Evaluating seismometer orientation of ChinArray-Ⅰ using teleseismic P-wave and Rayleigh-wave polarization

The accurate estimation of seismometer orientation is considered essential for seismological studies that depend on three-component seismic records. It is necessary to align the north-south horizontal component with the geographic north during station deployment to achieve accurate recording and analysis of seismic waveforms. Orientation deviation is influenced by a variety of factors. Ocean-bottom seismometers cannot achieve alignment with geographic directions due to limitations of the deployment method. Local magnetic anomalies or incorrect corrections for magnetic declination corrections can cause the magnetic compass fail to align to geographic north when seismometers are deployed on land. Ground subsidence and instrument movement during the operation stage can additionally cause orientation deviations. It is therefore considered an important step in enhancing the accuracy of subsequent research to employ seismological methods to estimate the orientation deviation during instrument deployment.

Because of the large surface deformation and high seismicity in Yunnan Province, many seismic studies have been carried out in this area using data recorded by China Earthquake Administration （CEA） permanent stations. However, the large distances between the stations of the permanent seismic stations can lead to the phenomenon of low resolution of the research results on a small scale. In order to conduct an in-deep study of the seismicity and deep structure of the region, 350 broadband transportable array were deployed in Yunnan and the surrounding areas of the southern section of the North-South Seismic Belt during 2011 to 2013, known as the ChinArray phase Ⅰ . Due to changes in the observation environment, the positions of two stations were shifted by a few kilometers in the middle of the deployment period during the observation period, so that a total of 352 stations were observed in the study area.

We estimate the seismometer orientations of ChinArray phase Ⅰ . Two independent polarization analysis methods, teleseismic P-wave and Rayleigh surface wave, are used for the study, and the two methods relies on the polarization characteristics of seismic waveforms. We selected P waves from 696 teleseismic events located in the epicentral distances range of 30 to 90 degrees, and Rayleigh waves from 1 928 events located within the range of 10 to 170 degrees of epicentral distances, and used both waveform data for orientation estimation, respectively.

By comparing the two methods, we found that the results were very consistent. The comparison shows that out of 348 stations that were measurable, 343 of them have an average difference of less than 10 degrees between the results obtained using the two methods, and only five stations have a difference of more than 10 degrees. There are some errors in the different measurement methods, considering that the means of orientation deviations for each station obtained from P-waves and surface-waves are similar, and that P-waves have a smaller overall dispersion, the P-wave measurement results were utilized in this thesis for a statistical analysis of the orientation deviation of the ChinArray phase Ⅰ . The statistical results indicated that orientation deviation measurements for 281 stations were less than 10 degrees, 21 stations have seismometer orientation deviations between 10 to 20 degrees, and the remaining stations encountered instrument failure or greater deviations. The results of this study are similar to previous analyses of teleseismic P-waves, but deviations greater than 5 degrees were observed at seven stations. It was further determined that the primary cause of these deviations was changes in the orientations of instruments at some stations during deployment, affecting five stations. For the other two stations, we analyze the noise level of the stations based on the power spectrum analysis method of probability density to detect the observation quality of the stations during the observation period, and find that the relevant anomalous measurements are due to the gain failures of the horizontal components.

In the current methodologies, the error range for measurements obtained from P-waves is typically smaller, while surface-waves measurement are relatively discrete. Surface wave-based measurements possess a higher temporal resolution when it comes to resolving time variations of orientations. High-quality Rayleigh surface-wave signals are easier to obtain than teleseismic P-wave, but the local subsurface structures lead to large error values. A broader range of epicentral distances can be chosen when we use surface waves to achieve better orientation coverage and stable results.

The results in this paper and a number of previous studies show that both teleseismic P-wave and Rayleigh surface-wave polarization methods can provide effective estimates of orientation deviation in seismometer placement after reliable data quality control. And the methods and outcomes of this research provide orientation correction information for the ChinArray phaseⅠand contribute to the data quality assessment of other transportable stations.