###### Abstract  
Currently, it is still challenging to detect earthquakes by using the measurements of Global Navigation Satellite System (GNSS), especially while only adopting single-frequency GNSS. To increase the accuracy of earthquake detection and warning, extra information and techniques are required that lead to high costs. Therefore, this work tries to find a low-cost method with high-accuracy performance. The contributions of our research are twofold: (1) an improved earthquake-displacement estimation approach by considering the relation between earthquake and ionospheric disturbance is presented. For this purpose, we propose an undifferenced uncombined Single-Frequency Precise Point Positioning (SF-PPP) approach, in which both the ionospheric delay of each observed satellite and receiver Differential Code Bias (DCB) are parameterized. When processing the 1 Hz GPS data collected during the 2013 Mw7.0 Lushan earthquake and the 2011 Mw9.0 Tohoku-Oki earthquake, the proposed SF-PPP method can provide coseismic deformation signals accurately. Compared to the results from GAMIT/TRACK, the accuracy of the proposed SF-PPP was not influenced by the common mode errors that exist in the GAMIT/TRACK solutions. (2) Vertical Total Electron Content (VTEC) anomalies before an earthquake are investigated by applying time-series analysis and spatial interpolation methods. Furthermore, on the long-term scale, it is revealed that significant positive/negative VTEC anomalies appeared around the earthquake epicenter on the day the earthquake occurred compared to about 4-5 days before the earthquake, whereas, on the short-term scale, positive/negative VTEC anomalies emerged several-hours before or after an earthquake. s 2022, 14, 4286. [[https://doi.org/10.3390/rs14174286](https://doi.org/10.3390/rs14174286)]([https://doi.org/10.3390/rs14174286](https://doi.org/10.3390/rs14174286))  
displacement inversion [10], coseismic slip distribution [11], and preseismic deformation [12]. Compared to other technologies like the InSAR-based method [5] and the gravity recovery and climate ex-periment (GRACE)-based method [13], GNSS has advantages such as low-cost hardware, wide-coverage signal, high-accuracy observation, and the capability of all-weather monitoring. In addition, GNSS-measured displacements will not suffer from drift, clipping, or instrument tilting errors. These characteristics make GNSS particularly valuable in earthquake monitoring [4]. In general, two GNSS high accuracy positioning methods, namely, Post Processing Kinematic (PPK) and Precise Point Positioning (PPP) can be used in coseismic displacement inversion. The PPK method is used as an efficacious technique, because it adopts the double-differenced mode, which removes common biases such as the satellite orbit-related biases, atmosphere-related errors, and receiver-dependent offsets. There are extensive researches on the PPK-based GNSS seismology. For example, [PERSON] et al. [14] processed high-rate GNSS data during the Mw6.0 Italy earthquake in 2016 by using the TRACK module of GAMIT/GLOBK and compared its solutions with that of PPP. The results illustrated that the seismic signals have impacted the noise characteristic of GPS-derived displacement waveforms. [PERSON] et al. [2] inverted the coseismic deformation of the 2016 Kaikoura (New Zealand) earthquake by processing the 1 Hz GNSS observations using both the TRACK module and Principal Component Analysis (PCA) spatial filtering. The challenge for PPK-based approaches is that the reference station may also be displaced in strong earthquakes, e.g., the Mw9.0 Tohoku-Oki one [4], even when they are several hundred kilometers away from the epicenter. Moreover, the PPK positioning accuracy will be significantly degraded when the baseline length increases because of the residuals of atmospheric delays after applying the double-differenced model. Compared to PPK, PPP [15] uses observations from a single station and precise satellite orbit/clock products afforded by the International GNSS Service (IGS) [16] centers.