Crustal structure of the Chugach Mountains, southern Alaska: A study of peg‐leg multiples from a low‐velocity zone

Abstract

A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high‐velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg‐leg multiples generated from within a low‐velocity zone between the second and third upper crustal high‐velocity layers. The third high‐velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low‐velocity zone that produces a seismic shadow zone wherein peg‐leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high‐ and low‐velocity layers. Synthetic seismogram modeling indicates that a low‐velocity zone with transitional upper and lower boundaries generates peg‐leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long‐offset (90–225 km) data. This may be due to a lower crustal waveguide whose top is the high‐velocity (7.6 km/s) layer and whose base is the Moho. A deep (∼54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg‐leg multiples have been interpreted on some long‐range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg‐leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide‐angle reflection profiles than previously recognized.