Discrimination of Seismic Events in the North Korean Test Site and Surrounding Area Using MDAC Spectral Ratio

ABSTRACT An event of ML 3.7 occurred near the Punggye-ri nuclear test site on 11 February 2022. It was stronger than the known historical earthquakes around the North Korean test site (NKTS). How to accurately identify whether this event and future events near NKTS are explosions or natural earthquakes is worth paying attention to. For moderate and small earthquakes, the regional P/S spectral ratio has the advantage of identifying explosions and earthquakes. However, the P/S spectral ratio depends on magnitude and ray path. Magnitude and distance amplitude correction (MDAC) was proposed and demonstrated to be an effective method to correct magnitude and path effect. Different from the traditional MDAC method, we use explosions to build the MDAC model to reduce the influence of the source radiation pattern. After building the MDAC model, we analyze the Pg/Lg and Pn/Lg spectral ratios of 6 nuclear tests and 60 earthquakes at the NKTS from 2013 to 2021. The results show that the spectral ratio residuals of the 6 nuclear tests are mainly distributed around 0 after correction, whereas the spectral ratio residuals of earthquakes are obviously <0 above 3 Hz and are clearly distinguished from explosions as expected. The Pg/Lg and Pn/Lg spectral ratios of the ML 3.7 earthquake near the NKTS on 11 February 2022 exhibit similar characteristics to historical natural earthquakes. We further discuss the influence of magnitude on P/S spectral ratios. In the NKTS, the differences in the P/S ratio between earthquakes and explosions are similar before and after magnitude correction at 4–8 Hz. Nevertheless, the magnitude correction increases by ∼27% of the difference between explosions and natural earthquakes at 6–9 Hz to better separate explosions from natural earthquakes.