Ellen M. Rathje1, Ismail Karatas2, Stephen G. Wright3, and Jeff Bachhuber4

         During the 1999 Kocaeli earthquake (Mw=7.4) in Turkey, coastal failures and sea

inundation were observed and were particularly concentrated along the margins of Izmit Bay and

Lake Sapanca, in pull-part basins created by stepovers in the fault rupture. Geotechnical site

characterization, geologic mapping, liquefaction evaluation, and slope stability analysis were

carried out to identify the principal contributing factors of the coastal failures. Results from this

study indicate that both liquefaction and tectonic subsidence contributed to the failures and sea

inundation within the pull-apart basins. Most of the liquefaction sites were situated at the

prograding nose of active delta fans, where the presence of steep slopes coupled with the loose

sediments found within young active delta fan deposits resulted in liquefaction-induced slope

failures and sea inundation.     Liquefaction in other coastal deposits outside of the actively

prograding delta fans caused limited lateral spreading and only minor sea inundation. Outside of

the delta fans, where soils were not liquefiable, tectonic subsidence associated with normal

faulting was the cause of the observed sea inundation. Generally, tectonic subsidence caused the

most severe sea inundation.       Based on these observations, the identification of regions

susceptible to both tectonic subsidence and liquefaction are important when evaluating seismic


KEYWORDS: Subsidence, Ground Failure, Liquefaction, Faulting, Coastal Failures

    Assistant Professor, Department of Civil Engineering, University of Texas, Austin, TX 78712.
    E-mail:, FAX: 1-512-471-6548
    Staff Engineer, GeoSyntec Consultants, Huntington Beach, CA.
    Brunswick-Abernathy Professor, Department of Civil Engineering, University of Texas,
    Austin, TX 78712
    Principal Engineering Geologist, William Lettis and Assoc., Walnut Creek, CA 94570
       The Kocaeli earthquake (Mw = 7.4) occurred on 17 August 1999 in the northwestern part

of Turkey along the North Anatolian fault. The bilateral strike-slip fault rupture involved

displacement on four distinct segments of the North Anatolian fault (Figure 1). These strike-slip

fault segments are separated by right-releasing stepovers, which accommodated significant

normal-slip displacement (up to 2.4 m) during the earthquake [1]. The stepovers in the fault

rupture coincide with distinct pull-apart basins that are filled with thick Quaternary deposits.

Severe coastal subsidence, ground failure, and sea inundation were observed within the pull-

apart basins located in coastal areas. This study focuses on identifying the main geotechnical

and geologic factors that contributed to the coastal failures and sea inundation in the coastal pull-

apart basins during the Kocaeli earthquake.

       The main coastal and near-shore submarine stepovers involved in the Kocaeli earthquake

fault rupture were the Karamursel, Golcuk, and Sapanca stepovers (Figure 1). Each of these

stepovers is associated with a distinct, sediment filled pull-apart basin and a topographically low-

lying area. Widespread subsidence and sea inundation were observed within each basin, causing

some of the most dramatic damage and destruction of coastal areas and facilities from the

earthquake.   Geotechnical site investigation, geologic mapping, liquefaction evaluation, and

slope stability analysis were carried out to investigate the localized failures and subsidence

observed at several sites within the coastal pull-apart basins. These sites are Degirmendere

within the Karamursel pull-apart basin; Golcuk, Yenikoy, and Seymen within the Golcuk pull-

apart basin; and Esme within the Sapanca pull-apart basin (Figure 1). The collected data were

evaluated to study the interaction and relative contribution of different mechanisms to the

observed coastal failures.


       The 1999 Kocaeli earthquake occurred on the western portion of the North Anatolian

fault, which is a major strike-slip fault extending over 1,500 km across northern and western

Turkey. This fault zone has experienced numerous past earthquakes (e.g., 1939 Erzincan, 1942

Erbaa, 1944 Bolu-Gerede; [2]), and the Kocaeli earthquake occurred on a portion of the fault that

was formerly a seismic gap. The earthquake ruptured a 126 km section of the fault which is

comprised of four distinct segments separated by three extensional pull-apart basins: the

Karamursel, Golcuk, and Sapanca basins [1]. Pull-apart basins formed as a result of extension

within the basins that is accommodated by vertical displacement along basin-bounding normal

faults (Figure 2) and causes structural downdropping and warping. In coastal lowland areas and

delta plains, this downdropping allows for sea inundation and forms topographic lows that are

loci for sedimentation.     The pull-apart stepovers in Turkey have been infilled with thick

Quaternary deposits over time, creating deep alluvial basins. The large dimension and thick

sediment infilling of these basins indicate that they are long-lived features in the displacement

history of the fault [1].

        Numerous prograding delta fans have formed along the coastal margins of the pull-apart

basins along the North Anatolian fault. These deltas are constructed at the discharge points of

creeks and rivers that transport sediment from the adjacent elevated areas and deposit their

sediment load into the topographic depocenter and water bodies at the basin margin. The

depositional processes within delta fans produce loose, saturated sediments that often contain

fine sand and silty sand layers that are susceptible to liquefaction. Additionally, the submerged

prograding delta noses typically are relatively steep (greater than 10 to 15 degrees) and quasi-

stable. The combination of loose sediments and steep unstable delta nose slopes makes delta fan

deposits prone to liquefaction-induced ground failure (i.e., lateral spreading, slope failures).

