Researchers in Italy have analyzed seismic signals from over 200 moderate to strong earthquakes, ranging from magnitude 4.0 to 7.4, and they conclude that the waves generated in the first few seconds of an earthquake (the primary, or P, waves) carry sufficient information to determine its magnitude and destructive potential.
Aldo Zollo and Maria Lancieri of the University of Naples and Stefan Nielsen of the National Institute of Geophysics and Vulcanology in Rome determined that the peak amplitudes of very early seismic signals recorded in the vicinity of an earthquake source correlate with the earthquake magnitude and may be used for real-time estimation of the size of the event. Surprisingly, the researchers say, earthquake magnitude can be estimated using just two seconds' worth of signal from the first recorded P and S (secondary) waves, that is, while the earthquake is still in progress and far from over. The study will be published later this month in Geophysical Research Letters.
Primary waves travel around six kilometers [four miles] per second, covering around 60 kilometers [40 miles] in 10 seconds. Secondary, or S, waves, which are usually more destructive, travel more slowly, around 3.5 kilometers [2.2 miles] per second, covering only around 17 kilometers [11 miles] in 10 seconds. Therefore, a city located around 60 kilometers [40 miles] from an epicenter would have around 15 seconds of lead time to prepare for an earthquake's impact, the time difference between the arrival of the first P wave at a recording station near the epicenter and the arrival of the S wave at the city itself.
In the study, the researchers looked into the entire active seismic belt of the Mediterranean region, which includes varying geological and tectonic systems and faults. They compared signals from both P and S waves from more than 200 earthquakes and found that stress release and/or slip duration on the fault in the very early stage of seismic fracture relates both to the observed peak amplitude of the early P wave and to the elastic energy available for propagation of the fracture.
Although relatively few magnitude 7 earthquakes have hit the study area in recent years, there have been many instances of quakes in the magnitude 6 range. (A magnitude 7 earthquake is over 30 times more energetic than one of magnitude 6.) Zollo notes that even magnitude 6 quakes can produce great damage, especially in urbanized areas and places where old structures were not built to current standards; this defines much of the Mediterranean basin and applies also in other areas.
The researchers say that installations as close as 50 kilometers [30 miles] from the epicenter could receive an earthquake warning 10 seconds prior to the arrival of the main body wave of an earthquake. Places further from the epicenter would have additional time, though still measured in seconds. To take advantage of this brief warning period, automated systems would have to be created that respond instantly to notification alert signals, and they would have to be carefully calibrated to avoid missed or false alarms.
Engineers note that in tall buildings, the higher floors sway much more than those near ground level, so that even a moderate earthquake could cause severe damage to a highrise, Zollo says. Therefore, even at 70-80 kilometers [40-50 miles] distance from its epicenter, a magnitude 6 quake could affect hospital operating rooms and other critical installations.
Closer to the epicenter, a magnitude 6 or higher earthquake can damage critical infrastructure, such as telephone lines, gas pipelines, highways, and railroads, as well as airport runways and navigation systems. These disruptions would have a domino effect in more distant areas, which could be mitigated by an early warning alert system, based on the earliest primary wave data to arrive at recording stations close to the epicenter.
The researchers note that earthquake early warning systems can also help mitigate the effects of such earthquake-induced disasters as fires, explosions, landslides, and tsunamis, which can in many cases be more devastating than the earthquake itself. Systems could be installed at relatively low cost in developing countries, where moderate sized earthquakes can cause damage comparable to that caused by much larger earthquakes in developed countries, they say.
The study was funded in part by the consortium Analisi e Monitoraggio del Rischi Ambientali (AMRA) scarl through the European Union-Seismic Early Warning for Europe (EU-SAFER) project.
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