Sensors 2008 , 8
scale. Thus, the magnitude and shaking intensity can be estimated for EEW purposes 3 sec after the P
arrival is detected (Allen, 1978). If
earthquake striking this site is high (Wu and Kanamori, 2005a, 2005b, 2007; Wu et al., 2007).
3. Discussion and conclusions
From our experience with the Japan, Taiwan and southern California data, if Pd exceeds 0.5 cm, the
PGV at the site most likely exceeds the damaging level, i.e., 20 cm/s. One possible approach for faster
warning is to monitor Pd, and issue a warning as soon as it has exceeded 0.5 cm. As shown in Figure
1, for the 2007 Niigata Chuetsu-Oki earthquake, at the nearest stations, NIG018 (∆=14 km), the
threshold value of Pd=0.5 cm was reached at 1.36 s from the arrival of P wave. If we issue a warning
at a threshold of Pd ≥ 0.5 cm, a warning will be issued at 1.36 s after the P arrival and several seconds
before the occurrences of PGA and PGV. This type of early warning approach will become effective
especially for close-in sites where warnings are most needed.
For any warning system, reliability is always important and it is desirable to have redundancy built
in the system to make it more robust. In this paper, we explore the feasibility of using several early
warning methods to increase the speed and reliability of early warning. In these methods, the
information from the initial part (up to a few seconds) of P wave is used to estimate the magnitude and
the strength of the impending ground motion at the same site. In view of the success of the Japan,
Taiwan, and Mexico warning systems, we believe that further enhancement of the system like the one
described here is worthwhile to make the overall system faster, more reliable, and robust.
Currently, MEMS (Micro Electro Mechanical Systems) acceleration sensors are well developed for
a wide range of applications from air bag systems, detecting industrial vibrations, and strong motion
recording (Holland, 2003). MEMS sensors are miniature sensors made in wafer fabrication facilities
similar to semiconductor foundries. Many types of commercial MEMS accelerometers exist and they
are inexpensive. Those accelerometers could be used for EEW purposes with the concept described in
this paper and are useful for future seismic hazard mitigation.
Acknowledgements
The authors wish to thank the two anonymous reviewers for their constructive comments which
helped improve the manuscript. This research was supported by the National Science Council of the
Republic of China (NSC95-2625-Z-002-028, NSC95-2119-M-002-043-MY3) with TEC contribution
number 00027. We would like to thank the NIED (Japan), Central Weather Bureau (Taiwan), and
Southern California Earthquake Center for providing us with seismic data.
References
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2. Allen, R. M.; Kanamori, H. The potential for earthquake early warning in Southern California.
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