        This study included areas of ground failure, subsidence, and sea inundation in each of the

three coastal pull-apart basins affected by the Kocaeli earthquake (Figure 1). The Karamursel

basin is a submarine basin within Izmit Bay that was identified by evaluation of bathymetric and

geophysical data. Displacement on the Golcuk fault, entering the east side of the basin, was

rapidly attenuated within the basin, and only negligible to no fault displacement was observed on

the Yalova fault that exits the west side of the basin and crosses the Hersek Peninsula [3]. The

coastal geology along the bay margin here consists mainly of Plio-Pleistocene sedimentary

bedrock overlain in some areas by Pleistocene marine terrace deposits and Holocene sediments.

The Holocene deposits consist of (1) alluvial fans and stream channel deposits that are incised

within older sediments and bedrock, and (2) actively prograding delta fans and modern beach

deposits along the narrow coastline. The active delta fans occur at the mouth of incised creeks

that originate in the hills immediately to the south of the coastline. Because of the close

proximity of the sediment source in the steep hills to the south, the delta fan sediments in this

area are relatively coarse (sand, fine gravel), poorly sorted, and laterally discontinuous.

       The Golcuk pull-apart basin is located east of the Karamursel basin and encompasses a

coastal area that extends east from the city of Golcuk (Figure 1). It is approximately 2 km wide

and 6 km long, and is located within a right-lateral step between the Golcuk and Sapanca fault

segments. The shoreline geometry is controlled both by local faulting and alluvial deposition.

Portions of the coastline are coincident with, and subparallel to, active splays of the North

Anatolian fault that ruptured during the Kocaeli earthquake. In other areas, the shoreline is

formed by coalescing alluvial deltas that prograde into Izmit Bay from alluvial plains, fans, and

bedrock hills to the south. Repeated tectonic subsidence within the Golcuk pull-apart basin has

formed a localized depocenter for delta progradation between the towns of Golcuk and Seymen.

The basin includes a broad, late Pleistocene to Holocene alluvial plain that is bordered by the

Golcuk normal fault at the west and south, and older Pleistocene fans that extend out from the

range south of Izmit Bay. Stream channels crossing the plain have shifted repeatedly over time,

forming broad delta fans at the discharge points with laterally interfingering packages of

sediment. In general, delta deposits become finer grained with increasing distance from the

mountain range front. The youngest, most actively prograding parts of the deltas occur at the

mouth of major streams and are defined by triangular-shaped cones of sediment that project as

much as 100 m seaward from the shoreline. These active delta lobes can be differentiated on

aerial photographs and consist of unconsolidated, loose sediments that are considerably less

consolidated than adjoining older parts of the delta. The creeks feeding the Golcuk basin delta

fans cross the wide alluvial plain and originate in the bedrock hills and older fans deposits further

inland than the streams feeding the delta fans that border the Karamursel basin. As a result, the

Holocene delta sediments within the Golcuk basin are significantly more fine grained (i.e., fine

sand, silt, and clay) than those bordering the Karamursel basin.

       The Sapanca pull-apart basin is located within a right-lateral step between the Sapanca

and Sakarya fault segments (Figure 1). The 60-m deep topographic depression caused formed by

the Sapanca pull-apart basin is infilled by Lake Sapanca and serves as a local, internally-drained

depocenter for sedimentation for streams that issue from the bedrock hills surrounding the basin.

The margins of much of the lake consist of coalescing late Pleistocene to Holocene alluvial fans

and terraces, and late Holocene beach deposits and prograding delta fans at the mouths of creeks.

Pliocene and Pleistocene bedrock and older alluvial fans occur inland from the current lake

margin and were deposited in a somewhat larger ancient tectonic basin that appears to have

narrowed along the active fault traces. The creeks feeding the delta fans bordering Lake Sapanca

originate in incised stream vallies close to the lake shore, and the fan sediments consist of

laterally-discontinouos, relatively coarse sand and gravel with some interfingering alluvial silt

and clay lenses and fine-grained lake deposits

       Bordering the Karamursel basin, the most severe sea inundation took place along a

Holocene delta fan at the mouth of Degirmen creek in the city of Degirmendere. The delta fan at

Degirmendere is located at the interface between a large Plio-Pleistocene alluvial fan complex to

the south, and a series of Pleistocene marine terraces that form a flight of relatively flat

topographic surfaces subparallel to the shoreline. The Holocene delta is incised into these older

deposits, and projects north into Izmit Bay. The shoreline delta deposits are comprised of

reworked Plio-Pleistocene fan and marine terrace sediments, and are relatively coarse and

laterally discontinuous. During the Kocaeli earthquake a large section of coastline, which

coincides with the Holocene delta fan, failed into Izmit Bay, extending approximately 300 m

along the coast and 75 m inland (Figure 3). A curvilinear, well-defined 1 to 2-m high head scarp

defined the landward extent of the failure zone and formed the post-earthquake shoreline. The

headscarp was located approximately 800 m south of the offshore projection of the Golcuk fault,

and did not appear to be associated with primary surface fault rupture. The headscarp shape and

location indicated that the coastal retreat was caused by a distinct slope failure rather than

tectonic subsidence or lateral spreading. Minor cracking extended an additional 100 m inland of

the coastal scarp to the approximate contact between late Holocene delta deposits and older

alluvial deposits. The cumulative amount of extension across the cracks in this zone suggest that

up to about 0.45 m of lateral seaward movement occurred behind the slide headscarp. No ejected

sand boils or other evidence of near-surface liquefaction was observed in the failure area;

however, some buildings inland of the failure experienced slight to moderate levels of

settlement. The pattern of cracking and lack of significant surficial differential movements

suggests that the slope failure was a deep slump or slide-type of failure rather than a classic

shallow liquefaction-induced lateral spread.

