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Abstract
Borehole strainmeters that are installed deeply into bedrock are capable of recording both continuous stress and strain measurements, and have consequently become an important tool for monitoring crustal deformation. A YRY-4 borehole strainmeter installed at the Guza Station recorded anomalous changes in borehole strain data preceding the Wenchuan earthquake on May 12, 2008 (UTC) (=8.0). We apply principal component analysis (PCA) to analyze borehole strain data from the Guza Station. The first principal component eigenvalues and eigenvectors are calculated. The fitted results of the cumulative number of anomalous eigenvalues demonstrate that an acceleration occurred approximately 4 months before the earthquake (from January 2008). The results of the combined eigenvalue and eigenvector analyses show that the spatial distribution of eigenvectors and accelerated occurrence of eigenvalue anomalies represents the stress evolution characteristics of the fault from a steady state to a sub-instability state in rock experiments. We tentatively infer that this process may also be linked to the preparation phase of a large earthquake.
Highlights
The first borehole dilatometer was invented by Sacks, of Carnegie Institution of Washington, and Evertson, of University of Texas, in September of 1968 [Sacks and Evertson, 1971]
The eigenvalues and eigenvectors of the first principal component of time series borehole strain data are used to analyze the characteristics of strain variation before the Wenchuan earthquake
The results of the analysis indicate that the borehole strainmeter of Guza station recorded the preparation phase of the Wenchuan earthquake, and principal component analysis (PCA) is capable of effectively extracting the features of crustal strain changes
Summary
The first borehole dilatometer was invented by Sacks, of Carnegie Institution of Washington, and Evertson, of University of Texas, in September of 1968 [Sacks and Evertson, 1971]. Thanks to the Plate Boundary Observatory (PBO) project, the development of the high-resolution borehole strainmeters in China has been accelerated [Qiu and Zhang, 2002]. The YRY-4 four-gauge borehole strainmeter, designed by Chi Shunliang, has been developed and there are more than 40 four-gauge borehole strainmeters sites in China [Chi et al, 2009]. Earthquakes will deform the Earth’s crust and borehole strain observation play an important role in studying this crustal deformation and the stress field. The observations have many advantages such as high resolution, broad band, and topographic-feature-independence, amongst others. High-resolution borehole strainmeters allow for the detection
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