2 Seismic Wave Propagation and Earth models

2. Seismic Wave Propagation and Earth models
Fig. 2.40 Records (above) of two regional earthquakes of Oct. 9, 1986 at Sierre (left) and of
July 7, 1985 at Langenthal, Switzerland together with the calculated reduced travel-time
curves (middle) and ray-tracing crustal models which best fit the observations (below),
redrawn and complemented from Anatomy of Seismograms, Kulhánek, plate 4, pp. 83-84, ©
1990 (with permission from Elsevier Science).
Sometimes, very strong onsets after Pg, well before Sn or Sg can be expected, may be related
to depth phases (e.g., sPmP; Bock et al., 1996). This may complicate proper interpretation of
the local phases as well and can usually not be solved in routine analysis. Also be aware that
in the case of sub-crustal earthquakes, which are common in subduction zones, none of the
crustal phases discussed above exist. In this case, the first arriving longitudinal and shear
wave onsets are termed P and S, respectively, as for teleseismic events (see Fig. 2.41).
Fig. 2.41 P- and S-wave onsets from a local earthquake in northern Chile at a depth of 110
km and a hypocentral distance of about 240 km (courtesy of B. Schnurr, 2001).
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