Krishna, Michael

Due to the slow wind day, we decided to take a look at an 6.7 magnitude earthquake that struck near Australia last night. All three STS seismometers (ETMX, EMTMY, and ITMY) and the BRS picked it up very clearly. The first plot in the attached pdf shows the time series of both the z component of the STS at ETMY and the BRS2 also at ETMY. The BRS shows clear coherence with the STS at the earthquake frequencies. The second plot shows the ASD of the same two signals along with the possible acceleration coupling of the BRS assuming a d value of 0.5 microns. The BRS signal is a factor of two or three above the possible acceleration coupling around the earthquake which suggests that it is seeing the real tilt component of the Rayleigh waves of the earthquake.

Along with studying how the BRS sees earthquakes, we used z components of the STS seismometers to estimate the velocity and direction of the primary wave. By measuring the phase between the seismometers at the end stations and the seismometer at the corner station, we were able to get the time delay of each arm. Using this we get an estimate for the velocity of 4700 +- 800 m/s and the angle of 71+-10 degrees from the X-arm. The third and fourth plots in the pdf show how the angle and velocity measurements change with different number of averages in the phase calculations. The fifth and sixth plots show how the measurements change over 125 second chunks of time.

Earthquake info: http://earthquake.usgs.gov/earthquakes/eventpage/us20005fsi#general

Edit: We realized that yesterday's calculations of the acceleration coupling had an extra factor of g so the BRS signal is actually a factor of 30 above the possible acceleration coupling. The first plot in the Update pdf shows the updated ASD of the z component of the STS at ETMY, the BRS, and the correct acceleration coupling. Also, using the ratio of the z component of the seismometer and the tilt signal we were able to get another estimate for the velocity which is consistent with the previous analysis.

We also decided to take a look at how the different STS seismometers reacted to the earthquake to ensure they were calibrated correctly. The second plot of the Update pdf shows the ASD of the z component of all three seismometers. The final four plots show the magnitude of the transfer function at the earthquake's primary frequency between the ETM seismometers and the ITMY seismometer with varying number of averages and over time of the earthquake. This suggests that the ETMX seismometer's calibration is ~7% less than ITMY and ETMY's calibration is ~15% less. Coincidentally, we had to add a factor of 0.85 to the BRS to get the gain to match the ETMY seismometer.