Analysis Of Borehole Strain Earthquake Precursory Observation Data Based On Wavelet Transform

Borehole strain monitoring is an important part of the earth dynamics observing system,which use strain sensor installed in borehole to observe the crustal strain.Borehole strainmeters are favorite instruments because of their higher resolution,good frequency response,the advantages of the anti-interference ability is strong,its order are widely used in seismic strain step,slow earthquake,earthquake precursor and volcanic eruption forecasting.The borehole strain monitoring network over China has been carried out with a large number of high quality observation data,it is possible to study the earthquake precursors,the earthquake prediction and so on.A study of the Wenchuan earthquake?Ms8.0?on May 12,2008 and the Lushan earthquakes?Ms7.0?on April 20,2013base on the method of discrete wavelet transform to decompose the high frequency part of Guzan stations borehole strain data,and calculate the detail coefficient of amplitude spectrum of data,and study the variation character;In order to study the frequency variation of the borehole strain data in the high frequency part,Using time-frequency analysis method base on continuous wavelet transform method to extract the high frequency anomaly in the frequency spectrum of borehole strain data,and contrast analyze the frequency distribution of strain anomaly in the different periods,and analyze the frequency distribution of weak strain anomaly and the correlation of two earthquake precursor strain anomaly frequency.The main research work as follows:Based on the borehole strain observation theory,the characteristics of borehole strain data in time and frequency domain are studied,the frequency distribution of solid tide and strain anomaly is compared,and the pre processing algorithm of borehole strain data is studied.It is proved that the first order difference method can effectively reduce the influence of the constant extraction of the corresponding variation of solid tide.Based on the discrete wavelet transform theory,we determine the10th Daubechies wavelet basis function with more obvious effect in frequency division of the borehole strain data;According to the statistical characteristics of the detail coefficients of each frequency band,the over-limit method is used to extract the over-limit anomalies.As for the Wenchuan earthquake?Ms8.0?,mainly concentrated between the 6.5×10^-5Hz to the 5.2×10^-4Hz until a few days before the Wenchuan earthquake?Ms8.0?.What's more,in the case of the Lushan earthquake?Ms7.0?,During two periods from October to December 2012 and April 2013,the over=limit anomalies had the significant amplitude changes in each frequency band.Based on continuous wavelet transform theory,the time frequency characteristics of the same earthquake signal,strain step and earthquake precursor anomaly are analyzed.In the large scale time range,the whole time frequency characteristic of the borehole strain data is studied,the strain anomaly is extracted in the time spectrum,and the frequency correlation of the strain anomaly and the ground earthquake precursor anomaly is analyzed.Partial strain anomalies are suspected precursory anomalies.For the Wenchuan earthquake?Ms8.0?,the strain anomaly is concentrated in the period from May 8,2008 to May 12,2008.For the Lushan earthquake?Ms7.0?,the strain anomaly is mainly concentrated in the period from April 1,2013 to April 20,2013,and the frequency correlation coefficient of the strain anomaly and the two earthquake precursor anomaly are all greater than 0.6.We speculate the extracted strain.We speculate that the strain anomaly extracted is a suspected precursor anomaly.