Strong and weak points of earthquake early warning systems.
Festa G., Zollo A., Colombelli S., Picozzi M., Elia L.
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IV - Geofisica e fisica dell'ambiente
Aula 32C-1 - Venerdì 21 h 09:00 - 12:00
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Few seconds are left between an earthquake rupture originating at depth and the strong ground shaking at the surface, possibly producing losses and damage. Earthquake Early Warning Systems -- EEWS -- try to estimate the devastating effects of an earthquake in this very short time window and aim to provide an alert before the ground shaking reaches the site to protect. EEWS usually are grouped into two classes. Regional EEWS are based on the robust estimation of the event location and magnitude, while the ground motion amplitude is provided by empirical attenuation relationships. While accuracy of information increases at a large distance from the event, the necessity to record few seconds of the early P wave, results in a blind zone -- a zone where the alert is not effective, because it arrives after the ground shaking -- of 20-30 km. Usually this is the region where most of the damage occurs for events of magnitude M $< 6.5$, such as the majority of Italian earthquakes. Moreover, the uncertainty in the peak amplitude estimate is driven by the GMPEs, owing to their simplified source and propagation description. Onsite systems provide directly the peak ground motion amplitude from the early P-wave recorded at the same site. Although they are less accurate, they can still provide a positive albeit small lead-time in the vicinity of the source. Moreover, under the hypothesis of linearity of the ground motion between P and S waves, they also contain a first-order effect of the source and propagation. Strengths and weaknesses of EEWS are discussed analysing the performances of the two EEWS on the major event of the central Italy 2016-2017 sequence. We will show that for the largest magnitude event, both systems predict well the ground shaking nearby the event source, with a rate of success in the 85%--90% range, within the potential earthquake damage zone. However, the time available for security actions, is significantly larger for the network-based system. Nevertheless, weaknesses of EEWS can be mitigated through a real-time characterization of nucleation process, the inclusion of extended source models and more accurate wave propagation.