       The pre-earthquake geometry at Degirmendere was constructed using a bathymetry map

[4], while the post-earthquake geometry was developed from bathymetric data provided by

Degirmendere city officials. Pre- and post-earthquake cross sections taken perpendicular to the

post-earthquake shoreline in the center of the failure zone are shown in Figure 4. The pre-

earthquake geometry shows that the offshore delta fan had a relatively steep slope, with a

maximum slope angle of about 18 degrees at a location about 100 m offshore. At a distance of

about 175 m from the coastline, the seafloor was almost flat. The post-earthquake geometry

shows a significant change in the shoreline and seafloor, with a steep (30 degree to vertical)

shoreline headscarp, and locally-steepened nearshore slope.       The seafloor slope inclination

offshore of the headscarp zone was reduced to about 5 degrees after the failure. Comparisons of

the pre- and post-earthquake profiles shows that the maximum thickness of sediment lost by the

landslide approached 25 m, and that the failure appeared to be relatively deep-seated.

       The geotechnical site investigation at Degirmendere consisted of 3 SPT borings and 2

CPT soundings (Figure 3) performed by the local firm ZETAS. These borings and soundings

covered a distance of about 50 m along the shoreline, and extended 10 to 15 m from the slide

headscarp and new shoreline. Rotary wash borings were performed with SPT measurements

taken every 1.5 m. The SPT were performed in accordance with ASTM D1586 [5], using a

safety hammer, rope and cathead, and AWJ drill rods. SPT energy measurements were not

performed. The CPT soundings were performed with standard CPT equipment manufactured by

A.P. van den Berg. Two SPT borings and two CPT soundings at Degirmendere were located

very close to the slide headscarp/shoreline, in an attempt to sample material that is similar to that

which was involved in the slide. However, because the delta sediments are laterally variable,

and the slide zone occurred mainly offshore, the collected subsurface data may vary somewhat

from that within the body of the slide.

       In situ data from the two CPT and two SPT performed along the shoreline at the mouth of

the creek in Degirmendere are shown in Figure 5. The subsurface materials encountered in these

borings consist primarily of medium dense sand and fine gravel in the top 30 m, with some

layers of silty clay and silty sand. The sand layers generally display CPT qc values between 10

and 20 MPa, while the SPT blowcounts are between 10 and 30. Some silty sand and silty clay

layers show qc values as low as 2-3 MPa, but these weaker layers were not found at consistent

depths within the different borings and soundings performed along the shoreline (Figure 5). The

discontinuity of layering is most likely the result of the complex depositional processes at work

along the coastline of this delta fan. The CPT and SPT data show that the sediments exhibit an

increase in resistance at about 10 m. Shear wave velocity measurements made at the site using

the Spectral Analysis of Surface Waves method [6] indicate a similar trend, with the shear wave

velocity increasing from about 200 m/s in the top 9 m to about 270 m/s at depths from 9 to 14 m.

       The CPT and SPT data were used to assess the liquefaction susceptibility of the soils at

Degirmendere [7]. The procedures outlined by Youd et al. [8] were used to evaluate liquefaction

susceptibility in terms of the cyclic stress ratio induced by the earthquake (CSR) and the cyclic

resistance ratio (CRR). The earthquake-induced CSR was computed as:

                            CSR =  / vo = 0.65 (PGA/g) (vo / vo) rd                      (1)

where PGA is the peak ground acceleration,  is the average shear stress, g is the acceleration of

gravity, vo and vo are total and effective vertical stress, respectively, and rd is the stress

reduction coefficient. The cyclic resistance ratio (CRR) was computed using measured in situ

test parameters and appropriate liquefaction correlations, as described by [8]. For the CPT data,

the CRR was evaluated only for layers where the soil behavior type index (Ic, [9]) was less than

2.6. This value of Ic represents the boundary between predominantly granular materials (sand to

silty sand) and predominantly fine-grained materials (silts and clays). Although some soils with

Ic greater than 2.6 may liquefy [8], this possibility was not considered in this study.

       For the liquefaction evaluation at Degirmendere, a PGA value of 0.3 g was used.

Because no strong motion station was located in the vicinity of Degirmendere, this PGA value

was inferred from the Yarimca Petkin (YPT) strong motion record that was recorded

approximately 4 km north of Degirmendere on the opposite site of Izmit Bay [10]. The YPT

station is situated on deep alluvium and recorded a PGA of 0.27 g (geometric mean of the two

horizontal components). Based on this recording, the PGA at Degirmendere was taken as 0.3 g,

and the computed CSR ranged from 0.25 to 0.35. The two CPT and two SPT in Figure 5

consistently predict that the soil from about 5 to 10 m (qc ~ 8-12 MPa, Ic = 1.5, N1,60 ~ 10-20)

was liquefiable during the Kocaeli earthquake, with factors of safety (FS) between 0.5 and 0.9.

At depths below 10 m, the various CPT and SPT data do not consistently predict the same

liquefaction potential. DN-CPT2 indicates highly liquefiable soils from 10 to 25 m (FS < 0.6),

while adjacent DN-SPT1 indicates all soils below 15 m have a FS greater than 1.1. DN-CPT1,

located 25 m east of DN-CPT2 and DN-SPT1, shows no soils below 10 m as liquefiable (FS >

1.5). Finally, DN-SPT2, located 25 m east of DN-CPT1, indicates dense sand from 10 to 25 m,

but liquefiable silty sand between 25 and 30 m (N1,60 ~ 10).

           The in situ test data indicate liquefiable soils are present within the deposit at

Degirmendere. However, these liquefiable soils are generally restricted to a depth of about 10 m.

The geometry of the failure zone, scarp shape and height, and cross section suggests that the

Degirmendere failure was deeper than 10 m, and that the shallow liquefiable layers encountered

in the borings cannot explain the deep-seated failure at Degirmendere. Therefore, slope stability

analyses were performed to gain further insight into the failure mechanism at Degirmendere.

