A way to detect the seismic precursor in granular medium is described and a model of propagation for precursive stress-strain signals is proposed.A strain sensor buried in a sandpit is used to measure a seismic precursor signal.The signal has been investigated and confirmed to originate from a specific earthquake.A comparison of simulated and experimental signals indicates that the signal results from the strain in the earth's strata.Based on the behavioral characteristics of granular materials,an analysis of why this method can be so sensitive to the seismic strain signal is undertaken and a model for the propagation of this stress-strain signal is proposed.The Earth's lithosphere is formed of tectonic plates,faults and fault gouges at their boundaries.In the case of the quasi-static mechanics of seismic precursory stress-strain propagation,the crustal lithosphere should be treated as a large-scale granular system.During a seismogenic event,accumulated force generates the stick-slip motion of adjacent tectonic plates and incrementally pushes blocks farther apart through stick-slip shift.The shear force released through this plate displacement causes soil compression deformation.The discrete properties of the sand in the sandpit lead to the sensitive response of the sensor to the deformation signal which enables it to detect the seismic precursor.From the analysis of the mechanism of the stress-strain propagation in the lithosphere,an explanation is found for the lack of signal detection by sensors installed in rocks.The principles and method presented in this paper provide a new technique for investigating seismic precursors to shallow-source earthquakes.
