Knowing the Depth of a Shallow Seismic Event, How Often Can We Find Depth Phases at Regional Distances?

Data from events in 2 U.S. Pacific Northwest earthquake sequences have been examined with a view toward improving the ability of small numbers of stations to accurately determine relative focal depth at near-regional distances. Most phase arrivals predicted by regional crustal studies were not observed. Accurate location of seismic events has emerged as an important goal for detection of potential violations of the Comprehensive Test Ban Treaty (CTBT). Accurate location serves two functions. First, a reliable location helps to identify natural seismicity and routine blasting activities. Second, if an event is found to be interesting from the CTBT monitoring standpoint, accurate location improves the ability of an on-site inspection team to actually make a firm determination of the nature of the event.