           Slope stability analyses using the program UTEXAS4 [11] were performed for both the

pre-earthquake and post-earthquake geometries at Degirmendere.            The subsurface materials

under pre-earthquake conditions were modeled as granular soils with an effective friction angle

of 35 degrees.       As expected for a slope consisting of granular materials under saturated

conditions, the critical slip surface indicated an infinite mode of failure along the steepest section

of the slope (offshore nose of the delta fan). The computed factor of safety was 1.75, indicating

that the slope was adequately stable under static conditions before the earthquake. Using drained

strengths, a minimum yield seismic coefficient (ky) of 0.1 was computed for the slope. This ky

corresponds to an infinite slip surface, which is not compatible with the observed deep-seated


           Slope stability analyses were performed next considering liquefied strengths for the

susceptible layers encountered in the borings and CPT soundings at depths of between 5 and 10

m, and between 25 and 30 m. A post-liquefaction residual strength (SR) of 40 kPa was assigned

to the shallow liquefiable layer based on an average N1,60 of 15 (FC~5%) and the

recommendations from Seed and Harder [12] and Baziar and Dobry [13]. The deeper liquefiable

layer was assigned an SR of 20 kPa based on N1,60 ~ 10 and FC ~ 25%. Analyses with residual

strengths assigned only to the shallow liquefiable layer predicted an infinite slope failure mode

because the shallow liquefiable layer coincides with the flatter part of the slope. Analyses with

residual strengths assigned to both the shallow and deep liquefiable layers predicted a deep-

seated failure with a factor of safety equal to 1.05. The yield seismic coefficient for this slip

surface (using drained strengths) is 0.19. Considering a PGA of 0.3 and reducing that value to

kmax = 0.2 to correct for averaging effects over the depth of the sliding mass [14] results in a

ky/kmax close to 1.0, suggesting that inertial effects alone, without soil strength reduction by

liquefaction, could not have triggered the failure at Degirmendere. The failure appears to have

initiated on a deep liquefied soil layer in the steeper part of the offshore delta nose, and may have

expanded laterally in a progressive mode. The zone of cracking inland of the failure scarp

suggests that this zone was quasi-stable and experienced some minor extensional movements in

response to formation of the slide headscarp and partial liquefaction of underlying soil layers.

       The Golcuk pull-apart basin is located on the south shore of Izmit Bay and encompasses

the coastal area that extends east from the city of Golcuk (Figure 6). The Golcuk basin is

bounded on the southwest by the Golcuk normal fault that accommodated between 0.5 and 2.4 m

of vertical displacement during the Kocaeli earthquake. The normal faulting produced global

subsidence of the basin, localized coastal subsidence, and sea inundation. The most dramatic sea

inundation from this earthquake occurred within the Golcuk basin, where approximately 0.5 km2

of the basin was inundated by the sea and another 0.75 km2 experienced substantial subsidence

but remained above sea level. The Golcuk normal fault follows a higher (3 to 6 m) paleoscarp in

Holocene and late Pleistocene deposits, suggesting that the fault has experienced at least several

previous episodes of normal displacement. Three sites of subsidence were investigated within

the Golcuk pull-apart basin: central Golcuk, Yenikoy, and Seymen (Figure 6).

Central Golcuk

       In central Golcuk, severe coastal subsidence occurred along the western margin of the

Golcuk pull-apart basin, near the Golcuk normal fault (Figure 6).           Here, shoreline retreat

extended inland over 300 m. Minor evidence of liquefaction was reported around the failed area

and local residents reported that settlements continued for weeks after the earthquake [15].

       The zone of intense sea inundation coincides with a Holocene delta fan that formerly

prograded as much as 100 m northward into Izmit Bay. The subsidence is concentrated in a

cone-shaped area at the nose of the delta fan, at the intersection between the normal fault and

shoreline. Approximately 1.5 m of vertical fault displacement occurred on the Golcuk normal

fault adjacent to this inundated area. Much of the subsidence here can be directly related to this

vertical downdropping. However, the magnitude and locally more-severe subsidence within the

Holocene delta fan suggests that other processes in additional to normal faulting contributed to

the inundation. Pre-earthquake aerial photographs of central Golcuk taken in 1974 indicate that

the subsidence and inundation zone coincides with an area that was an active delta in 1974.

Between 1974 and 1999, the creek that was the source of this delta was diverted into a channel,

and the active delta area was developed. Additionally, a bathymetry map of Izmit Bay [4]

indicates that the pre-earthquake slope offshore of the subsidence zone was steep (about 15 to 20

degrees), and formed by the rapidly accumulating delta sediments. Post-earthquake bathymetry

data suggest that the offshore area experienced some subsidence and that the seafloor slope was

flattened, although the data are not sufficient to differentiate the geometry of the offshore failure.

The concentrated, intense subsidence in this delta area and the flattened offshore delta nose

suggest that liquefaction-induced ground failure also contributed to the localized inland

subsidence here.

