Introduction

The earliest known earthquake detectors, called seismoscopes, were invented in China in the second century. The earliest designs could indicate an event had occurred and the direction of shaking; later designs also recorded the time [1]. Earthquake recorders, called seismometers, were not invented until 155 years ago. Seismometers, also called seismographs or accelerographs, record the time history of acceleration. Early designs recorded their data on smoked glass, smoked paper, rotating drums, film, or tape. Modern designs record data--generated by a set of three mutually orthogonal sensors called a triaxial accelerometer--digitally on solid-state memory. Seismometers called central recorders are capable of recording data from up to 15 accelerometers; an array of central recorders can handle any number of accelerometers.

Seismic switches, also called seismic triggers, which utilize seismic energy to actuate useful functions, were not invented until this century. With the advent of digital technology, users can now select the precise "g" forces at which they want their switches to trip. Today, seismic instruments are used to monitor or actuate a wide variety of functions and structures, including: fuel gases, toxic gases, pipelines, highrises, elevators, railways, bridges, dams, and nuclear-power plants. Several region-wide networks practice real-time seismology, whose goal is to provide immediate information on the magnitude, location, and time of each earthquake to their subscribers, who include emergency-management agencies, utilities, railways, and large corporations. This information enables these entities to make quick decisions on the optimal deployment of limited resources, helping to protect lives and property, and minimize business interruption.

Fuel gases

Seismic gas shutoff valves (SGSVs), designed to turn off gas service automatically in case of a strong earthquake, might be the first practical application of seismic triggers. The Magnitude 6.3, 1933 Long Beach earthquake--which generated the first accelerograph record for a local earthquake--caused fires at seven schools, prompting California to include in the Field Act a requirement that new schools be equipped with a SGSV. Because contemporary designs were overly sensitive to aseismic vibrations, school districts requested the requirement to be dropped after the Magnitude 7.7, 1952 Kern County earthquake, which tripped 68 (about 10%) of the Los Angeles School District SGSVs [2].

In 1981, a national standard for SGSVs was completed, and was adopted almost verbatim by California. In 1985, the U. S. Army Corps of Engineers began requiring a SGSV on all essential military facilities in seismic zones 3 or 4. In 1987, California began requiring SGSVs to be certified by its State Architect. In 1990, the General Service Administration mandated SGSVs on all federal office buildings in Region IX. In 1992, the American Society of Civil Engineers established a Standards Committee to rewrite the national standard. A prestandard should be available for public review this spring. California intends to use the new national standard, which might be completed this year, to replace its current standard.

SGSVs performed well during the Magnitude 6.7, 1994 Northridge earthquake [3]. Fortunately, numerous gas fires and explosions were averted because most residents were home to turn off their gas manually. However, Southern California Gas Company found a gas leak in the house line at 19.3% of the 841 structures where it was requested to reset a SGSV. The Los Angeles Fire Department expressed concern that if the earthquake had occurred during business hours, some of the 17,000 reported gas leaks could have caused neighborhood fires. Similar concerns were expressed following the Magnitude 6.6, 1971 San Fernando earthquake--that a Magnitude 8.4 earthquake centering on the Palmdale segment of the San Andreas Fault during a Santa Ana wind could cause numerous fires of the Bel Air-Brentwood type [4].

In 1995, Los Angeles began requiring a SGSV to be installed on all gas-serviced new construction and certain remodeled commercial buildings. Los Angeles uses California's standard, except that it also requires bracing and has raised the lower level for tripping from 8% to 20% g horizontal acceleration, at 1 and 2.5 Hz [5]. The author believes the International Association of Plumbing and Mechanical Officials will add an appendix soon to the Uniform Plumbing Code that will be similar to Los Angeles' regulation; and that many cities will adopt it during the next three years. About 180 mobilehomes burned due to the Northridge earthquake, prompting California to consider requiring mobilehome parks to install a SGSV by the year 1998 (seismic zone 4) or 2000 (seismic zone 3).

