Detection of Earth’s Free Oscillations Utilizing TianQin

The measurement of Earth’s free oscillations plays an important role in studying Earth’s large-scale structure. Space technology development presents a potential method to observe these normal modes by measuring intersatellite distances. However, the disturbance from Earth’s low-degree gravity field makes it challenging for low Earth orbit gravity measurement satellites such as Gravity Recovery and Climate Experiment and TianQin-2 to extract signals from Earth’s free oscillations directly. Here, we propose that by taking advantage of the high Earth orbit, the TianQin satellites can effectively avoid this disturbance, enabling direct measurement of Earth’s free oscillations. We derive an analytical waveform to describe the response of Earth’s free oscillations in TianQin. Based on this waveform, we use Bayesian analysis to extract the normal modes from numerical simulation data and perform parameter estimation. Our findings reveal that for a magnitude 7.9, Wenchuan-like earthquake, the resulting free oscillations will generate a signal with a signal-to-noise ratio of 73 in TianQin, and approximately 9 different modes can be distinguished. This result shows that TianQin can open a new window to examine Earth’s free oscillations and study Earth’s interior and earthquakes independently from ground-based gravity measurement.