Oblique fault movement during the 2016 Mw 5.9 Zaduo earthquake: insights into regional tectonics of the Qiangtang block, Tibetan Plateau

Abstract

The present east–west crustal extension of the Tibetan Plateau has been demonstrated through field investigations, satellite imagery, geodetic deformation, and earthquake focal mechanisms. Normal faulting earthquakes in the interior Tibetan Plateau are almost entirely confined to regions at elevations over 4000 m. However, our knowledge of the eastward extent of normal faulting in the plateau is still uncertain due to the limited occurrence of well-documented earthquakes. Based on a retrospective analysis of the 2016 Mw 5.9 Zaduo earthquake in the Tibetan Plateau, we consider the NE trending Zaduo-Shanglaxiu fault as the most likely rupture fault through a comprehensive analysis of relocated aftershock sequences, mapped active faults, and newly acquired strain rate tensor. We further determine seismogenic fault geometry using a Bayesian approach and sample with a Markov Chain Monte Carlo method. We interpret the Zaduo earthquake to reflect the release of slowly accumulated elastic strain accumulated mainly by gravitational forces rather than a delay triggering event from the 2010 Yushu earthquake. The viscoelastic calculations to estimate Coulomb stress changes over time indicate that long-term viscous flow in a weak mid-crust can load adjacent faults far more than static stress changes alone. Our results show that the Zaduo earthquake was a Mw 5.9 oblique normal faulting event that occurred in the easternmost part of the Tibetan Plateau, suggesting that the Qiangtang block at longitude ~ 95° E accommodates east–west extensional crustal deformation by small-scale oblique normal faults, which may act as the boundary of micro-blocks. This may also mean that the normal faulting in the Qiangtang block is expanding outwards, and a new rifting system may be formed, which requires more geological evidence.

Appendix. GNSS data processing

To obtain the interseismic crustal movement in the region of the Zaduo earthquake, we collected observation data from 31 campaign GNSS stations of the Crustal Movement Observation Network of China (CMONOC) dotted by black solid balls which were surveyed between 1999 and 2016, 16 continuous GNSS stations from the CMONOC and Qinghai Continuously Operating Reference System (QHCORS) described with red triangles and yellow squares, respectively (Fig. 1b). Continuous stations from the CMONOC and the QHCORS began operating in 2010 and 2013, respectively. The survey-model campaign stations started operations in 1998, with an occupation of at least four consecutive days in each survey. Meanwhile, the nationwide campaign GNSS stations were observed regularly in 2009, 2011, 2013, and 2015 before the 2016 Zaduo earthquake.

All the GNSS data were processed using the latest Bernese GNSS software (version 5.2) developed at the Astronomical Institute of the University of Bern. It is a scientific, high-precision, multi-GNSS data-processing software package that can simultaneously process GPS and GLONASS observation data and supports Galileo satellite navigation system data processing in its latest update version (Yu et al. 2019). Firstly, we processed the observation data along with more than 30 IGS stations in and around the Chinese mainland using a double-difference approach to generate daily solutions. Then, the daily loosely constrained station coordinates were transformed to the ITRF2014 framework (Altamimi et al. 2016) using IGS core reference stations to define the seven-parameter Helmert transformations. Finally, station coordinates and velocities were estimated from position time series. To clearly show interior deformation within the Qiangtang block, interseismic velocities were presented in Table S2, which has transformed into the stable Eurasian reference frame, and the strain field (Fig. 1b) was calculated using a modified least-squares method iterated over a 2D space with arbitrarily small increments to warrant solution continuity (Shen et al. 2015). The crustal strain field can better reflect the internal mechanism’s response to crustal deformation and reveal its possible correlations with seismic activity (Qu et al. 2018).