Earthquake Early Warning: Regional vs. On-site Warning Approaches

Maren Boese, Seismo Lab, California Institute of Technology, Pasadena

Earthquake early warning (EEW) systems serve as tools for co-seismic risk
reduction. They basically make use of differences in the propagation speed of
seismic and electromagnetic waves and issue warnings, if necessary, to
potential users before strong shaking at these sites occurs. The maximal
warning time of an EEW system is generally defined as the time span between the
P wave detection at the first triggered EEW sensor and the arrival of
high-amplitude S or surface waves at the user site. As these time periods
usually are extremely short, EEW systems must recognize the severity of
expected ground motions within a few seconds. Based on this information,
suitable actions for the damage reduction can be triggered and executed. EEW
systems mainly face two challenges: (1) They have to meet the claim of
extremely high reliability which means that both missed alarms and false alerts
have to be avoided; (2) the warning times should be as large as possible. Two
types of EEW systems are distinguished: (1) regional warning systems that are
based on station networks with stable but late estimates of seismic source
parameters (and therewith warnings), and (2) on-site warning systems that are
based on single sensor observations with fast but less reliable estimates. An
alternative to these two methods is proposed with the PreSEIS approach (B?se et
al., 2008) that combines seismic observations at several sensors, however, does
not require that seismic waves have arrived at more than one sensor before
estimates are issued.