Earthquake Early Warning Worldwide
A simple EEW system was already outlined in 1868 by the Californian physician J. D. Cooper in an article published by the San Francisco Daily Bulletin. Cooper proposed the installation of an array of seismic detectors from 10 to 100 km away from San Francisco. When large ground motions triggered a sensor, a signal would be telegraphed to San Francisco and would automatically ring a bell. Despite the simplicity and plausibility of this idea, the first EEW system was not realized until digital seismometry became more common in the late 1980s: to prevent derailment, the Shinkansen, Japan’s high-speed train, was automatically slowed down when a strong earthquake had been detected in the proximity of the railway. Today, not only Japan but also Romania, Taiwan, Turkey, Italy, Switzerland, China, and the US states California and Washington, to name just a few, are operating or testing EEW systems, sending alerts to test users or even the general public.
The available warning time of impending shaking depends on the distance of the receiver to the rupture and can range between 0 seconds (no warning possible) to minutes. As a simple example let us consider an earthquake of magnitude 7.5 for which the radius of the area which will experience strong ground shaking is roughly 55 km. S-waves, which mark the onset of the most energetic seismic waves, travel at a speed of ca. 3.5 km/s, and P-waves, which carry the first information about the earthquake, travel at a speed of ca. 6.5 km/s. For an earthquake that happened at a depth of 10 km, a seismometer directly at the epicentre will, therefore, detect the earthquake around 1.5 s after it occurred. Around 14 s later, the S-waves will have reached the furthest site for which strong ground shaking would be expected. Considering technical delays of around 4 s, a warning could be issued up to 10 s before the onset of strong ground shaking. In reality, things are much more complex. Mexico City, for example, which is built on a thick sedimentary basin, can experience very strong shaking from earthquakes off the Pacific coast several hundred kilometres away. Also, for large earthquakes, the rupture process itself has a strong influence on the areas that experience strong shaking. The SED is involved in developing and testing new EEW algorithms that capture some of this complexity to make warnings faster and more accurate. To learn more about SED’s EEW project called REAKT click here.
Earthquake Early Warning in Switzerland
In Switzerland we expect an earthquake with a magnitude 6 every 50 to 150 years. For earthquakes of this size the zone around the earthquake’s epicentre which experiences strong shaking is only on the order 20 to 30 km wide. As a result there is very little time between the first detection of the P-wave and the arrival of the destructive S-waves and possible warning times are short at best. Apart from automated procedures which can happen very fast (e.g. shutting down machinery or electrical equipment), such warnings can be used to increase the situational awareness. A train driver may be warned of potential landslides or officials can initiate emergency procedures. EEW can thus be seen as one end of the spectrum of SED’s alerting scheme, which we attempt to improve continuously.