The real-time continuous ground motion measurements used by ShakeAlert come from broadband and strong-motion seismic sensors operated by the California Integrated Seismic Network (CISN) in California and the Pacific Northwest Seismic Network (PNSN) in Washington and Oregon. Additional stations operated by many partner organizations also contribute to the system.
The target station density is 10 km (6 miles) spacing in urban areas, 20 km (12 miles) in seismic source areas that endanger population centers, and 40 km (25 miles) in other areas. The denser spacing in urban areas speeds up alerts, provides ground motion data for ShakeMaps, and allows for some station outages without major impact on system performance. When possible stations are placed within 5 km (3 miles) of major fault traces.
This optimal distribution of sensors has not yet been achieved and more stations are needed. Data from approximately 900 seismic stations are currently (1/2019) contributing to the ShakeAlert, about 600 are in California (CISN) and 300 are in the Pacific Northwest (PNSN). The planned density of 1,675 stations will be accomplished, as funding allows, by updating older existing stations and adding new stations. In addition, some currently contributing stations need to be upgraded to reduce data latency or relocated to reduce spurious signals due to cultural noise.
ShakeAlert seismic stations (12/2018).