New trenching results along the İznik segment of the central strand of the North Anatolian Fault (Turkey): an integration with preexisting data

Abstract

This paper provides a new contribution to the construction of the complex and fragmentary mosaic of the Late Holocene earthquakes history of the İznik segment of the central strand of the North Anatolian Fault (CNAF) in Turkey. The CNAF clearly displays lower dextral slip rates with respect to the northern strand however, surface rupturing and large damaging earthquakes (M > 7) occurred in the past, leaving clear signatures in the built and natural environments. The association of these historical events to specific earthquake sources (e.g., Gemlik, İznik, or Geyve fault segments) is still a matter of debate. We excavated two trenches across the İznik fault trace near Mustafali, a village about 10 km WSW of İznik where the morphological fault scarp was visible although modified by agricultural activities. Radiocarbon and TL dating on samples collected from the trenches show that the displaced deposits are very recent and span the past 2 millennia at most. Evidence for four surface faulting events was found in the Mustafali trenches. The integration of these results with historical data and previous paleoseismological data yields an updated Late Holocene history of surface-rupturing earthquakes along the İznik Fault in 1855, 740 (715), 362, and 121 CE. Evidence for the large M7 + historical earthquake dated 1419 CE generally attributed to this fault, was not found at any trench site along the İznik fault nor in the subaqueous record. This unfit between paleoseismological, stratigraphic, and historical data highlights one more time the urge for extensive paleoseismological trenching and offshore campaigns because of the high potential to solve the uncertainties on the seismogenic history (age, earthquake location, extent of the rupture and size) of this portion of NAFZ and especially on the attribution of historical earthquakes to the causative fault.

Segmentation and Holocene Behavior of the Middle Strand of the North Anatolian Fault (NW Turkey)

“…On the same segment, Civico et al. (2021) opened two trenches and evidenced one mid‐19th century event, one penultimate event after the late seventh century and two older events before the mid‐fifth century CE. West of Iznik Lake, Ikeda, et al.…”

“…South of the lake, Erginal et al (2021) reported a 50 cm high coseismic scarplet in beachrock deposits that they attributed to an eighth century rupture. On the same segment, Civico et al (2021) opened two trenches and evidenced one mid-19th century event, one penultimate event after the late seventh century and two older events before the mid-fifth century CE. West of Iznik Lake, Ikeda, et al (1989) proposed a mid-19th century event from archaeological remains found in a natural fault exposure.…”

“…To estimate this length, we rely on the magnitude estimated by historical seismology studies (see Table S1), from which a surface rupture length was derived using the relationship of Wells and Coppersmith (1994). The length of the vertical boxes corresponds to the dating of the events within 2σ uncertainty (Civico et al, 2021;Dikbas et al, 2018;Dogan, 2010;Gastineau et al, 2021;Ikeda, Suzuki, et al, 1991;Klinger et al, 2003;Özalp et al, 2013;Rockwell et al, 2009; see Table S2 for detailed data). and previous paleoseismological data shows that this approach is not sensitive enough to detect the smaller events (Mw 6.8-7) that are also known to occur along the MNAF.…”

Segmentation and Holocene Behavior of the Middle Strand of the North Anatolian Fault (NW Turkey)

“…To estimate this length, we rely on the magnitude estimated by historical seismology studies (see Table S1 in Supporting Information), from which a surface rupture length was derived using the relationship of Wells and Coppersmith (1984). The length of the vertical boxes corresponds to the dating of the events within 2σ uncertainty (Civico et al, 2021;Dikbas et al, 2018;Dogan, 2010;Gastineau et al, 2021;Ikeda, Suzuki et al, 1991;Klinger et al, 2003;Özalp et al, 2013;Rockwell et al, 2009; see Table S2 in Supporting Information for detailed data).…”

“…South of the lake, Erginal et al (2021) reported a 50 cm high coseismic scarplet in beachrock deposits that they attributed to an 8 th century rupture. On the same segment, Civico et al (2021) opened two trenches and evidenced one mid-19 th century event, one penultimate event after the late 7 th century and two older events before the mid-5 th century CE. West of Iznik Lake, Ikeda et al (1989) proposed a mid-19 th century event from archaeological remains found in a natural fault exposure.…”