Evan G., Pep C.

Summary:
We investigated when and why extra lines appear in the DARM spectrum near 1500-1520 Hz after the recent Montana earthquake. These lines appear associated with increased amplitude of the violin resonances, but coupling behavior is not firmly established. We would like the violin modes to come down in amplitude (better damping?) to verify that these lines could be mitigated.

Details:
Looking back over various spectra computed by the CW group (see here, for example), we find that the lines seem to have an amplitude behavior that changes in time. We wonder if they could be associated with changes in the violin resonances in time.

In the attached image, we plot three different times, June 19, July 19, and July 20, 2017. The 1500-1520 Hz band (upper left) shows the increased amplitude of the new lines; at the same time, we plot the violin resonances for the 500 Hz, 1000 Hz, and 1500 Hz bands. Note that the June 19 trace is quiet and we do not see evidence for additional lines. Also, there are no broad shoulders associated with this time. We speculate that the increased amplitude of the violin resonances is causing some strange mixing problem that creates additional lines in DARM.

After the Montana earthquake, the lines are much more stationary than we have ever observed lines in this band in the past. We had previously found that lines would be transient in this band. For example:
Present March 29
Gone April 4
Present June 28
Gone June 29
When looking for these lines, we caution that not all lines are always present and the amplitude of the lines is quite variable.

We also checked that the low frequency noise in DARM is not substantially different, so it's not clear that the broad shoulders are caused by up-conversion. However, since the fundamental violins are sometimes 2 orders of magnitude higher, it might be due to this. Again, better damping of the violins might help here. ;)