Some facilities have connected their SGSV to other life-safety functions. A Burbank hospital has connected its SGSV to its fire-alarm system (with a time delay) and its 2-of-5 logic, propane-gas-sensor system [6]. A Costa Rica hotel has connected its SGSV to its fire-alarm system. A building with a fire-suppression system in its kitchen could shut the fuel off outside while halon is being released if it connected its system to a SGSV. Some new-home builders are promoting the use of a SGSV in conjunction with carbon-monoxide sensors that would not only sound buzzers, but would also immediately turn off the most likely source of the gas.

Toxic gases

In 1988, the Santa Clara County Fire Chiefs Association developed a Model Ordinance for Toxic Gas Regulation, which was added to the Uniform Fire Code as an appendix that becomes Article 90 when adopted. This regulation requires seismic shutoff of toxic gases at 30% g or lower horizontal acceleration, at 2.5 Hz. The regulation was developed partly in response to the Bhopal disaster to reduce the potential hazards of toxic-gas releases in Silicon Valley [7], which include toxic-gas clouds, storm-drain run-off, groundwater contamination, and extreme fire and explosion hazards [8]. Santa Clara County, its ten cities, and Santa Rosa have adopted this regulation. Several semiconductor plants have installed fault-tolerant, 2-of-3 logic systems to minimize the likelihood of false triggers. It is recommended to use fail-safe systems capable of shutting off hazardous materials in response not only to seismic activity, but also to pressure loss, power loss, excess flow, and nearby leaks [9]. The reprographics industry has developed a similar standard affecting its members' ammonia lines.

Pipelines

The trans-Alaska pipeline uses a P-wave detection system to shut off the flow during strong earthquakes [10]. Many California water districts use seismic shutoff devices on their distribution lines. East Bay Municipal Utility District is installing a monitoring and dual-alarm system to monitor two aboveground concrete reservoirs and control their main valves; and a monitoring and dual-alarm system with 2-of-3 logic to control two large water mains near their crossings of the Hayward Fault.

Tokyo Gas uses 31 SGSVs to control its distribution lines in the Tokyo region [11], where about 1500 blocks were destroyed by fire following the M 7.8, 1923 Kanto earthquake. Los Angeles is considering whether to require the local natural-gas utility to install SGSVs in its distribution lines.

Highrises

In 1965, Los Angeles began requiring three accelerographs (i.e., earthquake recorders) to be installed in all buildings over 6 stories high with an aggregate floor area of at least 60,000 square feet, and in all buildings over ten stories high--in the basement, midportion, and near the top. In 1967, the International Conference of Building Officials added an appendix to the Uniform Building Code similar to Los Angeles' regulation, except it also requires accelerographs to be maintained. Many cities have adopted that regulation, with a few notable exceptions: San Francisco, Portland, and Seattle. The USGS installed and maintained required accelerographs for free until 1974. Subsequently, many building owners have allowed their instruments to become inoperative, partly because their building departments do not enforce the maintenance part of the provision [12]. In 1982, Los Angeles reduced the number of required accelerographs to one near the top (other cities still require three), but required it to be maintained.

Los Angeles is now considering whether to require three again, because the Northridge earthquake caused widespread damage to welds in steel-frame buildings, and also because there have been significant advances in accelerograph technology. Modern digital instruments are easy to maintain and are accessible by modem, which would enable building inspectors to have a set of records with them while making their initial post-earthquake inspections.

Elevators

Because the San Fernando earthquake caused widespread damage to elevators, in 1975 California began requiring elevators that travel faster than 150 feet/minute to install either a seismic switch or a counterweight derail-detection device, with retrofitting required by 1983. California allows an elevator seismic switch to trip at 15% g or lower acceleration in any direction. The seismic switch, usually located on the top floor, sends a contact closure to the elevator-control system, which causes operating elevators to proceed to the next floor, open, and shut-down until reset by a mechanic. California waives the requirement for a seismic switch if a registered engineer certifies that the guide system was designed and built to withstand the same seismic forces as the building; but recent earthquakes have taught that seismic-safety devices should always be required because buildings often perform better than their elevators.

