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ABSTRACT1 Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC 2 National Center for Research on Earthquake Engineering, Taipei, Taiwan, ROC * Corresponding author address: Prof. Kuo-Liang Wen, Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC; E-mail: wenkl@earth.ncu.edu.twThe horizontal-to-vertical spectral ratio has become increasingly popular in studies of site effect and determination of the predominant period of a site. In this study, this method is extended to identify nonlinear soil responses. To establish this phenomenon, borehole array records that already showed nonlinear site responses using spectral ratios between surface and borehole station pairs were analyzed. Moreover, in this study, the horizontal-to-vertical spectral ratio method was applied to weak and strong motion records from the same dataset. The results demonstrate that nonlinear site responses can be evaluated using horizontal-to-vertical spectral ratios of surface recordings at a single station.(Key words: Horizontal-to-vertical spectral ratio, Nonlinear site effect, Borehole array)

Hazards from gravity-driven instabilities on hillslope (termed ‘landquake’ in this study) are an important problem facing us today. Rapid detection of landquake events is crucial for hazard mitigation and emergency response. Based on the real-time broadband data in Taiwan, we have developed a near real-time landquake monitoring system, which is a fully automatic process based on waveform inversion that yields source information (e.g., location and mechanism) and identifies the landquake source by examining waveform fitness for different types of source mechanisms. This system has been successfully tested offline using seismic records during the passage of the 2009 Typhoon Morakot in Taiwan and has been in online operation during the typhoon season in 2015. In practice, certain levels of station coverage (station gap < 180°), signal-to-noise ratio (SNR ≥ 5.0), and a threshold of event size (volume >106 m3 and area > 0.20 km2) are required to ensure good performance (fitness > 0.6 for successful source identification) of the system, which can be readily implemented in other places in the world with real-time seismic networks and high landquake activities.

The National Center for Research on Earthquake Engineering, Taiwan, has developed an Earthquake Early Warning System (NEEWS). The NEEWS predicts peak ground acceleration (PGA) using an on‐site approach, whereas the Central Weather Bureau (CWB), Taiwan, uses a regional approach. Earthquake alerts are issued at the NEEWS stations once PGA reaches a preassigned PGA threshold, regardless of the approach used. An earthquake with a magnitude of 6.2 and a focal depth of 10.0 km struck Hualien, in eastern Taiwan, on 6 February 2018. It resulted in 17 fatalities and 285 injuries, 4 collapsed buildings, and damage to more than 175 buildings. During the earthquake, the system performance of 28 NEEWS stations was documented. In this study, we compare and discuss the accuracy of the PGA predictions, lead times, and classification performance of both approaches.

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The National Center for Research on Earthquake Engineering in Taiwan has developed an on‐site earthquake early warning system (NEEWS). The Meinong earthquake with a moment magnitude of 6.53 and a focal depth of 14.6 km occurred on 5 February 2016 in southern Taiwan. It caused 117 deaths, injured 551, caused the collapse of six buildings, and serious damage to 247 buildings. During the Meinong earthquake, the system performance of 16 NEEWS stations was recorded. Based on a preassigned peak ground acceleration (PGA) threshold to issue alarms at different stations, no false alarms or missed alarms were issued during the earthquake. About 4 s to 33 s of lead time were provided by the NEEWS depending on the epicenter distance. In addition, the directivity of the earthquake source characteristic and also possibly the site effects were observed in the diagram of the distribution of PGA difference between the predicted PGA and the measured PGA.