1:45 PM - 3:15 PM
[O05-P42] The research of Fukui earthquake fault (Ⅴ) ~How about the shake of Fukui plainwhen earthquakes occur?~
Keywords:Earthquake, Crack, Trapped Wave
We are studying about fault zone at the eastern margin of the Fukui plain, where Fukui Earthquake occurred. There are many earthquakes in Fukui prefecture because of the existence of many active faults. So we want to clarify underground structure of Fukui plain and mechanism of earthquakes’ shake.
It is known that when S wave arrives, the velocity vector of ground surface shows a certain direction remarkably. This direction is so called the fast-shake-direction of S wave. We can get this element from hodograph which makes velocity vector into graph. By accumulating information about the first shake direction at high school in Fujishima, Kanazu, Maruoka, Sakai and Mikuni, we can know their tendency and guess their characteristics.
From the figures, we analized first shake direction of S wave at each high school.
We learned that Fujisjima has some special underground structure from former study. In this year, we found the cases which shaking direction is irregular and we made two hypothesis. Firstly, the underground of Fujishima is stable. Secondary, closed crack NS direction exists under the school. Generally, the destruction incline to WNW-ESE.
According to the former study, there are cracks just under the surface of Kanazu. Also, We can’t see any irregular direction, so we can judge that crack was always closed when earthquakes occurred.
The result in Maruoka coincided with the direction of stress field and no phenomena shows crack’s existance. The result is as same as former study.
Waves in Sakai tend to incline to NW-SE. This result is different from former study, but there is irregular direction, so we judged that the crack hasn’t formed enough yet.
There is no tendency in Mikuni. Maybe, stress field around Mikuni is unstable land view of the Tozinbo.
Many cracks formed by fault movements. There are closed and opening cracks. If crack opens, then liquid like water is accumulated. We’ll explain the change of both cracks. We define that a premise crack is NS direction. Closed crack gives no influence on S waves with WE vibration direction, but waves with NS vibration direction are influenced by the friction of crack. On the other hand, When wave go through liquid parts of open crack, liquid can’t convey transversal waves, so their passage gets slower, while SN direction waves won’t change, so SN wave is recorded earlier we think.
Next, we explain about trapped wave. In fault zone, refractive index is larger than normal zone. For S wave to go through fault zone, an incident angle should be as small as possible, but the incident angle of waves which is parallel to the zone can’t be as small as critical angle. So waves reflect again and again like light in fiber and can’t go out from the zone. In S-coda, the wave whose size is as same as former one is recorded.
An earthquake case, we call them Trapped Waves.
We found the wave in two earthquakes. First case occurred near Neo village 2014.9.20. There was trapped wave in Sakai and Kanazu. We can see subtle succeeding wave in Fujishima, but it is maybe because the feature of the ground. The other case occurred around Ikeda town 2016.1.13. We can see trapped wave in Kanazu. Crack hasn’t formed enough in Sakai and velocity didn’t become slow enough, so we can’t see trapped wave. There is a kind of following wave in Maruoka, but it would be because Maruoka is nearest place to epicenter. Trapped wave didn’t occur in Fujisjima because Fujishima exists west side of fault zone.
It is known that when S wave arrives, the velocity vector of ground surface shows a certain direction remarkably. This direction is so called the fast-shake-direction of S wave. We can get this element from hodograph which makes velocity vector into graph. By accumulating information about the first shake direction at high school in Fujishima, Kanazu, Maruoka, Sakai and Mikuni, we can know their tendency and guess their characteristics.
From the figures, we analized first shake direction of S wave at each high school.
We learned that Fujisjima has some special underground structure from former study. In this year, we found the cases which shaking direction is irregular and we made two hypothesis. Firstly, the underground of Fujishima is stable. Secondary, closed crack NS direction exists under the school. Generally, the destruction incline to WNW-ESE.
According to the former study, there are cracks just under the surface of Kanazu. Also, We can’t see any irregular direction, so we can judge that crack was always closed when earthquakes occurred.
The result in Maruoka coincided with the direction of stress field and no phenomena shows crack’s existance. The result is as same as former study.
Waves in Sakai tend to incline to NW-SE. This result is different from former study, but there is irregular direction, so we judged that the crack hasn’t formed enough yet.
There is no tendency in Mikuni. Maybe, stress field around Mikuni is unstable land view of the Tozinbo.
Many cracks formed by fault movements. There are closed and opening cracks. If crack opens, then liquid like water is accumulated. We’ll explain the change of both cracks. We define that a premise crack is NS direction. Closed crack gives no influence on S waves with WE vibration direction, but waves with NS vibration direction are influenced by the friction of crack. On the other hand, When wave go through liquid parts of open crack, liquid can’t convey transversal waves, so their passage gets slower, while SN direction waves won’t change, so SN wave is recorded earlier we think.
Next, we explain about trapped wave. In fault zone, refractive index is larger than normal zone. For S wave to go through fault zone, an incident angle should be as small as possible, but the incident angle of waves which is parallel to the zone can’t be as small as critical angle. So waves reflect again and again like light in fiber and can’t go out from the zone. In S-coda, the wave whose size is as same as former one is recorded.
An earthquake case, we call them Trapped Waves.
We found the wave in two earthquakes. First case occurred near Neo village 2014.9.20. There was trapped wave in Sakai and Kanazu. We can see subtle succeeding wave in Fujishima, but it is maybe because the feature of the ground. The other case occurred around Ikeda town 2016.1.13. We can see trapped wave in Kanazu. Crack hasn’t formed enough in Sakai and velocity didn’t become slow enough, so we can’t see trapped wave. There is a kind of following wave in Maruoka, but it would be because Maruoka is nearest place to epicenter. Trapped wave didn’t occur in Fujisjima because Fujishima exists west side of fault zone.