Abstract: Studying the changes in site conditions before and after sand liquefaction is crucial for improving liquefaction discrimination methods. In a study utilizing the strong seismic observation data from the borehole array at the California Wildlife Park in the United States, we investigated the variation characteristics of the equivalent shear wave velocity and dominant frequency of the array site before and after liquefaction, triggered by the Brawley strong earthquake sequences. This analysis employed cross-correlation analysis and the Fourier spectral ratio method. The monitoring results of pore pressure under strong earthquake action indicate that the excess pore water pressure in the liquefied layer has reached the critical liquefaction state and rapidly dissipates after the earthquake, without any visible sand boil on the surface. The building up of excess pore water pressure causes a delay in the propagation time of shear waves among different observation positions of the borehole array. Furthermore, the dominant frequency of the array site evidently decreases during the sand liquefaction process. After the dissipation of pore pressure, the propagation time of shear waves and the dominant frequency of the site return to normal state within 4 days. This observed phenomenon holds significant reference value for establishing liquefaction criteria based on in-situ testing data at the site following liquefaction.

Key words: sand liquefaction, earthquake sequence, cross correlation analysis, time delay, shear wave velocity, predominant frequency