Remote Detection of the Electric Field Change Induced at the Seismic Wave Front from the Start of Fault Rupturing
(a) Monitoring records at the Hasaki site on 14 June, 2002, when a moderate earthquake with a magnitude of 5.2 (E1 in Figure 1) with depth 57 km occurred about 100 km west from the borehole observation site. The recorder was a low-speed analog machine recording at 1.5 cm/h. The focal parameters were chosen by the National Research Institute for Earth Science and Disaster Prevention (NIED). Three channels are shown corresponding to VLF/ELF, ULF, and DC bands. In the DC (black) and ULF (blue) bands there appeared two similar anomalies (A1, A2) and second type of variation (B), both of which are clearly different from the normal state. The fluctuating parts in the DC channel are enlarged in the ULF band. There are no anomalies found in the VLF/ELF. Anomalies A1 and A2 are arged to be immediate-term precursory phenomena to the earthquake, not of urban or natural origin, including meteorological or geomagnetic causes. (b) Monitoring records of the vertical electric field observed at the Hasaki site in a typical normal state on 12 June, 2002. Amplification parameters were chosen in the experimental period of approximately three months so that the background noise level would be less than 1/10 of the full scale in the paper recording.