The goal of this study is to identify Thai students' prior knowledge about particle motion when P-waves arrive. This existing idea significantly influences what and how students learn in the classroom. The data were collected via conceptual open-ended questions designed by the researchers and through explanatory follow-up interviews. Participants (n = 171) were freshmen in science, engineering, agricultural sciences, and medicine fields enrolled in a university in Thailand. The major categories of Thai students' prior knowledge about particle motion at P-wave arrival are (1) the belief that particles spread in all directions, like water waves, when P-waves arrive, (2) the belief that particles move forward with a sine wave motion, and that these particles travel with the propagating wave energy to the P-wave's final destination, (3) the belief that particles vertically move back and forth at P-wave arrival. These beliefs are the alternative conception held by more than three-quarter of our study population. The other held the scientific conception (category 4) that particles in a medium vibrate in the same direction as the propagating wave energy when Pwaves arrive, coupled with recognition that particles do not travel with the propagating energy. Recognizing the existence of this prior knowledge is vital to creating teaching strategies to promote the conceptual change approach, which is based on both historical Piagetian learning theory and the new trend "knowledge in pieces", about particle motion and seismic energy, in particular, as well as earthquakes, in general.