Seismic Risk Assessments Related to Early Warning Systems: a Case Study in NE Bulgaria

As urbanization progresses worldwide, earthquakes pose serious risks to lives and properties for urban areas near major active land-based seismic sources. Earthquake early warning (EEW) can be a useful tool for reducing earthquake risks, if the spatial relationship between cities and earthquake sources is favourable for such warnings, and the citizens and decision-makers are properly trained to respond to earthquake warning messages. Seismic warnings can be used to slow down high-speed trains, to shut down pipelines and gas lines to minimize fire hazards, to shut down manufacturing operations to decrease potential damage to equipment, or to save vital computer information to inhibit losses of data. The successful establishment and maintenance of EEWs consists of two key analyses: 1) rapid analysis of the strong ground motion records to be the alert for warning; and 2) analysis of the vulnerability of critical systems to define reasonable thresholds of switch off. This paper is targeted on the earthquake engineering contribution to EEW design and implementation based on available strong motion data. An illustration is given on how the available strong motion data base, coupled with fast real-time estimation of the strong motion record, can be used for setting the trigger levels and activation of the EEW. The paper also deals with the use of the accumulated expert knowledge on the seismic instrumentation and monitoring of structural systems that provide information on the dynamic behaviour of the systems during earthquakes and which might be used as background for near real-time damage mapping during an earthquake. Such mapping is of a particular importance for Bulgaria with its heterogeneous built environment and high seismic hazard scale.