        Three CPT soundings were performed in the vicinity of central Golcuk. Two of these

soundings (GL-CPT1, GL-CPT2) were performed west of the subsidence zone, while the other

(GL-CPT3) was performed adjacent to the subsidence zone, immediately east of the creek. Due

to rocky fill being placed to reclaim the subsided area after the earthquake, soundings could not

be located immediately within the subsidence zone. No SPT borings were performed in this


        The CPT sounding adjacent to the subsidence zone (GL-CPT3), along with the

interpreted subsurface profile, is shown in Figure 7. The top 10 m consist of loose, alluvial silty

sand (qc ~ 4-8 MPa, Ic ~ 1.5-2.0) with some interfingering alluvial and marine clay seams, which

is in turn underlain by about 11 m of alluvial and marine clay. Dense sand is found below the

clay layer (qc ~ 30 MPa). The CPT data were used to assess the liquefaction susceptibility of the

soils in central Golcuk using the procedures outlined previously. The PGA at Golcuk was

estimated as 0.3 g, based on the YPT recording on the opposite side of Izmit Bay. The CPT data

indicate that the soil from about 2 to 10 m is highly liquefiable, with a factor of safety of about

0.5. This result further suggests that liquefaction played a role in the subsidence in central


       The measured values of CPT tip resistance in the top 10 m from the three CPT soundings

in central Golcuk are shown in Figure 8. GL-CPT2 was performed about 200 m west of the

subsidence zone and GL-CPT1 was performed about 500 m west of the subsidence zone, in an

area comprised of older geologic materials (Figure 6). All of the soundings indicate loose,

liquefiable sand layers in the top 10 m, with some interbedded layers of silty clay. The similarity

in the soundings is surprising, because liquefaction was not observed near GL-CPT1 or GL-

CPT2. However, soundings GL-CPT1 and GL-CPT2 indicate thicker and more frequent silty

clay layers near the surface, which may have masked any evidence of liquefaction. Additionally,

the offshore slope in this area was flatter than at the delta nose in the subsidence area, suggesting

that steep offshore slopes near GL-CPT3 may have exacerbated the effects of liquefaction and

resulted in more intense surface effects.

       Yenikoy is located in the Golcuk basin east of the city of Golcuk on the eastern margin of

a large alluvial plain and deltaic peninsula that extends northward into Izmit Bay. Yenikoy is

underlain by interfingering Holocene alluvial deposits (Figure 6) consisting of coalescing

individual delta fans. The streams crossing the alluvial plain and older delta fans are relatively

small and have sources within the distant hills south of the coastline. The sediment carried by

these streams is relatively fine-grained, and the soils underlying the Yenikoy site have significant

percentages of silt and clay.

       A large zone of coastal subsidence and sea inundation occurred along the shoreline in

Yenikoy, with shoreline retreat ranging from about 50 to 350 m in this area. Both the shoreline

and inundation zone are subparallel to the trend of the Golcuk normal fault, located about 1.5 km

to the southwest. Bathymetry data [4] indicate that the offshore slope within 150 m of the pre-

earthquake shoreline is flat, averaging between 2 and 4 degrees. Post-earthquake bathymetry

suggest that the seafloor remained relatively flat, but possibly steepened somewhat near the

shoreline.   It is possible that tectonic tilting and minor secondary displacements on other

shoreline-parallel normal faults actually steepened the topography in this area. Watermarks on

subsided structures that formerly were at the shoreline suggest that the shoreline subsided

between 1 and 3 m. One CPT sounding was performed along the post-earthquake shoreline coast

in Yenikoy (Figure 6). The CPT data indicate that the top 17 m consists predominantly of silty

clay and clay (qc ~ 0.5 – 1.0 MPa, Ic ~ 3), with some dense sand layers (qc ~ 30 MPa). Some of

the silty and clayey layers contained shells and appear to be marine deposits that are

interfingered with the fine-grained delta sediments. None of the soils encountered at Yenikoy

are liquefiable, and no surficial evidence of liquefaction or ejected sand/silt was found in this

area. As a result, the subsidence at Yenikoy appears to be solely attributed to global tectonic

downdropping along the Golcuk normal fault, with possible additional vertical movement

associated with fault block tilting and secondary normal faults at the shoreline. The relatively

flat pre- and post-earthquake bathymetry profiles suggest that a large-scale, near shore slope

failure did not occur.

MKE Scrapyard-Seymen
       The town of Seymen is situated near the southeastern margin of the Golcuk pull-apart

basin (Figure 6), approximately 5 km east of central Golcuk. Coastal subsidence and sea

inundation, as well as distinct lateral spreading in some areas, occurred along a 1 km segment of

the coast. The distressed area in Seymen coincides with the latest Holocene prograding part of a

delta that has formed a small peninsula into Izmit Bay at the mouth of a creek.

       The two main areas of subsidence and ground failure in Seymen are the MKE Scrapyard

and an adjacent tea garden. The MKE Scrapyard is located at the nose of the Holocene delta fan

and shoreline retreat as large as 70 m occurred here, causing submergence of the shoreline

facilities within the MKE Scrapyard. A prominent 0.5 to 1-meter high, curvilinear scarp formed

the post-earthquake shoreline, and a secondary zone of cracking occurred within a 150 m wide

zone inland of the post-earthquake shoreline, across the nose of the delta fan. The lateral

displacements across cracks and joints in a concrete wall that traverses the zone of secondary

cracking were measured and indicated about 1.5 m of cumulative onshore lateral displacement in

the zone behind the offshore slide scarp.

       The pre-earthquake cross-section at the nose of the delta fan at the MKE Scrapyard was

developed from bathymetry data provided by ZETAS [16]. The maximum offshore slope angle

at the nose of the delta fan at the scrapyard was 22 degrees and the height of this slope was about

10 m. The maximum slope occurred approximately 25 m offshore, while the slope was flatter

further offshore with an angle of 6 degrees. To develop the post-earthquake offshore geometry,

water depth was measured using a sonar device (Lowrance X-85) that is typically used for

recreational boating applications. These data indicate that the inclination of the post-earthquake

offshore geometry at the delta nose is about 3 degrees.

       At the MKE Scrapyard, one CPT (SY-CPT5) was performed at the nose of the delta fan

in the area adjacent to the coastal slide and one CPT (SY-CPT4) and one SPT (SY-SPT1) were

performed approximately 150 m inland (Figure 6), along the wall where cracking was mapped.

