Use of Depth Phases in Event Screening

Accurate identification of depth phases is essential in improving the performance of the event screening process. In the first Release of event screening software, the rules for depth phases were insufficient, in that events that had inappropriate depth phase picks could satisfy the depth phase criteria. To strengthen the depth phase criteria so there is more confidence in the results, a new set of constraints were examined. The uncertainty in pP and sP onset times and the variability in the earth's velocity structure were the main factors that had to be accounted for. As a result, the existing criteria of a minimum moveout of 1.5 seconds and at least 3 associated depth phases, had to be extended to include minimum allowable pP-P and sP-P times and a SNR threshold of 2. The application of these new criteria is quite severe, resulting in no events less than 50km and only 20% of all depth phase solutions being screened out. On the otherhand one can be quite confident that no shallow events are screened out. Much depth phase information is being discarded using these stringent criteria. Research into an independent technique to determine whether the depth-phase solutions are reliable is being investigated. The relative amplitude method (Pearce algorithm), is being employed to search for double couple focal mechanism solutions that are consistent with the amplitudes of P, pP and sP phases associated with the event. The existence or non-existence of these solutions is an indicator of the validity of the depth phase picks.