tiedown tensions during 13jan14 earthquake
There was a 6.4 magnitude earthquake on 13jan14 at 00:01:05 ast.
- time: 00:01:05 AST
- earth quake:19.1385N, 66.8231W (82.19 Km
north-northwest of Arecibo)
- ao: 18.344289N 66.752778W
- Depth: 36 km
The puerto rican seismic network
accelerometer located at AO showed the earthquake:
The graph shows the tiedown tension vs time during
the earthquake (.ps) (.pdf):
- Page 1:
- The horizontal scale is seconds from the start of the
earthquake (at the epicenter). (+/- 1000 seconds).
- Top: telescope azimuth before and after the quake.
- 2nd: gregorian zenith angle during the quake.
- 3rd: tiedown tensions during the quake
- black: td12, red td4, green td8
- You can see the tensions begin to oscillate about 30
seconds after the quake (at epicenter).
- Bottom: total tiedown tension on the platform during the
- The tiedown jacks were stationary during this time period.
- The total tension will change as the temperature changes
(at midnight this happens very slowly).
- The average tiedown tension dropped by 800 lbs from before
to after the earthquake.
- Page 2: blowup of tiedown tensions
- tension in the 3 tiedowns
- total tension in the 3 cables
- The tension in the tiedowns went from 132.23 to 131.42 Kips
(a reduction of 800 lbs).
- Page 3 oscillation frequencies.
- data from times 28 to 151 seconds was transformed to
measure the oscillation freq.
- Data was sampled at 1 hz (making the maximum bandwidth .5
- The 3 colors are td 12 white, td4 red, td8 green.
- The strongest frequency was at .460 Hz i td 8.
- From previous hurricane measurements, this is probably
aliased from .54 hz.
- During hurricane georges a .57 hz freq was measured with
the tilt sensors. It was the pitch direction (az arm tilting
up and back).
- the azimuth was pointed at 49 degrees. this is about 10
degrees in azimuth from the td8 direction.
- .291, and .368 hz are also flagged. These were the
during a previous earthquake.
- 6.4 magnitude quake occurred 00:01:05 ast on 13jan14
- The azimuth arm was pointing within 10 degrees of td8
- The strongest oscillation was .54 Hz in td8
- The total tension in the tiedowns dropped by 800 lbs after the
- This has not been seen in previous earthquakes.
- Inspection of the cable from td 8 to the platform showed:
- the cable with the splice close to the tower had more than 8
strands broken were the cable hits the splice box (on the
- This is the same cable that had its hairpin cut during the
- The drop in tension was probably caused by the cable
relaxing when the strands popped.
up to oscillations