Fig. 1
From: Earthquake early warning: what does “seconds before a strong hit” mean?
Big-net EEW timeline of strong shaking arrival and gray zones for different station intervals. This example is illustrated for an event which occurs at a depth of 10 km. The vertical axis shows the elapsed time (s) from the earthquake origin time, and the horizontal axis the epicentral distance (D, in km). The travel time curves for P- and S-waves (denoted tP and tS) are drawn in blue and red as a function of epicentral distance, assuming 5.8 km/s for P-wave speed and 3.4 km/s for S-wave speed for the uppermost layer from the iasp91 model (Kennett and Engdahl 1991). A gray zone is the area where strong ground shaking arrives with S-wave at tS before an EEW alert at tA, (i.e., tS < tA), and its size depends on the network station interval (Δx) and system processing time. Here, gray zones are shown in different shades for different station spacings, Δx = 10, 20, 30, and 40 km, assuming the system processing time of 2 s after P-wave detection at three or more stations. The maximum Ds of the corresponding gray zones are ~ 11 km, ~ 17 km, ~ 23 km, and ~ 29 km, respectively. Outside gray zones, there is a warning time of tS - tA (> 0 s). The further the site is from the epicenter, the longer the warning time is. On the other hand, the ground shaking is stronger and more damage may be caused in the gray zone than outside the zone