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zmit and Düzce Earthquakes, Turkey
Earth Consultants International, Inc.
2522 N. Santiago Blvd.
Suite B, Orange, California 92867
(T.K.R., T.D.)
William Lettis & Associates, Inc.
25050 Avenue Kearny
Suite 108
Valencia, California 91355
(S.L.)
U.S. Geological Survey
345 Middlefield Road
MS977
Menlo Park, California 94025
(R.L.)
Institute of Geological and Nuclear Sciences
P.O. Box 30-368
Lower Hut, New Zealand
(R.L.)
William Lettis & Associates, Inc.
1777 Botelho Drive, Suite 262
Walnut Creek, California 94596
(W.L.)
California Institute of Technology
Pasadena, California 91125
(Y.K.)
Manuscript received 20 August 2001.
Surveys of offset linear cultural features that cross the surface ruptures of the 17 August and 12 November 1999 earthquakes on the North Anatolian fault in Turkey yield slip values as large as or larger than those recorded by near-field measurements in the same areas immediately after the earthquake. Using long, linear alignments of trees, fence lines, walls, and canals, we demonstrate as much as a 2-m increase in observed slip over the initial field measurements. On an average, we observed about 15% of the total lateral slip as off-fault deformation, with values ranging from 0% to 40% of the total slip. Part of this lateral deformation is accommodated by bending or drag in a zone as much as 30 m in width, although usually the zone varies between 5 and 20 m in width. This supports the idea that substantial nonbrittle, off-fault deformation is associated with ruptures in areas of alluvial fill. Alternatively, there may have been substantial afterslip, although none has been recognized on postearthquake repaired structures. Our observations suggest that postearthquake measurements of fault slip, using tape measures on offset geomorphic features, may underestimate the actual amount of slip in that event, where the nonlinearity of the offset feature does not allow recognition of the warping.
Surveys of multiple tree lines within groves of poplar trees, planted in straight lines across the fault prior to the earthquake, show surprisingly large lateral variations. In one grove, slip increases by nearly 1.8 m, or 35% of the maximum measured value, over a lateral distance of nearly 100 m. This and other observations along the 1999 ruptures suggest that the lateral variability of slip observed from displaced geomorphic features in many earthquakes of the past may represent a combination of (1) actual differences in slip at the surface and (2) the difficulty in recognizing distributed nonbrittle deformation.
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