11:30 〜 11:45
[SCG10-08] 地震発生直前の微小クラックの特性
キーワード：Microcrack, Earthquake Prediction, Nucleation Stage, Seismo-electric- signal, Electrokinetic effect, Tohoku Earthquake
At the last JpGU meeting we reported that a deep underground electric field measurement using special antenna could detect micro-cracks appearing in the nucleation stage of the Tohoku Earthquake (Fujinawa et al., 2013). Here we report several results of further analysis on the characteristic pulse-like phenomena. 1) Detection Distance: Some events of B-type variation have clear first and second phases (Figure 1). The S-P time is 25ms corresponding to about 180m of the epicentral distance. Majority of events have no apparent P phases due to the small strength of the P phase and/or large dissipation. The detection distance of P phase is about 200m. On the other hand the S phase of the frequency of some 100Hz and amplitude of 2mV suggests detection distance of some 10km, much larger than that of the acoustic emission signal of order several hundred meters by elastic observation. The characteristic electric field variation induced by crack through electro-kinetic mechanism have been discussed by systematic formulations ( Pride, 1994; Revil and Leroy, 2004). As to the wave mode, there are four kinds of wave, slow P and fast P wave (ordinary p wave), S-wave (ordinary S wave) and electromagnetic wave (EM). Events containing P phase have occasionally small forerunners at about the origin time possibly corresponding to (see Fig.1). 2) Correspondence to main shock:The seismological approaches (e.g., Kato et al., 2012) showed that there were two slow seismic slip events from mid-February to the Tohoku Earthquake and microearthquake activities around the foreshocks and mainshocks. Those activities were whole around the epicentral zone, about 300 km northeast from the observation site. The detection distance of the electric field change by the borehole antenna is at most 100 kilometer. Our observational evidence including temporal evolution of the microcrack activity and b value of 0.7 suggest that the micro-cracks of B-type are related to the nucleation process of the main shock, though they occurred at the edge of the giant rupture area. We propose that the nucleation process is not limited at around the asperity, but extends to whole rupture zone. More extensive monitoring of the microcrack of magnitude less than -5 can provide clue to this question.3) Intermittent CriticalityThere appeared undulation of microcrack activity after the most active period around 9th March , 2011. The undulation has been suggested to reflect the intermittent criticality indicating another phase of nucleation (Sornette and Sammis, 1995; Kapiris et al., 2005). As approaching to the main shock there appeared two kinds of events. One kind is a superposition of several smaller events. It is interpreted that small events substantially increased with the result of picking up more smaller events in the time interval of data length of 100ms. The second kind is like a long chain of small events. These feature suggest that microcrack activity has changed at the last stage of nucleation stage. 4) It is well known that the crust of the earth is elastic-porous medium filled with fluid as water. The research on the rupture of such kind of medium had a remarkable development in early 1990 contributing to interpret the mysterious seismo-electric phenomena associated earthquakes enabling systematic treatment and suggesting new method of geophysical prospecting. The formulation of Pride and Revil have been used to interpret the phase of faster propagation of EM signal with velocity much larger than the p-wave speed (Fujinawa et al., 2011), the ULF band anomalies associated the slow-slip (Han, 2013) . And, our electromagnetic method has been suggested to detect micro-cracks preceding natural earthquakes to identify the nucleation stage providing a break-through for the short term prediction method. The converted electromagnetic mode at the material contrast from elastic seismic wave has been proved to be profitable means to survey for oil and gas.