Research on amplification effect of loess site based on active source method

The Loess Plateau is prone to frequent strong earthquakes, with thick loess deposits developed across the region. Significant topographic and amplification effects are evident, particularly exemplified by the 2023 Jishishan M6.2 earthquake, where site amplification effects intensified seismic damage. Due to the lack of abundant strong-motion records to support research on loess site amplification effects, current studies predominantly rely on empirical statistics, laboratory experiments, and numerical simulations, with quantitative characterization of amplification effects based on field-measured data remaining scarce. By employing an underwater airgun active source experimental method, which leverages the high repeatability of active source signals in simulating ground motions, this study investigates the feasibility of applying the active source method to observe amplification effects at loess sites. The research preliminarily reveals amplification characteristics of loess plateau terrain and validates these effects using strong-motion records from the 2023 Jishishan M6.2 earthquake in Gansu Province. This work provides new technical insights for seismic effect observation in loess terrains through novel excitation sources. The results demonstrate that the seismic waves generated by the airgun exhibit high waveform consistency under conditions of stable excitation energy. The results indicate that the propagation distance of a single airgun source excitation is relatively long, with a maximum range of 151 kilometers. Through stack processing, this distance extends to approximately 350 kilometers. The amplification coefficient increases with greater soil layer thickness, though the trend varies significantly, for thicknesses below 10 meters, amplification effects are negligible. In the 10~30 meters range, amplification becomes pronounced, peaking at the 30-meter depth. Beyond 30 meters, the amplification coefficient stabilizes around 3.0, showing minimal incremental increase with further thickness growth. Within the loess tableland, the amplification effect decreases significantly when the horizontal distance from the edge exceeds four times the loess layer thickness. Analysis of strong-motion records from the 2023 Jishishan M6.2 earthquake in Gansu Province reveals a peak acceleration amplification coefficient of approximately 2.75. This value closely aligns with the amplification coefficient of 3.07 derived from active source observations, thereby validating the feasibility of the active source method for studying amplification effects in loess terrains. This approach thereby establishes an innovative in-situ technical methodology for investigating seismic ground motion effects in loess terrains, providing a critical scientific foundation for earthquake disaster risk assessment and mitigation strategies in such geologically sensitive areas.