Inland, the subsurface soils consist mainly of medium stiff, highly plastic clays and silts (PI 20-

40) including both alluvial and marine sediments. Some thin silty sand alluvial layers were

identified in the top 15 m, and a layer of medium dense silty sand (N1,60~17, 15-30% fines) was

encountered between 15 and 20 m. Liquefaction analyses using a PGA of 0.3 g indicate that the

factor of safety against liquefaction for this deeper silty sand layer is close to 1.0. The results

from the CPT performed at the nose of the delta fan along the shoreline at the scrapyard (SY-

CPT5) are shown in Figure 9. These data indicate that the top 10 m consist of loose, deltaic silty

sand (qc ~ 5 MPa, Ic ~ 2.2) with interbedded clay seams. The underlying soils consist mainly of

clays, which are in part marine in origin.

       The liquefaction susceptibility of the soils at the nose of the delta fan at the MKE

Scrapyard (SY-CPT5) was assessed using the procedures outlined previously and a PGA of 0.3

g. These analyses reveal that at the nose of the delta fan most of the soil in the top 10 m of the

profile is liquefiable, with a factor of safety less than 0.5 [7]. The elevation of this liquefiable

layer coincides with the location of the steep offshore slope, indicating that a liquefaction-

induced slope failure that daylighted at the base of the steep delta nose caused the localized sea

inundation at the MKE Scrapyard. It is possible that the failure occurred progressively, initiating

at the steep delta nose and propagating landward. The wide extensional zone behind the main

scarp is similar to the extensional zone observed behind the slide scarp at Degimendere, and

similarly suggests that this zone was quasi-stable and experienced limited lateral movement into

the slide zone possibly along partially liquefied layers.

Tea Garden-Seymen
       The Seymen tea garden is located approximately 300 m west of the MKE Scrapyard,

along the edge of the Seymen delta. Deformations characteristic of classical lateral spreading

were observed, but with only minor shoreline retreat and sea inundation.            Pre-earthquake

bathymetry in the tea garden area revealed an offshore slope of less than 2 degrees [16]. Post-

earthquake water depth measurements performed as part of this study revealed a similar offshore

geometry, indicating that the ground failure did not extend far offshore or did not involve large-

scale displacements of the seafloor. The ground cracking at the tea garden site was not mapped

during post-earthquake reconnaissance efforts, but a photograph taken from the air after the

earthquake [15] was used to estimate the cumulative lateral width of ground cracking. Using

landmarks (e.g., sidewalks, curbs) in the photograph that were measured in the field during this

study, the cumulative lateral crack width was roughly estimated to be between 2 and 4 m.

       One CPT and one SPT were performed along the shoreline at the Seymen tea garden.

The data from these CPT and SPT (SY-CPT2, SY-SPT2) are shown in Figure 10. The CPT and

SPT data indicate a 5-m thick layer of soft, silty clay at the surface, underlain by over 15 m of

loose silty sand (qc ~ 4-6 MPa, N1,60 ~ 6-12) with interbedded layers of silty clay. Below the

silty sand layer is highly plastic clay (PI 30-40). Liquefaction analyses using a PGA of 0.3 g

reveal that 5 to 10 m of the silty sand layer is highly liquefiable, with a factor of safety less than

0.5 [7]. This liquefiable layer is the most likely cause of the observed lateral spread cracking at

the tea garden. The 5-m thick silty clay layer at the surface presumably prevented any soil ejecta

from surfacing, and thus sand boils were not observed after the earthquake.

       The lateral spread displacement estimated from the photograph at the Seymen tea garden

was compared with the predicted lateral displacement from the empirical relationship of Youd et

al. [17] for sloping ground. The empirical relationship is a function of earthquake magnitude

(M), distance to the fault (R), ground slope (S), and the thickness (T15), fines content (F15), and

median gain size (D5015) of the liquefiable materials with N1,60 less than 15. These parameters

were estimated for the tea garden site using the CPT and SPT data, grain size distributions from

split spoon samples, and field observations (Table 1). Using these parameters, the Youd et al

[17] relationship for sloping ground predicts displacements between 7 and 14 m, which are much

larger than observed. However, the ground accelerations during the Kocaeli earthquake were

smaller than expected for a Mw = 7.4 event [10]. This effect can be taken into account by using

an attenuation relationship for acceleration (e.g., Abrahamson and Silva [18]) to compute the

equivalent distance for a specified acceleration level and using this distance in the Youd et al.

[17] relationship. For Mw = 7.4 and the estimated PGA of 0.3 g, the equivalent distance is about

20 km. Using 20 km in the Youd et al. [17] relationship results in a displacement prediction of

between 1.0 and 2.0 m, which is much closer to the estimated field displacements. The need to

use equivalent distance in the Youd et al. [17] relationship arises because these relationships use

magnitude and distance as indicators of earthquake intensity, rather than directly using peak

ground acceleration. It may be useful in the future to use peak ground acceleration directly as an

indicator of intensity in predictive relationships for lateral spread displacement rather than using

magnitude and distance as indirect indicators of intensity.

       The Sapanca pull-apart basin, which encompasses Sapanca Lake, is located within a

right-lateral step between the Sapanca and Sakarya fault segments (Figure 1). Right-lateral

displacement of 3 m was observed along the Sapanca fault segment where the fault enters the

north margin of the lake. Post-earthquake bathymetry indicates that the Sapanca fault segment

extends into the lake and displayed some normal displacement during the earthquake.

Significant right-lateral displacement (2 to 5 m) and minor vertical displacement (0.25 to 0.5 m)

were measured on the Sakarya fault segment on the southeast shoreline of Lake Sapanca.

Normal fault displacements were also documented in this area during the 1967 Mudurnu Valley

earthquake [19].

