# A CRITICAL EXAMINATION OF NEAR-FIELD ACCELEROGRAMS FROM THE SEA OF MARMARA REGION EARTHQUAKES

Sinan Akkar[^1] and Polat Gülkan[^2]

##### 

####### ABSTRACT

> In 1999, Turkey was struck by two major earthquakes that occurred 86
> days apart on the North Anatolian fault system. Both of these
> earthquakes had right-lateral strike slip mechanisms with moment
> magnitudes greater than 7. The number of strong motion records
> obtained from the Kocaeli earthquake (08/17/1999, M~w~ = 7.4) was 34.
> The second event, designated as the Bolu-Düzce earthquake (11/12/1999,
> M~w~ = 7.2), triggered 20 instruments. Among the records that we have
> from these earthquakes, seven are near-source ground motion data.
> These records were obtained from the cities of Gebze, Yarımca, İzmit
> (capital city of the province of Kocaeli), Adapazarı (capital of the
> province of Sakarya), Düzce (shaken strongly in both events) and Bolu.
> In many of these urban centers extensive structural damage was
> observed. While these near field data have greatly expanded the
> strike-slip near-source ground motion data base world wide for M~w~\>7
> they represent a blurred image of the actual severity of the ground
> motions in the epicentral area. Regrettably, sparseness of the
> national strong motion network, and the unrepresentative geologic
> conditions at the recording sites contributed to this deficiency.
> After decomposing the acceleration traces into fault-normal and
> fault--parallel components, we analyze the records with
> earthquake-resistant structural engineering criteria in mind. There
> appears to be no uniform support for the judgment that fault-normal
> direction should represent a greater damage causing potential when
> this potential is based on ground story drift ratio spectra. The
> component with the larger ground velocity correlates better with the
> component with the larger drift demand.

##### Introduction

The two Sea of Marmara region earthquakes in Turkey during 1999 rank
among the largest seismic events that have occurred in the eastern
Mediterranean Basin during the last one hundred years. The first of
these occurred on August 17 at 3:02 local time, and is a multiple
rupture event with M~w~ = 7.4 and M~s~ = 7.8. The second earthquake
(M~w~ = 7.2) occurred on November 12 at 6 pm. Their combined observable
fault rupture length is 175 km. We will refer to these earthquakes as
Events 1 and 2, respectively. In terms of their effects (ground
deformation patterns, damage, and losses) these earthquakes established
new thresholds. Unfortunately, only a patchwork of isolated strong
ground motion records were recovered during the main shocks of both
earthquakes. These records are useful, but the instruments lacked
precise time coordination. Their haphazard locations (instruments are
placed in institutional buildings, such as meteorological stations)
limited their usefulness. Further, all were stand-alone devices, and
were triggered on their own. The mixture of analog and digital sensor
outputs introduced another source of dissimilarity into the records
recovered. The deployment of the instruments in the epicentral area is
shown in Figure 1. The names of the affected cities or where the records
were taken have been mounted on the space image. The two different lines
describing the approximate location for the observed fault ruptures are
colored differently so that any directivity effects discussed later can
be estimated visually. We note that the near-field instruments (marked
with yellow in the figure) were actually several km removed from the
actual fault trace.

![](media/image1.jpeg){width="5.875196850393701in"
height="3.0354330708661417in"}

**FIGURE 1. LOCATION OF NEAR-FIELD STRONG MOTION INSTRUMENTS**

Gölcük and Yalova, both situated along the southern coast of the Bay of
İzmit, were two of the worst hit cities. They and many other smaller
settlement centers along the shore between them were the scenes of
horrible destruction, but no strong motion records have been recovered
there. For this reason, they are marked in red in Figure 1. This
unfortunate sparseness has created a void with respect to the question
of whether we have an adequate reflection of how the ground deformed in
the immediate vicinity of the fault rupture. There exists indirect
evidence that violent motions occurred. For example, parked trucks and
passenger busses were overturned at a filling station near İzmit during
Event 1 (Figure 2), and in Kaynaşlı, a small town 10 km east of Düzce
during Event 2 (Figure 3). The fault trace is visible immediately behind
the building with the rotated roof in Figure 3. There is other evidence
of the extremely strong ground shaking that must have been experienced a
close distances to the fault rupture. Widespread building damage such as
seen in Figure 4 of the central part of Gölcük is reminiscent of scenes
of bomb-ravaged urban areas in wars. Such damage must be attributed to
many factors inclusive of deficient building practices, but sights such
as this are not compatible with, for example, the damage in Erzincan,
700 km to the east of Ankara, during the M = 6.9, March 13, 1992
earthquake there when one component had a 0.5 g peak. Many parts of
Gölcük also settled by as much as 1.5 m, a displacement that was
accompanied by a 4 m horizontal slip. This is visible in Figure 5 where
the inundated entertainment park of the city is shown. Such complex
patterns of ground deformation must have caused unusually severe demands
from structures, but we have no records of these in the form of
acceleration traces recorded by closely spaced and better distributed
instruments.

