Simulation of Near-Fault 3D Ground Motion in Basins Within Complex Topography Using a Preconditioned Fast Multipole Indirect Boundary Element Method: A Case Study of the 2014 Ms 6.5 Ludian Earthquake in Yunnan, China

This study proposes a preconditioned fast multipole indirect boundary element method (FMIBEM) for efficient and accurate simulation of 3D ground motions in near-fault intermountain basins. The method comprehensively models physical processes, including fault rupture, seismic wave propagation, topographic, and basin effects. Following a thorough validation of the method's accuracy and efficiency, it is applied to simulate ground motions in the 0.01–4.00 Hz frequency range from the 2014 Ms 6.5 Ludian earthquake in Yunnan, China, to elucidate the physical mechanisms driving the observed damage distribution. The earthquake occurred in a region characterized by significant topographic complexity, where non-horizontal, irregularly shaped sedimentary basins are embedded within undulating valleys. This setting represents a highly intricate multi-domain seismic wave scattering scenario. Numerical simulations successfully reproduce prominent near-fault phenomena such as directivity effects and the fling-step effect, while also highlighting the coupling between these near-fault effects and topographic amplification as well as basin effects. The results indicate that mountainous areas near the fault amplify ground motions, with stronger shaking typically occurring at mountain peaks compared to valleys. However, the directivity effect can reverse this trend, causing ground motions in valleys to surpass those on mountain tops. Simulation results from the 2014 Ms 6.5 Ludian earthquake demonstrate that the Longtoushan basin (an intermountain layered sedimentary basin) exhibits prominent amplification effects. The PGV and PGA amplification factors within the basin domain reached up to 17.28 and 18.01, respectively, when incorporating low-velocity basin sediments compared to the bedrock model devoid of basin structures. Due to the high impedance contrast between the basin and its surroundings, seismic energy becomes trapped within the basin, leading to higher amplitude ground motions and prolonged shaking durations compared to adjacent areas. In combination with the near-fault fling-step effect, this resulted in significant structural damage and even the collapse of many buildings in Longtoushan Town. This study provides valuable insights into the mechanisms of seismic damage in complex near-fault environments and offers scientific guidance for seismic hazard zoning and the seismic design of structures in such regions.