Submitted on: 05 January 1999.
Revised on: 09 April 1999.
Accepted on: 27 May 1999.
___________________Geofisica Internacional---INST GEOPHYSICS UNAM
Submitted on: 18 June 2009.
Revised on: 12 October 2009.
Accepted on: 12 November 2009.
___________________Geofisica Internacional---INST GEOPHYSICS UNAM
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Submitted on: 30 July 2002.
Revised on: 01 November 2002.
Accepted on: 22 December 2002.
___________________Geofisica Internacional---INST GEOPHYSICS UNAM
Submitted on: 13 December 1998.
Revised on: 11 February 1999.
Accepted on: 23 February 1999.
___________________Geofisica Internacional---INST GEOPHYSICS UNAM
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