Sendai Quake
Thinking about Japan. Interpreting beach balls. This was the initial beach ball (moment tensor solution) from the USGS. It's been updated since as more stations have reported in.
This one is not good for Japan. Yellow up. White down. In this case it works out to be a map view of the sea surface an instant after the quake.
These beach ball diagrams require a little mind bending to understand.
When the seismic waves from an earthquake reach a number of dispersed monitoring stations the initial P-wave motions are used to reconstruct the motion at the epicenter. A compressive wave at a seismometer indicates that initial motion is toward the station. Dilation first motions at the seismometer indicate that initial motion at the epicenter is away from station.
As more stations report, quadrants of dilation and compression can be mapped out on a sphere where the position on the sphere relates to the the shortest path between the seismometer and the epicenter. Sites near the epicenter generally relate to points on the top of the sphere and distant sites relate to points on the bottom of the sphere. As the quadrants are constrained they are used to define the orientation of the fault and the potential for tsunamis due to upward and downward seafloor movement. In this case the fault is an extremely steep reverse fault (aka thrust fault) along the subduction zone between Pacific Plate and a Japanese microplate. It's represented by the line cutting the beachball from the SW to NE.
The beach ball is basically a little cartoon version of the fault plane and relative motions along it.
The Japanese microplate (represented by the yellow quadrant) moved almost straight up during the quake and the Pacific Plate (represented by the white quadrant) moved almost straight down. Vertical motions of the sea floor are pretty much the worst case scenario for a tsunami. Water that's pushed up over the Japanese microplate began moving out laterally immediately after the earthquake. The hole created over the Pacific plate pulled large volumes of water in. The disturbance generated a series of waves which propagate across the ocean basin.
A rough version of this moment tensor solution was created a few tens of seconds after the quake. It's a major component of automated tsunami warning systems and an input to expert warnings and tsunami models created in the minutes afterwards.
Sendai Quake
Thinking about Japan. Interpreting beach balls. This was the initial beach ball (moment tensor solution) from the USGS. It's been updated since as more stations have reported in.
This one is not good for Japan. Yellow up. White down. In this case it works out to be a map view of the sea surface an instant after the quake.
These beach ball diagrams require a little mind bending to understand.
When the seismic waves from an earthquake reach a number of dispersed monitoring stations the initial P-wave motions are used to reconstruct the motion at the epicenter. A compressive wave at a seismometer indicates that initial motion is toward the station. Dilation first motions at the seismometer indicate that initial motion at the epicenter is away from station.
As more stations report, quadrants of dilation and compression can be mapped out on a sphere where the position on the sphere relates to the the shortest path between the seismometer and the epicenter. Sites near the epicenter generally relate to points on the top of the sphere and distant sites relate to points on the bottom of the sphere. As the quadrants are constrained they are used to define the orientation of the fault and the potential for tsunamis due to upward and downward seafloor movement. In this case the fault is an extremely steep reverse fault (aka thrust fault) along the subduction zone between Pacific Plate and a Japanese microplate. It's represented by the line cutting the beachball from the SW to NE.
The beach ball is basically a little cartoon version of the fault plane and relative motions along it.
The Japanese microplate (represented by the yellow quadrant) moved almost straight up during the quake and the Pacific Plate (represented by the white quadrant) moved almost straight down. Vertical motions of the sea floor are pretty much the worst case scenario for a tsunami. Water that's pushed up over the Japanese microplate began moving out laterally immediately after the earthquake. The hole created over the Pacific plate pulled large volumes of water in. The disturbance generated a series of waves which propagate across the ocean basin.
A rough version of this moment tensor solution was created a few tens of seconds after the quake. It's a major component of automated tsunami warning systems and an input to expert warnings and tsunami models created in the minutes afterwards.