Japan launched a new earthquake warning system in 2007.
It uses sensors that pick up the faster-moving and less destructive P-waves produced by an earthquake and transmits a warning broadcast on television and radio before the destructive S-waves and surface waves arrive, giving recipients of the message, precious seconds to seek shelter. (P waves are generally about 1.7 times faster than S waves)
The system gives people as much as a 50 second warning before an earthquake occurs in places far from the epicenter. The system is only really effective for places a decent distance away from epicenter.
Eight earthquake warnings were issued the first year the system was in operation. The main problem with them is that few people actually were aware of the warnings. Japanese companies have begun developing cell phones ad other devices that can pick up the warnings better. Japan also has a tsunami warning system that issues warnings on television.
Practical uses of the detection of P-waves
Shinkansen bullet trains
The Tohoku Shinkansen is a high-speed passenger train (bullet train) that connects Tokyo to the northern city of Morioka in Honshu, Japan.
The line is protected by a seismic early warning system, which includes two sets of accelerometers: one set is deployed along the line (wayside system), while the other comprises eight accelerometers placed along the eastern coast of Honshu (coastal system). The coastal system is designed to protect the train against earthquakes with origin in the highly active offshore subduction zone.
It causes trains to automatically stop when the ground acceleration exceeds a preset limit.
Earthquake prepared elevators
Otis, the elevator maker, had equipped a lot of their Japanese elevators with safety seismic equipment.
About half the elevators Otis maintains in Japan—including most in high-rise buildings and regions with severe earthquake risk—are equipped with seismic detectors. At the first vibration of the quake (P-wave), these devices return the elevators to the ground floor so passengers can exit, then block them until Otis can check their safety.
The detectors did their job. Some 16,700 elevators in the areas affected by the quake were shut down by the emergency systems. Otis, which had worldwide revenues of $11.58 billion in 2010 and manufactured about 40,000 of the 80,000 elevators it services in Japan, didn’t receive any report of trapped or injured passengers. “All the elevators operated as they were supposed to” said a company spokesperson.
* Many factories in earthquake-prone areas have machines that automatically shut down when they sense vibrations from a quake.
* The Japanese gas company has installed meters that shut off the gas supply in the event of tremor.
* Nuclear power plants and other dangerous factories are also automatically shut down. (!?!?-ER)
Important remark : P-waves and S-waves arrive close together at places close to the epicenter, where warning would only be a matter of a couple seconds at most. As a result, such equipment will only be reliable with an epicenter at least 100 km (around 15-20 secs distance for P-waves, and around 30-40 secs for S-waves) from a populated area. Practically speaking it will be mainly used for subduction triggered earthquakes. Other countries able to use these systems are among others South American countries, Philippines, Indonesian, New Zealand etc. However, only 2 seconds is required to shut down essential facilities like power systems for hospitals and other services, so different systems are applicable if used for life safety versus life continuation. Therefore even at a much closer distance, early warning systems are extremely useful.
Something more about the speed of P-waves and S-waves
The speed of an earthquake wave is not constant but varies with many factors. Speed changes mostly with depth and rock type. P waves travel between 4 and 13 km/sec. S waves are slower and travel between 3.5 and 7.5 km/sec.
Generally spoken the destructive S-wave is approx. 50-60% of the velocity of the P-wave.
Make the calculation yourself with an average of 8 km/sec. and a distance from the epicenter of 240 km. The P-wave will then arrive 30 sec. after the rupturing. The destructive S-wave, traveling at 4.8 km/sec (60% of 8) will arrive after 50 seconds. The 20 seconds time lapse (in this example) is for a lot of instruments enough to safely stop.
Credits : A part of this article is derived from Jeff Hayes and MIT