Earthquake Early Warning Systems

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Volume XXXII Number 5May 2008Earthquake Early Warning Systems:IAn Investment that Pays off in Secondsn October 2007, Japan unveiled a national earthquakeearly warning system tasked with providing thegeneral public with a few seconds of warning before theonset of strong earthquake ground shaking. This articledefines earthquake early warning systems and describeshow they can improve the way modern societies mitigatethe effects of damaging earthquakes.From the start, it should be made clear that earthquake early warning is not earthquake prediction. Auseful earthquake prediction requires specific information about the timing, location, and size of a future– an invited commentearthquake. It must be reliable enough that useful actionsto protect life and property (evacuation, shutting downelectric and gas lines, etc.) can be initiated based on thewarning information. To date, earthquake prediction isa “holy grail” of seismology; it continues to be a hotlydebated topic and active area of research. In general, theverdict is still out on whether earthquake science can beadvanced to the stage where useful and reliable prediction statements can be made (e.g., “We expect a magnitude 6 earthquake to initiate within the Los Angelesmetropolitan area on September 24, 2015”).

If it’s not prediction, then what is it?While it may not be possible to predict when andwhere the next damaging earthquake might occur, it ispossible to estimate the effects of strong ground shaking on surrounding areas while an earthquake is stillrupturing. This is possible because earthquakes producedifferent types of waves that travel at different speeds. P(or primary) waves travel at approximately 6.5 kilometers(4 miles) per second and are the first waves to arrive atseismic monitoring instruments in a given region. Theyhave relatively low amplitudes and are less likely to causedamage to buildings, but they carry important information about the size and location of an earthquake. S (orsecondary) waves travel more slowly at approximately3.5 kilometers (2.2 miles) per second and arrive after theP-waves, but they cause stronger levels of shaking andcan bring down buildings during an earthquake.Earthquake early warning systems (EEWS) estimatethe expected maximum shaking, based on informationextracted from the early arriving P-waves, and send thisinformation to regions farther away from the earthquakesource (or epicentral) region. Because information cantravel at the speed of light (much faster that the damaging earthquake waves), there can be up to tens of secondsof warning time between the availability of the shakingestimates and the onset of damaging ground motions.The difference between P- and S-wave arrival times(also known as the S-P time) is proportional to thedistance of a given site from the earthquake epicenter.Regions in the immediate vicinity of the earthquakeepicenter (the blind zone of an early warning system)will have very small S-P times and, therefore, little or nowarning time. The farther a given location is from the epicentral region, the more warning time is available. Thus,early warning systems can be most effective for providingwarning for large earthquakes that start at some distancefrom a site. For example, Los Angeles could have about aminute of warning before a repeat of the 1857 Fort Tejonearthquake, which initiated in the vicinity of Parkfieldand ruptured about 185 miles southward. However, thecity would have little or no warning from a large earthquake on the Puente Hills fault system, which runs undermetropolitan Los Angeles.Different Flavors of Early WarningA fundamental ingredient in the operation of anEEWS is a modern network of seismic instrumentscapable of measuring earthquake ground motions andprocessing and analyzing incoming waveform data inreal-time.While the technological ingredients have only become available in recent decades, the concept of earthquake early warning has been around for a while. Shortlyafter the 1868 Hayward Fault earthquake (magnitude 7.0),J.D. Cooper wrote an editorial in the San Francisco DailyEvening Bulletin proposing a system that would ring abell over City Hall if ground motions exceeding a certainlevel were detected along earthquake faults. Cooper’sarticle identified a number of requirements that continue2Natural Hazards Observer May 2008to define today’s earthquake early warning systems: (1)systems with more dense instrumental networks perform better, (2) no warning will be available for regionstoo close to the earthquake source, (3) broadcasting ofwarnings and the actions in response to the warning information should be automated. (A 2005 article by HirooKanamori gives a recent review and history of researchefforts in this area. See References.)“Early warning systems can be mosteffective for providing warning forlarge earthquakes that start at somedistance from a site.”There are numerous “flavors” of modern earthquakeearly warning systems. “Front-detection” systems areappropriate for areas where the damaging earthquakesconsistently originate from a known region and the targetwarning area is at some distance away. For instance,many earthquakes that cause damage to Mexico Citytypically initiate along the Guerrero coastal subductionzone, about 185 miles from Mexico City. Thus, a relatively straightforward system consisting of a series ofaccelerometers installed along the coast works fairly well.These accelerometers send information via radio link togovernment offices in Mexico City when a certain levelof ground motion has been exceeded at more than threestations. Because of the considerable distance betweenthe source region and Mexico City, warning times on theorder of 75 seconds are possible. Although Mexico City issituated a considerable distance from the source region,ground motions there can be abnormally large (and earlywarning information very useful) because the city is builton an ancient lake bed. The resonance of these lake sediments can produce amplified ground motions capable ofcollapsing high-rise buildings. Such a front-detection system is also possible for the city of Bucharest (Romania),which is located about 109 miles from the Vrancea regionthat is the source of damaging earthquakes.Single-station approaches use P-wave informationat a given site to predict the maximum ground motions(from the S-waves) at the same location. The uncertaintyin the predicted ground motion amplitudes may belower for the single-station approach, as it requires only arelationship between P-wave derived quantities and peakS-wave amplitudes. Such approaches have the potentialto provide rapid warning information for regions in theblind zone of network-based approaches and have beenproposed for nuclear plants and structural control applications.Finally, network-based approaches provide earlywarning for widespread regions that have numerous potential earthquake sources (for example, Japan and California). The network-based approach uses stations thatare part of a seismic monitoring network, which estimatethe magnitude and location of an earthquake as quicklyas possible. Such systems predict the maximum expectedground motions throughout the region of interest. The

