Modelling of pulse-like velocity ground motion during the 2018 M_w 6.3 Hualien earthquake, Taiwan

Creators: Lin, Yen-Yu; Kanamori, Hiroo; Zhan, Zhongwen; Ma, Kuo-Fong; Yeh, Te-Yang

Abstract

The 2018 February 6 M_w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s⁻¹) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M_w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9Vs speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake.

Additional Information

© The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Received: 19 February 2020; Revision received: 04 June 2020; Accepted: 15 June 2020; Published: 19 June 2020. We appreciate the helpful comments from the Editor, Prof. Jean Virieux, two referees, Mr Hugo Samuel Sánchez-Reyes and Anonymous. We thank Dr Hsin-Hua Huang and Dr Shiann-Jong Lee for providing us with the Taiwan 3-D velocity structure and the earthquake source parameters determined with the Real-Time Moment Tensor Monitoring System, respectively. We thank the Central Weather Bureau for providing us with the TSMIP records and source parameters, including the location and focal mechanism of the 2018 Hualien earthquake. We thank Prof. Yih-Min Wu at National Taiwan University for providing the Palert records. We thank Dr Che-Min Lin at the National Center for Research on Earthquake Engineering who provided us with the shallow structure for Hualien City. We thank Dr Chung-Han Chan, Mr Jia-Cian Gao, and Dr Chun-Hsiang Kuo for helpful discussion in seismic hazard assessment. This research was supported by the Taiwan Earthquake Research Center (TEC), funded through the Ministry of Science and Technology(Taiwan) with grant number MOST 108–2116-M-008–025-MY2. The TEC contribution number for this article is 00159. This work is financially supported by 'Earthquake-Disaster & Risk Evaluation and Management Center, E-DREaM' from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Z. Zhan is supported by the U.S. Geological Survey under grant number G19AP00030. The strong-motion records used in this study were obtained from the National Taiwan University (NTU), the Institute of Earth Sciences (IES) of Academia Sinica and the Central Weather Bureau (CWB). The P-alert records used in this study are available to the public and can be downloaded from the NTU cloud disk (http://palert.earth.sinica.edu.tw/db/, last accessed October 2019). The strong-motion records from IES and CWB used in this study can be obtained upon request from IES and CWB. The damage records used in this study are at http://fault.moeacgs.gov.tw/MgFault/index.php/10–2017-06–02-07–05-29/130–1070330 (last accessed October 2019). The moment tensors determined with Broadband Array in Taiwan for Seismology (BATS) solution are available at http://bats.earth.sinica.edu.tw (last accessed October 2019). The Central Weather Bureau (CWB) website can be accessed at http://www.cwb.gov.tw/eng/index.htm (last accessed October 2019). Seismic Analysis Code (SAC) is available at http://ds.iris.edu/files/sac-manual/(last accessed October 2019). Frequency-Wavenumber (FK) synthetic seismogram package is available at http://www.eas.slu.edu/People/LZhu/home.html (last accessed October 2019).

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