North Korea nuclear test seismogram - crustal phases

Figure from the Nevada Seismological Laboratory

Back on January 17, I had an entry on crustal phases. We should do a short review to understand the nomenclature shown in the diagram in the last blog for the seismic phases seen on China seismograms after the Korea nuclear test.

You can find a listing of seismic phases here.

Note in particular the phases shown in the previous quake observed for the NK nuclear test:

• Pg - at short distances, an upgoing P wave from a source in the upper crust; otherwise, a wave refracted from a mid-crustal discontinuity
  
• Pn - a P wave refracted along the crust-mantle boundary
  
• Sg - at short distances, an upgoing S wave from a source in the upper crust
  
• Sn - an S wave refracted along the crust-mantle boundary

Rayleigh and L waves are surface waves.

Lancaster earthquake - just a phase?

We're still doing an extended tutorial on the Lancaster earthquake.

Go down the page there, and click on Phase (Arrival Time) Data.

This shows the phases recorded at different seismic station. We know already that there are three physically different types of seismic waves - P, S, and L. But phase not only indicates which type of wave, but what path it has traveled.

You can find a listing of seismic phases here.

Note in particular the phases for the Lancaster quake:

• Pg - at short distances, an upgoing P wave from a source in the upper crust
• Pn - a P wave refracted along the crust-mantle boundary
  
• Sg - at short distances, an upgoing S wave from a source in the upper crust
  
• Sn - an S wave refracted along the crust-mantle boundary

from http://www.isc.ac.uk/doc/analysis/2003p13/index.html#crustal

I think the preceding e you find for some phases indicates an "emergent" phase, one that is not too sharp, but builds up more gradually. Confirmation, anyone?