A multiple time-window inversion of 53 high-sampling tsunami waveforms on ocean bottom pressure, GPS, coastal wave, and tide gauges shows a temporal and spatial slip distribution during the 2011 Tohoku earthquake. The fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface. Very large, approximately 25 m, slip off Miyagi on the deep part, at a location similar to the previous 869 Jogan earthquake model, was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in Sendai and Ishinomaki plains. Huge slip, up to 69 m, occurred on the shallow part near the trench axis 3 min after the rupture initiation. This delayed shallow rupture extended for 400 km with more than 10 m slip, at a location similar to the 1896 Sanriku tsunami earthquake, and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern Sanriku coast, 100 km north of the largest slip. The average slip on the entire fault is 9.5 m and the total seismic moment is 4.2 × 10 22 Nm (Mw = 9.0). The large horizontal displacement of seafloor slope is responsible for 20 to 40 % of tsunami amplitudes. The 2011 deep slip alone can reproduce the distribution of the 869 tsunami deposits, indicating that the 869 Jogan earthquake source could be similar to the 2011 earthquake at least in the deep plate interface. The large tsunami at the Fukushima nuclear power station is due to the combination of the deep and shallow slip, or triggering of shallow slip by the deep slip, which was not accounted in the previous tsunami hazard assessments.

Can the magnitude of a giant earthquake be estimated from paleoseismological data alone? Attempts to estimate the size of the Jogan earthquake of AD 869, whose tsunami affected much of the same coast as the 2011 Tohoku tsunami, offers an excellent opportunity to address this question, which is fundamental to assessing earthquake and tsunami hazards at subduction zones. Between 2004 and 2010, examining stratigraphy at 399 locations beneath paddy fields along 180 km of coast mainly south of Sendai, we learned that a tsunami deposit associated with the AD 869 Jogan earthquake had run inland at least 1.5 km across multiple coastal lowlands, and that one of the lowlands had subsided during the Jogan earthquake and an earlier earthquake as well. Radiocarbon ages just below/above sand deposits left by the pre‐Jogan tsunamis suggested recurrence intervals in the range of 500 to 800 years. Modeling inundation and subsidence, we estimated size of the Jogan earthquake as moment magnitude 8.4 or larger and a fault rupture area 200 km long. We did not consider a longer rupture, like the one in 2011, because coastal landform and absence of a volcanic ash layer make any Jogan layer difficult to identify along the Sanriku coast. Still, Sendai tsunami geology might have reduced casualties by improving evacuation maps and informing public‐awareness campaigns.

Tsunami accompanied with the Sumatra earthquake of 26 December 2004 affected many countries around the Indian Ocean. Thailand located approximately 500 km east of its source, was also severely suffered from the tsunami. From 24 February through 4 March 2005, we surveyed the damaged areas in Thailand from south of Phuket Island up to the border of Myanmar including four islands. The whole coastal area facing the Andaman Sea could be covered. We measured 37 points in total, the tsunami heights are less than 10 m, except at a few locations. We found that the largest tsunami height reached up to 19.6 m at Ban Thung Dap of Phra Thong Island. During our survey, we also collected five paper copies of analog tide gauge records. In addition that we could detect two other tide gauge records from the web-site. Therefore, totally seven tide gauge records were obtained in Thailand. All of the recorded tsunami waveforms indicate that sea level initially withdrew with duration in 30 to 60 min, followed by the rising-up. This phenomenon corresponds to the eyewitnesses' accounts of the survivors who experienced the tsunami.

Tsunami fragility (fragility curve, or fragility function) is a new measure, we propose, for estimating structural damage and fatalities due to tsunami attack, by integrating satellite remote sensing, field survey, numerical modeling, and historical data analysis with geographic information system (GIS). Tsunami fragility is expressed as the structural damage probability or fatality ratio related to hydrodynamic features of tsunami inundation flow, such as inundation depth, current velocity and hydrodynamic force. It expands the capability of estimating potential tsunami damage in a quantitative manner.