Penn State - Earthquake Seismology

Earthquake Ground Motion Animations

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.

Click here for more information on EarthScope; click here for information on the status of the transportable array.

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


Vertical component displacement seismograms used in the animation. The animation illustrates the signal power (which is proportional to the square of the ground velocity). The two red lines show iasp91 P and PP times, and the two blue lines show the predicted arrival times of S and SS. Open image in separate window to enlarge.

Slowness Spectra

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.

Vertical Component Ground-Velocity Slowness Spectra

The P wavetrain (500-1400s)

S wavetrain (1400-2250s)

Short-arc Rayleigh Waves (2250-3500s)

Rayleigh-wave coda (3300-3800s)

Long-arc S waves (4000-6000s)

The 251 Stations Used (see note above)

Penn State Earthquake Seismology

Department of Geosciences