In 1981, an appendix similar to California's regulation was added to the National Safety Code of Elevators and Escalators, except it requires a seismic switch and only affects new installations and major renovations. A study done after the Loma Prieta earthquake found that the advantage of a seismic switch is that the required inspection can discover damaged doors, hoistways, or rails that would not be detected by the alternative "ring-on-a-string" system unless the counterweight derailed or the system false triggered. Seismic switches are even more desirable outside California, where elevator codes do not require retrofitting [13].

Railways

About 25 years ago, Japan National Railways installed a system of seismic switches designed to cut off the power to its Bullet Train. The system has evolved into a seismic network capable of providing advance warning roughly proportional to the distance between the epicenter and the railway. The Teito Rapid Transit Authority uses a three-station seismic system to stop Tokyo subways during strong earthquakes [14].

In 1977, the Bay Area Rapid Transit Authority (BART) installed a seismic switch at each of 8 stations selected based on their proximity to the San Andreas, Hayward, Calaveras, or Concord Faults. Each switch will trip an audio alarm in the station-agent's booth if the acceleration reaches at least 10% g in any direction. The agent then notifies Central Control, which will radio train operators to either remain stopped or to proceed slowly to the next station and unload. Only two of these switches have ever tripped--both by the Loma Prieta earthquake; nonetheless, the entire system was inspected for damage before service was restored [15]. There are no accelerographs in either of BART's tubes beneath San Francisco Bay, although recording and analyzing the tubes' motions could prove useful in evaluating damage following future earthquakes [16]. BART is instrumenting three new stations with accelerographs that will send two adjustable alarm levels over BART's SCADA system and also record vibrations. The final phase of BART's seismic-instrumentation program would be to connect the seismic signals with the train-control system, and decide whether to adjust the sensing level or add more switches [17].

Los Angeles County's Metropolitan Transportation Authority is installing a monitoring and dual-alarm system in eleven Red Line Subway Stations. The system was installed in 1995 at two stations scheduled to open this spring. At 10% g or greater acceleration in any direction, the recorders will start and the low-level alarms will trigger signals at the Central Command Facility (CCF), which will notify train operators to slow down and unload at the next station. At 20% g or greater acceleration, the high-level alarms will trigger signals at the CCF, which will notify train operators to slow to a crawl and stop at the next station; and also turn on large fans to vent the tunnels of potential gases.

The Skytrain Light Rail Commuter System in Vancouver, Canada, has 14 seismic sensors that provide two adjustable alarm levels. The low-level alarm requires the operator to perform certain emergency procedures. The high-level alarm automatically causes the system's central computer to initiate emergency procedures; overriding operator response [18].

The Puerto Rico Highway and Transportation Authority is administering the construction of Tren Urbano, which will be an urban rail system with six dual-alarm seismic triggers; three at grade and three at elevated stations.

Bridges

In 1981, California's Strong Motion Instrumentation Program (CSMIP) installed 26 seismic sensors on the Vincent Thomas Suspension Bridge. Records produced during the Magnitude 5.9, 1987 Whittier Narrows earthquake (centered 25 miles away) indicated the deck moved about 4 inches vertically during the shaking [19]. Records produced during the Northridge earthquake (centered about 35 miles away) showed that the bridge's motions exceeded those caused by the 1987 earthquake [20]. Besides certain bridges, CSMIP also monitors selected buildings, dams, power plants, and utilities as part of a project begun in 1972 to obtain data for design engineers. CSMIP releases reports within days or weeks of significant earthquakes. CSMIP deploys over 400 instruments, but its goal is 1,015 [21].

Dams

Many California dams are seismically instrumented, although they are not required to be. CSMIP has instrumented several dams based on size and proximity to active faults, but most accelerographs on dams were installed by the dams' owners. It is the responsibility of each dam's owners to do their own inspections following any earthquake of sufficient magnitude based on the dam's proximity to the epicenter, although inspectors from California's Division of Safety of Dams (CDSD) will inspect all dams within a defined radius of the epicenter following a large earthquake. Whenever a dam-owners' inspection finds significant distress, the CDSD will also do an inspection. Twelve of the 22 dams operated by the CDSD for the California Water Project have accelerographs; some also have a seismic trigger connected to shut down water flow [22].