       The topographic depression caused by the Sapanca pull-apart basin serves as a

depocenter for sedimentation, with Holocene delta fans prograding into the lake at the mouths of

many creeks. The severe subsidence observed along the margins of Lake Sapanca during the

Kocaeli earthquake was mainly concentrated at the nose of delta fans. The most significant

failure occurred along the south margin of the lake at the Hotel Sapanca, and a smaller failure

occurred along the north margin of the lake within the town of Esme [15]. Both of these sites are

situated on Holocene delta fans. The Hotel Sapanca site experienced as much as 50 m of

shoreline retreat [15], and geotechnical investigations by other researchers found liquefiable silty

sand and sand in the top 10 m of the site with qc < 10 MPa, N60 < 10 [20], and a factor of safety

against liquefaction of less than 0.5.

       The Esme site is located on a Holocene delta fan along the north shore of Lake Sapanca

(Figure 11). This delta is located near the base of a bedrock hill front, which is approximately

500 m to the north. Bedrock is mapped as tertiary sedimentary rock, and the delta sediments are

derived from erosion of this bedrock. The close distance between the hill front and delta reduces

the amount of reworking and grain size sorting that occurs in the delta sediments. As a result,

the near surface delta deposits are relatively coarse, consisting mainly of sand with silt and

gravel lenses. The active portion of the delta has constructed a narrow nose that extends into

Lake Sapanca, and this is the area that experienced inundation during the Kocaeli earthquake.

The inundated area extended approximately 50 m along the coast and shoreline incursion was

estimated as approximately 35 m back from the pre-earthquake shoreline. Onshore ground

cracking was purely translational and extended approximately 150 m inland, but no soil ejecta

was observed at the site [15].        The normal faulting that was observed in post-earthquake

bathymetry data occurred approximately 750 m offshore from Esme and most likely did not

contribute to the failure.

        Pre-earthquake bathymetry data were not available for Lake Sapanca, and therefore the

offshore slope at Esme before the earthquake is unknown. The post-earthquake geometry of the

Esme site was developed using water depth measurements collected as part of this study. These

data indicate that the post-earthquake offshore slope of the delta nose near the shore is relatively

flat, with a slope angle of approximately 7 degrees. Approximately 25 m offshore the slope

angle increases to 15 degrees. The onshore portion of the delta exhibits a low gradient, on the

order of 2 to 4 degrees.

        The site investigation at Esme included one SPT boring and three CPT soundings (Figure

11).   One SPT boring (ES-SPT1) and one CPT sounding (ES-CPT3) were located at the

shoreline within the ground failure zone, while another CPT (ES-CPT4) was located

approximately 100 m inland along the creek that is actively depositing the delta sediments. The

third CPT sounding was performed 1.5 km west of the failure in a non-failed portion of the delta

fan. The in situ data from ES-SPT1 and ES-CPT3 are shown in Figure 12. The top 10 m

consists of loose sand to silty sand (qc ~ 3-6 MPa, N60 ~ 5-13), with the fines content varying

from less than 10% to about 30%. This sand layer is underlain by a 5-m thick nonplastic silt

layer and a medium-dense, silty sand layer (qc ~ 20 MPa, N60 ~ 20). Stiff, silty clay (PI 10-20) is

found below 20 m.            The CPT performed inland (ES-CPT4) indicated similar subsurface

conditions, but the layers were somewhat thinner due to the effect of topography on deposition.

        The CPT and SPT data were used to assess the liquefaction susceptibility of the soils at

Esme. No strong motion station was situated at Esme, but the Sakarya station (SKR) is located 5

km east of Esme. The SKR station is situated on shallow soil and recorded a PGA of 0.4 g.

Based on this recording and considering the effect of the soil conditions on ground shaking, the

PGA at Esme was estimated as between 0.3 and 0.5 g. These values were used to estimate the

earthquake-induced CSR and the CRR values were computed from both SPT and CPT data. The

liquefaction susceptibility analysis indicates that most of the top 10 m of silty sand is highly

liquefiable with factors of safety well below 0.5. The medium dense sand between 15 and 20 m

also is liquefiable (qc ~ 20 Mpa, N60 ~ 20) with factors of safety between 0.75 and 1.0. These

liquefiable layers are the most likely the cause of the coastal failure and sea inundation at Esme.

       Severe coastal subsidence and sea inundation were observed within the coastal pull-apart

basins during the 1999 Kocaeli earthquake in Turkey.             Geotechnical and geologic site

investigations were performed at several coastal sites to assess the failure mechanisms that

caused the observed subsidence. The collected information was used to evaluate the two most

likely causes of the onshore sea inundation: tectonic subsidence associated with normal faulting

and liquefaction-induced ground failure.

       A summary of the observations from the coastal sites described in this paper is given in

Table 2 and discussed below. The Degirmendere site, within the Karamursel pull-apart basin,

experienced up to 75 m of shoreline retreat at the nose of a delta fan. Due to the active

progradation of the delta, the offshore slope at Degirmendere was relatively steep (18 degrees).

The near-surface soils (top 10 m) were liquefiable but could not explain the observed deep-

seated slope failure. Slope stability analyses indicated a factor of safety of 1.05 for the slope,

assuming that a deeper soil layer (25-30 m deep) liquefied.          These analyses indicate that

liquefaction of deeper soils most likely caused the failure at Degirmendere.

       Within the Golcuk pull-apart basin, four sites of coastal subsidence were investigated. In

central Golcuk, sea inundation extended as far as 300 m inland at the nose of an active delta fan.

