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Earthquake Pre Awareness & Seismic Risk Reduction

Abstract:

The writing about global perspective is to provide guidance that will assist communities throughout the world in ongoing efforts to improve their capability for adopting and using real time communicating systems for the benefit of all sectors of the community.

Introduction

Natural disasters and in particular earthquake disasters are events, which are difficult to handle. Despite the fact that the technology exists to protect the population from earthquake catastrophes, there have been great problems in drawing public attention to the ever growing seismic risk in areas of low to moderate seismcity, where destructive earthquakes are very rare.

It has been realized that alarm, rapid response and early warning systems could be very beneficial in reducing loss of lives during a natural disaster. In particular, this has been a challenging task for earthquake protection as the pre-warning times are only a few seconds. Moreover, research in earthquake prediction has shown that we are still far away from the accurate prediction of the time, location and magnitude of strong earthquakes, which would be necessary to start with a timely evacuation of the endangered area.

However, with the earthquake rapid response system operated by the National Weather Bureau in Taiwan, it was possible to get an accurate picture about the effect of the September 21, 1999 Chi-Chi earthquake within a few minutes, which allowed the swift launching of rescue operations. The efficient emergency management of this catastrophe has certainly saved quite a number of lives and has received international attention. This system is based on several hundred seismic stations distributed over Taiwan, which are connected to a central recording station. The system can eventually be upgraded by suitable software and continuous communication with the seismic field stations to provide pre-warning for urban areas and critical facilities. However, for nuclear power plants, high speed trains, tunnels and highways, pipelines etc. dedicated systems may be more appropriate because of the lower complexity and the interface problems with the organizations, businesses and authorities, which should be integrated in such a system.
At present, the earthquake early warning and early response system for Istanbul, Turkey, which is currently under development, may be the most advanced one for a large urban area. The early warning part of this system will first be operated on an experimental basis because experience with such sophisticated systems is still scarce.

Design Background:

Even in countries where destructive earthquakes are rather frequent, most of the existing buildings have either not been designed against earthquakes or by methods which today are considered as obsolete. Therefore, the earthquake safety of these buildings is unknown and in many cases may be inadequate. A thorough seismic safety review of public buildings in Switzerland has shown that about 20% would have to be strengthened. As Switzerland is a country with low to moderate seismicity and because of the generally good quality of the constructions it may be concluded that the percentage of seismically deficient buildings – especially in the poorer countries – may be much higher than 20%. This applies in particular to the urban areas of the countries of the former Soviet Union of the Caucasus and Central Asia where specific construction methods have been used (standardised buildings made of prefabricated elements), which are located in highly seismic areas. Recent earthquakes have clearly shown systematic deficiencies in widely used building designs. Therefore, the percentage of structurally deficient buildings may easily exceed 50% in many seismic areas despite the fact that in some of these countries the earthquake regulations are very advanced as for example in Colombia.

In low to moderate seismic areas it must be expected that an (Maximum Credible Earthquake) MCE with a return period of say 10’000 years will cause large-scale destruction in buildings designed for a 475 year return period. This situation can be easily understood as in a high seismic area the earthquake loads during an MCE may exceed the design loads by say 20 to 50%, whereas in low seismic areas the MCE earthquake loads may exceed the design loads by factors of 2 to 4. Therefore, we may conclude that in low to moderate seismic areas there exists a substantial seismic risk as with the present regulations based on a return period of 475 years, the buildings – this applies in particular to the brittle masonry type of buildings – are essentially under-designed. The ductility of reinforced concrete and steel buildings ensures that they will not collapse as easily as the very vulnerable masonry buildings and other structures with seismic deficiencies (soft storey, etc.). Because of these seismic risk considerations, the new earthquake regulations in the US are now using an approach in which the seismic actions are determined based on a return period of 2500 years. The seismic actions are then multiplied by a factor of 2/3. This results in seismic design loads, which are equivalent to a return period of ca. 475 years in the high seismic areas of California (i.e. no change of the current practice) and in loads, which correspond to a return period of ca. 1250 years in the moderate seismic zones of the Middle West and the East coast of the USA.

