Geology topics
William Lettis
Publications and source records attributed to William Lettis.
Lateral offsets on surveyed cultural features resulting from the 1999 İzmit and Düzce earthquakes, Turkey
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.
The surface rupture and slip distribution of the 17 August 1999 Izmit earthquake (M 7.4), North Anatolian fault
The 17 August 1999 İzmit earthquake occurred on the northern strand of the North Anatolian fault zone. The earthquake is associated with a 145-km-long surface rupture that extends from southwest of Düzce in the east to west of Hersek delta in the west. Detailed mapping of the surface rupture shows that it consists of five segments separated by releasing step-overs; herein named the Hersek, Karamürsel-Gölcük, İzmit-Sapanca Lake, Sapanca-Akyazi, and Karadere segments from west to east, respectively. The Hersek segment, which cuts the tip of a large delta plain in the western end of the rupture zone, has an orientation of N80°. The N70°-80°E-trending Karamürsel-Gölcük segment extends along the linear southern coasts of the İzmit Gulf between Karamürsel and Gölcük and produced the 470-cm maximum displacement in Gölcük. The northwest-southeast-striking Gölcük normal fault between the Karamürsel-Gölcük and İzmit-Sapanca segments has 2.3-m maximum vertical displacement. The maximum dextral offset along the İzmit-Sapanca Lake segment was measured to be about 3.5 m, and its trend varies between N80°E and east-west. The Sapanca-Akyazi segment trends N75°-85°W and expresses a maximum displacement of 5.2 m. The Karadere segment trends N65°E and produced up to 1.5-m maximum displacement. The Karadere and Sapanca-Akyazi segments form fan-shape or splaying ruptures near their eastern ends where the displacement also diminished.