Event 1 caused widespread damage along the southern coast of the Bay of
İzmit. The epicenter of this earthquake is calculated to lie directly
south of İzmit near a village called Kullar. The rupture then advanced
bilaterally, triggering a North Anatolian Fault segment in Arifiye,
south of Adapazarı, and then following a step-over traced a north-east
line to Gölyaka, a rural settlement about 15 km south west of Düzce. Its
western propagation bisected Gölcük and smaller settlements in the
region. Its point of termination in the west is underwater, near Hersek
peninsula (Koçyiğit, *et al*., 1999) Event 2 originated at the eastern
end of the rupture in Event 1, and propagated to the east, with Bolu in
its line of sight. In a strict sense, the Bolu record is not near field
because it was recorded at 20 km to the nearest traced fault. It
contains the largest PGA value (0.8 g) among the seven records studies.
A recording reportedly made in Kaynaşlı during Event 2 was not available
to us. It is said to display a peak acceleration of 1.1 g.

This paper will examine the available near source records obtained from
these earthquakes in terms of damage-causing potential as used in
earthquake structural engineering. Our objective is to part the veil
through an examination of records from the closest points to the
affected region. The database we examine is composed of seven records
from the six cities identified in Figure 1.(Düzce had the misfortune of
experiencing very strong shaking during both earthquakes.) Tables 1 and
2 summarize information concerning the accelerograms. Only the very
strong motion parts of some of these traces were used. The complete
accelerograms from Gebze, Yarımca and Adapazarı contain evidence of
waves arriving from subsequent ruptures during Event 1. These effects
are not considered in this paper, although it is expected that longer
duration ground shaking must have exacerbated both structural damage and
ground deformation and soil liquefaction observed, particularly in and
around Adapazarı.

![](media/image2.jpeg){width="4.709027777777778in"
height="2.8680555555555554in"}

**FIGURE 2. OVERTURNED VEHICLES AT FILLING STATION IN IZMIT DURING EVENT
1**

![](media/image3.jpeg){width="4.54375in" height="3.029166666666667in"}

**FIGURE 3. FILLING STATION, KAYNAŞLI WITH FAULT TRACE IN BACKGROUND,
EVENT 2**

![](media/image4.jpeg){width="4.815277777777778in"
height="3.201388888888889in"}

**FIGURE 4. CENTRAL AREA OF GOLCUK FROM THE AIR**

![](media/image5.jpeg){width="5.190277777777778in"
height="2.9805555555555556in"}