Furthermore, based on seismic risk considerations, and besides the above problem of the return period of the design earthquake in areas of low to moderate seismicity, it is also necessary to impose higher seismic safety standards for
(i) urban and highly industrialized areas,
(ii) facilities with a high damage potential such as nuclear power plants, large dams, natural gas and oil facilities, offshore platforms, pipelines, facilities of chemical and petrochemical industry etc.,
(iii) vital facilities (lifeline systems), which must be operable during and after and earthquake,
(iv) transportation systems such as high speed trains, traffic flow on bridges and in tunnels, and
(v) expensive machines, which are vulnerable when in operation during an earthquake such as
(a) turbo-generators, which are vulnerable to differential support movements and out-of-balance forces, and
(b) industrial robots, which are vulnerable to ground shaking when in operation with their arms extended, etc.

In general, it is possible to apply higher seismic safety standards to new buildings and installations. However, this does not cover the existing buildings and installations and there are also seismic design limitations with respect to (ii), (iv) and (v). In those cases, the optimum earthquake safety can be achieved by a combination of seismic design and early warning.

In the case of early warning, one has to be aware that due to the high speed of the damaging shear waves of about 3.5 kilometres per second, the maximum pre-warning times in areas with well-defined fault zones can be as high as 60 to 80 seconds (Mexico City). In other areas, where the active faults are not known, such as in areas of low to moderate seismicity, the warning time may be less than 5 seconds. There may also be events with almost zero warning times, depending on the seismic network used for the early warning system. As damaging earthquakes have durations of several seconds, a zero warning time can still be very beneficial, as a system can be put into a safe state before most of the damaging seismic waves have arrived at the site.

As far as earthquake protection is concerned, the objectives of modern earthquake-resistant building codes are:
(i) To protect human lives and to prevent injuries;
(ii) To minimize economic losses (structural and non-structural damage in buildings, damage of infrastructure, etc.);
(iii) To maintain vital services and to minimize operation/production interruptions;
(iv) To protect the environment and cultural heritage.

The first priority in earthquake protection is to save lives. For life saving the following phases of an earthquake event can be distinguished:
(i) Several years before an earthquake: Measures: seismic design and strengthening of buildings and installations; preparation of emergency plans, to conduct programs for earthquake preparedness of population, installation of earthquake early warning, seismic alarm and earthquake rapid response systems. Note: practically all people and buildings or facilities can be covered.
(ii) A few seconds before an earthquake: Safety system: earthquake early warning systems; warning provided by earthquake early warning system with pre-warning times of zero to maximum 90 seconds: evacuation of buildings; shut-down of critical systems (nuclear and chemical reactors); stop high-speed trains. Note: very few people can be evacuated for pre-warning times of less than 30 seconds, however, critical facilities can be put into a safer position.
(iii) During an earthquake: Safety system: seismic alarm systems; alarm released by a seismic alarm (or early warning) system will provide signal for shut-down of critical systems (nuclear and chemical reactors); alarm signals can be used to initiate emergency stop of high-speed trains and vulnerable machines and industrial robots. Note: critical facilities can be put into a safer position.
(iv) Immediately after an earthquake: Safety system: earthquake rapid response systems; information provided by an earthquake rapid response system: within seconds after an earthquake damage maps based on the spectrum intensity can be made available, which show the damaged areas and form the basis for efficient rescue operations. Note: injured people trapped in damaged buildings may be rescued in time.

This qualitative analysis shows, that earthquake early warning, seismic alarm and earthquake rapid response systems can be very beneficial. Moreover, these systems are inexpensive as discussed below.

What are the differences between an early warning, an alarm and a rapid response system?

(i) Early warning system: An earthquake early warning system is the most sophisticated system of the above three systems and requires seismic stations (strong motion instruments, which can provide real-time spectrum intensities and peak ground accelerations) close to the source of earthquakes and continuous communication between the seismic stations and a central processing station. The early warning system can also be used as alarm and rapid response system if there are seismic stations located in critical buildings and distributed uniformly in an urban area. Typically such a system would consist of a number of seismic stations close to a potential source zone. But for vulnerable facilities with a large damage potential such as nuclear power plants where seismic source zones are not known accurately a seismic fence or array of instruments may be placed around a nuclear power plant within a radius of say 30 to 60 km in order to achieve a reasonable pre-warning time.
Main applications: urban areas; high speed trains; highways; gas distribution systems; nuclear power plants; offshore platforms and facilities of petrochemical industry; pipelines; industrial facilities (robots); commando centres, radio stations and rescue units; telecommunication centres; power generation facilities, etc.

