Paleoseismology of the North Anatolian fault: a case study of the Mudurnu Valley segment, Turkey : R. Demirtas, Individual Studies by Participants at the International Institute of Seismology and Earthquake Engineering, 31, 1995, pp 119–130

Abstract

Turkish-German Earthquake Research Project excavated four trenches across the surface ruptures of the 1957 and 1967 earthquakes on the Mudurnu Valley segment in the years 1993 and 1994, respectively. Mudurnu Valley segment forms the segment of the North Anatolian fault, in northwestern Turkey under the interaction of strike-slip and NE-SW tensile stress. The Abant earthquake of 1957 (Ms=7.0) and the Mudurnu Valley earthquake of 1967 (Ms=7.1) ruptured this segment for 40 km and 60 km in this century. There was no information about the age of the faulting events ruptured this segment prior to these last two earthquakes in historical records. In these exploratory trenches, the apparent geometry of the faults was high angle south-side up reverse faulting. This compressional features seems to be derived from subtle restraining bend of the excavated area. A strong earthquake probably caused a huge landslide damming up the flow of the Mudurnu river in the vicinity of trench site. Then, a temporary lake formed during the period between 8200 yr. B.P. and 2500 yr. B.P. A rapid sedimentation occurred in this lake and Taskesti formation deposited. The sediments exposed on the walls of the trenches are classified into 4 main lithologic units. This units represent the sediments deposited in a fluvial-lacustrine environment. Based on the 14 C dating, we assumed that unit b must have deposited in this lake between 3995 yr. B.C. and 4335 yr. B.C. Unit a deposited in a meandering river before 4335 yr. B.C. and Unit c and unit d are composed of slope debris material derived from the upthrown block of the fault after 3995 yr. B.C. In this study, we revealed clear evidence for at least three or four previous large earthquake events produced surface faulting prior to the last two earthquakes of 1957 and 1967 based on stratigraphy and structures appeared on the trench walls. Radiocarbon dates show that all faulting events prior to the last two earthquakes occurred on the Mudurnu Valley segment between some time 4335 B.C. and 2500 B.P. years ago. Furthermore, our excavation has demonstrated that the average recurrence interval of this segment is longer than 150 years.