Características de las variaciones estacionales de la velocidad sísmica del subsuelo superficial y sus factores influyentes en el área de Tianjin, norte de China, utilizando datos de ruido sísmico ambiental

It is feasible to continuously monitor the temporal changes of shallow subsurface media by utilizing ambient seismic noise. Here, the authors analyzed three-year continuous records of 117 earthquake early warning stations in the Tianjin area in the North China Plain to explore the seasonal variation of shallow subsurface seismic velocity. They examined the daily horizontal-to-vertical spectral ratio (HVSR) curves and found that 25 stations exhibit clear seasonal variations in peak frequency and peak magnitude within the frequency range of 5-20 Hz. Subsequently, autocorrelation is applied to the seismic noise of these 25 stations in the 5-20 Hz frequency range to calculate the relative velocity changes. Ten stations show the same variation pattern in the peak frequency of HVSR curves and relative velocity changes based on autocorrelation. This pattern exhibits characteristics where the peak frequency and seismic velocity increase rapidly in November, decrease quickly in February of the following year, and change relatively little in other seasons. By comparing with meteorological data, the authors infer that the freezing of the soil layer when the temperature drops below 0°C leads to an increase in shear wave velocity and peak frequency in winter. The results suggest that studies on crustal structure detection need to consider temperature effects.