4. Geotechnical and Geological Aspects
4.1 Geotechnical Aspects
A preliminary reconnaissance of the geotechnical conditions of Duzce, Kaynasli, and Bolu urban areas was done during the Turkey Expedition in December 1999. Widespread building damage was observed in
Duzce and Kaynasli. Minor damage was noted in downtown Bolu, but heavy in the areas of recent expansion of the city towards the north and east.
4.1.2 Observations in Duzce
In Duzce, heavy damage occurred in old and new buildings alike. There were no areas of concentrated damage, but damage was rather uniformly widespread. All buildings inspected in Duzce had shallow foundations, about 3 m below the ground surface, which allowed for the construction of a full basement.
The subsurface soil conditions were investigated in two locations, both in downtown. The first location was an open excavation for a new building in Haydar Gördebil Street about 100 m north of the river. Figure 18 shows the excavation. Observe in the figure the vertical cut in the alluvial, and the sand lenses which are noticeable by their gray color and the small failure at the bottom-center of the excavation. A trench was excavated at the corner between Mehmet Akif St. and Namik Kemal St., up to a depth of 4.1 m. The soil conditions were similar in both locations.
The soil deposits in Düzce can be classified as alluvial deposits of stiff brown clay with lenses of dense sand and silty sand in the upper 4 to 5 m. The presence of the water table between 2 to 3 m below the ground surface explains the generalized practice in the area of buildings with a basement. The depth of the water table is determined by the river, which at the time of the investigation, had a water level similar to the subsurface observations.
Figure 18. Limited soil exploration in downtown Duzce
4.1.3 Observations in Bolu
In Bolu, old buildings performed much better than new buildings. This was clearly related to the spatial location of the structures. In downtown Bolu, a lithified Pleistocene conglomerate outcrops in many locations, with minimum soil cover and very small weathering depth. This outcrop was followed along
Sehit Kanni St., Cukurçesme St., Parkalan St., Atatürk Blvd., and Izzet Baysal St., which delimit the old city. Inspection of the buildings in Bolu focused on downtown (the old part of the city), and the expansion
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area of the city towards the East and North. It was evident from the inspection that as the buildings were located further from the center of the city, they suffered heavier damage. This was particularly evident in the northern part of Bolu, north of the E-80 highway, where the damage was extensive and involved new buildings, some of them recently finished. All buildings across the city have shallow foundations between 1 to 2 m below the ground surface. In downtown Bolu, the buildings have the foundations on rock, but outside the old city, the foundations are in soil deposits of alluvial origin.
A trench was dug North of E-80, close to the intersection between Çamaklar and Ulubatli St. Figure 19 shows the trench. The soils are of alluvial origin. A clay layer is found down to a depth of 3.5 m, on top of a gravel layer. The small failures at the bottom of the trench occur in the gravel layer, and the water at the bottom of the trench determines the location of the water table.
No signs of liquefaction were detected in Düzce or in Bolu. This is consistent with the stiff clay found below the ground surface. However, the depth of exploration was limited to about 4 m, and no conclusions can be drawn about conditions below this depth.
Figure 19. Excavation in Bolu
It is clear that different soil conditions in Düzce, and Bolu, have produced different effects on buildings.
The data gathered from this expedition is highly relevant, but limited to relatively few observations.
Furthermore, no deep investigation was possible at that time. Additional data needs to be collected for the following reasons:
(1) Damage to the buildings should be evaluated in the light of the soil conditions.
(2) A detailed analysis of the damage caused by the 12 November earthquake requires detailed field data.
(3) Data from such an event can be used for future assessments or extrapolated to other locations at risk.
4.2 Geological Data
In many ways the recent earthquakes in Turkey have the potential of advancing markedly the state of knowledge of damage to man-made structures caused by ground rupturing along an earthquake fault zone.
