The sand layer strain records are a new-style strain data with some physical basis and prediction significance, whose measure principle is based on granular media theory and whose observational instruments have been installed in the sand layer with very fine particles. The evolution characteristics of sand layer strain observational data and the abnormal forms of earthquake precursory at different distances at Changping and Tongzhou stations have been systematically analyzed to consider that the sand layer strain records are a comprehensive strain response in the crust at different distance, and including the change compositions with extremely complex and meaningful information with very rich. The clear precursor change features and impending informations were remarkably demonstrated before some earthquakes with different epicentral distances and magnitudes. Specifically, there were some unloading variations with large-scales and large amplitudes, and can reflect the precursor images of seismogenic processes to the large earthquakes, which may reveal certain physical models and precursor evidences of seismogenic processes. According to the above reasons and facts, the sand layer strain with the novelty, the unique physical property and certain precursory significance have obvious differences with the other traditional strain.
Based on different scales, granular media is abound in the natural world, such as planets in the universe, the Earth's interior plates, sands on the beach, fault gouges in earthquakes rupture zone, everyone in society, even in the fault surface of the earthquake inside the earth, and so on. Since the 1990s, the theoretical study on granular media began to gradually rise, followed by the rapid development and became one of the best international hot (Sornette, et al. 1994; Jennifer, et al. 2005; Zhao, et al. 2007; Wu, et al. 2007; Peng, et al. 2007). However, this theory in earthquake research and practical prediction involved very little.