This event is an interesting example of variations in propagation of a phase seismologists call Lg.
Lg is a "regional phase" that propagates at the remarkably common velocity of 3.5 km/s. It is believed to be a guided phase that propagates completely within the earth's crust. This is like surface waves, which are also guided waves, but the physical process is thought to be quite different. Surface waves are waveguide modes that are "tuned" to low frequency. We see these surface waves as the most prominent low-frequency signal seen on all of these seismograms.
The first two seismographs compare the record at PPBLO (Indiana University, Bloomington) in two forms.
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This is the unfiltered record which is "broadband" because the signal has both slowly changing energy (low frequency) and some shorter scale length energy (high frequency). Click on thumbnail for larger image. |
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This is the same data on the same time scale with the low frequecy information removed (0.5-2 Hz band). The prominent secondary phase seen on the filtered data is Lg. Click on thumbnail for larger image. |
Note that at PPBLO it arrives at almost the same time as the surface waves. This is because both propagate at a velocity of approximately 3.5 km/s.
The next two seismographs show national scale variations in Lg propagation that are quite striking.
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This is the filtered data (same time scale). Notice the Lg is very clear on PPREH, which is in New Mexico, but is hardly visible in PPUHS in Oregon. Click on thumbnail for larger image. |
For reasons that are not fully understood Lg does not propagate well north out of the Gulf of California, but it does propagate east out of the Gulf of California to be seen throughout most of the rest of North America.
This is an extemely good example of this phenomenon!