| Summary: | The aims of the present study were to use Langmuir Probe payload electron density data and Plasma Analyzer Package payload O<sup>+</sup> density data from the Zhangheng-1 electromagnetic satellite to statistically analyze anomalies in electron and oxygen ion densities before strong earthquakes (<i>M<sub>s</sub></i> ≥ 6.0) in western China and its neighboring areas. The goal was to investigate the physical mechanisms underlying electron and oxygen ion generation by evaluating the correlations between such anomalies and the seismic activity before the 6.4-magnitude earthquake in Yangbi, Yunnan, China, on 21 May 2021. Nine (75%) of the twelve earthquakes that occurred during the study period and were not affected by magnetic storms were preceded by anomalous electron or oxygen ion densities of 1.1–4.5 × 10<sup>10</sup>/m<sup>3</sup> and 2.8–6.0 × 10<sup>10</sup>/m<sup>3</sup>, respectively. The anomalies were generally observed within the two weeks preceding the earthquakes and were associated with most strike-slip and thrust earthquakes, which were mainly located on the southeastern and northwestern edges of the Tibetan Plateau—but not normal fault earthquakes. The anomalies were likely the result of acoustic-gravity waves generated by slow vibrations of the Earth’s surface reaching the ionosphere, where they cause oscillations in ionospheric electron and ion densities. In addition, the association between ionospheric anomalies and strong earthquakes was confirmed by the observation of other atmospheric anomalies before the Yangbi earthquake.
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