recently activated national earthquake early warning system in Japan is the largest network-based early warningsystem currently in operation.How useful is a few seconds of warning time?An equally fundamental component to an earlywarning system’s success is an informed and well-prepared user community capable of efficiently using theinformation. A year before their national early warningsystem began broadcasting warning information, theJapan Meteorological Agency launched an extensivepublic outreach and education campaign to familiarizethe public with the system and insure that the publicwas informed of how to react to information from thesystem. The campaign focused on simple personal safetymeasures that could be taken in response to a warning.However, if user systems are able to automate decisionsand actions based on early warning information, morenumerous applications are possible.High speed trains in Japan have been automaticallyslowed down and stopped by early warning systemssince the 1990s. Other automated responses include stopping elevators at the closest floor, opening fire stationdoors (so that fire trucks don’t get stuck due to jammeddoors), and saving data on computers. These applications are relatively easy to implement, as the cost offalse alarms is negligible. More complicated applicationsinclude diverting airport traffic, inserting control rods innuclear plants, stopping high precision manufacturingprocesses, and interfacing with active structural controlsystems to change the dynamic response characteristicsof buildings. For these applications, the cost of falseor missed alarms is significantly higher, putting morestringent requirements on the reliability of the warninginformation.In general, the reliability of early warning estimatesat any given time is dependent on the amount of availableobservations, and there is a trade-off between reliabilityand available warning time. Using the earliest estimatesfrom an EEWS increases the available warning time, butat the cost of having to deal with uncertain estimates;later estimates are more reliable but provide less warningtime. Increasing the number of real-time stations in anearly warning network increases the reliability of warninginformation and the available warning times.Earthquake Early Warning in the United StatesThe California Integrated Seismic Network is currently implementing, testing, and evaluating the real-timeperformance of two network-based approaches and onesingle-station approach in northern and southern California. There are about 250 stations with real-time capabilities throughout the state. In contrast, approximately 1,000stations distributed over an area roughly the same sizeas California contribute to the Japanese early warningsystem. A significant amount of investment is necessaryto upgrade and expand the existing networks in California to an early warning-ready system. Such investmentsare likely to pay off in the long term, considering thatearthquakes rank among the most costly natural disastersof the twentieth century.While an early warning system would not preventall earthquake-related losses, it would allow people afew seconds to respond and take simple personal safetymeasures, which could significantly reduce the numberof casualties. If an early warning system in Californiacould prevent just 1% of the damage from a repeat of themagnitude 7.9 earthquake in San Francisco in 1906 (whichwas estimated by the California Office of EmergencyServices to cost 122 billion), it would pay for itself manytimes over. That sounds like a pretty good investment tome.Georgia Cua (georgia.cua@sed.ethz.ch)Swiss Seismological Service, Zurich, SwitzerlandReferences1. Kanamori, H. 2005. Real-time seismology and earthquakedamage mitigation. Annual Review of Earth and PlanetarySciences 33: 195-214.2. Nakamura, Y. 1988. On the Urgent Earthquake Detectionand Alarm System (UrEDAS). Proceedings of the 9th WorldConference of Earthquake Engineering, Japan Association forEarthquake Disaster Prevention, Tokyo, Kyoto: 673-678.The early warning algorithms being implemented and tested inCalifornia are the ElarmS, Virtual Seismologist, and Amplitude/Period monitor algorithms. Details about these algorithmscan be found in the following publications:1. Allen, R.M., and H. Kanamori. 2003. The potential for earthquake early warning in southern California. Science 300:786-789.2. Cua, G., and T. Heaton. 2007. The Virtual Seismologist (VS)method: A Bayesian approach to earthquake early warning. InEarthquake Early Warning Systems. P. Gasparini, G. Manfredi,and J. Zschau (eds). Springer Heidelberg.3. Wu, Y.M., and H. Kanamori. 2005. Experiment on an onsiteearly warning method for the Taiwan early warning system.Bulletin of the Seismological Society of America 95: 347-353.Web ResourcesJapan Meteorological Agency, Earthquake Early rthquake Early Warning in Japan (compiled by MasumiYamada, Kyoto University)www.eqh.dpri.kyoto-u.ac.jp/ masumi/eq/ews.htmU.S. National Earthquake Hazards Reduction Programwww.nehrp.govU.S. Geological Survey Earthquake Hazards Programhttp://earthquake.usgs.govNatural Hazards Observer May 20083

