借鑒日本的地震早期報警係統

日本從去年起啟動地震早期報警係統。下麵這篇文章技術性強一些，但是有一點物理基礎的人均可讀懂，隻需耐心一點。沒有時間翻譯，抱歉：）

日本在地震預報上花費人力財力最多，最終采取的卻是這麽一個預警係統。其基本出發點是利用地震波傳播的速度和時間賽跑，在地震發生後幾秒鍾內，破壞性最強的剪切波和麵波還未到達之前，給人們發出警報，有機會逃生。強震麵前，一秒之差是生死之差。當然有其局限性，但中國是否可以借鑒？

Earthquake Early Warning Starts Nationwide in Japan

By M. Hoshiba, O. Kamigaichi, M. Saito, S. Tsukada, and N. Hamada

EOS, Transaction, American Geophysical Union, Vol 89, pp 73-74

When an earthquake occurs, a certain amount of time elapses before destructive seismic energy hits nearby population centers. Though this time is measured on the order of seconds, depending on the proximity of the rupture to a given city or town, a new public safety program in Japan is taking advantage of the fact that seismic energy travels slower than electronic communication.

In this program, the Japan Meteorological Agency (JMA) rapidly determines the hypocenter (earthquake epicenter and focal depth) and magnitude of the earthquake by using real-time data from stations near the hypocenter. The distribution of strong ground shaking is anticipated quickly, and then the information is delivered immediately to government officials, representatives from various industries, members of the news media, and individuals before strong ground shaking reaches them. For example, on receiving the warning, the control room of a railway company can send an emergency notice to all train drivers to stop their trains immediately, elevators in buildings can be triggered to stop at the nearest floor and open their doors automatically, and surgeons can temporarily suspend their surgical operations to avoid risk to patients on operating tables.

This innovative new service, called Earthquake Early Warning (EEW), started nationwide in Japan and became fully operational in October 2007. This service is definitely different from earthquake prediction. Although it is currently impossible to be aware of earthquakes before their occurrence (earthquake prediction), EEW operates with the assumption that it is possible to warn people located at a certain distance from the hypocenter before strong ground shaking reaches them.

Even though the interval between the delivery of EEWs and the time when strong shaking reaches people is relatively short (counted in seconds), EEWs can be a useful and powerful tool for mitigating an earthquake disaster by giving people enough time to take appropriate safety measures in advance of strong shaking.

Determining Hypocentral Parameters and Anticipating Seismic Intensity

Earthquakes occur when stressed rock moves through brittle rupture. Two types of seismic waves are radiated from the hypocenter: One is the P wave, which travels at about 7 kilometers per second, and the other is the S wave, which travels at about 4 kilometers per second.

EEW technology not only takes advantage of the relatively slow velocity of the seismic waves as compared with instantaneous electronic communication, but it also uses the difference in arrival time between P and S waves. The S wave is slower than the P wave, but the amplitude of the S wave is usually 3–10 times larger than that of the P wave. This generally means that stronger shaking is observed along the S wave.

The hypocenter and magnitude of an earthquake are determined as quickly as possible using only early parts of the P waves at a few stations close to the hypocenter. Using information about the hypocenter and magnitude, the arrival time of the S waves and seismic intensities (that is, intensity of strong ground shaking) at each place can be rapidly anticipated. If predicted seismic intensities surpass a certain threshold, a warning is disseminated and broadcast.

It is important to rapidly determine earthquake parameters so that EEWs can inform users about an earthquake before the arrival of the larger-amplitude S waves. The technique for quickly determining the hypocenter and magnitude is based on a combination of several methods developed by a joint research project between JMA and Japan’s Railway Technical Research Institute (RTRI) [Odaka et al., 2003; Tsukada et al., 2004; Kamigaichi, 2004], and by Japan’s National Research Institute for Earth Science and Disaster Prevention (NIED) [Horiuchi et al., 2005], all of which aim to determine, within a few seconds, the hypocentral parameters using records of a single or a few stations.

For this quick determination, JMA collects waveform data from more than 1000 stations not only of its own seismic observation network but also of NIED’s High Sensitivity Seismograph Network (Hi-net [see Okada et al., 2004]). This combination provides a spacing of approximately 20 kilometers between each seismic station throughout Japan, greatly aiding in the rapid determination of earthquake parameters. The anticipation of seismic intensity is based on the empirical methods of Si and Midorikawa [1999], Matsuoka and Midorikawa [1994], and Midorikawa et al. [1999], in which peak ground velocity at the surface is predicted at first using the hypocentral distance, focal depth, magnitude, and amplification of seismic wave amplitude at a given site estimated from geological conditions. From the peak ground velocity, the seismic intensity is empirically estimated. A step-by-step procedure is adopted to improve the accuracy of the estimation as available data increase with elapsed time. Accordingly, EEWs are issued repeatedly with improved reliability.

Dissemination and Broadcast of EEWs

After the estimations of seismic intensity are made, EEWs are disseminated automatically to EEW users. Information disseminated includes updated warnings and occasional cancellation reports. EEW users are divided into two categories: online limited users and other general users. Online limited users are organizations that obtain EEWs by computer communication technology to control automatically something related to their businesses. These users are assumed to understand adequately the characteristics and limitations of EEW technology. Online limited users include railway companies, elevator companies, and manufacturing industries. By contrast, general users comprise most individuals and organizations who receive the warnings and then decide how best to mitigate their risk.

For online limited users, an EEW describes information on the hypocentral parameters (latitude, longitude, focal depth, origin time, and magnitude), anticipated maximum seismic intensity, and earliest arrival time of S waves for districts where seismic intensity is predicted to be equal to 4 or greater on the JMA scale (http://www.jma.go.jp/jma/kishou/know/shindo/explane.html). Seismic intensity 4 on the JMA scale approximately corresponds to VI or VII on the modified Mercalli scale.

