Preliminary Study on the Duration of High-Frequency Seismic Waves in Northern Taiwan from Deep Regional Earthquakes

¹ Tsunami and Disaster Mitigation Research Center (TDMRC), Universitas Syiah Kuala, Banda Aceh 23111, Indonesia
² Research Center for Marine Sciences and Fisheries, Universitas Syiah Kuala, Banda Aceh 23111, Indonesia
³ Department of Marine Sciences, Faculty of Marine and Fisheries, Universitas Syiah Kuala, Banda Aceh 23111, Indonesia
⁴ Institute of Earth Science, Academia Sinica, Taipei 115, Taiwan
⁵ Agency for Meteorology, Climatology, and Geophysics (BMKG), Jakarta 10610, Indonesia

^* Corresponding author: haekal.azief.haridhi@usk.ac.id

Abstract

The high-frequency seismic waves are usually generated by shallow earthquakes and is observed at a location near the earthquake source. Seismic energy tends to attenuate as it propagates through the Earth’s surface and interior; however, this is not the case at the subduction zone. A regional earthquake that occurred within the subducting slab was observed to have high-frequency energy, although it is from the deep (i.e. > 100 km) and with moderate magnitude (i.e. Mw 5). This phenomenon resulted from the slab effect on focusing the earthquake signal or known as the guided wave. Taiwan has a complicated tectonic feature, where Taipei city, its capital, sits above the oblique subduction of Philippine Sea Plate (PSP), i.e. the southern Ryukyu subduction zone, enhancing its exposure to strong shaking resulting from the deep regional earthquakes. In this study, we use six regional earthquakes at the southern Ryukyu subduction zone recorded by the Formosa Array (FM Array). The FM Array is a dense seismic network with a total of 140 stations located in the northern part of Taiwan, with the distance between stations approximately 5 kilometers. With high-resolution data, we could identify the distribution of the guided wave phenomenon through body wave dispersion of deep regional earthquake signals, where only stations above 10 – 20 km from the plate interface showed dispersion. An Hp/Lp ratio with a 5-second moving window is applied to the dispersed signal. The results indicate that the high-frequency signal has a longer duration at stations that sit 10 to 20 km above the plate interface. This study suggests that the slab effect will produce an anomalous seismic intensity at the subduction region and should be considered in the Ground Motion Prediction Equation (GMPE) so that the damaged to buildings resulting from strong shaking could be reduced.