There are three usable regional depth phases, sPg, sPmP, and sPn, and their corresponding reference phases, Pg, PmP, and Pn. The differential time between each depth phase and its reference phase can be used to estimate earthquake focal depth. We have developed a method to determine focal depth for moderate and small earthquakes by using a regional depth-phase modeling (RDPM) method. We used a default focal mechanism to generate the differential times for all earthquakes. To estimate the reliability of the modeled focal depths, we compared our solutions with those obtained by other methods and found the consistency is good. Because the focal depths estimated by RDPM are model dependent, we tested the extent of the dependency and found that a 10% error in the crustal model may generate a 10%–15% error in the modeled depth. The absolute error is determined by the error in the crustal model and the focal depth itself. We found that earthquake location errors have only a small effect on the modeled focal depths. By analyzing synthetic and observed waveforms, we found a distance window within which sPmP and PmP are well developed, and, within the window, the P portion of the waveform is relatively simple and sPmP and PmP are easy to identify. We also demonstrated that the assumptions of sPmP and PmP are correct, that regional depth phases are not developed or not discernible in some regions, and that regional depth phases have special features that can be used to identify those phases.