Paleoseismological studies on Düzce fault and geological data on the seismogenic sources in the vicinity of Düzce area

Abstract

In 1999, two earthquakes ruptured the Northern Almaclk strand (NAIS) of the North Anatolian Fault that includes the Diizce and Aksu faults, producing surface rupture along the northern border of the Almaclk block. Paleoseismological trenching is performed at four sites along the Aydmpmar and Mengencik segments of four-structural-segmented Diizce fault. Six dated-events from eleven trenches that cross cuts the fault zone provided insights on its seismogenic behavior. The excavations from five trenches at three sites expose evidence for six earthquakes that occurred since B. C. 1740. A shallow of watertable, unfavorable trench materials and lack of financial support to perform 3-D trenching made the identification anu characterization of individual paleoearthquakes difficult. However, by integrating date constrains of events from the trenches that were performed along the Diizce fault including previous studies along the easternmost segment, a periodical recurrence model seems to be concordant with the radiocarbon age data. Seven serial surface-rupturing earthquakes including the 1999 Diizce event along the Diizce fault are defined. The model suggests that the Diizce fault is behaving as a single seismic segment at least for the last serial rupture in Holocene time. The additional two older events are showing an irregularity in the serial. Hitherto unrecognized earthquakes probably would eliminate this irregularity. A recurrence interval is estimated by also considering the two older earthquakes. This final sequential model suggests that 1999-type earthquakes repeat each 355±35 year (% 70 probability). The slip rate that was calculated from 350 cm average slir of the 1999 event and the recurrence period is about 9.5±1 mmlyr having % 70 probability. This value is in agreement with geodetic measurements (10 mmlyr) from independent studies. Because short time has elapsed since the 1999 earthquake that occurred along the Diizce fault, the Diizce fault does not appear to have an important seismogenic potential in near future. The Diizce plain is one of the major basins of the Marmara region. There is no active faulting excluding Diizce and Aksu faults that is detected in the basin and surrounding borders according to this study. In addition to this pull-apart and then continuing active formation of the Diizce basin hypothesis is not validated by rigorous data. Therefore, considering these data, it may be suggested that Diizce area is not under a severe seismogenic threat for very near future.