Field Detection of Microcracks to Define the Nucleation Stage of Earthquake Occurrence

Figure 1

Schematic diagram of multiple component electromagnetic field observations [25, 28] of the full option. The electric field is measured by a pair of horizontal dipoles of 8 m in length with a span of 20~50 m and using a vertical monopole of 600–1,200 m surrounded by wire with a radius about 10–20 m buried at a depth of about 1 m. The magnetic field is measured by three induction-type magnetometers (BF-6, EMI) installed perpendicularly to each other. Triggered data in three electrical and magnetic fields are transmitted to a central station once a night. The present observation at Hasaki was conducted by the simplest model using only borehole sensor with continuous data acquisition.