By means of wavelet decomposition and the help of a modified Overrun Rate Analysis (ORA), the tiny anomaly can be extracted from the raw data. Generally, a band-passed signal obtained with wavelet decomposition can be divided into two parts: normal “background” and abnormal “bad points”. An approach to locate the objective as well as exact distinction between the two parts has been developed for the determinate threshold of ORA. According to the analysis, the extent of strain anomaly along the seismic fault of the Wenchuan earthquake 2008 is much larger than previously recognised. The anomaly is characterized by small pulses in consistence with peaks of Earth Tide, providing a clue to search for its mechanism.
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