**FIGURE 5. GROUND SETTLEMENT IN GOLCUK**

**TABLE 1. 17 AUGUST 1999 NEAR-FIELD RECORDS**

<table>
<colgroup>
<col style="width: 8%" />
<col style="width: 19%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 6%" />
<col style="width: 6%" />
<col style="width: 5%" />
<col style="width: 10%" />
<col style="width: 4%" />
<col style="width: 6%" />
<col style="width: 6%" />
<col style="width: 8%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><p>Station</p>
<p>(Ref.)</p></td>
<td><strong>Recording Site Description<sup>1</sup></strong></td>
<td><strong>R<sub>close</sub><sup>2</sup> (km)</strong></td>
<td><h1 id="comp">Comp</h1></td>
<td><p><strong>PGA</strong></p>
<p><strong>(cm/s<sup>2</sup>)</strong></p></td>
<td><p><strong>PGV</strong></p>
<p><strong>(cm/s)</strong></p></td>
<td><strong>PGD (cm)</strong></td>
<td><strong>t<sub>eff</sub><sup>3</sup> (s)</strong></td>
<td><strong>t<sub>dur</sub> (s)</strong></td>
<td><strong>T<sub>g</sub> (s)<sup>4</sup></strong></td>
<td><strong>EPA (cm/s<sup>2</sup>)</strong></td>
<td><h6 id="epapga">EPA/PGA</h6></td>
<td><p><strong>PGV/PGA</strong></p>
<p><strong>(s)</strong></p></td>
</tr>
<tr class="even">
<td rowspan="4"><p>Sakarya</p>
<p>(Adapazari)</p>
<p>(SKR)</p></td>
<td rowspan="4">The recording station is relatively isolated
storage-type building on a gentle hillside. The geological process would
be erosion and soil formation of underlying bedrock. Grading has
excavated into a hillside 40m west of station. Exposed bedrock in
hillside is limestone. Structure is too small, and would not have
foundation excavated to bedrock –records might show effects of a shallow
thin soil layer.</td>
<td rowspan="4">3.2</td>
<td>EW</td>
<td>399.37</td>
<td>79.80</td>
<td>198.64</td>
<td>12.620 (19.41%)</td>
<td rowspan="4">65.0</td>
<td>1.08, 1.86</td>
<td>313.92</td>
<td>0.79</td>
<td>0.20</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
</tr>
<tr class="even">
<td>FN</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
<td>-</td>
</tr>
<tr class="even">
<td rowspan="4"><p>Yarımca</p>
<p>(YPT)</p></td>
<td rowspan="4">Local topography is flat, due to being on a river delta
with the geological process being dominated by sediment accumulation.
Soil apperared to be clay/silt at the surface. Fine-grained materials
are expected as the site is relatively far from nearest topography.</td>
<td rowspan="4">3.28</td>
<td>EW</td>
<td>226.05</td>
<td>84.70</td>
<td>167.57</td>
<td>12.255 (40.85%)</td>
<td rowspan="4">30.0</td>
<td>3.5</td>
<td>196.20</td>
<td>0.87</td>
<td>0.375</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>315.63</td>
<td>79.60</td>
<td>65.32</td>
<td>12.435 (41.45%)</td>
<td>1.5, 3.0</td>
<td>195.22</td>
<td>0.62</td>
<td>0.252</td>
</tr>
<tr class="even">
<td>FN</td>
<td>311.51</td>
<td>77.98</td>
<td>71.10</td>
<td>12.455 (41.52%)</td>
<td>1.5, 3.0</td>
<td>196.20</td>
<td>0.63</td>
<td>0.250</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>222.83</td>
<td>84.04</td>
<td>165.50</td>
<td>12.225 (40.75%)</td>
<td>3.5</td>
<td>195.22</td>
<td>0.88</td>
<td>0.327</td>
</tr>
<tr class="even">
<td rowspan="4"><p>İzmit</p>
<p>(IZT)</p></td>
<td rowspan="4">The recording station occupies inside a switchback in
the paved street climbing a steep hillside. The geological process would
be erosion and soil formation of underlying bedrock. An old stone fence
next to the garden is presumed to be constructed from local rock. It
appeared to be made of gray, tightly cemented sandstone/limestone
mix.</td>
<td rowspan="4">4.26</td>
<td>EW</td>
<td>222.73</td>
<td>54.28</td>
<td>129.28</td>
<td>9.860 (32.87%)</td>
<td rowspan="4">30.0</td>
<td>-<sup>5</sup></td>
<td>224.65</td>
<td>1.01</td>
<td>0.244</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>163.88</td>
<td>32.04</td>
<td>47.64</td>
<td>9.200 (30.67%)</td>
<td>-</td>
<td>153.04</td>
<td>0.93</td>
<td>0.196</td>
</tr>
<tr class="even">
<td>FN</td>
<td>164.31</td>
<td>30.91</td>
<td>43.47</td>
<td>9.265 (30.88%)</td>
<td>-</td>
<td>155.0</td>
<td>0.94</td>
<td>0.188</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>218.79</td>
<td>54.83</td>
<td>129.34</td>
<td>9.810 (32.70%)</td>
<td>-</td>
<td>223.67</td>
<td>1.02</td>
<td>0.251</td>
</tr>
<tr class="even">
<td rowspan="4"><p>Gebze</p>
<p>(GBZ)</p></td>
<td rowspan="4">Local topography has rolling hills, with geological
process being soil formation and slow erosion. The foundation of the
recording station is almost certainly excavated to rock. Rocky soil near
the recording station. Float collected from soil is reddish
sandstone.</td>
<td rowspan="4">7.74</td>
<td>EW</td>
<td>140.56</td>
<td>34.72</td>
<td>103.70</td>
<td>4.765 (15.88%)</td>
<td rowspan="4">30.0</td>
<td>-</td>
<td>146.17</td>
<td>1.04</td>
<td>0.247</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>264.17</td>
<td>45.59</td>
<td>82.58</td>
<td>6.015 (20.05%)</td>
<td>-</td>
<td>179.52</td>
<td>0.68</td>
<td>0.172</td>
</tr>
<tr class="even">
<td>FN</td>
<td>240.24</td>
<td>37.01</td>
<td>112.51</td>
<td>5.635 (18.78%)</td>
<td>-</td>
<td>201.11</td>
<td>0.84</td>
<td>0.154</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>136.75</td>
<td>46.27</td>
<td>97.71</td>
<td>5.270 (17.57%)</td>
<td>-</td>
<td>121.64</td>
<td>0.89</td>
<td>0.338</td>
</tr>
</tbody>
</table>

