Station Correction of EEW System in Ilan, Taiwan

Abstract:

The Central Weather Bureau (CWB), Taiwan had already used regional EEW system sent predicted intensity and warning time to many school users. Due to leveling of intensity was roughly divided into seven grades according to peak ground acceleration (PGA) in Taiwan, the intensity warning message is not applicable for many other EEW end users, the accuracy of the estimated PGA still need to improve.

The earthquake data used in this study is recorded by Taiwan Strong Motion Instrumentation Program (TSMIP) from 1991 to 2012 in Ilan, Taiwan. A total of 1945 records were used for this study. Earthquake sorting threshold was set as focal depths less than 35 km and magnitude M_w ≥5.0 in this study. First 3 seconds window were used after P waves arriving, and applied two kind of filtering including a 0.075Hz high pass and a 0.2~5Hz band pass Butterworth filter to remove the low-frequency drift and possible high frequency noise. We computed τ_c and P_d from the above mentioned filtering displacement signals after the P arrival.

Two pairs of linear relations had showed in each station between τ_c, P_d, magnitude (M_w), and hypocenter distance (R) that could be corrected individually. Regional results were applied in first step base on Wu (2005, 2006). Due to site responses are varies for different stations, it’s not suitable using same linear relations. Correction of individual stations was used in second step to improve accuracy of prediction of M_w and R. Third, PGA prediction from site correction based ground motion prediction equation (Jean et al., 2006) indicated that the corrected parameters of EEW in this study had improved the accuracy of ground motion (PGA) prediction. Finally, one target event (2002 Hualien earthquake, M_w=7.1) was used for verification of this correction method.