Challenges in Source Parameter Estimation: Insights from the 2017 Pohang Earthquake and Its Aftershocks

Minkyung Son, Esteban J. Chaves, & Chang Soo Cho

Published September 8, 2024, SCEC Contribution #13838, 2024 SCEC Annual Meeting Poster #038

The 2017 Pohang earthquake (Mw 5.4), the second-largest instrumentally recorded event in the Korean Peninsula, occurred near fluid injection points. To better understand its rupture processes, we estimated corner frequency, rupture radius, and stress drop for the mainshock and seven significant aftershocks (Mw > 3.5). We applied the Empirical Green's Function (EGF) approach to locally recorded seismograms to isolate earthquake source characteristics by removing attenuation effects. By systematically varying frequency bands, window lengths, and cross-correlation criteria, we investigated the uncertainty in corner frequency estimates and how different EGF selections influenced the results. The results showed that variations in cross-correlation criteria and upper frequency band limits significantly impacted the distribution and mode of corner frequency estimates, sometimes producing distinct higher and lower frequency groups. Despite this variability, the shapes of spectral ratios and source time functions consistently suggested a complex rupture process for the mainshock, involving the sequential rupture of two or more asperities. Additionally, one aftershock (Mw 3.6), located approximately 0.97 km from the mainshock hypocenter, exhibited a double-peak source time function, indicating near-simultaneous rupture of two fault segments within a possible fault jog that shared the rupture area with the mainshock. In contrast, six other aftershocks followed simpler rupture models. Our findings highlight the challenges in source parameter estimations: 1) the variation in corner frequency estimate distributions emphasizes the difficulty in obtaining consistent stress drop values, and 2) the inferred complexity in rupture processes suggests the need for tailored approaches when estimating rupture characteristics, even within a single earthquake sequence.

Key Words
Pohang earthquake, corner frequency, stress drop, empirical Green's function apporach

Citation
Son, M., Chaves, E. J., & Cho, C. (2024, 09). Challenges in Source Parameter Estimation: Insights from the 2017 Pohang Earthquake and Its Aftershocks. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology