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Earthquake Early Warning Systems: an Investment That Pays Off in Seconds – an Invited Comment

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Earthquake Early Warning Systems: an Investment That Pays Off in Seconds – an Invited Comment Volume XXXII • Number 5 May 2008 Earthquake Early Warning Systems: An Investment that Pays off in Seconds – an invited comment n October 2007, Japan unveiled a national earthquake earthquake. It must be reliable enough that useful actions Iearly warning system tasked with providing the to protect life and property (evacuation, shutting down general public with a few seconds of warning before the electric and gas lines, etc.) can be initiated based on the onset of strong earthquake ground shaking. This article warning information. To date, earthquake prediction is defines earthquake early warning systems and describes a “holy grail” of seismology; it continues to be a hotly how they can improve the way modern societies mitigate debated topic and active area of research. In general, the the effects of damaging earthquakes. verdict is still out on whether earthquake science can be From the start, it should be made clear that earth- advanced to the stage where useful and reliable predic- quake early warning is not earthquake prediction. A tion statements can be made (e.g., “We expect a mag- useful earthquake prediction requires specific infor- nitude 6 earthquake to initiate within the Los Angeles mation about the timing, location, and size of a future metropolitan area on September 24, 2015”). If it’s not prediction, then what is it? to define today’s earthquake early warning systems: (1) systems with more dense instrumental networks per- While it may not be possible to predict when and form better, (2) no warning will be available for regions where the next damaging earthquake might occur, it is too close to the earthquake source, (3) broadcasting of possible to estimate the effects of strong ground shak- warnings and the actions in response to the warning in- ing on surrounding areas while an earthquake is still formation should be automated. (A 2005 article by Hiroo rupturing. This is possible because earthquakes produce Kanamori gives a recent review and history of research different types of waves that travel at different speeds. P efforts in this area. See References.) (or primary) waves travel at approximately 6.5 kilometers (4 miles) per second and are the first waves to arrive at seismic monitoring instruments in a given region. They Early warning systems can be most have relatively low amplitudes and are less likely to cause effective for providing warning for damage to buildings, but they carry important informa- large earthquakes that start at some tion about the size and location of an earthquake. S (or “ distance from a site. secondary) waves travel more slowly at approximately 3.5 kilometers (2.2 miles) per second and arrive after the ” P-waves, but they cause stronger levels of shaking and There are numerous “flavors” of modern earthquake can bring down buildings during an earthquake. early warning systems. “Front-detection” systems are Earthquake early warning systems (EEWS) estimate appropriate for areas where the damaging earthquakes the expected maximum shaking, based on information consistently originate from a known region and the target extracted from the early arriving P-waves, and send this warning area is at some distance away. For instance, information to regions farther away from the earthquake many earthquakes that cause damage to Mexico City source (or epicentral) region. Because information can typically initiate along the Guerrero coastal subduction travel at the speed of light (much faster that the damag- zone, about 185 miles from Mexico City. Thus, a rela- ing earthquake waves), there can be up to tens of seconds tively straightforward system consisting of a series of of warning time between the availability of the shaking accelerometers installed along the coast works fairly well. estimates and the onset of damaging ground motions. These accelerometers send information via radio link to The difference between P- and S-wave arrival times government offices in Mexico City when a certain level (also known as the S-P time) is proportional to the of ground motion has been exceeded at more than three distance of a given site from the earthquake epicenter. stations. Because of the considerable distance between Regions in the immediate vicinity of the earthquake the source region and Mexico City, warning times on the epicenter (the blind zone of an early warning system) order of 75 seconds are possible. Although Mexico City is will have very small S-P times and, therefore, little or no situated a considerable distance from the source region, warning time. The farther a given location is from the epi- ground motions there can be abnormally large (and early central region, the more warning time is available. Thus, warning information very useful) because the city is built early warning systems can be most effective for providing on an ancient lake bed. The resonance of these lake sedi- warning for large earthquakes that start at some distance ments can produce amplified ground motions capable of from