16:24 〜 16:42
[SIT19-10] Seismic Reverse Time Migration (RTM) Imaging of 2024 Noto Earthquake (M7.6) Rupture Zone
キーワード：2024 Noto Earthquake, Reverse Time Migration, Surface Related Multiple Elimination
The 2024 Noto earthquake (M7.6) caused strong ground motion across Noto Peninsula and triggered a tsunami along the eastern coast of the Sea of Japan. In order to illustrate the geological settings and the seismogenic fault geometry, a multi-channel seismic (MCS) reflection survey was conducted shortly after the earthquake in March 2024 using research vessel Hakuho-maru (KH-24-E1 cruise), collecting 14 seismic 2D lines offshore the northeast coast of Noto Peninsula. The MCS data was recorded with a recording length of 5 or 6 seconds and a sampling interval of 2 milliseconds, using a 1200 m long streamer cable with 48 channels at 25 m group spacings. This data was previously processed for both time domain and depth imaging. The main time domain processing steps included trace editing, deghosting, surface-related multiple elimination, predictive deconvolution, stacking velocity analysis, and post-stack time migration. The depth domain processing included several rounds of reflection traveltime tomography for interval velocity model building, and Kirchhoff pre-stack depth migration (KPSDM). Despite the challenging nature of the dataset due to strong noises, limited offsets, and low fold coverage, the KPSDM sections showed fundamental features of the subsurface geology in the survey area. Active reverse and strike-slip faults with significant deformations were initially imaged on the depth migrated section. However, due to the limitations of the Kirchhoff migration method, the reflections from the steep fault plane could not be illuminated in the KPSDM results.
To solve this issue, and implement the full information carried in the seismic waveforms, we applied reverse time migration (RTM) on the previously pre-processed seismic shot gathers of line K1 using the KPSDM-derived velocity model. In contrast to KPSDM, which mostly relies on ray tracing imaging, RTM takes advantage of seismic wave equation solution and incorporates the actual wave propagation trajectories to build the subsurface image. This allows RTM to overcome the dip limitations and go beyond the levels that KPSDM can image a dipping reflector. Additionally, RTM yields a better reflectivity than KPSDM, which is closer to the true-amplitude reflectivity of the subsurface. We could identify clear reflections from the dipping fault planes in the seismogenic zone offshore the Noto Peninsula and trace the faults up to the seafloor. This area suffered from very weak amplitudes in the previous KPSDM section and appeared as a wide blank zone in the seismic depth image. Without applying RTM, these amplitude blanking zones might have resulted in a doubtful interpretation of the seismic images. For example, previous studies have shown that Sea of Japan is host to considerable amount of gas seepages into the sea water, and this may cause new interpretation pitfalls when the current KPSDM depth imaging is not accurate. In the RTM results, however, we can see that a major part of the seismic depth section is recovered by high amplitude reflections. Although some parts still show low-amplitude zones, which may still be attributed to local gas seepage, we strongly suggest that a long-offset seismic dataset and RTM imaging can recover most of the missing reflections in this area.
To solve this issue, and implement the full information carried in the seismic waveforms, we applied reverse time migration (RTM) on the previously pre-processed seismic shot gathers of line K1 using the KPSDM-derived velocity model. In contrast to KPSDM, which mostly relies on ray tracing imaging, RTM takes advantage of seismic wave equation solution and incorporates the actual wave propagation trajectories to build the subsurface image. This allows RTM to overcome the dip limitations and go beyond the levels that KPSDM can image a dipping reflector. Additionally, RTM yields a better reflectivity than KPSDM, which is closer to the true-amplitude reflectivity of the subsurface. We could identify clear reflections from the dipping fault planes in the seismogenic zone offshore the Noto Peninsula and trace the faults up to the seafloor. This area suffered from very weak amplitudes in the previous KPSDM section and appeared as a wide blank zone in the seismic depth image. Without applying RTM, these amplitude blanking zones might have resulted in a doubtful interpretation of the seismic images. For example, previous studies have shown that Sea of Japan is host to considerable amount of gas seepages into the sea water, and this may cause new interpretation pitfalls when the current KPSDM depth imaging is not accurate. In the RTM results, however, we can see that a major part of the seismic depth section is recovered by high amplitude reflections. Although some parts still show low-amplitude zones, which may still be attributed to local gas seepage, we strongly suggest that a long-offset seismic dataset and RTM imaging can recover most of the missing reflections in this area.