High-resolution reflection seismic imaging of fault systems in Metropolitan Seoul, South Korea

Abstract

<p>The Korean peninsula is considered a stable intraplate setting with few large magnitude earthquakes and in terms of seismic risk, a low-risk region. However, the peninsula is crosscut by crustal-scale fault systems, some of which may be active or have a potential of reactivation. After the Tohoku-Oki earthquake (Mw 9.0, 2011) offshore Japan, subsequent larger magnitude seismic events were registered along some of the fault systems in the South Korean portion of the peninsula. Following these, seismic risk in the area has been given more attention with several initiatives to study the current state of the seismicity in the region and to better understand the geometry and role of these crustal-scale fault systems.</p><p>To provide information on the geometry of subsurface structures causing the seismic events, a number of locations were identified for high-resolution reflection seismic imaging. In November 2020, the first active-source seismic profiles in the region (P1 and P2) were acquired with a total length of approximately 14 km with the aim to better understand the correlation between one of the main faults, the Chugaryeong fault system, and the seismicity in the area. A novel data acquisition survey consisting of a 120-unit micro-electromechanical sensors (MEMS-based) seismic landstreamer and 290 wireless recorders was employed to allow both near-surface and deep imaging of structures. Profile P1, 5 km long, was acquired on the outskirt of Seoul and P2, 9 km long, was acquired in the central part of the city. Acquiring P2 was a significant challenge given that the Seoul metropolitan area is densely populated. Difficulty to obtain good geophone-ground coupling and anthropogenic noise severely degraded the data quality. Nonetheless, final seismic sections from both profiles show encouraging results, particularly along P1 where much deeper imaging was possible (up to 9 km depth). An integrated processing work flow was required to take advantage of both the landstreamer and wireless data and this proved to be instrumental for improved imaging and subsequent interpretation.</p><p>Along P1 a clear correlation between seismic event clusters and reflection intersections (at two depth intervals of 4.5-5 km and 8-9 km) is observed, suggesting that seismic triggering is coupled to the fault intersections at depth. P2 shows strong westerly-dipping reflections with similar characteristics to the ones seen along P1, but only visible from the near surface to around 1200 m depth. It was not possible to map these faults deeper along P2, probably due to the noise conditions, thus no correlation between fault intersections and seismicity could be made. The encouraging reflection seismic results from both profiles, motivated the acquisition of a much longer profile (P3, 40 km) crossing three major fault systems in 2021. It lies in between P1 and P2 and a similar acquisition strategy was used as before. Preliminary results are ready and these are currently being interpreted together with other seismological and geological information from the area.</p>