M. Coughlin, K. Venkateswara

Michael noticed the earthquake on October 14th (M7.3 off of Nicaragua) was nicely visible in the seismometers and the BRS. I have attached the timeseries plots. I've also attached ASD plots of about 2 hours of data during the earthquake. GPS time = 1097293900.

The first ASD plot shows the T240X and the BRS RY out signals. Our guess for the horizontal displacement coupling was about 2e-4 rad/m. Multiplying this number with the T240 displacement output gives the red curve in the second plot. This seems to explain the signal in the BRS very well. The plot also shows the approximate estimate of the expected tilt signal assuming

theta ~ (2pi/wavelength) * X  

where wavelength = velocity / frequency (assuming sound velocity of 5 km/s). This gives the light blue curve, which is a factor of ~5 below the displacement coupling in the 50-100 mHz range.

The third plot also shows the residual between the BRS RY out and the displacement coupling which is close to the light blue curve, but without knowing the displacement coupling in an independent measurement, we cannot estimate the tilt component. The fourth plot shows the coherence between the two instruments.

This is a good confirmation of the expected horizontal displacement coupling in the BRS and a measurement of the distance between the COM and the pivot, which appears to be 30 +/-5 micrometers. If |d| had been <3 micrometers, this would have enabled a direct measurement of the tilt component of earthquakes.