Abstract: This paper has introduced the precursory regularity by means of analyzing the exam­ples of success and failures in earthquake prediction. For example, if an earthquake with magnitude of more than 6. 0 occurs in an area where the network has a certain capability to monitor the earthquake, we can observe more or less precursory phenomena. The bigger the earthquake is, the more the precursory anomalies occur prior to the earthquake, the wider the precursory anomalies are distributed, the longer the anomalies last, and the larger the anomalous amplitude becomes. Meanwhile, the expression form of the precur­sory complexity is emphatically and systematically generalized and arranged. It mainly in­eludes as follows: Uncertainty of relation between anomalies and earthquakes, mainly ex­pressed as "There is the anomaly, but no earthquake follows" and "Earthquakes occurred without any anomaly prior to them"; Different precursory features in time and space are shown in different regions; The precursor even in the same region are not repetitional; It is difficult to distinguish the precursor from aftereffect; Complexity of the precursory ge­netics; Variety of the short-term and imminent precursory anomalies; Non-synchronism of precursory changes in the initial and turning point; Inhomogeneity and multi variability of precursors in spatial distribution; Uncertainty of the relation between the amplitude of precursory anomaly, earthquake magnitude and epicentral distance; Different characteris­tics of anomalies in active period and quiet period. Combined with earthquake cases the above-mentioned phenomena are briefly explained in this paper.

Key words: Earthquake precursor, Seismic focus, Fracture mechanics, Precursory monitoring, Field and source, Precursory complexity