The depth phase is typically an attribute (characteristic) phase pP-a P wave reflected from the Earth’s surface at a point comparatively near the hypocenter. At distant seismograph stations, the pP goes around with the P wave by a time span that changes gradually with distance but quickly with depth. This time interval, pP-P (pP minus P), is used to calculate the depth-of-focus tables. Taking into account and applying the time difference of pP-P as read from the seismogram and the distance between the epicenter and the seismograph station, the depth of the quake can be identified from the issued travel-time curves or depth tables.