We use converted body‐wave phases from local earthquakes to constrain depth to basement and average attenuation relations for the Seattle basin in Washington and the Tualatin basin in Oregon. P‐, P‐to‐S‐(Ps), S‐to‐P‐(Sp), and S‐wave arrivals are present in three‐component recordings of magnitude 2.5–4.0 earthquakes at seismic stations located in these basins. Based on their relative travel times, these phases are attributed to body‐wave conversions at the basement‐to‐basin contact or to high‐impedance interfaces within the basins. Depth to basement values are calculated using the differential travel times between direct and converted phases, as well as average P‐ and S‐wave velocity values. We also identify a high‐impedance layer in the Tualatin basin that likely represents a laterally extensive deposit of volcanic materials embedded between the basement contact and the Columbia River Basalt Group. In addition, the average Q_P-Q_S attenuation relation is calculated for each station by taking the spectral ratio of converted phases to their parent body‐wave arrivals. For the Seattle basin, our analysis yields an average Q_P value of 73 and an average Q_S value of 60 for seismic waves with frequencies between 2 and 25 Hz. In the Tualatin basin, a much reduced Q_P–Q_S relation suggests that average body‐wave attenuation is likely higher than in the Seattle basin. The converted phase techniques presented here provide a reliable way to develop estimates of basin depth and attenuation structure for undercharacterized regions using simple passive source seismic records.