2. Simulation Of Tsunami
Around Japan, the severe earthquake might occur in the vicinity of the coast.
In that case, the calculation began after the earthquake occurs with the latest computer, and because the
tsunami raids a Japanese coast at once after the earthquake occurs, the tsunami warning cannot be
announced by the time the tsunami reaches.
Then, the fault with the possibility to generate the tsunami is set beforehand, the numerical simulation of
the tsunami is done, and the result is accumulated as a tsunami forecast data base.
1. The horizontal position and depth of fault
2.Size of fault
3.The direction of fault
4.Inclination of fault
5.Direction and size of slipping
It is necessary to provide it. The direction of the fault has been decided referring to a past earthquake.
To deal even if the earthquake of what size occurs in what place, a lot of simulations are done about the horizontal
position of the fault, depth, the size of the fault, and the size of slipping.About the inclination and the direction of
slipping, of the fault, it is a setting that most greatly generates tsunamis, and the inclination assumes 45° to be a pure
reverse fault .
3. The fault is about 1,500 places to horizontal direction, and it thinks about four kinds, and one fault
requests of diastrophism of these in bottom of the sea of 6 kinds and magnitudes to depth to and
from 0-100km.
This is handed over to the tsunami spread calculation.
4. tsunami spread calculation
In general, it can be thought that the bottom of the sea diastrophism that generates the tsunami becomes
a ruggedness that the vertical movement of the ocean crust is caused in the surface of the sea
immediately after the occurrence of the earthquake the state as it is because diastrophism is completed
before it has the extension of tens of kilometers or more, and the tsunami begins to extend.
The surface of the sea ruggedness pattern obtained thus is assumed to be an initial source of wave of the
tsunami, and the appearance where this is transmitted in every direction is calculated.
The computational domain is delimited in detail like the lattice in length and breadth, and as the method of
the math calculation, time of height and the speed of the tsunami in each lattice is chased and calculated
according to the equation of the tsunami spread.
It calculates for all faults like this, and the appearance of the temporal variation of the tsunami that
appears in the coast is reproduced.
5. Forecast of height of tsunami in coast
There is no height of the tsunami that becomes the standard of the tsunami warning expected in the coast because it uses height in the coast calculated by the simulation as it is. It is
because of depth's in the vicinity of the coast being thought to be a fall of the repeatable precision of the tsunami in the place where geographical features shallowly becomes
complex, too because the size of the computational grid is made constant. It takes a great amount of time to calculate the whole country, and it is not realistic though there is a
method of doing a very detailed make the computational grid detailed in the vicinity of the coast to solve this calculation.
Then, height in the coast is presumed by using "Green law" from the height of the tsunami in being the offing combination in no still very inclusion of the error margin.
The speed of the tsunami slows if a tsunami offshore (point where depth is deep) comes to the place where depth of the coast is shallow, and the interval between the previous wave
and the wave in the back shortens. However, the energy conserved in the person wave must be equal.
The wave surface is a coastline and the height of the wave rises as a result to the extent that the interval between the wave and the wave shortens when entering into going side by
side. This is a green law.
6. Making of warning and warning of each
forecast district in Japan
The Meteorological Agency is informing you of height and the attainment expectation time of the tsunami of each
tsunami forecast district where the coast in the whole country was divided into 66 expected that the tsunami
warning and the warning are announced.
These tsunami forecast districts are set considering it for the emergency in the related disaster prevention
organization such as municipalities when warning and the warning are announced after the feature of how of a
tsunami different according to geographical features to appear is investigated.
Here, it explains how the Meteorological Agency is making warning and the warning of the forecast district.
In the tsunami warning and the warning of the forecast district, it announces judging "Tsunami", "Tsunami", and
"Tsunami advisory" based on the highest in that value among the predictive values of the height of the tsunami in
two or more points in the forecast district, and unifying the maximum height.
What applies a green law to the tsunami simulation calculation result to a point (forecast point) left for being the
offing combination from the coast at about 15km based on the method of above-mentioned "Forecast of the height
of the tsunami" and converts it into height in the coast is used for the tsunami height presumption of an individual
point.
7. Attainment time of tsunami
to forecast district
The computational error of the attainment expectation time of the tsunami grows in the vicinity of the coast where
depth is shallow.
Then, the tsunami attainment time to the forecast district is calculated by adding time that the tsunami spreads from
there to the coast at the attainment time in the forecast point it is the offing combination of obtaining because of the
simulation.
Gravitational acceleration is used, and at this time, g and depth are assumed to be h, and the tsunami uses
transmitting in the sea at the speed of √gh.
How to the forecast district to request the attainment expectation time is as follows.
① The point where the spread time from the forecast point is equal is connected from the depth data around
the forecast point.
② The spread chart of the tsunami is made by repeating ①.
③ The spread time of the tsunami from the forecast point to the coast can be read from the tsunami spread
chart.
④ The earliest one is made the attainment expectation time to the forecast district about all the forecast
points in the forecast district for the attainment expectation time to the coast sooner or later.
The attainment expectation time to the tidal observatory is used to announce for the spread time from each tidal
observatory to the source of wave of the tsunami.
8. Comparison with past tsunami recode
The observed value and the predictive value of a past tsunami are compared by using the tsunami forecast data base
that does as mentioned above and makes it.
When the height of the tsunami of 139 cases where the tsunami has been observed so far was seen, the average of the
ratio of the observed value in the tidal observatory and the predictive value of the data base became about 1.2, and
became good on average the observed value when reproducing it.
On the other hand, the predictive value to the tsunami forecast district announced by the tsunami warning and the
warning is about 1.8 times on the average because it adopts the maximum predictive value in the forecast district
(Refer to < height of the tsunami in the forecast district >) compared with the observed value.
However, the tsunami can become a tsunami that is bigger than the height of the expectation of the tsunami told by
the tsunami warning and the warning according to circumstances because there is a character that height changes
greatly because of local geographical features.
Leaving of shelter to the height at once from the coast at once if the tsunami warning is announced if the warning is
announced is important.
