Field Detection of Microcracks to Define the Nucleation Stage of Earthquake Occurrence
Figure 6
Monitoring records of normal state and active state. (a) DC channel record on March 31, 2011 corresponding to the normal state without any short period fluctuation of large strength except for a diurnal variation due to the earth and ocean tide. There were no conspicuous environmental noises to make it difficult to detect anomalous signals. (b) DC channel record on March 25 with pulse-like fluctuations around 1:00 and 23:00. The signals are very similar to those found in the analog record at several cases of crustal activities [25, 26]. The strength of the signal is about twice as large as the earth tide. (c) The AC channel record on March 4 corresponding to the normal state except during the time period 17:00–19:00. During the calm state there were no large variations except for the atmospherics and environmental noises comprising a horizontal belt-like part. (d) AC channel records on March 9 corresponding to the most active day. The amplitude of the pulse has a distribution, and the positive polarity dominates compared to the negative polarity because of the pulse-like signals are of B type having B-1 and B-3 types of unbalanced polarity. The peak value is diminished because of averaging procedure for dairy monitoring.