TABLE 1. 17 AUGUST NEAR-FIELD RECORDS (Continued)

<table>
<colgroup>
<col style="width: 8%" />
<col style="width: 19%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 6%" />
<col style="width: 6%" />
<col style="width: 5%" />
<col style="width: 10%" />
<col style="width: 4%" />
<col style="width: 6%" />
<col style="width: 6%" />
<col style="width: 8%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><p>Station</p>
<p>(Ref.)</p></td>
<td><strong>Recording Site Description<sup>1</sup></strong></td>
<td><strong>R<sub>close</sub><sup>2</sup> (km)</strong></td>
<td><h1 id="comp-1">Comp</h1></td>
<td><p><strong>PGA</strong></p>
<p><strong>(cm/s<sup>2</sup>)</strong></p></td>
<td><p><strong>PGV</strong></p>
<p><strong>(cm/s)</strong></p></td>
<td><strong>PGD (cm)</strong></td>
<td><strong>t<sub>eff</sub><sup>3</sup> (s)</strong></td>
<td><strong>t<sub>dur</sub> (s)</strong></td>
<td><strong>T<sub>g</sub> (s)<sup>4</sup></strong></td>
<td><strong>EPA (cm/s<sup>2</sup>)</strong></td>
<td><h6 id="epapga-1">EPA/PGA</h6></td>
<td><p><strong>PGV/PGA</strong></p>
<p><strong>(s)</strong></p></td>
</tr>
<tr class="even">
<td rowspan="4"><p>Düzce</p>
<p>(DZC)</p></td>
<td rowspan="4">Topography of the area is flat, and soil appeared to be
silty-clay at the surface. The active geological process is sediment
accumulation, robably in a river flood plain. Clearly in a basin.</td>
<td rowspan="4">17.06</td>
<td>EW</td>
<td>375.55</td>
<td>49.61</td>
<td>108.57</td>
<td>10.960 (40.32%)</td>
<td rowspan="4">27.18</td>
<td>1.36</td>
<td>318.82</td>
<td>0.85</td>
<td>0.132</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>330.51</td>
<td>60.59</td>
<td>63.81</td>
<td>10.330 (38.00%)</td>
<td>2.7</td>
<td>243.29</td>
<td>0.74</td>
<td>0.183</td>
</tr>
<tr class="even">
<td>FN</td>
<td>390.08</td>
<td>67.65</td>
<td>99.03</td>
<td>10.155 (37.22%)</td>
<td>1.8</td>
<td>342.37</td>
<td>0.88</td>
<td>0.173</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>267.78</td>
<td>48.47</td>
<td>51.53</td>
<td>10.765 (39.61%)</td>
<td>1.1</td>
<td>203.07</td>
<td>0.76</td>
<td>0.181</td>
</tr>
</tbody>
</table>

> ^(1)^ Anderson, *et al*., (2000)
>
> ^(2)^ Shortest distance between the observed fault rupture and the
> recording station.
>
> ^(3)^ Bommer, *et al*., (1999)
>
> ^(4)^ Dominant ground period where the smoothed Fourier amplitude
> spectrum is maximum.
>
> ^(5)^ No clearly identifiable dominant ground period is established
> for these records.

