2025 Volume 78 Pages 73-86
This study reports the tsunami source modeling of an MJMA 7.1 earthquake which occurred beneath Hyuganada Sea, off Miyazaki, Japan on 8 August 2024, using tsunami waveforms recorded by the recently-implemented offshore seafloor observation network called Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis (N-net) together with the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) installed in the Nankai Trough subduction zone, off the Pacific coast of southwestern Japan. We first processed the ocean-bottom pressure gauge data from the N-net offshore system and DONET to extract tsunamis, which showed tsunamis up to a few centimeters were recorded. The station NAE18 of N-net, nearest to the epicenter, observed the pressure offset change corresponding to the permanent seafloor vertical uplift of approximately 5 cm. We then forwardly simulated tsunamis based on centroid moment tensor solutions and finite fault models of the previous studies, indicating the good observation performance of the N-net ocean-bottom pressure gauges. We finally conducted the inversion analysis of the tsunami waveforms to estimate the tsunami source distribution, which extends in a dimension of approximately 40 km×20 km along the strike and dip directions, respectively. From the comparison with the past studies, the rupture area of the 2024 earthquake was likely to overlap with the southern half of the rupture area of the MJMA 7.0 earthquake in 1961, while the rupture areas of the MJMA 6.7 and 6.9 earthquakes in 1996 seemed separated from the 2024 event. This study demonstrates that the use of the near-field tsunami waveforms from N-net significantly enhanced the constraint on tsunami source estimation as well as the finite fault modeling off the coast of the western part of the Nankai Trough subduction zone. N-net also has the potential to improve tsunami monitoring and prompt evaluation for the “Nankai Trough Earthquake Extra Information.”
