Earthquake
Outbreak and cause of the earthquake
On the surface of the earth, the plate which is the layer of the board-formed rock approximately thickness 100km moves. These plates clash with each other and produce a distortion. An earthquake is generated in reaction when this is restored.
As shown, there are four plates for a rotation of Japan.
It is continental plate … North America plate and Eurasia plate
It is marine plate ... Philippine Sea plate and Pacific plate
I had it in North America plate and a border of the Pacific plate by an earthquake generated at the position (the north side) of the blue cross of the figure on Tohoku district Pacific offing earthquake … March 11, 2011
There is it to the border of Philippine Sea plate and the Eurasia plate by an earthquake considered to get up by the earthquake that may be generated in the Nankai trough earthquake … future near the position (the south side) of the blue cross.
※The green arrow of the figure shows a plate movement direction.
This figure is the figure which watched a plate from the just beside.
Earthquake to be generated right under the area where a person such as the red arrow neighborhood of the figure of inland earthquake … lives in
Earthquake to be generated on the border of the plate such as the blue arrow neighborhood of the figure of plate border type earthquake …
The depth (depth of the seismic center) of the earthquake occurrence spot is deepened to understand that I watch the distribution of the earthquake occurrence spot as I go to the Sea of Japan side from the Pacific side.
※The black arrow shows the movement direction of the plate.
Ditch in the trench … bottom of the sea
Mountain range in the ridge … bottom of the sea
Spot (Point O saying in a figure) where the seismic center … earthquake was generated
Spot (Point B saying in a figure) of the surface of the earth right above the epicenter … cause
I call it the depth of the epicentral distance … seismic center
Distance in a straight line from the distance … seismic center from the seismic center
※There is a great many that I use the distance from the seismic center in the figure by the calculation of this later earthquake. By the issue of entrance examination, depth (BO) of the seismic center is identified as distance (AB) to the epicenter, and there is the problem not to come loose when I do not demand the distance from the seismic center using a Pythagorean theorem (I learn it in 3 mathematics). In that case, you should understand that you use the following formulas.
Take the square root of statement
I can find the distance from the seismic center in the above-mentioned expression.
When an earthquake is generated, two different waves (P wave, S wave) of the speed occur at the same time from the seismic center and open concentrically.
Speed is fast in a wave causing P wave (Primary wave) (primary wave (longitudinal wave)) … preliminary tremors. The speed is quite 6~8km/ second
Speed is slow in a wave causing master of S wave (Secondary wave) (side wave) … motion required. The speed is quite 3~5km/ second
The first small rolling in case of the preliminary tremors … earthquake
Big rolling after main motion … preliminary tremors
※The problem to ask the keyword phrase of these four shows frequent questions and I should answer in anything or am easy to hesitate.
Is a problem sentence; “the wave called …?” If is asked と; P wave or S wave. “The rolling called …?” You should judge preliminary tremors or main motion for wind answering if asked about と whether it is a wave, or it is rolling.
Time of the interval before main motion coming after … preliminary tremors began in preliminary tremor duration
With seismic intensity … observation spot のゆれの size, there are 0,1,2,3,4,5 weak a little less than five, a little less than six, a little less than six, 7 ten classes
The size of the scale of the magnitude … earthquake. If 1 becomes big, the energy of the earthquake doubles 32. If 2 becomes big, approximately 1,000 double with 32*32.
Calculation (list type) of the earthquake
[exercise]
The next list settled the time when the preliminary tremors in each spot and main motion began. I fill the blanks of the list, and, at the outbreak time of the earthquake, find P wave, the speed of the S wave.
| Distance from the seismic center | The start time of preliminary tremors | The start time of the main motion | |
| Point A | 48km | 12:45:34 | 12:45:40 |
| Point B | 80km | 12:45:38 | ① |
| Point C | ② | ③ | 12:48:58 |
[elucidation]
- At first the most important one is, “the distance from the seismic center is proportional to preliminary tremor duration” by the calculation of the earthquake.
At first let’s obtain the data called six seconds for 48km than data of Point A. Next
At Point B for 80km? Because the wind of the second does not know preliminary tremor duration, I make a proportional expression here.
48km ：Six seconds = 80km: Because the product of x internal terms and the product of the outer term are equal
48x=480
x=10
Thus, as for the preliminary tremor duration of Point B, add 10 at 12:45:38 for ten seconds
①The の answer is 12:45:48.
- “It is a difference of the time with a difference of the distance between two spots, and the speed of P wave and the S wave is found only in breaking it”.
Because there is a difference of the distance of 32km, the P wave is four seconds late behind data of Point A and Point B, and it may be said that the S wave is late for eight seconds. Thus
The P wave: /4 32km second = 8km/s
The S wave: It becomes /8 32km second = 4km/s.
- At the outbreak time of the earthquake, it is found “only in breaking it in the time when the S wave reaches it whether it is P wave at the distance from the seismic center of some spot”.
It is 48/8 = six seconds in the P wave going ahead through the distance of 48km with the speed of 8km/s than data of Point A
Six seconds pass after an earthquake was generated and understand that P wave reaches Point A.
Thus, because it is six seconds ago when preliminary tremors arrive at Point A that an earthquake was generated,
It is -6 second = 12:45:28 at 12:45:34.
Because it is Point C, but main motion performs outbreak (12:48:58) next 30 seconds later after an earthquake performs outbreak (12:45:28)
= 120km this becomes the distance from the seismic center of Point C for 4km/s (the speed of the S wave) X 30 seconds. This is an answer to ②.
In addition, because the P wave takes 15 seconds when I go ahead through the distance of this 120km at 8km/s
Because P wave arrives at Point C 15 seconds after an earthquake occurrence
It is +15 = 12:45:43 at 12:45:28. This is an answer to ③.
[solution]
By the way, I can demand ②③ at high speed in a trick-like solution after having understood ①.
As shown, in fact, a difference of the time when I looked lengthwise and the difference of the distance are proportional to even which line.
So because y is established = 4 at 8:10, and x is established = 32 at 8:10, too,
x=40,y=5
So, in ②, the person who is quick in a calculation is relieved of 120km ③ for wind called 12:45:43 in around ten seconds.
There are not most of the books using this elucidation technique and thinks whether you will almost tell me in the private supplementary school.
Please use it if good.
| Distance from the seismic center | The start time of preliminary tremors | The start time of the main motion | |
| Point A | 48km | 12:45:34 | 12:45:40 |
| Point B | 80km | 12:45:38 | 12:45:48 |
| Point C | 120km | 12:45:43 | 12:48:58 |
The outbreak time of the earthquake: 12:45:28
The speed of the P wave: 8km/s
The speed of the S wave: 4km/s