TABLE 2. 12 NOVEMBER NEAR-FIELD RECORDS

<table>
<colgroup>
<col style="width: 6%" />
<col style="width: 21%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 5%" />
<col style="width: 9%" />
<col style="width: 4%" />
<col style="width: 6%" />
<col style="width: 6%" />
<col style="width: 8%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><p><strong>Station</strong></p>
<p><strong>(Ref.)</strong></p></td>
<td><strong>Recording Site Description<sup>1</sup></strong></td>
<td><strong>R<sub>close</sub><sup>2</sup> (km)</strong></td>
<td><h1 id="comp.">Comp.</h1></td>
<td><h6 id="pga">PGA</h6>
<p><strong>(cm/s<sup>2</sup>)</strong></p></td>
<td><h6 id="pgv">PGV</h6>
<p><strong>(cm/s)</strong></p></td>
<td><strong>PGD (cm)</strong></td>
<td><strong>t<sub>eff</sub><sup>3</sup> (s)</strong></td>
<td><strong>t<sub>dur</sub> (s)</strong></td>
<td><strong>T<sub>g</sub><sup>4</sup> (s)</strong></td>
<td><strong>EPA (cm/s<sup>2</sup>)</strong></td>
<td><h1 id="epapga-2"> EPA/PGA</h1></td>
<td><h1 id="section-1"></h1>
<h1 id="pgvpga"> PGV/PGA</h1>
<p><strong>(s)</strong></p></td>
</tr>
<tr class="even">
<td rowspan="4"><p>Düzce</p>
<p>(DZC)</p></td>
<td rowspan="4">See Table 1.</td>
<td rowspan="4">8.23</td>
<td>EW</td>
<td>503.32</td>
<td>86.05</td>
<td>170.12</td>
<td>11.965 (46.19%)</td>
<td rowspan="4">25.90</td>
<td>0.85</td>
<td>371.80</td>
<td>0.74</td>
<td>0.171</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>401.77</td>
<td>65.76</td>
<td>88.04</td>
<td>12.650 (48.85%)</td>
<td>0.43, 0.74</td>
<td>363.95</td>
<td>0.91</td>
<td>0.164</td>
</tr>
<tr class="even">
<td>FN</td>
<td>404.19</td>
<td>62.59</td>
<td>93.22</td>
<td>12.855 (49.64%)</td>
<td>0.43, 0.74</td>
<td>363.95</td>
<td>0.90</td>
<td>0.155</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>493.21</td>
<td>84.25</td>
<td>165.25</td>
<td>11.640 (44.95%)</td>
<td>0.85</td>
<td>372.78</td>
<td>0.76</td>
<td>0.171</td>
</tr>
<tr class="even">
<td rowspan="4"><p>Bolu</p>
<p>(BOL)</p></td>
<td rowspan="4">The recording station is relatively isolated adjacent to
flat agricultural land. It is near the lowest point in cross sectiın
across the valley from north to south. The geological process is one of
sediment accumulation, nad fine-grain materials are expected as it is
relatively far from the nearest significant topography. Soil at the
surface is a silty clay. The recording station is located in a localized
pocket of the worst damage in Bolu. It is likely that this station is on
the softest, deepest sediments in the Bolu valley.</td>
<td rowspan="4">20.41</td>
<td>EW</td>
<td>805.90</td>
<td>66.92</td>
<td>21.27</td>
<td>12.53 (24.63%)</td>
<td rowspan="4">50.87</td>
<td>0.74, 1.12</td>
<td>463.03</td>
<td>0.57</td>
<td>0.083</td>
</tr>
<tr class="odd">
<td>NS</td>
<td>739.46</td>
<td>58.25</td>
<td>40.29</td>
<td>12.78 (25.12%)</td>
<td>0.53, 0.92</td>
<td>581.73</td>
<td>0.79</td>
<td>0.079</td>
</tr>
<tr class="even">
<td>FN</td>
<td>755.92</td>
<td>66.88</td>
<td>20.99</td>
<td>12.86 (25.28%)</td>
<td>0.74, 1.12</td>
<td>423.80</td>
<td>0.56</td>
<td>0.088</td>
</tr>
<tr class="odd">
<td>FP</td>
<td>801.60</td>
<td>56.82</td>
<td>40.76</td>
<td>12.74 (25.04%)</td>
<td>0.53, 0.92</td>
<td>604.30</td>
<td>0.75</td>
<td>0.071</td>
</tr>
</tbody>
</table>

> ^(1)^ Anderson, *et al*., (2000)
>
> ^(2)^ Shortest distance between the observed fault rupture and the
> recording station.
>
> ^(3)^ Bommer, *et al*., (1999)
>
> ^(4)^ Dominant ground period where the smoothed Fourier amplitude
> spectrum is maximum.
>
> ^(5)^ No clearly identifiable dominant ground period is established
> for these records.

