CISN Earthquake Early Warning: ShakeAlert Hybrid Branch

The California Integrated Seismic Network (CISN) is developing an integrated, statewide earthquake early warning (EEW) system for California. In summer 2009 the CISN completed a three-year proof-of-concept study, analyzing three EEW algorithms for viability in California: (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. The study successfully detected earthquakes and accurately predicted the resulting ground shaking. As of December 2010 the CISN EEW team is halfway through a second three-year project to build an end-to-end prototype early warning system capable of delivering warning to a small group of test users. This new system is called CISN ShakeAlert. An area of ongoing research is the Hybrid Branch: a new, integrated algorithm to calculate event magnitude and location in realtime. The Hybrid Branch takes advantage of the best aspects of each of the original test algorithms. The Hybrid Branch will be able to rapidly recognize and assess an event using only a single station’s P-wave data, as OnSite does, but it will also combine data from multiple stations in a network-based approach, as Virtual Seismologist and ElarmS do. This will give the Hybrid Branch the speed of a single-station EEW method with the reliability of a multi-station method. One of the challenges of the Hybrid Branch is how to progress from a single station description of a given event to a multi-station view of the same event. The authors use a Bayesian approach to combine event information and adapt to changing data availability. Output from the Hybrid Branch will be sent to the ShakeAlert Decision Module, which consolidates event information from a variety of sources and generates earthquake alerts.