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Nonlinear Site Response Observed by the NDHU Downhole Array During the April 2, 2024 Taiwan M7.4 Earthquake

Description: 

At the end of April 2^nd, 2024, an earthquake with a magnitude of 7.4 occurred in Hualien County in eastern Taiwan. The largest earthquake in Taiwan in a quarter century induces extremely strong ground motion with a maximum PGA of 1.5 g and PGV over 80 cm/sec in the nearfield. In two months after the mainshock, thousands of aftershocks, including seven earthquakes larger than M_L 6.0, occurred in a northeast-striking linear region of about 70 km. The NDHU downhole array consisted of four accelerometers installed at depths of 0, 20, 30, and 70 meters. It is just 5.2 km from the epicenter of the M7.4 earthquake and observed a PGA of 271 gal. This study used a large number of downhole records during the earthquake sequence to analyze the local nonlinear site responses of the ground motion. For small input ground motions with peak acceleration lower than 10 gal, the PGA observed at the ground surface is about twice that observed at the depth of 70 meters. Based on the spectral ratio of surface to downhole horizontal accelerations, the predominant frequency of the soil amplification is at 2.0 Hz, which agrees with the theoretical transfer function of the soil profile. The second and third modes of soil amplification also obviously appear at 5.5 Hz and 8.4 Hz, respectively. As the input ground motion increases, the amplification of peak acceleration from downhole to surface gradually decreases, and the dominant frequencies also become lower. For the M 7.4 mainshock, the peak acceleration at the ground surface is 1.5 times the downhole input ground motion of 181 gal. The predominant frequency of the soil amplification moves to 1.5 Hz. The nonlinear site responses depressed the peak acceleration and predominant frequency of ground motions observed at the NDHU downhole array. The significant nonlinearity allows us to conduct further research on soil behavior.

Session: Why Ignore the Structure? Soil-structure Interaction and Site Response at Local and Regional Scales [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Che-Min

Student Presenter: No

Invited Presentation: 

Poster Number: 12

Authors