Earthquake precursors in Japan: Precursor time and detectability

Abstract Earthquake precursor data amounting to 418 in number accumulated in Japan are analyzed. Most data are obtained under the nation-wide earthquake prediction programme. The precursor time ( T ) versus main shock magnitude ( M ) relationship is different for different groups of precursors. A linear relation such as: log T = a + bM where a and b are constants, seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide-gauge observation, ground tilt observed by a water-tube tiltmeter, anomalous seismic activity and geomagnetic field change. Although a similar relationship seems to hold for the disciplines of earth-currents, radon concentration, underground water etc., the precursor time is substantially smaller for these disciplines than for the former disciplines. The above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity; precursors probably associated with diffusion of underground water into the dilatant region seem to follow. For a number of precursor disciplines such as foreshock, ground tilt observed by a pendulum tiltmeter, resistivity by a variometer and so on, no dependence of T on M is found. The relationship between M and the logarithmic epicentral distance ( D ) measured in kilometers is different from discipline to discipline. Relying on empirical and theoretical studies of coseismic strain step, it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 −7 -10 −6 . Precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 −9 −10 −8 . Precursors of other disciplines seem to correspond to a strain of 10 −8 −10 −7 . It is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger. In some favourable cases, a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area.