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Numerical modelling of SH L_g waves in and near continental margins

Regan, J. and Harkrider, D. G. (1989) Numerical modelling of SH L_g waves in and near continental margins. Geophysical Journal International, 98 (1). pp. 107-130. ISSN 0956-540X. http://resolver.caltech.edu/CaltechAUTHORS:20140317-141448325

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Abstract

The effect of transition regions between continental and oceanic structures on the propagation of L_g waves from continental sources is examined. In particular, the attenuation due to variations in layer thickness in such transition regions is calculated and explained for a suite of simple models. The measured attenuation, due to the geometry of the transition regions between the oceanic and continental structures within a partially oceanic path with source and receiver in a continental structure, is at most a factor of four for frequencies from 0.01 to 1 Hz. This is inadequate to explain the observed extinction of L_g along such paths. This extinction has previously been attributed to the effects of the transition region geometry. The method used to calculate the results presented in this study is developed and its validity and accuracy are demonstrated. Propagator matrix seismograms are coupled into a Finite Element calculation to produce hybrid teleseismic SH mode sum seismograms. These hybrid synthetics can be determined for paths including any regional transition zone or other heterogeneity that exists as part of a longer, mostly plane-layered, path. Numerical results presented for a suite of transition models show distinct trends in each of the regions through which the wavefield passes. The wavefield passes through a continent-ocean transition region, then a region of oceanic structure, and finally through an ocean-continent transition region. When an L_g wavefront passes through a continent-ocean transition, the amplitude and coda duration of the L_g wave at the surface both increase. At the same time, much of the modal L_g energy previously trapped in the continental crust is able to escape from the lower crust into the subcrustal layers as body waves. The magnitude of both these effects increases as the length of the transition region increases. When the wavefront passes through the region of oceanic structure further energy escapes from the crustal layer, and produces a decrease in L_g amplitude at the surface. The rate of amplitude decrease is maximum near the transition region and decreases with distance from it. When the wavefield passes through the ocean-continent transition region a rapid decrease in the L_g amplitude at the surface of the crust results. The energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer. Some of the body wave phases produced when the wavefield passes through the continent-ocean transition region are incident on the continental crust in the ocean-continent transition region. These waves are predominantly transmitted back into the crust. The other body wave phases reach depths below the depth of the base of the continental crust before reaching the ocean-continent transition and, thus, escape from the system.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1111/j.1365-246X.1989.tb05517.xDOIArticle
http://gji.oxfordjournals.org/content/98/1/107PublisherArticle
Additional Information:© 1989 Royal Astronomical Society. Accepted 1989 January 11. Received 1988 December 21; in original form 1988 June 27. J. Regan conducted part of this research while a Canadian Government Laboratory Visiting Fellow at Geophysics Division, Geological Survey of Canada, Ottawa. This research was supported in part by the Advanced Research Projects Agency of the US Department of Defense and was monitored by the US Air Force Geophysics Laboratory under contract F19628-87-K-0028; Division of Geological Sciences Contribution No. 4623; Geological Survey of Canada Contribution No. 20988.
Funders:
Funding AgencyGrant Number
Advanced Research Projects AgencyUNSPECIFIED
US Air Force Geophysics LaboratoryF19628-87-K-0028
Subject Keywords: synthetic seismograms; L_g waves; continental margins; finite element; attenuation
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences4623
Record Number:CaltechAUTHORS:20140317-141448325
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20140317-141448325
Official Citation:Regan, J. and Harkrider, D. G. (1989), Numerical modelling of SH Lg waves in and near continental margins. Geophysical Journal International, 98: 107–130. doi: 10.1111/j.1365-246X.1989.tb05517.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44360
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:17 Mar 2014 22:31
Last Modified:28 Aug 2014 18:51

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