##### Strong Motion Networks in Turkey: History and Performence

The national strong motion accelerograph network in Turkey was initiated
in 1973. This network is operated by the General Directorate of Disaster
Affairs. Initially, analog acceleration records were installed as they
were the then-available technology (İnan, *et al*., 1996). Later the
system has been supported by the addition of digital instruments. As of
May, 2000, this system is comprised of some 140 instruments, about
evenly divided between analog and digital types. The instruments are
placed inside institutional buildings such as meteorology stations or
local ministerial offices for safety, phone hookup and ease of
maintenance. Figure 6 shows their locations.

Additional instruments are deployed in Turkey by other agencies and
universities. For example, a number of historic religious edifices in
İstanbul such as the Saint Sophia Museum, Süleymaniye Mosque have been
instrumented with strong motion sensors on account of their cultural
importance. A recently ended program managed jointly by the General
Directorate of Disaster Affairs and Japan International Cooperation
Agency has established a network in nine provinces in northeastern
Turkey between Ankara and Samsun on the Black Sea coast. The purpose of
this network is to arrive at quick estimates of losses and casualties if
a major earthquake should strike the subject area. The suspension
bridges across the Bosphorus have been the subject of health monitoring,
and have been outfitted by accelerographs operated by the General
Directorate of State Highways. The Scientific and Technical Research
Establishment of Turkey (TÜBİTAK) has funded research programs that have
enabled the setting up of small local networks or distributed single
instruments designed for specific purposes. Small, specific-purpose
clusters of instruments deployed by Middle East Technical University
(ODTÜ), İstanbul Technical University (İTÜ) or Boğaziçi University (BÜ)
operate as stand-alone systems mostly in the İstanbul metropolitan area.
The General Directorate of State Hydraulic Works (DSİ) operates single
strong motion recording systems in or near major dams they have built.
Clearly, the number of instruments is very meager for a country with the
size and seismicity of Turkey.

All of the accelerograms in Tables 1 and 2 are from the national network
except for the YPT record which has been downloaded from data obtained
by a sensor operated by BÜ. All but the Gebze and İzmit records came
from digital instruments.

![](media/image6.wmf){width="5.038194444444445in"
height="2.963888888888889in"}

###### FIGURE 6. STRONG MOTION NETWORK OF TURKEY

##### General Observations on the Ground Motion

Development of indigenous attenuation relations for ground motion
parameters in Turkey has not yet occurred in Turkey due to meager data.
When numerical data of peak ground acceleration (PGA) is evaluated from
Tables 1 and 2, they would seem to be on the low side for these
magnitudes and distances. Certainly, peaks of 0.3-0.4 g from Event 1 are
not consistent with the structural performance for most of the cities
cited in Table 1. But neither is the 0.8 g peak recorded during Event 2
in Bolu where the percentage of collapsed buildings was much less than
in İzmit. This conclusion is not applicable to the derived velocity and
displacement values. Traditionally, instrument sensors have been
oriented in the NS and EW directions in Turkey. The tables have entries
for both of these horizontal components, as well as the fault-normal
(FN) and fault-parallel (FP) directions as explained below.

One objective of this study has been to test the applicability of the
theory that motions normal to the direction of fault rupture tend to be
more severe than the orthogonal horizontal component (Somerville *et
al*., 1997). For this purpose, the radial line extending from the
closest point on the fault rupture to the recording station defines the
FN direction. The horizontal component perpendicular to this then
becomes the FP component. Because of their azimuths from the observed
fault rupture this definition is nearly coincident with the designation
of FN as the geographical north except for GBZ and BOL. It deviates from
true north for DZC for Event 1 when the rupture terminated about 17 km
southwest of the city. These directions are illustrated in Figure 7, and
are the basis of the directions quoted in Tables 1 and 2. The effective
duration in the tables is based on the bracketed Arias intensity
(Bommer, *et al*., 1999). The start of the strong ground shaking is
taken as the time when the Arias intensity reaches 0.1 m/s, and the end
is defined as the time when the remaining energy in the entire record
equals 0.125 m/s. Clearly, any acceleration record whose Arias intensity
is less than 0.135 m/s is not considered as strong ground motion. The
"characteristic" ground motion period is defined as the period(s) where
the smoothed Fourier amplitude spectrum becomes maximum.