Disaster Mitigation.Second in a SeriesFManaging Lahars the New Zealand Way:A Case Study from Mount Ruapehu Volcanoew hazards have such well-defined attributes thatspecific preparations can be made in advance of ananticipated event. The break-out lahar (volcanic mudflow)threat from Mount Ruapehu’s summit crater lake in NewZealand was one situation, however, where the likelihood, location, magnitude, and approximate timeframeof occurrence were all relatively well-constrained. As a result, a specific emergency response to the predicted laharcould be planned and mitigation measures employed.When the lahar did occur in March 2007 there was noloss of life, reported injuries, or major damage to infrastructure. Here, we present the mitigation, warning, andplanned response measures that were in place at the timeof the lahar.Mount Ruapehu, New ZealandMount Ruapehu is an active volcano situated in the centerof the North Island, New Zealand, near the southern limitof the Taupo Volcanic Zone. Historic activity has beendominated by relatively frequent but small eruptionsthrough the acidic Crater Lake that occupies the activevent. Larger magmatic eruptions occurred in 1945 and1995–1996. Volcanic hazards are dominated by basesurges and ballistic fall-out, which are confined to thesummit area, primary eruption-triggered and secondaryrain-triggered lahars in catchments draining the mountain(principally the Whangaehu River, the natural outlet toCrater Lake, and the route of more than 47 lahars sincerecords began in 1861), and more widespread ash falls.Pyroclastic flows, surges, and debris avalanches also occur but are less frequent.Because Ruapehu is located within the TongariroNational Park World Heritage Area, the mountain doesnot pose an immediate threat to established towns, whichare located outside the park boundaries. The inception ofthis park in 1887 has provided rather fortuitous land useplanning for one of New Zealand’s most active volcanoes.Although two small ski areas (Whakapapa and Turoa) arelocated on the northern and western slopes of the volcano, residential ski lodges are located beyond the rangeof historic ballistics and away from known lahar paths. Inaddition skiers on the slopes are alerted to any eruptioncaused lahars by an automated eruption detection andwarning broadcast system.During 1995–1996, volcanic activity at Mount Ruapehu emptied the summit crater lake and deposited about25 feet (7.5 meters) of tephra (sand, gravel, and bouldersized volcanic material) across the former hard rock outletarea. Over the following decade, the crater lake refilledto a level where it threatened to overtop and breach thisfragile barrier. Based on rising lake levels, it was likely4Natural Hazards Observer May 2008that a breach would happen in late 2006 or 2007. Once thedam failed, it was predicted that up to 63.5 million cubicfeet (1.8 million cubic meters) of hot, acidic water wouldbe released into the steep gorge of the upper WhangaehuRiver, forming a lahar.Although no human settlements were specificallyat risk, some key infrastructure was threatened, including electricity transmission pylons; road, rail, and farmbridges and fiber optic cables carried across them; andstate highway, district, and farm roads. Lives were alsoin danger if people were present in the lahar path as itcame down the mountain (for example, along roads andbridges) on the walking track around Mount Ruapehu orat the Tangiwai memorial site.The precedent of the 1953 Tangiwai disaster, NewZealand’s worst volcanic tragedy, played a key role in informing emergency planning. This event was precipitatedby a situation almost identical to the one that developedin 2007—eruptions in 1945 had emptied Crater Lake andconstructed a fragile barrier of volcanic debris over theformer outlet. This dam failed without warning on Christmas Eve 1953, generating a lahar that critically damageda rail bridge minutes before the arrival of the WellingtonAuckland express train. Unable to stop in time, the engineand most of the carriages plunged into the lahar-swollenriver, and 151 lives were lost. Given this historic event,authorities aimed to avert a similar tragedy by planningand taking action in advance of another dam-break lahar.Preparing for a Lahar from Mount Ruapehu’sCrater LakeIn preparation for the anticipated lahar, the Departmentof Conservation (as manager of the park) produced anEnvironmental and Risk Assessment for Mitigation of theHazard from Ruapehu Crater Lake in April 1999. The reportpresented 24 options in six categories, which included thefollowing: Allow lahar to occur: develop alarm and responsesystem, improve land use planning, but no engineeringintervention at crater or in lahar flood zones Allow lahar to occur but intervene in lahar flood zonesto reduce its size and/or confine it Prevent or reduce lahar by hardening or perforating the1995–1996 tephra barrier at the crater Prevent or reduce lahar by excavating a trench throughthe 1995–1996 tephra barrier at the crater Prevent lahar and reduce lake volume by excavatingtrench into underlying rock at outlet Defer, prevent, or reduce lahar via other options (e.g.,siphoning, barrier truss)

