Bob, 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 oceans. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>