**6. Possible factors that determine the development of the regional depth phases**

Many factors determine the development of regional depth phases. The key factors are the crustal structures at the free surface and at the Moho. We tested the effect of crustal structure at the free surface on the development of the regional depth phases. We divided the first layer of the default crustal model into two parts, making the first layer 1-km thick. We changed the velocities in the new first layer gradually to obtain new crustal models (Table 2). For each new crustal model, we generated synthetics with depth 12 km at distance 2.20°. Fig. 20 shows that on traces 020, 030, 040, 050, 125, and 225, the depth phase *sPmP* is not discernible. On traces 010, 325, and 425, *sPmP* is not prominent. On traces 001, 005, 525, and 625, *sPmP* is clear. Based on this test, we may say that if there is a sedimentary layer in the source region (reflecting sites) and the layer is sufficiently thick, the *sPmP* phase is not developed.

Fig. 20. Synthetic waveforms generated with the new crustal models listed in Table 2 at distance 2.20º and depth 12 km. Trace 125 was generated with Model 1 in Table 2; trace 225 with Model 2; trace 325 Model 3; trace 425 Model 4; trace 525 Model 5; trace 625 Model 6. Trace 001, 005, 010, 020, 030, 040 and 050 were generated with Model 1, but the thickness of the first layer was changed in steps to 0.01, 0.05, 0.1, 0.2, 0.3 0.4, and 0.5 km, successively.

### **7. Outline of the procedure to identify the regional depth phases**

There are distance windows in which regional depth phases are developed. Phase *sPg* is developed well within 100 km, *sPmP* is developed well within about 200 to 300 km, and *sPn* is developed at more than 300 km. Figs. 3, 4, and 5 show these distance windows. Because time differences *sPg*–*Pg*, *sPmP*–*PmP,* and *sPn*–*Pn* are not sensitive to station distance, we can align all records generated by an earthquake by station distance to identify these regional depth phases.

The procedure has the following steps:


If two pairs (one pair = regional depth phase and its reference phase) on two records at two stations have similar differential times, the pairs may be treated as candidates for modeling.

We can model one pair or two pairs using RDPM to obtain the focal depth. The modeling procedure is shown in Fig. 9. If only one record is available at a distance window, the period feature (*sPg*, *sPmP,* and *sPn* have longer periods than their respective reference phases) may be used for the identification. The feature that *Pn*, *PmP,* and *sPmP* have relative positions on the record (see Fig. 7) can also be used to identify *sPmP* and *PmP*. Dineva *et al.* (2007; their Fig. 9) provided an excellent example for regional depth phase identification.

If more than one station has regional depth phase records for the same earthquake, the focal depth solutions obtained at different stations should be similar (see Ma and Atkinson, 2006; their Table 1).
