Environmental Response Removal And Pre-earthquake Anomaly Extraction Of Borehole Strain Data

China is one of the most affected countries by the earthquake disaster in the world.The earthquake not only damaged people’s lives and property safety,but also triggered a series of secondary disasters such as landslides,mud-rock flows and collapses.Earthquake prediction is one of the difficult scientific problems in the world which needs to be explored for a long time.Earthquake precursor research is the key to earthquake prediction.Earthquake is the result of instability and rupture of underground rock strata after the concentrated stress exceeds the strength limit.The borehole strain gauge can directly observe the crustal strain.The pre-earthquake anomaly of borehole strain is very important for the study of earthquake precursor.In borehole strain observation,the strain response caused by solid tide,air pressure,water level and other environmental factors will drown out the pre-earthquake strain anomalies.Therefore,it is of great social significance to study the environmental response and anomaly extraction method of borehole strain data,and obtain the earthquake related strain anomaly accurately and reliably from the strain observation data.This paper based on the environmental observation data,the strain response caused by environmental factors was solved and removed to obtain the crustal strain.The longterm background of crustal strain after the removal of environmental influence was analyzed,and the negentropy anomaly was extracted.In order to extract small anomalies in the locking stage effectively,the combined extraction method of weighted over-limit anomalies from multiple stations was studied.The main research contents of this paper are as follows:(1)Solution and removal of strain response of environmental factors.Through Time Series Decomposition,the long-term trend of strain data and the strain response of solid tide were obtained and removed.Multi-channel Singular Spectrum Analysis was used to solve the strain response of removing air pressure and water level,and the crustal strain was obtained.Through the frequency analysis of the strain response of the solid tide and the correlation analysis of the air pressure water level and their strain response,it is proved that the influence of the solid tide,air pressure and water level on the borehole strain data can be effectively removed.(2)Single station seismic negentropy anomaly extraction.The probability distribution characteristics of long-term crustal strain were analyzed,and the negentropy anomalies associated with earthquakes were extracted according to the background characteristics of Gaussian distribution.The accumulation of negentropy anomalies of the Lushan earthquake showed a linear increase--acceleration increase--a small amount of anomalies--acceleration increase.Combined with the fracture evolution process of rock stress loading(initial micro-fracture--extensional fracture--stress lock--earthquake eruption),it was inferred that the abnormal acceleration increase of the Lushan earthquake 4 to 6 months before the earthquake may be related to the extensional fracture.Only a few anomalies were extracted in the locking stage before the earthquake.Compared with Benioff strain accumulation method and traditional strain difference method,the effectiveness of the method and the reliability of pre-earthquake negentropy anomaly are verified.By studying the negentropy accumulation of Lushan,Wenchuan and Jiuzhaigou earthquakes,it is found that the negentropy anomaly before the earthquake is related to the epicentral distance and magnitude,but the acceleration time of the negentropy anomaly accumulation can correspond to the acceleration time of the Benioff strain accumulation,which verifies that the extracted negentropy anomaly before the earthquake may be related to the seismogenic process.(3)Multi-station weighted over-limit anomaly extraction.In order to extract the anomalies of the locking stage before the earthquake,the difference of the data of multiple stations was analyzed,and the similar strain components of multiple stations were obtained by Multivariate Variational Mode Decomposition.According to the first law of geography,a weighted over-limit anomaly extraction method is defined to extract the anomalies in the pre-earthquake locking stage.The analysis of four earthquakes in Sichuan-Yunnan region shows that the abnormal growth occurs in different degrees within 30 days before the earthquake,and there is a certain correlation between the weighted over-limit anomaly before the earthquake.By analyzing the weighted over-limit anomaly in the locking stage of the Lushan earthquake,it is found that the weighted over-limit anomaly increases significantly before the earthquake.It is speculated that this anomaly may be the precursor signal of the Lushan earthquake,which verifies that the weighted over-limit anomaly can be extracted to the anomaly in the locking stage more comprehensively.