(ii) Alarm system: In the case of a seismic alarm system the seismic stations (strong motion instruments) are located in the buildings or facilities, where the alarm signal is needed, e.g. in a nuclear power station. Continuous communication between the seismic station and the alarm station is also needed for a seismic alarm system.
Main application: urban areas; high speed trains; highways; gas distribution systems; nuclear power plants; offshore platforms and facilities of petrochemical industry; pipelines; industrial facilities (robots, chip factories); commando centres, radio stations and rescue units; telecommunication centres; power generation facilities, etc.

(iii) Rapid response system: For an earthquake rapid response system a large number of seismic stations (strong motion instruments) is needed, which are distributed uniformly over an urban area. The stations do not need a continuous communication with a central station. The stations may be equipped with mobile phones, which will send SMS messages to the central station a few seconds after the end of an earthquake. The messages sent may contain information about the peak ground acceleration and the spectrum intensity, which will be the basis for the automatic preparation of damage maps. Main application: urban areas; commando centres, radio stations and rescue units; telecommunication centres; power generation facilities, etc.

Early warning and alarm systems are mainly used to shut down or to initiate the shut down of vulnerable systems and dangerous processes and their scope is roughly the same. Rapid response systems are mainly applicable to larger urban or industrial areas, where catastrophe management is an important public task.

Concept of seismic early warning, alarm and rapid response systems

Present technology in seismic instrumentation and telecommunications permits the implementation of a system for earthquake early warning. Such a system is capable of providing from a few seconds to a few tens of seconds of warning before the arrival of strong ground shaking caused by a large earthquake.

An earthquake early warning and rapid response system can provide the critical information needed
(i) to minimize loss of lives and property, and
(ii) to direct rescue operations.

The basic features of a seismic alarm system are shown in Fig. 1 (Heaton, 1985). Ground motions recorded by an array of seismograph stations and/or locally processed real-time data (spectrum intensity, response spectra, Fourier spectra etc.) are telemeter to a central processing site. The main parameters of an earthquake, i.e. the location, time of origin, magnitude, focal mechanism, amplitude of ground shaking and reliability estimates are computed. That may take about one minute from the time an earthquake has been detected. Although, the knowledge of the seismic parameters is desirable it is not essential for early warning or for issuing an alarm in critical facilities. The earthquake parameters are needed for rapid response systems but only if the affected urban area is not equipped with a dense array of seismic stations.

If the site is far from the source zone, which is monitored by seismic stations, then tens of seconds may be available before shaking begins. This time may be used to receive further information about the size of the earthquake. In this way, users at large epicentral distances take action only for the large earthquakes that present a real hazard. However, in practice it is more appropriate to take immediate action if an alarm is issued, i.e. if the trigger level for the acceleration is exceeded in several redundant stations and/or the spectrum intensity exceeds a predefined value. For earthquake early warning and alarm systems there is usually insufficient time to compute the hypocenter, focal parameters and the magnitude of an earthquake, as this time is needed for the more complex alarm decision making process. The benefits of an early warning system increase with increasing pre-warning time. The same applies to a seismic alarm, which must be issued at the very beginning of an earthquake. An alarm may also be worthless for critical facilities that could be damaged by ground shaking, if it is released after an earthquake. However, for secondary earthquake effects such as landslides, release of hazardous materials, flood waves due to a dam break, fires etc., which may develop shortly after an earthquake, a delayed alarm is still very beneficial.

Immediately after the occurrence of an earthquake, the seismic stations provide information regarding the strength of shaking in different locations. This information can be used to estimate areas of substantial damage, so that emergency services can be allocated promptly and properly. Because the strong motion instruments in the array would have a large dynamic range, the seismic network may routinely record ground motions from numerous small earthquakes and teleseismic events. The routine use of a seismic network for studies of small events would help to ensure that the system operates properly when relatively rare large events occur.