In late 2001, the Minister of Conservation decided toallow the lahar to occur without direct intervention at thecrater rim, while also installing an alarm warning systemand requiring that emergency management response andcontingency plans were developed by relevant agencies.State-of-the-art warning system hardware known asERLAWS (Eastern Ruapehu Lahar Alarm and WarningSystem) was installed by the Department of Conservationand a consortium of local government bodies, emergencyservices, and infrastructure agencies. The warning hardware would detect failure of the tephra dam and confirmthe creation of a lahar by sequential triggering of sensorsin the upper Whangaehu Gorge and would then transmita warning signal to agencies. Agencies would then puttheir response plans into action.In addition, a series of preemptive engineering measures were implemented, including: raising and strengthening the State Highway 47 roadbridge at Tangiwai to withstand the forces of a lahar constructing an embankment at the mouth of theWhangaehu Gorge to reduce the risk of the lahar spilling into channels to the north, from which it could thencross State Highway 1 (the main north-south highway)and enter the Tongariro River installing gates and warning lights and signs on roadscrossing the Whangaehu River and other potentiallahar channelsTwo groups were organized to develop responseplans to the lahar. The Southern Ruapehu Lahar PlanningGroup was responsible for developing a response planfor the southern side of the mountain, where the mainrisk from the lahar was. The Northern Ruapehu LaharPlanning Group was responsible for planning a responsefor the northern side, which included the possibility ofa large lahar overflowing to the north and entering theTongariro River and Lake Taupo. The planning groupscomprised a range of organizations, including local government bodies, the police, and other emergency serviceswith input from other agencies involved in the response,such as the Department of Conservation, GNS Science,and transport and energy infrastructure companies. At anational level, the Ministry of Civil Defence & EmergencyManagement assisted with planning but was not directlyinvolved in developing any plans.The response plans defined agencies’ roles andresponsibilities during the event and outlined a timelinefor expected response. The response involved

Earthquake Early Warning Systems: An Investment that Pays off in Seconds I n October 2007, Japan unveiled a national earthquake early warning system tasked with providing the general public with a few seconds of warning before the onset of strong earthquake ground shaking. This article defines earthquake early warning systems and describes

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