Abstract: Earthquake precursor observation provided the earthquake information generally including the locality of anomaly, the variable of anomaly, the duration of anomaly, the effective extension of time after anomaly completion, therefore, the earthquake synthetical information variable S-value is proposed to describe the earthquake anomaly. Taking synthetical mathematic expression of the earthquake anomaly information as base, the smooth filter, disappearing cycle, tendency item as well as the data processing method, such as the subordinate function method, are used to compute and extract the value S from 67 sets of observational data in Northwest China region. The results show that the S value synthetical information during the period of May 1986 to March 2005, 10 earthquakes with M_S≥5.0 in the Northwest China region appears very high anomaly (S=0.154).

Key words: Subordinate function, S-value, Precursory anomaly, The Northwest China region