This area coincides with an area close to the Golcuk normal fault that experienced approximately

1.5 m of vertical fault displacement. The pre-earthquake offshore slope was as steep as 20

degrees and highly liquefiable soils were encountered in the top 10 m of the site. Consequently,

it appears that both tectonic subsidence and liquefaction played a role in the sea inundation in

central Golcuk. In Yenikoy, the sea inundation reached as far as 350 m inland. However, no

liquefiable soils were encountered here and the offshore slope was flat (less than 4 degrees).

Therefore, the coastal subsidence in Yenikoy is solely attributed to normal faulting within the

Golcuk pull-apart basin. In Seymen, the MKE Scrapyard experienced 70 m of sea inundation at

the nose of an active delta fan. The offshore slope was as steep as 22 degrees and liquefiable

soils were found in the top 10 m. Only minor tectonic subsidence was observed in this area, and

therefore, the sea inundation is mostly attributed to a liquefaction-induced slope failure at the

delta nose. At the tea garden in Seymen, lateral spread deformations were observed with only

minor sea inundation. The site is situated away from the nose of the active delta fan in Seymen,

with an offshore slope of less than 3 degrees. Liquefiable soils were identified in the top 15 m of

the deposit at the tea garden and are the most likely cause of the lateral spreading. Lateral spread

deformations estimated from Youd et al. [17] agreed well with those estimated in the field, after

accounting for the smaller than expected ground motions from the Kocaeli earthquake.

       In the Sapanca pull-apart basin, the two main sites of coastal failure (Esme, Hotel

Sapanca) were situated at the nose of active delta fans. The Esme site experienced up to 35 m of

shoreline retreat, while the Hotel Sapanca experienced up to 50 m of inundation. Because no

pre-earthquake bathymetry was available for Lake Sapanca, it was not possible to estimate the

offshore slopes at either of these sites. However, highly liquefiable soils were encountered at

both sites and are the most likely cause of the failures and sea inundation.

       The field investigation summary in Table 2 indicates that the largest inland extent of sea

inundation and subsidence occurred at sites that experienced more than a meter of tectonic

subsidence due to normal faulting.       In the case of central Golcuk, liquefaction may have

enhanced the extent of subsidence. The sites with moderate sea inundation (25 to 100 m of

shoreline retreat) predominantly were situated at the nose of delta fans with quasi-stable steep

slopes and liquefiable soils.    Most of these sites did not experience significant tectonic

subsidence, but did undergo localized deformation. Finally, the site with the least sea inundation

(Seymen-Tea Garden) was not situated at the nose of a delta fan, but classic lateral spread

deformations occurred due to the presence of liquefiable soils and sloping ground.

       The collected field observations, field data, and associated analyses indicate that both

tectonic subsidence and liquefaction-induced slope failures can cause significant sea incursion in

coastal pull-apart basins. During the Kocaeli earthquake, tectonic subsidence caused up to 350

m of coastal retreat and subsidence. Additionally, liquefaction-induced coastal slope failures

caused shoreline retreat of between 25 and 75 m, while lateral spreading caused only minor sea

inundation. Whereas tectonic subsidence was broadly distributed within the pull-apart basins,

particularly near to the basin-bounding Golcuk normal fault, liquefaction and slope failures were

largely restricted to the most active prograding parts of Holocene delta fans. It is important to

note that these prograding portions of delta fans are readily identifiable by geologic mapping and

aerial photograph analyses. Conversely, subsurface conditions encountered in SPT borings and

CPT soundings often appeared to be relatively similar in both failed zones in latest Holocene

deposits, and adjacent non-failed zones in older Holocene or even late-Pleistocene deposits. This

comparison suggests that careful geologic mapping and age estimation of units may be very

useful in distinguishing problematic areas.

       Based on this study, when siting facilities in coastal areas, it is important to consider

coastal failures that can cause tens to hundreds of meters of shoreline retreat. In these cases, a

study should focus on identifying both: (1) the extent of extensional pull-apart basins and the

locations of basin-bounding normal faults, as well as (2) more localized areas susceptible

liquefaction and liquefaction-induced slope failures, including actively prograding delta fans.


       Financial support was provided by the United States Geological Survey under grants

01HQGR0042 and 02HQGR0059 to the University of Texas and grants 01HQGR0043 and

02HQGR0030 to William Lettis and Associates. This support is gratefully acknowledged. The

site investigation was performed by ZETAS Corporation of Istanbul, Turkey with the help of Dr.

Turan Durgunoglu and Mr. Turhan Karadayilar. Jason Holmberg of William Lettis & Associates

prepared many of the figures for this paper.

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    Harder Jr., M.E. Hynes, K. Ishihara, J.P. Koester, S.S.C. Liao, W.F. Marcuson, III, G.R.
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    Workshops on Evaluation of Liquefaction Resistance of Soils. ASCE Journal of
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             Table 1. Parameters used in Youd et al. (2002) relationship to predict
                 lateral spread displacement at the tea garden site in Seymen.
                                   Parameter         Estimate
                                       M               7.4
                                     R (km)           1, 20
                                     S (%)             1-3
                                     T15 (m)           5-10
                                    F15 (%)            20
                                   D5015 (mm)          0.4

                  Table 2. Summary of field investigations of coastal failures.

                     Maximum                         Maximum                       Vertical
                                                                   FS against
                   inland extent     Delta Nose?     pre-event                     tectonic
                   of inundation                       slope                      subsidence
  Degirmendere          75 m            Yes             18          0.5-1.0          None
                       300 m            Yes             20           ~0.5            1.5 m
    Yenikoy            350 m             No             4            N/A             1-3 m
                        70 m            Yes             22           <0.5            Minor
                       <5m               No             3            <0.5            Minor
      Esme              35 m            Yes              ?            <0.5            None
  Hotel Sapanca         50 m            Yes              ?            <0.5            Minor


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