image: Schematic illustration of the tidal force affecting the seismic wave velocity variation in a fault zone. (Image by USTC)
Credit: USTC
Seismological research is directly related to the incubation, occurrence, and evolution of earthquakes. Scientists seek to reveal potential earthquake precursors by monitoring the stress state of fault zones, thus providing bases for earthquake prevention and mitigation. It is hard to measure the stress in-situ, while changes in seismic wave speed can reflect changes in the zone. Hence, temporal variation monitoring of wave speeds becomes an important means of detecting medium changes. Moreover, there is little high-resolution observation on how tidal forces affect the changes in seismic wave speed within the fault zone over time.
In a study published in National Science Review, a research team led by Professor YAO Huajian from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) revealed the significant impact of tidal forces on the changes in seismic wave speeds in the fault fracture zone by monitoring the temporal changes in seismic wave speeds.
The research team utilized continuous noise data collected from a dense seismic array located in Anninghe fault zone, Qinghai-Tibet Plateau, southwest China, combined with seismic interferometry technology, to calculate the seismic wave velocity changes in the underground medium of the Anninghe fault zone area.
They reported that the wave velocity changes have distinct diurnal and semi-diurnal cycles, and these periodic changes are more pronounced within the fault fracture zone.
In addition, after removing the environmental factors, there are also significant monthly periodic changes within the fault fracture zone. The researchers compared the results of wave velocity changes with theoretical tidal strain, and found that both showed good correlation in diurnal, semi-diurnal, and monthly periodic components, indicating that this periodic change is mainly affected by tidal forces.
We applied the standard spectral ratio (SSR) method to the observed east-west ground motions from the selected local and teleseismic earthquakes. It showed that due to the amplification effect, the amplitude spectrum ratio in the fault fracture zone was significantly higher than that in other areas, indicating that the degree of medium fragmentation in this area was higher.
The tidal force affected the seismic wave velocity variation by causing the periodic opening and closing of micro-cracks in the underground medium. Generally, the wave velocity decreased when the cracks were open and increased when the cracks were closed.
Since the degree of medium fragmentation in the fault fracture zone was higher and the micro-cracks were more developed, this area was more sensitive to the tidal force, resulting in more significant observed wave velocity changes.
In this study, the monitoring method based on continuous ambient noise successfully captured the significant impact of tidal forces on the structure of the fault fracture zone, revealing the disturbance of the tidal force on the internal stress field of the fault zone during the period without earthquake occurrence.
This study provides an important means for constructing a time-varying geophysical comprehensive observation system for active fault zones.