CISN ShakeAlert: Specification of the Prototype Earthquake Early Warning System

The California Integrated Seismic Network (CISN) earthquake early warning (EEW) project is developing components of a single, unified, prototype EEW system for the state of California. In summer 2009 the CISN concluded a three-year project testing three EEW algorithms: (1) Onsite, run by the California Institute of Technology, (2) Virtual Seismologist, run by the Swiss Seismological Service, and (3) ElarmS, run by the University of California at Berkeley. Currently all three algorithms process data in realtime from across California, and all have successfully identified earthquakes and predicted ground shaking a few seconds before it was felt. Output and performance statistics are collected at a centralized database run by the Southern California Earthquake Center. This work demonstrated that a statewide "ShakeAlert" EEW system could offer advanced warning of ground shaking, and identified the network upgrades and software that would be required. The CISN is beginning another three-year project, to integrate the three algorithms into a single prototype system capable of providing alerts to a small group of test users. A single waveform-processing module will filter data from all early-warning-capable seismometers in California. An event detection module will then integrate data from across the state. The module will combine single-station and network-based methods, providing a rapid initial warning which is then updated as additional stations contribute data. Uncertainty information will also be provided at all times. An alert decision module will receive all estimates and uncertainties, and determine what information to send to users. The decision to send an alert or not is based on data availability, uncertainties, and user preference, with the knowledge that increased accuracy comes at the cost of reduced warning time. The CISN is collaborating with potential users to develop useful ShakeAlert products and procedures.