A magnitude 8 earthquake coming from Chile tripped all the BSC-ISIs on 04/01/14. Sheila's wd plots show that the ISIs tripped on the actuator watchdogs. We suspected that the servo loops were trying to compensate for the common mode seen by the BSC-ISIs at both ends of the arms, thus saturating the actuators. We studied the ground motion recorded by the ground STS at the corner station (BS-STS) and the ground T240 installed at EX (EX-T240) at the GPS time provided by Sheila, in order to find out.

On the attached document:

• p1:  The time series recorded along X,Y and Z* show the three consecutive waves of the earthquake, coming one after the other: P-wave, S-wave, and then the surface waves.
• p2:  The common motion is highly dominant along the degrees of freedom which are the most affected:
  • Z for P-wave. Amplitude peak corresponds to actuator trip time.
  • X and Y for S-wave
• p3: The ground sensors show a high level of coherence below 0.1Hz, We expect to see most of the common motion there.
• p4: Most of the motion is seen below 0.5Hz. X and Y are affected the most .
• p5-7: The differential motion is smaller than the common motion by up to a factor 6, in the frequency region of interest.

We could considerably reduce the amount of signal sent to the BSC-ISI actuators during a major earthquake (factor as big as 6), and thus reduce the risk of saturating the actuators, by removing the common mode from the signal we feed to the servo loops of the BSC-ISIs.

Data, plots and scripts are commited to the seismic SVN:
/ligo/svncommon/SeiSVN/seismic/Common/Data/2014_04_01__Chile_Earthquake_Data/

Note 1: X and Y had to be swapped on EX data, and a minus sign was also added to the new x-axis EX data. It is likely that EX T240 is mis-oriented.
Note 2: The calibration of EX-T240 was still the one that JeffK made, at the time we looked at. RichM updated it since.