Sunday, September 22, 2019

P-Alert Strong Motion Network

 Taiwan is one of the leading countries in the development ofearthquake early warning (EEW) systems. Two types of EEW systems are in operation globally. One type is an onsite EEW system, which determines earthquake information from initial P-waves and predicts the most severe ground shaking of the S-wave trains that arrive immediately afterwards. The other type is a regional EEW system, which is also referred to as a front-detection EEW system. A regional EEW system considers that the speed of signal communication is much faster than the propagation of a seismic wave. Thus, instruments are installed in the earthquake source region and give an early warning to distant cities. The Central Weather Bureau (CWB) has been the major agency that has been developing the EEW system in Taiwan since 1993. The National Taiwan University (NTU) also commenced developing the EEW system for research purposes using low-cost accelerometers (P-Alert) in 2010 with the support of the Ministry of Science and Technology (MOST). A total of 600 stations were deployed and configured in 2018. A P-Alert system can provide earthquake information within approximately 15 seconds of an earthquake occurrence. Thus, this system may provide an early warning for cities that are located at a distance greater than 50 km from the epicenter. Additionally, the P-Alert system also has an onsite alert function that triggers a warning sound once the incoming P-waves are greater than a certain threshold. It can provide a lead time of two to three seconds before the time of peak ground acceleration (PGA) for the regions that are close to the epicenter. A high resolution shaking map was produced within one to two minutes after the earthquake occurred via the P-Alert system. The high shaking regions of the shaking map can indicate the locations of damage and casualties and help to estimate damage. The earthquake rupture direction is potentially identifiable according to the delicate shaking map and the strong motion records of the P-Alert system.

In the past decade, progress has been made towards implementation of earthquake early warning (EEW) in Japan, Taiwan, and Mexico (e.g. Nakamura 1988; Espinosa-Aranda et al. 1995; Wu et al. 1998; Wu … Teng 2002; Odaka et al. 2003; Horiuchi et al. 2005, 2006a). Two approaches have been adopted: (1) regional warning and (2) Onsite warning. The first approach is the traditional seismological method in which data from a seismic network are used to locate an earthquake, determine the magnitude, and estimate the ground motion in the region involved. In (2), the beginning part of the ground motion (mainly P wave) observed at a site is used to predict the ensuing ground motion (mainly S and surface waves) at the same site. In this case, it is not necessary to locate the event and estimate the magnitude, although in some systems the hypocentre and magnitude are also determined in order to estimate the strength of ground shaking in the region (Nakamura 1988; Odaka et al. 2003).
The regional approach is more comprehensive, but usually takes a longer time and cannot provide early warnings at distances close to the epicentre. The early warning system in Taiwan is a typical example. The Taiwan Central Weather Bureau (CWB) uses a regional warning system that requires an average of 22 s to determine earthquake parameters with magnitude uncertainties of ±0.25. It provides a warning for areas beyond about 70 km from the epicentre. Taiwan's system has been in operation since 2002 with almost no false alarms and successfully reported many M >; 6 events (Wu … Teng 2002; Wu et al. 2006a). With the advancement of new methodology and more dense seismic networks, regional systems are beginning to be able to provide early warnings to areas close to the epicentre (Tsukada … Ohtake 2002; Kamigaichi 2004; Horiuchi et al. 2005).


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