Details

Autor(en) / Beteiligte

• Braile, L.W
• Chiang, C.S
• Barazangi, Muawia
• Brown, Larry

Titel

The Continental MohorovičIć Discontinuity: Results from Near‐Vertical and Wide‐Angle Seismic Reflection Studies

Ist Teil von

• Reflection Seismology: A Global Perspective, 1986, p.257-272

Ort / Verlag

Washington, D. C: American Geophysical Union

Erscheinungsjahr

1986

Quelle

Wiley Online Library All Obooks

Beschreibungen/Notizen

• The continental Mohorovičić discontinuity (Moho) has been recognized worldwide from refracted (Pn) and wide‐angle reflected (PmP) phases from earthquake and explosive sources. Studies utilizing Pn and PmP arrivals have generally interpreted the Moho to be a simple discontinuity in velocity between lower crustal and upper mantle rocks which is laterally continuous and which exists virtually everywhere at the base of the continental crust. Recently, deep seismic reflection profiling data have shown that the near‐vertical reflection signature of the Moho is more complex. The reflection data have been interpreted to indicate that the Moho consists of a transition zone of several km thickness and that it is laterally variable and even discontinuous or non‐existent in places. We have examined near‐vertical and wide‐angle seismic reflection record sections and performed model studies in an attempt to understand this disparity in interpretation and to try to resolve the actual seismic structure of the Moho. Part of the discrepancy in interpretation is due to distinct differences in characteristics and information content of near‐vertical and wide‐angle reflection data. Differences in frequency content, angle of incidence, station spacing and processing methods can lead to disparate Interpretations from the two data sets even for Identical Moho models. We have utilized near‐vertical and wide‐angle reflection/refraction synthetic seismogram studies for one‐ and two‐dimensional velocity models to investigate the seismic signature of a variety of Moho transition zone models. For all Moho velocity structures, the wide‐angle reflection (PmP) and the refraction (Pn) arrivals have simple and short duration waveforms. Travel times and amplitudes of the wide‐angle observations are relatively insensitive to details of the velocity structure of the Moho. However, the Pn and PmP arrivals are excellent indicators of the existence and depth of the Moho and its general lateral continuity. Furthermore, the amplitude‐distance characteristics of the Pn and PmP phases can be used to estimate the thickness of the Moho transition zone. Near‐vertical incidence reflections from Moho transition zones are primarily sensitive to fine structure of the transition. Smooth velocity transitions have low reflectivity for near‐vertical data. Constructive and destructive interference of reflections from laminated Moho structures produces a Moho signature consisting of bands of strong reflections which vary laterally. Where observed, these laminated reflectors provide information on the nature of the transition zone and its approximate thickness. However, band‐limited, near vertical reflection data do not always reliably indicate the existence or structure of the Moho. Comparison of observed near‐vertical and wide‐angle reflection data with the results of synthetic seismogram model studies suggests that the continental Moho is normally a transition zone of about 0 to 5 km thickness In which the velocity generally increases with depth, but also may contain thin laminations of high and low velocity material. The gross structure of the Moho is laterally continuous and appears to exist virtually everywhere as the base of the continental crust. However, the fine structure of the Moho, where it exists as a transition zone of thin laminations, is laterally variable resulting in the discontinuous character of Moho reflections on near‐vertical incidence record sections.