Deformation Observation Evidence of Sub—Instability State

It is proved by rock mechanical test that variation of stress-strain curve from almost linear to nonlinear exists before principal rupture occurs and that the periodicity of rock rupture, that is the four stages of long term, medium term, short term and instant term is shown .There is a sub-instability period between peak value of stress and rapid instability point. The process, during which the relation of stress and strain may appear nonlinear fluctuation ,and which is the key stage of the instability entering of fault, of sub-instability is the stalemate from strain accumulation to strain release .(MA Jin,2012).

Sampling rate of previous seismic precursor observation data was low and the transmission was hysteretic. It was difficult to gain image of sub-instability. Wide use of observation techniques of broadband and high sampling rate in recent years offers condition to obtaining of deformation observation evidence for study of sub-instability. The process of quasi-linear stage, stage of linear deviation before instability, stage of sub-instability after the peak and stage of near instability is recorded clearly by Linfen broadband tiltmeter before the 2010 Hejin earthquake of M4.8 . Observation data from April, 2009 to July, 2009 is based on almost linear variation; Nonlinear variation deviating linear appears in August; Data begins to drop slowly after the peak reaches in October and that is stage of sub-instability; Data appears quickening drop in December and turns flat on Dec.21, 2009.35 days later the earthquake of M4.8 occurs 89km away from the observation station. It is illustrated that sub-instability can be proved through deformation observation data.