Source Characterization for Earthquake Early Warning

Abstract

Earthquake Early Warning Systems (EEWS) are real-time, seismic monitoring infrastructures that are able to provide a rapid notification of the potential damaging effects of an impending earthquake. This objective is achieved through the fast telemetry and processing of data from dense instrument arrays deployed in the source region of the event of concern (regional EEWS) or surrounding/at the target infrastructure (front-detection or site-specific EEWS). A regional EEWS is based on a dense sensor network covering a portion or the entire area that is threatened by earthquakes. The relevant source parameters (event location and magnitude) are estimated from the early portion of recorded signals (initial P-waves) and are used to predict, with a quantified confidence, a ground-motion intensity measure at a distant site where a target structure of interest is located. Site-specific (or on-site) EEWS consist of a single sensor or an array of sensors deployed in the proximity of the target structure that is to be alerted and whose measurements of amplitude and predominant period on the initial P-wave motion are used to predict the ensuing peak ground motion (mainly related to the arrival of Sand surface waves) at the same site. Frontdetection EEWS is essentially a variant of the on-site approach, where a barrier-shaped, accelerometric network is deployed between the source region and the target site to be protected. The alert is issued when two or more nodes of the array record a ground acceleration amplitude larger than a default threshold value. For typical regional distances, the peak acceleration at the barrier nodes is expected to be associated with the S-wave train, so that the distance between the network and the target is set to maximize the lead time (i.e., the time available for warning before the arrival of strong ground shaking at the target sites), which is, in this case, the travel time of S-waves from the barrier to the target site. EEWS have experienced a very rapid improvement and a wide diffusion in many active seismic regions of the world in the last three decades (Fig. 1). They are operating in Japan, Taiwan, Mexico, and California. Many other systems are under development and testing in other regions of the world such as in Italy, Turkey, Romania, and China. Most of these existing EEWS essentially operate in the two different configurations described above, i.e., regional and on-site, depending on the source-tosite distance and on the geometry of the considered network with respect to the source area. The “front-detection” EEWS such as the barrier-type, Seismic Alert System (Espinosa-Aranda