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Since the inception of 62 borehole seismic arrays deployed by Central Weather Bureau(CWB) in Taiwan until the end of 2018, a large quantity of strong-motion records have beenaccumulated from frequently occurring earthquakes around Taiwan, which provide anopportunity to understand the site effects (e.g., amplification) caused by the subsurfacematerials. Each borehole array includes two force balance accelerometers, one atthe surface and other at a depth of a few ten-to-hundred (30-492) meters, as wellas one broadband seismometer is below the borehole accelerometer. In general,the background seismic noise level are lower at the borehole stations than surfacestations, facilitating to detect microseismicity. For a few earthquakes, the seismogramsrecorded by the borehole stations are smaller than surface stations due to the differentgeological conditions. The resulting of site-specific spectral decay parameter κ0(Kappa value) has been derived by our pervious study, and relative higher κ0 could beobserved at surface station, indicating a strong attenuation effect. In this study,we used 147 earthquakes with local magnitude (ML) > 4.0 to investigate the siteamplification at materials between downhole and surface stations. Result showsthe amplification factors ranging from 1.1 to 10, provide additional parameter tounderstand the site effect. These amplification factors have a strong relationship withVs30 (the average S-wave velocity of the top 30 m of strata), showing the linearcorrelation coefficient of -0.6. Furthermore, we conduct a series investigation ofborehole-based and surface-based ML values for a purpose of providing the comprehensiveearthquake catalog and additional seismic records to study earthquake early warning. [ABSTRACT FROM AUTHOR]

On February 6, 2018, a Mw 6.4 earthquake struck near Hualien city, in eastern Taiwan with a focal depth of 10.4 km. The earthquake caused strong shaking and severe damage to many buildings in Hualien. The maximum intensity during this earthquake reached VII (> 0.4 g) in the epicentral region, which is extreme in Taiwan, and capable of causing damage in built structures. Seventeen people died and approximately 285 were injured. Taiwan was one of the first countries to implement an earthquake early warning (EEW) system that is capable of issuing a warning prior to strong shaking. In addition to the official EEW run by the Central Weather Bureau (CWB), a low-cost EEW system (P-Alert) has been deployed by National Taiwan University (NTU). The P-Alert network is currently operational and is capable of providing on-site EEW as well as a map of expected ground shaking. In present work, we demonstrate the performance of the P-Alert network during the 2018 Hualien earthquake. The shake maps generated by the P-Alert network were available within 2 minutes and are in good agreement with the patterns of observed damage in the area. These shake maps provide insights into rupture directivity which are crucial for earthquake engineering. During this earthquake individual P-Alert stations acted as onsite EEW systems and provided 2-8 s lead-time in the blind zone around the epicenter. The co-seismic deformation (Cd) is estimated using the records of P-Alert stations. The higher Cd values (Cd > 2) in epicentral region are very helpful for authorities to respond for the purpose of damage mitigation. [ABSTRACT FROM AUTHOR]