Primary microseism is the less studied seismic background vibration of the Earth. Evidence points to sources caused by ocean gravity waves coupling with the seafloor topography. As a result, these sources should be in water depth smaller than the wavelength of ocean waves. Using a state-of-the-art ocean wave model, we carry out the first global-scale seismic modeling of the vertical-component power spectral density of primary microseisms. Our modeling allows us to infer that the observed weak seasonality of primary microseisms in the southern hemisphere corresponds to a weak local seasonality of the sources. Moreover, a systematic analysis of the source regions that mostly contribute to each station reveals that stations on both the East and West sides of the North Atlantic Ocean are sensitive to frequency-dependent source regions. At low frequency (i.e., 0.05 Hz), the dominant source regions can be located thousands of kilometers away from the stations. This observation suggests that identifying the source regions of primary microseisms as the closest coasts can be misleading.