Los Angeles County Public Works is instrumenting the Devil's Gate Dam in Pasadena with a central recorder and three triaxial accelerometers as part of a rehabilitation project. The Metropolitan Water District will be instrumenting three dams that will be built during the next several years near Hemet, California, with over a dozen accelerographs.

Nuclear-power plants

All nuclear-power plants in the United States have earthquake monitors. Most of these plants have already replaced their analog instruments with digital systems. Some now use systems that automatically calculate the values necessary to determine whether a plant can remain in operation; a decision that must be made within four hours.

Real-time seismology

In 1990, Caltech and the U.S. Geological Survey (USGS) began upgrading the Southern California Seismic Network (SCSN) to broadcast reports via pagers to subscribers within 5 minutes of an earthquake, alerting them to the magnitude, location, and time. When the Northridge earthquake occurred, there were 18 subscribers (government agencies, utilities, and railways) to CUBE (Caltech-USGS Broadcast of Earthquakes). Due to hardware and software problems, CUBE was unable to provide information during the first hour; nonetheless, a study to assess the value of CUBE's Northridge data to its subscribers indicates that it was still the first data upon which actions were taken. In 1995, California-wide broadcasts became available through the cooperation of CUBE and REDI (UC Berkeley's Rapid Earthquake Data Integration), and are being used by California's Office of Emergency Services, Caltrans, and utilities and railways with statewide responsibilities. CUBE subscribers now have software that maps the ground-acceleration data broadcast by the SCSN in real time [23].

Mexico City operates the world's only public real-time warning system, which provided 72 seconds of warning before vibrations arrived from the Magnitude 7.2, 1995 Ometepec earthquake. Only minor damage occurred in Mexico City, although there was considerable damage near the epicenter [24].

Other applications

Seismic switches are also used for annunciators, cranes, lockdown facilities, generators, mainframe computers, conveyors, pumps, saws, presses, and electrical services for buildings. Over a dozen California schools have connected a seismic switch to an annunciator that will repeatedly broadcast the message "Duck, cover, and hold!" over their P.A. system. Similar systems are used in many large buildings in Tokyo. Under certain conditions, these types of systems can be triggered by the initial P wave of a damaging earthquake and provide a warning before the arrival of the usually more damaging S waves, which travel slower. Although prone to false triggers, warning systems have great potential for enhancing safety by making it possible to provide a few seconds for people to take precautions, such as ducking under a desk or moving away from machinery.

Conclusions

The past 20 years have seen many improvements in seismic monitoring and actuation technology, which have greatly increased the ability of emergency managers and design engineers to decrease the risks from earthquakes to life, property, and commerce. The public has already benefitted in many ways that they are unaware of, including the reduction of the risk from fire following earthquake, and the use of seismic data in evacuation decisions for highrises and also for neighborhoods below dams and regions surrounding nuclear-power plants. As the 1990s are the International Decade for Natural Disaster Reduction, the author expects this trend will continue.

