A discussion on the largest recorded epicentral distance for seismic phase P_diff diffracted on core-mantle boundary

The largest epicentral distance of Chile M_S8.8 earthquake reaches 179.7° recorded by Gansu Digital Seismic Network. Through artificially reading observational first-arrival travel time of the Chile earthquake， we calculated the average observational slowness of the first arrival phase is 4.104 s/°， which is basically consistent with theoretical slowness (4.439 s/°) of P_diff. The results show that the P_diff can recorded at the epicentral distance 179.7° due to particular location of Gansu Digital Seismic Network and southeastern Gansu temporary observation array； the origin observed travel time lags behind the IASP91 theoretical travel time obviously； and P_diff travel times observed from different directions are affected by complex morphology of the core-mantle boundary.