Strain partitioning, transfer and implications for ongoing processes of the intra-continental graben formation in the NW margin of the Ordos block, China: insights from densified GNSS measurements

GNSS data and processing

1. Intensive observations

In this study, in addition to GNSS data surveyed around the Ordos block provided by Hao et al. (2021), we collected GNSS data surveyed west of 103°E from Phases I and II of the Crustal Movement Observation Network of China (CMONOC), which has conducted measurements every one or two years from 1999 to 2019. In particular, we also collected data from 70 GNSS sites deployed by the National Geodetic Control Network of China (NGCNC). These sites were distributed in the northwestern boundary of the Ordos block and the interior of the Alashan and Ejina blocks and first measured 72 hours of data in 2014 or 2015. From September to October 2021, we conducted the second set of measurements for these GNSS sites of 96 continuous hours.

2. Data processing

The GAMIT package (Herring et al., 2015a) was employed to process the double-differenced carrier phase observations, and daily loosely constrained solutions were obtained. The primary geophysical models and parameters used in the processing, such as the FES2004 tidal loading model, GPT2 global pressure and temperature model (Lagler et al., 2013), and VMF1 mapping function (Boehm et al., 2006), are the same as those in Hao et al. (2021). Instead of incorporating global daily solutions provided by the IGS analysis centers, we also processed data from ~70 evenly distributed global International Terrestrial Reference Frame (ITRF) core tracking GNSS sites using the GAMIT package with the same models. We estimated daily coordinates and uncertainties by constraining the daily regional solutions with global solutions, and the daily free network solutions were transformed into the ITRF2014 (Altamimi et al., 2017) reference frame by utilizing the GLOBK package (Herring et al., 2015b). For campaign GNSS sites, the weighted least-squares adjustment was used to solve the linear velocity with respect to the ITRF2014.

To analyze the differential crustal motion between blocks, we selected the stable Ordos block as the regional reference frame and transformed the GNSS velocity field referenced to the ITRF2014 into the Ordos-fixed frame through Euler rotation. The absolute Euler pole of the Ordos block is located at 75.083°N ± 0.850° and 139.692°W ± 3.701°, with an angular rotation rate of 0.352 ± 0.006°/Ma (Hao et al., 2021).

References

Altamimi, Z., Métivier, L, Rebischung, P., Rouby, H., Collilieux, X., 2017. ITRF2014 plate motion model.Geophys. J. Int. 209:1906–1912

Boehm, J., Werl, B., and Schuh, H. (2006). Troposphere mapping functions for GPS and very long baseline interferometry from European Centre for Medium-Range Weather Forecasts operational analysis data. Journal of Geophysical Research, 111, B0240. doi:10.1029/2005JB003629.

Hao, M., Wang, Q., Zhang, P., Li, Z., Li, Y., Zhuang, W., 2021.“Frame wobbling” causing crustal deformation around the Ordos block. Geophysical Research Letters 48, e2020GL091008. https://doi.org/10.1029/2020GL091008.

Herring, T.A., King, R.W., McClusky, S.C., 2015b.GAMIT reference manual, global Kalman filter VLBI and GPS analysis program, Release 10.6.Massachusetts Institute of Technology, Cambridge.

Herring, T.A., King, R.W.,McClusky, S.C., 2015a.GAMIT reference manual, GPS analysis at MIT, Release 10.6.Massachusetts Institute of Technology, Cambridge.

Lagler, K., Schindelegger, M., Böhm, J., Krásná, H., and Nilsson, T. (2013). GPT2: empirical slant delay model for radio space geodetic techniques. GeophysicalResearchLetters, 40, 1069-1073. doi: 10.1002/grl.50288.