Winnetka deformation zone: Surface expression of coactive slip on a blind fault during the Northridge earthquake sequence, California. Evidence that coactive faulting occurred in the Canoga Park, Winnetka, and Northridge areas during the 17 January 1994, Northridge, California earthquake

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Measurements of normalized length changes of streets over an area of 9 km{sup 2} in San Fernando Valley of Los Angeles, California, define a distinctive strain pattern that may well reflect blind faulting during the 1994 Northridge earthquake. Strain magnitudes are about 3 {times} 10{sup {minus}4}, locally 10{sup {minus}3}. They define a deformation zone trending diagonally from near Canoga Park in the southwest, through Winnetka, to near Northridge in the northeast. The deformation zone is about 4.5 km long and 1 km wide. The northwestern two-thirds of the zone is a belt of extension of streets, and the southeastern one-third ... continued below

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Medium: P; Size: 77 p.

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Cruikshank, K.M.; Johnson, A.M.; Fleming, R.W. & Jones, R.L. December 31, 1996.

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  • Cruikshank, K.M. Portland State Univ., OR (United States). Dept. of Geology
  • Johnson, A.M. Purdue Univ., West Lafayette, IN (United States). Dept. of Earth and Atmospheric Sciences
  • Fleming, R.W. Geological Survey, Denver, CO (United States)
  • Jones, R.L. City of Los Angeles, CA (United States). Dept. of Public Works

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Description

Measurements of normalized length changes of streets over an area of 9 km{sup 2} in San Fernando Valley of Los Angeles, California, define a distinctive strain pattern that may well reflect blind faulting during the 1994 Northridge earthquake. Strain magnitudes are about 3 {times} 10{sup {minus}4}, locally 10{sup {minus}3}. They define a deformation zone trending diagonally from near Canoga Park in the southwest, through Winnetka, to near Northridge in the northeast. The deformation zone is about 4.5 km long and 1 km wide. The northwestern two-thirds of the zone is a belt of extension of streets, and the southeastern one-third is a belt of shortening of streets. On the northwest and southeast sides of the deformation zone the magnitude of the strains is too small to measure, less than 10{sup {minus}4}. Complete states of strain measured in the northeastern half of the deformation zone show that the directions of principal strains are parallel and normal to the walls of the zone, so the zone is not a strike-slip zone. The magnitudes of strains measured in the northeastern part of the Winnetka area were large enough to fracture concrete and soils, and the area of larger strains correlates with the area of greater damage to such roads and sidewalks. All parts of the pattern suggest a blind fault at depth, most likely a reverse fault dipping northwest but possibly a normal fault dipping southeast. The magnitudes of the strains in the Winnetka area are consistent with the strains produced at the ground surface by a blind fault plane extending to depth on the order of 2 km and a net slip on the order of 1 m, within a distance of about 100 to 500 m of the ground surface. The pattern of damage in the San Fernando Valley suggests a fault segment much longer than the 4.5 km defined by survey data in the Winnetka area. The blind fault segment may extend several kilometers in both directions beyond the Winnetka area. This study of the Winnetka area further supports observations that a large earthquake sequence can include rupture along both a main fault and nearby faults with quite different senses of slip. Faults near the main fault that approach the ground surface or cut the surface in an area have the potential of moving coactively in a major earthquake. Movement on such faults is associated with significant damage during an earthquake. The fault that produced the main Northridge shock and the faults that moved coactively in the Northridge area probably are parts of a large structure. Such interrelationships may be key to understanding earthquakes and damage caused by tectonism.

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Medium: P; Size: 77 p.

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OSTI as DE99000964

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  • Other Information: PBD: 1996

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  • Other: DE99000964
  • Report No.: USGS-OFR--96-698
  • Grant Number: FG02-93ER14365
  • DOI: 10.2172/677055 | External Link
  • Office of Scientific & Technical Information Report Number: 677055
  • Archival Resource Key: ark:/67531/metadc710304

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  • December 31, 1996

Added to The UNT Digital Library

  • Sept. 12, 2015, 6:31 a.m.

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  • April 18, 2016, 6:49 p.m.

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Cruikshank, K.M.; Johnson, A.M.; Fleming, R.W. & Jones, R.L. Winnetka deformation zone: Surface expression of coactive slip on a blind fault during the Northridge earthquake sequence, California. Evidence that coactive faulting occurred in the Canoga Park, Winnetka, and Northridge areas during the 17 January 1994, Northridge, California earthquake, report, December 31, 1996; Denver, Colorado. (digital.library.unt.edu/ark:/67531/metadc710304/: accessed January 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.