Case_Study

The NAFS represents the boundary between the westward-moving Anatolian plate and the relatively stable Eurasion plate to the north. The North Anatolian Fault Sustem is an approximately 1500-km- long and 10 to 100-km- wide dextral shear zone trending NW, and then E-W and SW between Karliova in the east and northern Aegean Sea in the west (Figure 1)

Figure 1. Simplified map showing the study area and major tectonic structures in Turkey and adjacent regions. AKT: Aksu Thrust, CAFS:
Central Anatolian Fault Sysyem, DSFS: Dead Sea Fault System, EAFS: East Anatolian Fault System, ESCV: Erciyes andesitic-basaltic
stratovolcano complex, GYFS: Goksu-Yakapinar Fault System, IAESZ: Izmir-Ankara-Erzincan Suture Zone, KMATZ: Kyrenia-Misis-Andrın
Fold-Thrust Fault Zone, KTJ: Karliova Triple Junction, MTJ: Maras Triple Junction, LSFZ: Lake Salt Fault Zone, NAT: North Aegean Through,
NAFS: North Anatolian Fault System, NS: Northern Strand of NAFS, SF: Salanda Fault, SS: Southern Strand of NAFS, 1. Erciyes Pull-Apart Basin,
2. Erzincan Pull-Apart Basin, 3. Malatya Pull-Apart Basin. Large black arrows indicate the direction of plate motion and Large white arrow indicates
the escaping direction of Anatolian platelet (Kocyigit 1996)

Starting from the Karliova triple junction, the NAFS expressed as a fairly simple single trace east of Bolu. To the west of Dokurcun, the NAFS splits into two major branches. The southern branch traverses the Lake Abant and Mudurnu Valley areas. The Duzce Fault is the northern branch of the NAFS and is the source of the 12 November 1999 Duzce earthquake (M=7.2)

Figure 2. Map of northwestern Turkey showing the study area and ruptures of the 17 August 1999 and 12 November 1999 Duzce earthquakes. ( from C.Hitchcock et al, 2003)

The 12 November 1999 Duzce earthquake (M=7.2) produced a 40- km long surface rupture between Golyaka in the west and the Bolu mountains in the east (Figure 3). The rupture was oriented approximately east-west with average right-lateral offset of 3 m and vertical displacement of up to 3.5 m.

Maximum right-lateral offset is 5±0.3 m near the village of Guven. Surface rupture occurred within a narrow zone of deformation typically between 2 and 5 m wide.

In order to obtain detailed information on the style and frequency of surface-faulting eartquakes, an exploratory paleoseismic trenches were excavated on the Duzce Fault.

The trenches were excavated 1 to 2 m deep and 5 to 20 m long, across the fault traces.

In conjuction with the trench investigations at these sites, surface rupture related features at each site have been mapped prior to trenching.

Age estimates of the surfical deposits exposed within the trenches are based on stratigraphic relationships and radiocarbon analyses on selected charcoal fragments collected from the deposits

Located in an open field, approximately 500 m east of Trench 1.

The site is chracterized by an 8.5-m-wide zone of en-echelon south facing scarps and open fissures produced by the 1999 Duzce earthquake.

An older, broad, south-facing scarp also is present at the site indicating pre-1999 fault activity.

The trench site was selected because of the presence of the older south-facing scarp, well-dfined fissure fills along the trace, and of the adjacent offset fence.

Within Trench T-2, interbedded alluvial deposits, including sand and gravel flodd deposits and silty overbak deposits from the nearby Asarsu River, are warped into a broad south-facing scarp and are locally faulted (Figures 4&5)

Figure 4. Duzce Fault, Trench T-2, east wall ( from C.Hitchcock et al, 2003)

Total vertical structural relief on the basal faulted gravel across the trench is about 1.5 m, likely reflecting a buried fault scarp including vertical offset from the 1999 earthquake and at least one prior earthquake.

Evidence for one or more prior earthquakes also includes the presence of buried fissure fills.

Trench-T2 provides direct stratigraphic evidence for at least one and probably two pre-1999 earhquakes

Figure 5. Duzce Fault, Trench T-2, west wall ( from C.Hitchcock et al, 2003)

Charcoal sample DU-T2-9 was collected from an unfaulted deposit overlying the faulted fissure fill and yielded a calibrated age of 1412 ybp.

Thus, the stratigraphic relatioships and age results suggest at least one and probably two pre-1999 earthquakes in the past 1821 years.

It can be inferred that the penultimate occurred some time between 1400 and 1800 years ago.

Location is on the Duzce Fault, in a small orchard at a bend in the old road to Bolu, about 1 km east of Trench 2.

At this location, a prominent east-west trending, south-facing, pre-existing 2-m-high scarp is preserved near the bend in a small stream.

Rupture associated with the 1999 earthquake is located near the middle of the scarp and consists of a single vertical scarp with approximately 0.5-m down to the sout vertical offset. Trench T-3 is an approximately 6-m long, 1.5-m-deep trench across the pre-existing scarp and the 1999 Duzce Fault rupture (Figure 6).

The trench site was selected because of the presence of a pre-existing fault scarp, suggesting that the previous earthquakes had ruptured in the same location.

Figure 6. Duzce Fault, Trench T-3 (from C. Hitchcock et al, 2003)

Within Trench T-3, several distinct packages of colluvium are in fault contact with bedrock and are locally internally faulted.

The 1999 fault rupture is expressed in the trench as a near-vertical fault that extends to the ground surface with an associated fissure fill.

The stratigraphic relationships and age dates at Trench-3 document four possibly five surface faulting events in the past approximately 2100 years.

The youngest event occurred in 1999, the oldest event occurred about 2100 years ago to form the unit 6 colluvial wedge. The penultimate (second oldest) event occurred about 300 years ago. The third and fourth(?) events occurred some time between about 2100 and 300 years ago.

Based on results from Trench-T2, at least one may be both of these events occurred between about 1800 and 1400 years ago.

It is concluded that from these relationships, the recurrence intervals between surface faulting events is at least 300 years, and may range from 300 to a maximum of 800 years.

Using the amount of vertical separation as a proxy for the style and amount of surface displacament, it appears that the penultimate and pre-penultimate events at the site were in similar in size to the 1999 Duzce earthquake.