a site. For example, Los Angeles could have about a collapsing high-rise buildings. Such a front-detection sys- minute of warning before a repeat of the 1857 Fort Tejon tem is also possible for the city of Bucharest (Romania), earthquake, which initiated in the vicinity of Parkfield which is located about 109 miles from the Vrancea region and ruptured about 185 miles southward. However, the that is the source of damaging earthquakes. city would have little or no warning from a large earth- Single-station approaches use P-wave information quake on the Puente Hills fault system, which runs under at a given site to predict the maximum ground motions metropolitan Los Angeles. (from the S-waves) at the same location. The uncertainty in the predicted ground motion amplitudes may be Different Flavors of Early Warning lower for the single-station approach, as it requires only a A fundamental ingredient in the operation of an relationship between P-wave derived quantities and peak EEWS is a modern network of seismic instruments S-wave amplitudes. Such approaches have the potential capable of measuring earthquake ground motions and to provide rapid warning information for regions in the processing and analyzing incoming waveform data in blind zone of network-based approaches and have been real-time. proposed for nuclear plants and structural control ap- While the technological ingredients have only be- plications. come available in recent decades, the concept of earth- Finally, network-based approaches provide early quake early warning has been around for a while. Shortly warning for widespread regions that have numerous po- after the 1868 Hayward Fault earthquake (magnitude 7.0), tential earthquake sources (for example, Japan and Cali- J.D. Cooper wrote an editorial in the San Francisco Daily fornia). The network-based approach uses stations that Evening Bulletin proposing a system that would ring a are part of a seismic monitoring network, which estimate bell over City Hall if ground motions exceeding a certain the magnitude and location of an earthquake as quickly level were detected along earthquake faults. Cooper’s as possible. Such systems predict the maximum expected article identified a number of requirements that continue ground motions throughout the region of interest. The 2 Natural Hazards Observer • May 2008 recently activated national earthquake early warning sys- earthquakes rank among the most costly natural disasters tem in Japan is the largest network-based early warning of the twentieth century. system currently in operation. While an early warning system would not prevent all earthquake-related losses, it would allow people a How useful is a few seconds of warning time? few seconds to respond and take simple personal safety measures, which could significantly reduce the number An equally fundamental component to an early of casualties. If an early warning system in California warning system’s success is an informed and well-pre- could prevent just 1% of the damage from a repeat of the pared user community capable of efficiently using the magnitude 7.9 earthquake in San Francisco in 1906 (which information. A year before their national early warning was estimated by the California Office of Emergency system began broadcasting warning information, the Services to cost $122 billion), it would pay for itself many Japan Meteorological Agency launched an extensive times over. That sounds like a pretty good investment to public outreach and education campaign to familiarize me. the public with the system and insure that the public was informed of how to react to information from the Georgia Cua ([email protected]) system. The campaign focused on simple personal safety Swiss Seismological Service, Zurich, Switzerland measures that could be taken in response to a warning. However, if user systems are able to automate decisions References and actions based on early warning information, more numerous applications are possible. 1. Kanamori, H. 2005. Real-time seismology and earthquake High speed trains in Japan have been automatically damage mitigation. Annual Review of Earth and Planetary slowed down and stopped by early warning systems Sciences 33: 195-214. since the 1990s. Other automated responses include stop- 2. Nakamura, Y. 1988. On the Urgent Earthquake Detection and Alarm System (UrEDAS). Proceedings of the 9th World ping elevators at the closest floor, opening fire station Conference of Earthquake Engineering, Japan Association for doors (so that fire trucks don’t get stuck due to jammed Earthquake Disaster Prevention, Tokyo, Kyoto: 673-678. doors), and saving data on computers. These applica- tions are relatively easy to implement, as the cost of The early warning algorithms being implemented and tested in false alarms is negligible. More complicated applications California are the ElarmS, Virtual Seismologist, and Ampli- include diverting airport traffic, inserting control rods in tude/Period monitor algorithms. Details about these algorithms nuclear plants, stopping high precision manufacturing can be found in the following publications: processes, and interfacing with active structural control systems to change the dynamic response characteristics 1.
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