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Subject: regional P phases
From: sean@...........
Date: Tue, 8 Aug 2000 13:29:57 -0500 (CDT)


Regarding multiple P phases for near regional quakes:

These are generally seen for "local" quakes recorded by high-gain
regional networks for events within 100 to 200 km of the station.

They are called Pn, Pg, and P*, and are the result of the P wave
energy propagating along different horizontal layers from the 
hypocenter to the station. Pn is propagating below the Mohorovicic
discontinuity at about 7.76 km/sec, and arrives first. Pg is described
by Jeffreys as propagating in the "granitic" layer below the Conrad
discontinuity at 6.5 km/sec, and P* is in the thin layer above it 
propagating at 5.6 km/sec. Rarely a phase Ps in the sedimentary layer
is seen at 4.7 km/sec.

There are comparable S-phases: Sn at 4.36 km/sec; Sg at 3.74 km/sec;
and S* at 3.36 km/sec. These are generally difficult to observe because
of the surface wave arrivals.

At distances generally greater than about 200 km only Pn is seen, 
since the energy of the other phases is rapidly dispersed. This is the 
usual "P" that we work with. These phases should not be confused with 
the teleseismic phases of deep or distant quakes, like pP or PcP.

A discontinuity is where there is an abrupt change in the velocity
of seismic waves caused by a major change on the rock properties.
The Mohorovicic discontinuity, or Moho, usually lies about 20 km
deep, but is deeper under mountain chains, even old worn down ones
like the Ozark uplift where it is 40 km deep, and shallower under 


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Larry Cochrane <cochrane@..............>