Hydraulic fracturing (fracking) is widely used to extract oil and gas from shale formations, but it can trigger small earthquakes, raising safety concerns. In this study, we used naturally occurring ground vibrations (ambient noise) to examine how underground structures changed during hydraulic fracturing operations. By analyzing data recorded from sensors installed in shallow boreholes near hydraulic fracturing wells, we measured changes in seismic wave velocities during two stages of fluid injection. Our results reveal a reduction in seismic wave velocity at the depth of hydraulic fracturing activity. This can be caused by an increased fluid migration and rock damage during cross-well injections that significantly weakened underground structures, contributing to increased earthquake activity. This research highlights that monitoring subtle changes in the Earth's underground using ambient noise can help detect structural changes early. Such monitoring provides a cost-effective and practical tool to better understand and potentially manage earthquake risks associated with hydraulic fracturing.