density concentration, meteorological parameters, and so on.
The circle in the Figure * indicate the 60 km radius and it gives us the empirical detectable distance from source region of M6 earthquake. If a M>6 earthquake happen in the circle, the ULF geomagnetic sensor is expected to register the some anomalous changes.