For an earthquake early warning system the seismic stations should be located close to the source zones where damaging earthquakes may occur. This is the case for the early warning system for Mexico City, early warning systems in Japan and Taiwan. In all three cases the potential source zones are located offshore.

General benefits of earthquake early warning, alarm and rapid response systems

These systems are very effective when large numbers of people may be lost in a single incident, i.e. collapse of a stadium during a performance, failure of a high-rise building with high occupancy, derailment of a train, dam failure with downstream flooding or a nuclear reactor accident with uncontrolled release of radioactive substances.

The main benefits of an earthquake early warning and rapid response system are as follows:
• Earthquake early warning systems help to reduce the loss of lives but they do not help to reduce economical losses due to damage to buildings and infrastructure.
• They are efficient tools in urban areas where a significant portion of the buildings and infrastructure are deficient seismically.
• In cases where the seismic source zone is clearly known and sufficiently far away (Mexico City), large segments of the population can be warned by radio, television, etc. In Mexico City, public schools and government agencies are directly connected with the alarm system. Operation of critical facilities and processes can be stopped.
• When sufficient warning time is available, critical facilities and systems can be put into safe position and fast hazardous processes can be stopped in time.
• Early warning systems also form part of alarm systems and can be used to alarm the population where rapid response is needed. A typical example would be to issue the so-called water alarm, i.e. alarming the population living in the downstream region of a large dam.
• Early warning systems are useful for facilities and processes where rapid response can contribute to the reduction of the seismic risk. These are nuclear power plants, high speed trains, gas mains, highways etc.

A modern earthquake early warning system for an urban region shall include
(i) monitoring of the seismic activity in the potential seismic source zones, and
(ii) monitoring of the ground motion within the urban area.

The signals obtained from (i) are used for early warning of the population and to shut down critical facilities and processes, and those of (ii) are basically used to determine the areas with major damage, and to initiate rapid response measures.

The seismic stations can send their signals through radio, the mobile phone network or the internet. However, this may cause unnecessary delays. It has to be studied on a case by case basis if inexpensive public telecommunications systems can be used or if rather expensive dedicated lines have to be used for selected stations.

Reliable hardware for earthquake early warning systems is available. Solutions for telecommunications and data transfer also exist. However, the main problems are
(i) to distinguish a strong earthquake from other signals, in order to avoid false alarms as all alarms have to be automated due to time constraints,
(ii) to determine to which agencies and facilities the alarm shall be forwarded, and
(iii) to determine how the alarm shall be implemented in critical facilities.

Implementation aspects of early warning and rapid response systems

A seismic early warning system has excellent prospects where the seismic source zones are far away from urban centres. Unfortunately this is the exception. Many cities are already located within a seismic source zone. In this case, the benefits of an earthquake early warning system are not that obvious and public acceptance may me limited. Moreover, early warning systems require more advanced technology than rapid response systems. Time plays a lesser role in rapid response systems than in early warning systems. Hence, it is possible to use inexpensive mobile phones for data transfer in rapid response systems, whereas continuous data transfer via radio is needed in early warning systems.

The same type of seismic sensors can be used for early warning and rapid response systems, if they are located in an urban area.

As earthquake early warning systems are still under development, it is recommended to install first a rapid response system and to expand that eventually to include the features of an early warning system (continuous communication, dedicated software).

Conclusions

Earthquake early warning and alarm systems are low-cost solutions for the reduction of the seismic risk of important facilities such as high speed trains, nuclear power plants, pipelines etc. They are also feasible for large cities, which are exposed to earthquakes occurring at known faults. For all urban and highly industrialized areas earthquake rapid response systems are recommended, which can be upgraded into early warning and alarm systems. The technology for such systems is available.

As with the current seismic codes used in most parts of the world, the seismic risk in urban areas of low to moderate seismicity is still very high, earthquake early warning, alarm and rapid response systems are recommended worldwide.

A state-of-the-art alarm and rapid response system for an urban area may cost about USD 5 millions.

The installation of an earthquake early warning, alarm and rapid response system shall not be a substitute for the seismic strengthening of critical facilities such as seismically under-designed nuclear power plants.

Decision makers have recognized the great advantages of earthquake early warning systems. Several systems have already been installed during the last couple of years. Most of them are in Japan. Many more systems are under discussion or in the planning phase.




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