Indeed, prior to these earthquakes, there have been only a few earthquake ruptures that have been recognized to be broad zones, rather than narrow “fault lines” (Lawson, 1908; Tchalenko and Ambraseys,
1970; Philip and Meghraoui, 1983; Johnson et al., 1994, 1997). Furthermore, damage to engineering structures during earthquakes generally is attributed to ground shaking or foundation failure. Only at
Landers, California (Lazarte et al., 1992) has it been documented that buildings can be seriously damaged
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or destroyed within the broad zones of ground rupture that accompany strike-slip faulting as well as by ground shaking. In all cases the faults have been strike-slip, so the Turkish earthquakes provide an excellent opportunity to build on what we know about details of zones of deformation and fracturing along strike-slip faults. This information should allow the identification of different kinds of structural damage within the rupture zones.
It is now more widely recognized in the geological community that faults that activate during earthquakes produce not fault lines, but rather broad deformation zones, ranging from a few to a thousand meters in width and containing one or several faults and generally high ground strains. It is, however, quite unclear how much earthquake damage is caused by ground shaking, by soil failure or subsidence, and by faulting itself. Damage due to shaking and soil liquefaction is well known, but structural damage due to permanent ground deformation along faults has hardly been recognized. Instead, such damage is attributed to shaking or soil failure. What is needed is a collaborative, multi-disciplinary analysis of causes of damage to structures in the vicinity of the deformation zones along fault ruptures in Turkey. The deformation zones would be determined by detailed mapping by geologists. At the same time, teams of earthquake engineers, geotechnical engineers and geologists would relate collaboratively the building damage within deformation zones to ground shaking, soil failure or large ground deformations.
The fault that ruptured during this earthquake is part of the North Anatolian fault system, one of the largest active strike-slip faults in the world. In strike-slip fault systems, deformation occurs where one crustal block moves laterally with respect to an adjacent block. There generally is also oblique displacement on strike-slip faults (Wilcox et al., 1973; Sylvester, 1988), though, as appears to be the case with the segment of the North Anatolian fault that ruptured during the 12 November 1999 earthquake. During the field work in December 1999, the Purdue Expedition was able to trace the rupture zone for about 25 km.
4.2.1 Characteristics of the fault trace
One of the most interesting observations during the field work was that there were distinct changes in deformational style and width of the rupture zone along strike.
Figure 20. Characteristics of the fault trace- vertical offset
Oblique displacements produced a component of extension along the western segment, predominantly strike-slip displacement along the central segment, and a component of thrusting along the eastern segment.
A general question relating to these observations is whether there is a correlation between faulting style along the rupture zone and the width of the zone of building damage caused by ground rupture.
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Figure 21. Rupture zone near Kaynasli
Some of the kinds of ground rupturing in Turkey are shown in Figures 20 and 21. Near the western end of the segment, close to the village of Hamamyani, the exposed fault splays into at least five fracture segments forming a fault zone about 1 km wide. Each of the segments has a normal component of displacement down to the northeast. In Figure 20 the disrupted cement tiles and fences document the vertical offset.
There was about 1. 5 m of right-lateral offset and about 0.3 m of vertical and horizontal offset across the fracture.
There is a need to map sample areas such as this, in order to determine how much ground deformation there is between large fractures such as that shown in Figure 20. The preliminary field observations suggest that buildings in this area have experienced relatively minor damage. This may indicate that the fractures are relatively widely spaced, which would reduce the likelihood of a fracture occurring under a building.
Along the central part of the fault zone the belt of rupture was only a few meters wide and appeared to be a single rupture. Estimates of building damage along this central segment are difficult due to a lack of buildings in the proximity of the rupture zone.
Along the eastern segment of the fault rupture, in the vicinity of Kaynasli, the belt of rupture is extremely well exposed and is located within a highly populated area. In this area, the rupture zone appears to consist of several segments over a belt ~ 0.5 km wide. Offset fences along this segment indicate a minimum of 1.5 m of strike-slip displacement. In addition to strike-slip displacement, several of these segments may have undergone shortening with top to the southeast thrusting based on our preliminary observations and discussions with local residents. Building damage in this area is extensive, with much of the damage concentrated near the rupture zone. For example, Figure 21 shows an open fracture in the foreground. The fracture accommodated about 1.5 m of horizontal offset, with the block on the right moving about 1.5 m toward the observer relative to the block on the left. The opening of the fracture suggests that the rightlateral offset was oblique to the trend of the open fracture. The compressive, thrust-like features in the background suggest that, along that part of the fracture, the soil on the two sides have been pushed together.
