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Sensors 2008, 8
acceleration signals are recursively converted to ground velocity and displacement. A P-
wave trigger is constantly monitored. When a trigger occurs, τ
earthquake magnitude and the on-site ground-motion intensity could be estimated and the
warning could be issued. In an ideal situation, such warnings would be available within 10
sec of the origin time of a large earthquake whose subsequent ground motion may last for
tens of seconds.
Keywords: earthquake, early warning system, seismic hazard mitigation.
1. Introduction
Because of the extreme complexity involved in the earthquake processes, reliable earthquake
prediction is not currently possible (Kanamori et al., 1997). Present technological advances in seismic
instrumentation and in digital communication and processing permit the implementation of a real-time
earthquake monitoring system. From the point of view of seismic hazards mitigation, earthquake early
warning (EEW) is becoming a practical tool to reduce the loss caused by a damaging earthquake
(Kanamori et al., 1997; Teng et al., 1997; Wu and Teng, 2002; Allen and Kanamori, 2003).
The idea of an earthquake early warning system was proposed more than one hundred years ago by
Cooper (1868) for San Francisco, California. About a hundred years later, Japan Railways Company
designed an EEW system in 1965 and started operation in the following year (Nakamura, 1988). In the
past decade, progress has been made towards implementation of earthquake early warning in Japan,
Taiwan, Mexico, Southern California, Italy, and Romania (e.g., Nakamura, 1988; Odaka et al. 2003;
Allen and Kanamori, 2003; Horiuchi et al, 2005; Wu et al., 1998, 1999, 2006, 2007; Wu and Teng,
2002; Wu and Zhao, 2006; Espinosa-Aranda et al., 1995; Zollo et al, 2006; Böse et al., 2007). In
particular, the systems developed at the National Research Institute for Earth Science and Disaster
Prevention (NIED) (Horiuchi et al., 2005) and the Japan Meteorological Agency (JMA) (Kamigaichi,
2004; Tsukada et al., 2004) were integrated in June, 2005. The system was successfully activated
during the 2007 Noto Hanto (Peninsula) and the 2007 Niigata Chuetsu-Oki earthquakes, and provided
accurate information regarding the source location, magnitude and intensity at about 3.8 s after the
arrival of P wave at nearby stations. Thus, it provided early warning before arrival of strong shaking.
Currently, there are many seismic networks using real-time strong motion signals for earthquake
monitoring (Wu et al., 1997, 2000, 2001; Hauksson et al., 2001). In this paper, we describe the
Pd methods developed for earthquake early warning purposes.
τ
2.
and P
method
c
d
Determinations of magnitude and the strength of shaking from the initial P wave are two important
elements for earthquake early warning. Strength of shaking can practically be represented by peak
gound acceleration (PGA), peak ground velocity (PGV), and peak ground displacement (PGD). Figure
1 shows a strong motion record of a M
represents acceleration, and after once and twice integration the signal can be coverted to velocity and
6.6 earthquake in Japan. Generally, strong motion signal
w
and Pd are computed. The
c
2
τ
and
c
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