![](media/image7.png){width="4.670833333333333in"
height="2.797222222222222in"}

###### FIGURE 7. COMPONENT DIRECTIONS FOR THE RECORDS

##### Derived Velocity and Displacement Records

In deriving the velocity and displacement signals we used two different
procedures: the "standard" procedure (SCM) applied to 70 mm film analog
records of early generation instruments (Trifunac and Lee, 1973), and
the "alternative" procedure (ACM) developed to mimic better the physical
environment of near-field earthquakes (Chen, 1995). Integration of two
different sets of velocity signals yielded two different generations of
displacement signals but Tables 1 and 2 list ACM values, and only these
are exhibited in the figures. The reason for not utilizing the SCM
results for derived quantities was the apparent inconsistency of the
permanent offset values with field observations. The ACM is judged to be
superior in recovering the long period components of ground
deformations. As samples, Figures 8-10 show the time variations for the
EW components of acceleration, velocity and displacement at five
stations during Event 1. The NS component of the instrument in SKR
malfunctioned, so its record could not be decomposed into other
directions. The FN component of the motions in Düzce and Bolu during
Event 2 is shown in Figures 11-13.

Most of these traces exhibit typical features from near-field records:
large permanent displacement offset, and large pulse-like velocity
coinciding approximately with when the permanent displacement is
attained. Examination of both these figures and the entries in Tables 1
and 2 show that there appears to be no particular consistency for the
larger velocity peak being associated with the fault-normal component.
Again, most of these records have very large ground velocity peaks, and
permanent displacement offsets. The inferred rupture propagation
direction in Event 1 makes all records in Figure 8, with the possible
exception of İZT, prime candidates for forward directivity effects, and
this is supported by the shapes of the velocity signals in Figure 9 as
well as the PGV/PGA ratios in Table 1. For Event 2, Bolu is station
where, in spite of the relatively large PGV value, the long period
velocity pulse is absent. The coherent long-period waves cause the
PGV/PGA ratio to become larger, thus making the constant acceleration
part of response spectra longer. The structural implications of this
observation are discussed separately.

##### Response Spectra

An interesting display of what two successive very strong earthquakes
recorded 86 days apart can cause in a city occurred in Düzce. Hundreds
of buildings, already weakened during Event 1 collapsed raising the
total number to 600. We compare the response spectra in Düzce in Figure
14. These plots are contain curves of the unreduced smooth design
spectra according to UBC97 and the Turkish Earthquake Code dated 1998
(TEC98). Note that the UBC near-source factors were computed for the
distances in Tables 1 and 2 so that minor differences exist between the
two parts of Figure 14.

In terms of general properties, the soil classification pairs SC-Z2 and
SD-Z3 in these codes are similar. It is noted that a good deal of
similarity exists between the individual plots, except for the emergence
of a peak at about 0.8 in Event 2. This figure demonstrates that similar
fault mechanisms and soil conditions cause stable ground motions to be
experienced at a given site. The small building housing the instrument
in Düzce may have modified the free-field motion but the records are
believed to be accurate replications of the actual motions experienced
by many structures in the city. Adjustments required in UBC97 for
extending the constant acceleration part follow the tenor of the ground
motion more closely, but for systems with period in the range of 1 s or
longer TEC98 is clearly inadequate. Given the general geologic
conditions in Düzce that lies in a river plain, and the likelihood of
severe prior damage in most buildings, the engineering message conveyed
by Figure 14 is that the observed destruction was not unexpected.

Better appreciation of this observation is possible when spectral
displacement curves of the ground motions are examined. The spectral
displacement can also be utilized in deriving relative interstory
drifts, so plots of this spectral quantity for all components of the
ground motions considered in this paper are given in Figure 15. For
purposes of comparison the unreduced spectral displacement derived for
the best matching soil type according to UBC97 is mounted on each frame.
The two DZC and the BOL records are observed to represent the largest
spectral displacement. We will examine later how drift demands on
idealized structural systems can be derived from these curves.

![](media/image8.wmf){width="3.1346456692913387in"
height="2.6570866141732283in"}

**FIGURE 8. EW ACCELERATION SIGNALS OF EVENT 1**

![](media/image9.wmf){width="2.984027777777778in"
height="2.6569444444444446in"}

**FIGURE 9. EW VELOCITY SIGNALS OF EVENT 1**

![](media/image10.wmf){width="2.9216535433070865in"
height="2.589763779527559in"}

**FIGURE 10. EW DISPLACEMENT SIGNALS FOR EVENT 1**

![](media/image11.wmf){width="3.1366141732283466in"
height="2.7007874015748032in"}

###### FIGURE 11. FN ACCELERATION SIGNALS OF EVENT2

![](media/image12.wmf){width="3.0744094488188978in"
height="1.4744094488188977in"}