Again, this feature suggests that the direction of slip is different than the traces of the fracture in the foreground and the fracture underlying the thrusts in the background.
An example of damage due to faulting is shown in the apartment building under obvious distress in the background right of Figure 21.
In order to determine the kinds of rupturing in the three areas of the fault that ruptured during the 12
November 1999 earthquake it is necessary to map the area of interest
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4.2.2 Damage distribution in relation to fault rupture
One of the most intriguing observations made by the reconnaissance team was that building damage appeared to be highly concentrated on the north side of the rupture zone. This phenomenon is best observed in the village of Kaynasli where construction is extensive on both sides of the well exposed fault rupture. In this area, buildings on the south side of the fault show relatively minor damage, whereas buildings on the north side of the fault show extensive damage. Similar relations were documented in several areas along the fault rupture.
The occurrence of the main break on one side or the other of a fault rupture zone has been documented in normal faults in sandstone (Aydin and Johnson, 1978) as well as in surface ruptures along strike-slip zones active during the 1906, San Francisco, California, earthquake (Lawson, 1908) and the 1992 Landers,
California, earthquake (Johnson et al., 1994, 1997). At Landers, the main rupture was along the west side of the rupture belt in some places and along the east side in others. The main rupture, though, always occurred at the edge of the rupture belt. Thus, the occurrence of the main rupture in Turkey on one side of the belt or the other was expected. It is important to document these relations in Turkey with detailed mapping. The observation, however, is quite significant to earthquake ruptures in general. It is clear that one should not focus merely on damage due to the main rupture; one should identify the entire zone of rupture. Much of the damage may be in buildings within the belt.
The 17 August and 12 November earthquakes provide an excellent opportunity to study zones of deformation and fracturing along strike-slip faults. The identification of the deformation zones and the data on internal deformation within the zones need to be collected immediately because they have the potential of changing hazard maps, building codes, legislation and the general practice of earthquake engineering.
The results of gathering data on the damage to buildings along main ruptures are crucial to future questions of siting of critical structures near faults. For example, if the rupture zone is 200 m wide and the main rupture was on the east side, as was common at Landers (op cit.), a building sited within 200 m west of the main rupture zone will be severely damaged, whereas a similar building 2 m east of the rupture zone will remain undamaged by ground fracturing. However, in such a study, damage due to several causes, such as ground shaking, foundation failure and ground rupture must be carefully distinguished.
4.2.3 Relationship between geologic setting and damage
As discussed in more detail in Section 4.1, the stratigraphy/soils vary greatly between the towns of Duzce, and Bolu. Many of the towns along the North Anatolian fault system are located within sedimentary basins. These sedimentary basins were attractive locations for settlement and ideal for farming because they contained fertile soils, major river systems and lakes, and low topography. In general, sedimentary basins form along strike-slip faults as a result of localized crustal extension. These types of basins are often referred to as "pull-apart" basins because they form between parallel overlapping strike-slip fault segments.
During the early stages of formation, pull-apart basins are often filled by mud-rich lake deposits. As the basin drainage network develops, sand-rich fluvial (river) deposits become more important during the later stages of basin formation. Pull-apart basins along the eastern part of the North Anatolian fault system (e.g.,
Erzincan, Susehri, Niksar and Lake Hazar basins) have been studied in some detail (Hempton and Dunne
1984), whereas basins along the western part of the fault system (our study area) have received little attention.
The initial observations from this reconnaissance mission suggest that the specific location of towns may have been one factor that influenced building damage. The heavily damaged town of Duzce, for example, appears to be built in a relatively young pull-apart basin that was filled by mud-rich lacustrine deposits.
The relatively undamaged parts of the village of Bolu, in contrast, are built on a bedrock high within a small pull-apart basin. The more heavily damaged areas of Bolu appear to be built on mud-rich lake deposits.
What is desperately needed in the areas affected by the 12 November 1999 earthquake are detailed geological maps and cross sections that will help define the locations of sedimentary basins and the deposits that filled these basins
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