Both plots below show a significant "drop" in the middle of their domain. In X-direction (top-plot) the tilt of the wall (structure) where the sensor is mounted suddenly "drops" at the same moment in time the earthquake waves pass the structure. At the same moment the Y-direction (bottom-plot) also drops, although significant less then the graph of the top plot. After the event both graphs do not return to their pre-event level.
Earthquake resulting in damage
Below two simplified animations are showing a transversal surface wave passing a building. The solely purpose of these animations is to help you understand our data and should not be interpreted as scientific correct.
Subsidence / Tilt
Conclusion: The part of the structure where the sensor was mounted, never returned to the same position as before the event. In the X-direction the tilt of the wall shifted by minus 4.7x10-3 degrees. The shift in the Y-direction is less than 2x10-3 degrees. Since the wall did not return to its original position plastic deformation occurred.
The graph below shows the total arc (the difference between the highest measured value and the lowest measured value) during the passage of the quake.
Both spikes represent the maximal readings during the quake. This means the total rotation from "left to right"as the quake passes.
Conclusion: During the earthquake, in X-direction the wall rotated 0.02 degrees. And in Y-direction 0.025 degrees.
Below the Peak Ground Accelerations (PGA) in respectively X-, Y- and Z-direction. These measurements are te Peak Accelerations within sample periods from 5 seconds measured with a sample rate from 400Hz each channel.
Conclusion: During the, by gas exploration induced, earthquake the following Peak Ground Accelerations where recorded:
- In X - direction:0.24 m/s2
- In Y - direction:0.085 m/s2
- In Z - direction:0.34 m/s2
Given the local conditions, this was a relatively severe earthquake, that rotated and translated the wall in all directions. Based on the tilt meters it is obvious plastic deformation occurred.
Due to the fact that vibrations and accelerations are showing clear spikes at the same time as the "drop" in tilt, one is allowed to conclude that the earthquake is the direct cause of the deformation.
At first glance a tilt drop of only 0.0047 degrees seems insignificant. But take into account that a sensor is often mounted in the foundations of a building. It is very likely that the rooftop is 8 meters (the radius) or higher above the foundations. In that case the top of the building shifted 0.66 mm. That by itself doesn't have to lead to visible damage. But at the fifth quake this sums up to 3.3 millimeters. That will rupture brick walls.
Stiff deformation = Number of deformations x radius x tangent(post-event tilt - pre-event tilt)
Stiff deformation = 5 x 8000 x tan(0.4962 - 0.4915)
Stiff deformation = 3,3 millimeter.