Main shocks of natural earthquakes are known to be accompanied by preshocks which evolve following the modified Ohmori’s law in average over many samples. Individual preshock activity, however, is far less systematic for predictive purposes. On the other hand, the microcracks in laboratory rock experiments are always preceded to final rupture. And, previous investigations of field acoustic emissions showed that the activity increases prominently before and after the main shock. But there is no detection of any phenomena to identify the nucleation stage. Here we show that a special underground electric field measurement could detect microcracks. Pulse-like variations were classified into three groups (A, B, C) by frequency. The B-type is suggested to define the nucleation period: activity increases sharply following the modified Omori’s law before the main shock and there is no activity afterward. The B-type is subgrouped into three types possibly corresponding to crack-rupture modes. The variations are supposed to be induced by crack occurrence through electrokinetic effects in the elastic-porous medium. The detection distance is suggested to be several orders larger than that of the acoustic emission due to the effective smallness of dissipation rate, and the waveform can be used to infer the rupture mode. 1. Introduction The identification of anomalous phenomena in each stage of seismic cycle is a prerequisite for understanding physical processes to be applied to earthquake forecasting (e.g., Mogi [1], Rikitake [2], and Scholz [3]). For the short-term prediction, numerous observations by seismic, geodetic, hydrologic, and electromagnetic approaches have been tried to investigate various phenomena to find precursory phenomena and to build physical and statistical models [1, 3]. Among all, the preshock has been considered to be the most direct and plausible phenomenon for identifying the earthquake nucleation stage. Previous investigations of preshock activity of natural earthquakes or microcracks activity in laboratory experiments (e.g., [1, 3], Jones and Molnar [4], Ohnaka [5], and Maeda [6]) have provided essential evidence as accelerating activity. Those evidences have been utilized to build the physical and statistical models of nucleation process [1, 5]. These models have been further developed: for instance, superimposed undulation of foreshock activity was utilized to infer dynamical critical phenomena to estimate more exact final rupture time (e.g., Sornette and Sammis [7], Kapiris et al. [8], Eftaxias et al. [9], and many papers referred in
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