Strong Ground Motion Recorded by High-Rate GPS during the 2021 Ms 6.4 Yangbi, China, Earthquake

The 2021 Ms 6.4 Yangbi earthquake, Yunnan, China, was recorded by the Dali Global Positioning System (GPS) network comprising 37 permanent stations within 100 km of the epicenter. All of these GPS stations recorded 1 Hz data, and 12 of them also recorded 5 Hz data. Using sophisticated data processing strategies, especially the method to overcome ionospheric and multipath errors, near-field waveforms of epoch-by-epoch displacement of all of the GPS stations within an epicentral distance of similar to 50 km were derived from high-rate GPS observations with a root mean square error of 3.8, 4.2, and 8.2 mm for the east-west, north-south, and up components, respectively. The peak ground displacement of up to 14 cm in the horizontal direction and 4.2 cm in the vertical direction and the largest coseismic displacement of 3.1 cm in the horizontal direction and 3.2 cm in the vertical direction were observed for the stations within 6-9 km of the epicenter. The waveforms, with dominant periods between 7 and 10 s, present a systematic change in shape as a function of distance from the source, which demonstrates that the high-rate GPS observations can provide reliable relativedisplacement response spectra at the periods needed in the design of large structures for resisting strong earthquakes. Comparisons of the GPS displacement time series at 1, 2.5, and 5 Hz suggest that 5 Hz data are able to capture strong ground information that are of interest to both earthquake seismologists and engineers. We conclude that the displacement waveforms and motion trajectories from high-rate GPS observations provide unique additional information of the near-field strong ground motion that is valuable to seismologists in unraveling the dynamic process of fault rupture and to engineers for designing large structures with very long period response.