EEWs are disseminated to the limited users based on a specific pattern. At first, an initial warning is disseminated when the estimated earthquake magnitude exceeds a specific threshold. The threshold currently is 3.5. To avoid a false alarm, these estimations are performed after noise discrimination using the first 2 seconds of the waveform data.

Next, the warnings are updated if further calculations suggest different earthquake parameters. As of now, updates will be issued if new data differ from information in previous warnings by 0.2 degrees for latitude and longitude or 20 kilometers for the depth of the hypocenter. Updates will also be issued if estimations of the earthquake’s magnitude or estimations of seismic intensity are raised by 0.5 or lowered by 1.0.

Ultimately, a final warning is disseminated either when a certain time has passed after the first detection of the P wave, or when the estimated value of magnitude becomes stable.

For general users, EEWs include information on an epicenter location name, such as the name of the prefecture for inland earthquakes or the name of the sea area for offshore earthquakes, the origin time of the event, and the locations of districts with predicted seismic intensity equal to 4 or greater on the JMA scale.

EEWs will be delivered to general users through various methods such as television, radio, e-mail via cellular telephone, and Internet. Using the Internet, a Japanese electronics company even developed a tissue-boxsized EEW device that sounds an alarm, indicates the anticipated intensity, and counts down the seconds to the S wave arrival. The method of issuing warnings to general users is based on a pattern similar to that used for limited users.

First warnings are disseminated when the seismic signals are detected at two or more stations and the anticipated maximum seismic intensity is equal to or exceeds “5Lower” (seismic intensity “5Lower” on the JMA scale approximately corresponds to VII–VIII on the modified Mercalli scale).

Warnings are updated when the seismic intensity is anticipated to be equal to 5Lower or greater at districts where the intensity was less than 4 in the first warning. In the updated warnings, the newly added districts are described.

A cancel report is disseminated to all users in the case of false alarm. For example, it is automatically disseminated to the online limited users when there is no P wave detection at stations other than the first station, indicating that the detection of a P wave at the first station is false.

This information dissemination system was tested for 29 months between February 2004 and June 2006. During that time, thresholds of magnitude and seismic intensity were tentatively 3.5 and 3, respectively, for issuing the first warning. In total, EEWs were issued 855 times, including 26 false alarms due to mechanical troubles, lightning strikes, and human error. All of the false alarms occurred when estimations of seismic parameters were made using only a single station.

Application of EEWs

EEWs are useful and powerful tools for reducing the risk of a major disaster in the event of an earthquake, by making people aware that strong ground shaking will soon occur. EEWs are expected to be effective for online control of traffic and lifeline systems, and for emergency action.

In addition to triggering trains to slow down, elevators to stop, or hospitals to suspend surgical operations, other examples of the application of EEW include transferring important data from computers to disks immediately and shutting down electronics automatically. Further, people receiving general EEWs can choose to take emergency precautions (for example, taking shelter under a desk, or keeping away from glass windows) at homes, schools, offices, halls, and shopping malls.

In addition, this technique can be directly applied at JMA to quicken the dissemination of tsunami warnings, potentially allowing such warnings to be issued within 2 minutes of an offshore earthquake.

Technical Limits of EEWs

Though EEWs are expected to be a powerful tool for mitigating earthquake disasters, the system has several technical limitations.

For inland shallow earthquakes, EEWs are too slow for places near the epicenter, where the S wave arrives less than a few seconds after the occurrence of the earthquake. For earthquakes of more than a magnitude of 7, rupture usually continues for more than 10 seconds, with a first warning possibly being disseminated in the middle of the rupture. In the case of such large earthquakes, the estimation of magnitude and seismic intensity may be underestimated in some cases.

Additionally, when more than two earthquakes occur repeatedly at short intervals, it is difficult to separate them automatically. The estimation of the hypocenter and magnitude is not processed appropriately, so the error in the anticipation of seismic intensity becomes large.

Case Study: A 7.2 Earthquake off Miyagi Prefecture

During the testing period, an M = 7.2 earthquake occurred off the coast of Miyagi prefecture, Japan, on 16 August 2005. The first warning was disseminated 4.5 seconds after the first detection of the P wave at the closest seismic station. The dissemination was earlier by 16 seconds than the arrival of the larger-amplitude S wave at the city of Sendai where seismic intensity of “5Upper” was observed (Figure 1). EEW users were aware of the strong shaking in advance of it reaching the city.

For the limited online users, EEW delivery service was started in August 2006; general users were able to receive warnings beginning in October 2007. Because EEW is a new and innovative service, JMA is making an effort to familiarize general users with it to avoid confusion caused by poor understanding. Though EEWs to general users have not been issued as of 25 November 2007, the effects of EEW on damage reduction will be evaluated in the near future.

Acknowledgments

The authors thank S. Horiuchi, K. Abe, O. Hiroi, and M. Kikuchi for their comments and contributions and the National Research Institute for Earth Science and Disaster Prevention for providing the waveform data to JMA’s EEW project. The work is supported by a joint research effort between JMA and RTRI, and also the project entitled “Research Project for the Practical Use of Real-Time Earthquake Information Networks” sponsored by Japan’s Ministry of Education, Culture, Sports, Science and Technology.

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Author Information
Mitsuyuki Hoshiba, Osamu Kamigaichi, Makoto Saito, Shin’ya Tsukada, and Nobuo Hamada, Japan Meteorological Agency, Tokyo; E-mail: [e-mail]