We investigate the rupture process of the M9.0 Tohoku‐Oki mega‐thrust earthquake using the relatively low‐frequency strong‐motion records (0.01–0.125 Hz) observed at 36 K‐NET and KiK‐net stations, the epicentral distances of which range from 120 km to 400 km. The fault model is a rectangular plane, the length and width of which are 510 km along the Japan Trench and 210 km along subducting direction of the Pacific Plate, respectively. We perform the multi‐time‐window inversion analysis with a 30 × 30 km2 subfault. The derived slip model has one large slip area. This area extends from the region around the hypocenter to the shallow part of the fault plane and further to the north and south along the trench axis, located far off southern Iwate, Miyagi, and northern Fukushima prefectures. The seismic moment is 4.42 × 1022 Nm (Mw 9.0) and the maximum slip is 48 m. The slips near the coast are relatively small, except off Miyagi prefecture, which experienced a slip greater than 5 m. The shallow large slip area, which continuously ruptured from 60 s to 100 s after the initial break, radiated seismic waves rich in very‐low‐frequency content (<0.02 Hz). The rupture after 100 s propagating to the southern fault area, contributes to the distinct phases observed for Fukushima and Ibaraki prefectures. The relationship between the proposed rupture model and the feature of the acceleration waveforms is not straightforward and suggests the frequency dependency of the seismic wave radiation.

National Research Institute for Earth Science and Disaster Resilience (NIED) integrated the land observation networks established since the 1995 Kobe earthquake with the seafloor observation networks established since the 2011 Tohoku earthquake and tsunami as MOWLAS (Monitoring of Waves on Land and Seafloor) in November 2017. The purpose of MOWLAS is to provide comprehensive, accurate, and rapid observation and monitoring of earthquake, tsunami, and volcano events throughout Japan and its offshore areas. MOWLAS data are widely utilized for long-term earthquake forecasting, the monitoring of current seismic activity, seismic and tsunami hazard assessments, earthquake early warning, tsunami warning, and earthquake engineering, as well as earthquake science. Ocean bottom observations provide an extension of observations to areas where no people are living and have the advantage of increasing lead time of earthquake early warning and tsunami warning. The application of recent technology advancements to real-time observations as well as the processing of MOWLAS data has contributed to the direct disaster mitigation of ongoing earthquakes. These observations are fundamental for both science and disaster resilience, and thus it is necessary to continue ceaseless operation and maintenance.

A great earthquake, named the 2003 Tokachi-oki earthquake, occurred in the southern Kuril subduction zone on 26th September 2003, 4:50 JST (41.7797 • N, 144.0795• E, 42 km depth; Japan Meteorological Agency). Its ground motion was recoreded at 655 stations of the nationwide strong motion networks, K-NET and KiK-net. A maximum peak ground acceleration of 988 cm/s 2 (gal) was observed at station HKD100 and amplitudes greater than 200 cm/s 2 were observed over a wide area of eastern Hokkaido. We used a multi-line linear waveform inversion method to estimate the rupture process from the strong motion data of supplied by 15 stations. We assumed a fault plane model of 140 km × 160 km with strike and dip angles of our fault model are N246• E and 18• , respectively, placed on the estimated upper boundary of the subducting Pacific Plate. The estimated total slip distribution consisted of three major slip areas; (a) around the hypocenter, (b) the northwest part of the fault with the maximum slip of 5.9 m, and (c) the northeast edge of the fault plane. The major asperity (b) was composed of two large slip areas with different slip rate functions: the duration of moment release in the sourtheast part is longer than 15 sec, but in contrast most of the seismic moment of the northwest part was released in a short period of less than 10 sec. Our estimation of the total seismic moment was 2.9 × 10 21 N·m which corresponded to Mw = 8.2.

ABSTRACT:After the 1995 Hyogoken-Nanbu earthquake, NIED (National Research Institute for Earth Science and Disaster Prevention) installed the K-NET that uniformly covers all Japan with more than one thousand strong-motion accelerometers on the ground surface. In addition to the K-NET, NIED constructed an uphole/downhole observation network, KiK-net, with approximately 669 stations. Each KiK-net station has a borehole of 100 m or more in depth and strong-motion seismographs have been installed both on the ground surface and at the bottom of the boreholes. The velocity profiles and geological information as well as the observed records are widely accessible on the website.

In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction as in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments.