Solomon Islands - 01 April 2007, 20:39:56 UTC (Mw = 8.1)
The animation shows the vertical ground velocity in the 100-50s period range observed across the western United States by the EarthScope transportable array and generated by the 01 April, 2007 Solomon Islands great earthquake. Each circle represents a seismometer and the colors change to reflect variations in the signal amplitude crossing the array. Near the end of this animation you can see the waves that traveled the long way around Earth to reach the western United States (they propagate from NE to SW). Based on their travel time and phase velocity, those phases are most likely body waves such as SSS, SSSS, SSSSS up to SSSSSSSSS. Reverberating core phases ScS3, ScS4, etc. are also arriving in the time window. Station O01C is located at the Eel River Conservation Camp, Redway, CA. (Note - I replaced the animation with a different scaling to enhance the body waves on 05 August, 2007, I still have the old one if you like that better.)
For more information on the earthquake, click here. A plot of the displacement seismograms used in the animation is shown near the bottom of this page. For the animation, I use velocity seismograms corrected for instrument sensitivity only - this enhances the shorter period components of the signal.
Selected seismic wave traveltimes (from Phillip Crotwell's version of taup)
Gcarc = 54deg / Depth = 15 km Phase Time Slowness P 788.92 4.617 PcP 789.57 4.431 PP 1008.21 7.888 PPP 1126.93 8.824 SKS 1421.56 5.664 S 1451.01 8.912 ScS 1453.10 8.301 SS 1820.97 14.379 SSS 2039.46 15.637 PPP 2333.02 4.663 SSS 4285.94 9.291
We can use the TA stations as a large (1500x1000km) seismic array to estimate wave phase velocity (the velocity at which a wave sweeps across a horizontal surface - Earth's surface in this case) and direction of approach, or azimuth. Some examples for the Solomon Islands earthquake are shown below. Warm colors, such as red, indicate substantial relative power arriving with a particular direction, which can be read like a compass (North at the top). The distance a spectral peak is from the center the slower the wave propagates across the array. The circles show velocities of 32, 16, 8, 6, 4, 3, 2 km/s respectively, starting from the center of each image.
The color range for each image is self-scaled. The bandwidths used for body waves adn the coda are 100-25s, for the surface waves are for 200-25s. The large event size resulted in a source duration of about 60s, so much of the short period energy is filtered away by the earthquake rupture. Using a Cartesian coordinate system is an approximation, but the distortion is not too large, based on the images below.
I made a mistake when I created these in that I left out the BK and CI components of the TA and used only stations with formally labeled in the TA network. But I used 251 stations computing the spectra. I recomputed the coda spectrum using 307 waveforms (including the CA stations) and it changed only slightly. Sorry.