References

1.	Dewey, J., and Byerly, P., 1969, The early history of seismometry (to 1906): Bulletin of the Seismological Society of America, v. 59, pp. 183--227.
2.	Strand, C. L., 1988, A catalog of post-earthquake gas leaks and related incidents, with a review of the case for mandating earthquake gas shutoff valves: Unpub. ms., 58 pp.
3.	Strand, C. L., 1995a, Gas leaks, gas-related fires and performance of seismic gas shutoff valves during the Northridge earthquake, in O'Rourke, M. J., ed., Proceedings of the Fourth U.S. Conference on Lifeline Earthquake Engineering: Technical Council on Lifeline Earthquake Engineering of the American Society of Civil Engineers, New York, pp. 692--699.
4.	Duke, C. M., and Moran, D. F., 1972, Earthquakes and city lifelines, in The San Fernando earthquake of February 9, 1971 and public policy: Special Subcommittee of the Joint Committee on Seismic Safety, California Legislature, pp. 53-71.
5.	Strand, C. L., 1995b, Los Angeles modifies seismic gas shutoff valves regulation: Western HVACR News, October, p. 5.
6.	Flig, A. Y., 1993, New developments in earthquake disaster prevention and non-structural hazard mitigation, in Gould, P. L., ed., Proceedings, 1993 National Earthquake Conference: Central United States Earthquake Consortium, Memphis, Tennessee, v. 2, pp. 307--316.
7.	Perkins, J. B., Selvaduray, G., and Wyatt, G. E., 1991, Review of emergency preparedness programs for preventing or controlling toxic gas releases in earthquakes: Association of Bay Area Governments, Oakland, 64 pp, 4 app.
8.	Perkins, J. B., and Wyatt, G. E., 1990a, Hazardous materials problems in earthquakes: Background materials: Association of Bay Area Governments, Oakland, p. 97.
9.	Perkins, J. B., and Wyatt, G. E., 1990b, Hazardous materials problems in earthquakes: A guide to their cause and mitigation: Association of Bay Area Governments, Oakland, p. 44.
10.	National Research Council, 1991, Real-time earthquake monitoring: National Academy Press, Washington, D. C., 52 pp.
11.	National Research Council, 1991, Real-time earthquake monitoring: National Academy Press, Washington, D. C., 52 pp.
12.	Siegel, G. W., 1989, Earthquake instruments and building safety: Building Standards, July-August, pp. 9 and 20.
13.	Schiff, A., 1990, Performance of elevators, in Benuska, L., ed., Loma Prieta earthquake reconnaissance report: Earthquake Spectra, Supplement to v. 6, pp. 364-377.
14.	National Research Council, 1991, Real-time earthquake monitoring: National Academy Press, Washington, D. C., 52 pp.
15.	Frank Oklesson, personal communication, 1996.
16.	Werner, S. D., and Schiff, A. J., 1990, Railways and buses, in Benuska, L., ed., Loma Prieta earthquake reconnaissance report: Earthquake Spectra, Supplement to v. 6, pp. 270-274.
17.	Burns, J. S., 1978, Seismic sensing and alarm at BART: Unpub. ms., Presented at APTA Rapid Transit Conference, Chicago, Bay Area Rapid Transit District Report #BA-ENG-78-E2, 19 pp.
18.	Flig, A. Y., 1993, New developments in earthquake disaster prevention and non-structural hazard mitigation, in Gould, P. L., ed., Proceedings, 1993 National Earthquake Conference: Central United States Earthquake Consortium, Memphis, Tennessee, v. 2, pp. 307--316.
19.	Anonymous, 1989, State's strong motion program records shake, rattle and roll at active earthquake locations: California Consulting Engineer, Winter, 3 pp.
20.	Shakal, A, Huang, M., Darragh, R., Cao, T., Sherburne, R., Malhotra, P., Cramer, C., Sydnor, R., Graizer, V., Maldonado, G., Petersen, C., and Wampole, J., 1994, CSMIP strong-motion records from the Northridge, California earthquake of 17 January 1994: California Department of Conservation, Sacramento, California Strong Motion Instrumentation Program Report No. OSMS 94-07, 308 pp.
21.	Anonymous, 1989, State's strong motion program records shake, rattle and roll at active earthquake locations: California Consulting Engineer, Winter, 3 pp.
22.	David Kessler, personal communication, 1996.
23.	Eguchi, R., Goltz, J. D., and Seligson, H. A., 1995, The application of new technologies, in Schiff, A. J., ed., Northridge earthquake: Lifeline performance and post-earthquake response: American Society of Civil Engineers, New York, Technical Council on Lifeline Earthquake Engineering Monograph No. 8, pp. 275--287.
24.	Todd, D., 1995, Mexico City's earthquake early warning system: EERI Newsletter, v. 29, no. 11, p. 11.