10:45 AM - 11:30 AM
[O01-01] Development of earthquake and tsunami studies in the last decade since the 2011 Tohoku disaster
★Invited Papers
Keywords:East Japan earthquake and tsunami disaster , Giant earthquake, Tsunami
The East Japan Earthquake and Tsunami Disaster on 11 March 2011 caused nearly 20,000 casualties, mostly due to the tsunami, and caused fatal accident of Fukushima Dai-ichi Nuclear Power Station.
The Tohoku earthquake (M 9.0) was gigantic size, the largest among instrumentally recorded ones in Japan. The aftershock area is 500 km long and 200 km wide. This earthquake was recorded and studied in most detail in the world. The GPS network on Japanese Island recorded large coseismic displacements. Marine geodetic, pressure and bathymetry measurements, started before 2011, revealed the large seafloor movement. The tsunami was first recorded on offshore pressure gauges before it reached coast, and the data were sent to Japan Meteorological Agency (JMA) through submarine cable. Types and locations of smaller earthquakes in Tohoku region have changed after 2011, indicating that the stress field changed. Postseismic displacements on land and seafloor are still continuing, after ten years.
In the Tohoku region, similar tsunamis were recorded in the past. On Sanriku coast, the 1896 Meiji Sanriku tsunami caused more casualties than the 2011 tsunami. Ground shaking by earthquake was weaker, but the tsunami heights were similar to the 2011 event. Such an earthquake is called “tsunami earthquake,” and due to large slip near the trench axis. In Sendai plain, a Japanese chronicle depicted earthquake damage, tsunami inundation and casualties in AD 869 (Jogan era). The distribution of the tsunami deposit is similar to the 2011 inundation area. Geological studies clarified that similar earthquakes repeated with 450 to 800 year interval.
Our analysis of high-resolution tsunami waveforms has shown the temporal and spatial distribution of slip on faults. It shows that the 2011 Tohoku earthquake consists of an interplate earthquake (M 8.8) similar to the 869 event and a tsunami earthquake (M 8.8) similar to 1896 event. They occurred almost simultaneously, or the former triggered the latter.
On 11 March 2011, JMA issued tsunami warning 3 min after the earthquake, but the size (M) was underestimated as 7.9, hence the forecasted tsunami heights were 3 to 6 m. JMA updated the forecasted heights, based on the offshore tsunami data, but it did not reach all the coastal residents, because of a power failure and early evacuation. Currently, 150 seismic and bottom pressure sensors were installed off Tohoku coast, and the data enable JMA to issue earthquake early warning and tsunami warning more quickly and accurately. By using the observed tsunami data and numerical simulation, tsunami data assimilation method is developed as a tsunami forecast method.
The Earthquake Research Committee (ERC) of the government made a long-term forecast that the probability of an M 7.5 earthquake off Miyagi in 30 years from 2010 would be 99 %. This is based on the 37 year interval of previous earthquakes in historical records. The ERC now also uses geological data to forecast giant earthquakes with longer recurrence intervals. The Central Disaster Management Council, based on lesson of the 2011 tsunami disaster, decided to consider two types of future tsunamis: More frequent L1 tsunamis with lower tsunami heights, and less frequent L2 tsunamis with larger tsunami heights. For L1 tsunami, hardware must be constructed to protect lives, properties and economy. For L2 tsunami, saving people’s lives is the highest priority with software measures.
For tsunami assessment at nuclear power stations, deterministic tsunami hazard assessment based on past recorded tsunamis were used to design tsunami heights. After 2011, probabilistic tsunami hazard assessment, considering aleatory and epistemic uncertainties, has been introduced.
Importance of education has been pointed out by Torahiko Terada, in his essay “Tsunami and Humankind.” He wrote that the lessons from the 1896 Sanriku tsunami was not utilized to reduce damage in 1933.
The Tohoku earthquake (M 9.0) was gigantic size, the largest among instrumentally recorded ones in Japan. The aftershock area is 500 km long and 200 km wide. This earthquake was recorded and studied in most detail in the world. The GPS network on Japanese Island recorded large coseismic displacements. Marine geodetic, pressure and bathymetry measurements, started before 2011, revealed the large seafloor movement. The tsunami was first recorded on offshore pressure gauges before it reached coast, and the data were sent to Japan Meteorological Agency (JMA) through submarine cable. Types and locations of smaller earthquakes in Tohoku region have changed after 2011, indicating that the stress field changed. Postseismic displacements on land and seafloor are still continuing, after ten years.
In the Tohoku region, similar tsunamis were recorded in the past. On Sanriku coast, the 1896 Meiji Sanriku tsunami caused more casualties than the 2011 tsunami. Ground shaking by earthquake was weaker, but the tsunami heights were similar to the 2011 event. Such an earthquake is called “tsunami earthquake,” and due to large slip near the trench axis. In Sendai plain, a Japanese chronicle depicted earthquake damage, tsunami inundation and casualties in AD 869 (Jogan era). The distribution of the tsunami deposit is similar to the 2011 inundation area. Geological studies clarified that similar earthquakes repeated with 450 to 800 year interval.
Our analysis of high-resolution tsunami waveforms has shown the temporal and spatial distribution of slip on faults. It shows that the 2011 Tohoku earthquake consists of an interplate earthquake (M 8.8) similar to the 869 event and a tsunami earthquake (M 8.8) similar to 1896 event. They occurred almost simultaneously, or the former triggered the latter.
On 11 March 2011, JMA issued tsunami warning 3 min after the earthquake, but the size (M) was underestimated as 7.9, hence the forecasted tsunami heights were 3 to 6 m. JMA updated the forecasted heights, based on the offshore tsunami data, but it did not reach all the coastal residents, because of a power failure and early evacuation. Currently, 150 seismic and bottom pressure sensors were installed off Tohoku coast, and the data enable JMA to issue earthquake early warning and tsunami warning more quickly and accurately. By using the observed tsunami data and numerical simulation, tsunami data assimilation method is developed as a tsunami forecast method.
The Earthquake Research Committee (ERC) of the government made a long-term forecast that the probability of an M 7.5 earthquake off Miyagi in 30 years from 2010 would be 99 %. This is based on the 37 year interval of previous earthquakes in historical records. The ERC now also uses geological data to forecast giant earthquakes with longer recurrence intervals. The Central Disaster Management Council, based on lesson of the 2011 tsunami disaster, decided to consider two types of future tsunamis: More frequent L1 tsunamis with lower tsunami heights, and less frequent L2 tsunamis with larger tsunami heights. For L1 tsunami, hardware must be constructed to protect lives, properties and economy. For L2 tsunami, saving people’s lives is the highest priority with software measures.
For tsunami assessment at nuclear power stations, deterministic tsunami hazard assessment based on past recorded tsunamis were used to design tsunami heights. After 2011, probabilistic tsunami hazard assessment, considering aleatory and epistemic uncertainties, has been introduced.
Importance of education has been pointed out by Torahiko Terada, in his essay “Tsunami and Humankind.” He wrote that the lessons from the 1896 Sanriku tsunami was not utilized to reduce damage in 1933.