Case_Study

Case Study 2 :

Geological evidence for the last two faulting events on the southern branch of the North Anatolian Fault System in the Mudurnu Valley, western Turkey

East of Adapazari plain NAFS follows the Mudurnu Valley, and consists of a single strand, which it is tentatively called Mudurnu Fault (Figure 1)

The westward extension of the Mudurnu Fault goes into the Adapazari plain and is traceable for about 15 km along the boundary between the mountainous region of pre-Tertiary rocks on the south and dissected hills underlain by Pliocene to Quaternary nonmarine sediments on the north.

Figure 1. Active faults in the western part of the North Anatolian Fault System. Active faults are shown by solid lines
(hatched on downthrown ; with small solid circles where surface ruptures associated with earthquakes appeared).
ISFZ= Izmit-Sapanca fault zone; IMF= Iznik-Mekece fault; MF= Mudurnu fault; The Izmit-Sapanca fault zone is shown
by oblique hatches, where many folds and faults of short length develop in Pliocene-Quaternary basin fills. (From Ikeda et al, 1991)

The surface ruptures associated with the 1967 Mudurnu Valley earthquake, occurred continously for 55 km from 31.20 E to 30.60 E (Figure 2), but further west to Sapanca Lake (Ambraseys and Zatopek, 1969).

Maximum strike separation was 1.9 m right-lateral; the maximum vertical separation was 1.2 m.

Figure 2. Surface rupture zone in the lower Mudurnu valley associated with the Mudurnu Valley earthquake of July 22, 1967, The surface rupture zone is shown by thick solid lines.
(From Ikeda et al, 1991)

The surface rupture associated with the 1967 and 1957 earthquakes follow in places tectonic scarps and nothces that cut geomorphic surfaces probably of Holocene and late Pleistocene ages (Figure 3)

This suggest that, in recent geologic times, the Mudurnu fault has moved with the same sense and has used the same trace as the surface rupture associated with the 1967 and 1957 earthquakes.

Figure 3. A sketch map of the trench site, showing detailed geometry of the
surface rupture associated with the 1967 earthquake. Note that the 1967 rupture
follow exactly the same trace (within a range of about several m) as an old fault
that is indicated by the distribution of Neogene siltstone and Pre-Tertiary rocks in
the west of this figure. (From Ikeda et al, 1991)

The trench was excavated on a Holocene fluvial terrace formed by the Mudurnu River (Figure 4)

Relative heights of the terrace are as low as 2 m above the present flood plain of the Mudurnu River.

Quite small ephemeral streams flowing flowing eastward from the hills have deposited fine alluvium onto the terrace

Figure 4. Trench site at Cakaloglu in the Mudurnu Valley, looking to the northwest. Arrows indicate the surface
rupture zone associated with the Mudurnu Valley earthquake of 1967. The trench is near the left arrow.
(From Ikeda et al, 1991)

Figure 5. Sketch of trench exposures. The original sketch was made on a scale 1:10. Cobbles are approximately to scale. Horizontal scale ;
distance (x) from the southern end of the trench; vertical scale ; depth (d) (no vertical exaggeration ). Unit numbers are indicated,
with letters for subdivisions. (From Ikeda et al, 1991)

Unit 1 : is the upper most layer, composed of dark brown, slightly humic, loamy soil.

Unit 2 : is composed mainly of yellowish brown silt intercalated with a few distinct sand layers.

Unit 3: consists of thin alternating layers of clay, silt, sand and granule gravel. Unit 3 thins northward, indicating deposition against a gentle, south-facing slope that is interpreted as an existing fault scarp

Unit 4: consists of subrounded cobble gravel deposited as channel bottom sediments.

In summary, lithologic characteristics of sediments in the trench indicate the changes in sedimentary environments:

Stage 1 : Trench site was on a flood plain of the Mudurnu River, where the cobble gravel (Unit 4) was deposited as channel bottom deposits.

Stage 2 : The main course of the stream was fixed away from the trench site, probably because part of the flood plain in the previous stage was slightly emerged to form the low terrace (Figure 3)

Stage 3 : Since that time, the terrace has been subjected to deposition of overbank silt during large floods and occasional deposition of proximal materials from the hills on the west. Deposits of units 1 and 2 are the sediments of this stage.

When a fault scarp forms across the basin it forms a warping of the surface and sediments (Figure 6) (b) . The scarp interrupts the drainage and the post-earthquake sediments onlap the scarp showing a clear discordancy with the pre-events deposits in the fault zone (c). This discordancy (indicated with a star) corresponds to an event horizon. A successive earthquake produces a new fault scarp and the process starts again (d) (from Pantosti et al., 1993).

Figure 6. Piano di Pecore basin faulting and filling process. Figure 6a. Deformed sediments on the western exposure pf the Mudurnu trench. Unit numbers are assigned. Arrows indicate a minor fault.

(From Pantosti et al, 1993) (From Ikeda et al, 1991)

Timing of events

Sample E09-01 was derived from a horizon just below the boundary between units 3 and 4.

The 14C date of this sample is 250±50 yr B.P., and is calibrated dendrochronologically. The calibrated age of sample E09-01 is most likely to be 1650±20 A.D. It is probable that the penultimate event occurred shortly after 1650 (±20) A.D.

Concluding Remarks

Analyses of the stratigraphy and structures of sedimentary sequence in the trench have revealed the last two faulting events (including the Mudurnu Valley earthquake of 1967) that occurred on the southern portion of the NAFS.

Ambraseys and Finkel (1988), on the basis of an extensive survey of historical documents, have revealed that the Anatolian earthquake of August 17, 1668 caused severe damage within a narrow band about 100 km wide and 600 km long, running along the NAFS from Bolu in the west to Erzincan in the east.

The penultimate even that was identified from the trench investigation is correlated with the Anatolian earthquake of 1668.

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