###### FIGURE 12. FN VELOCITY SIGNALS OF EVENT2

![](media/image13.wmf){width="3.0255905511811023in"
height="1.7783464566929135in"}

###### FIGURE 13. FN DISPLACEMENT SIGNALS OF EVENT2

![](media/image14.wmf){width="4.945275590551181in"
height="3.9283464566929136in"}

![](media/image15.wmf){width="4.843700787401575in"
height="4.080708661417323in"}

###### FIGURE 14 PSA PLOTS FOR RECORDS IN DÜZCE, EVENTS 1 AND 2

![](media/image16.wmf){width="4.2362204724409445in"
height="1.8826771653543306in"}

![](media/image18.wmf){width="4.208267716535433in"
height="2.0783464566929135in"}

![](media/image20.wmf){width="4.200787401574803in"
height="2.0866141732283463in"}

######  {#section-2}

######  {#section-3}

###### FIGURE 15. SPECTRAL DISPLACEMENT CURVES

##### Drift Spectra

It is possible to observe that the near source records in Figure 8 do
not have the appearance of a broad-band process which is characteristic
of far-field events. These signals seem to contain a number of
long-period pulses that translate into longer period and coherent
velocity and displacement pulses with large peaks. When structures are
subjected to such ground motions, the customary build-up of vibrational
response with several modes dominating the global response may not occur
before one of the coherent velocity and displacement pulses propagating
through the structure as waves causes a large local deformation and the
associated damage (Iwan, 1997). Emergence of performance-based design
criteria favors a renewed expression of structural requirements in terms
of permissible displacements. Essentially, the process of structural
design in this complementary approach is to determine the displacements
first, and then to compare these with allowable limits.

Member forces follow only after the associated displacements have been
found. A suitable analytical vehicle utilized to model structural
behavior of multistory frames is the shear beam: imagine a large number
of rigid laminates of length H connected via laterally flexible links.
Let one end of it be connected to the ground. If this assembly is
subjected to a displacement history of z(t) and its time derivative the
velocity history of v(t) at the ground level, then these signals will
travel within the beam as damped waves with speed c where

> $c = \frac{4H}{T}$ (1)

where T is the period. The equation governing the lateral displacement
u(t) is

$u(t) = \text{exp}( - \alpha t)f(y \pm \text{ct})$ (2)

The constant α can be related to the viscous damping ratio ζ as

$\alpha = \frac{2\text{πς}}{T}$ (3)

The traveling waves will be continuously reflected from the top (free)
end and the ground, and the shear deformation ∂u/∂y at the base will be
(Iwan, 1997)

$\frac{\partial u(T,\varsigma)}{\partial y} = \underset{\forall t}{\text{max}}\frac{1}{c}\left| v(t) + \frac{2\text{πς}}{T}z(t) + 2\sum_{n = 1}^{N \leq 2\frac{t}{T}}{( - 1)^{n}\text{exp}( - n\text{πς})\left\lbrack v(t - n\frac{T}{2}) + \frac{2\text{πς}}{T}z(t - n\frac{T}{2} \right\rbrack} \right|$
(4)

The shear deformation is equivalent to the lateral drift of the ground
story divided by the height of the ground story columns. This is called
the ground story drift ratio. The plot of equation (4) as a function of
period and damping constitutes the drift spectrum. Paradoxically, the
major contribution to ∂u/∂y comes from the ground velocity v(t), and not
z(t) (Akkar and Gülkan, 1999). For most structural types c is typically
125-200 m/s, so it is easy to see that a single velocity pulse with a
peak in the range of 50 cm/s can generate a drift ratio greater than
0.02. For the quality of materials and workmanship available for
reinforced concrete buildings in Turkey, it is likely that columns will
develop end yielding at drift ratios below 0.01. The rapid decay of
capacity when this action is repeated over several significant cycles
can easily cause the types of abject collapse in Figure 4.

Figure 16 demonstrates the different character of near-field ground
motions. Three similar magnitude earthquakes with similar peak
accelerations (except for Bolu) are considered in the comparison. The
influence of the coherent large velocity pulses is exhibited through
greatly enhanced ground story drift ratio demands. This is the most
important requirement a near-source ground motion imposes on structural
frames.

![](media/image22.wmf){width="5.031102362204725in"
height="3.444488188976378in"}

###### FIGURE 16. COMPARISON OF DRIFT SPECTRA FOR NEAR AND FAR FIELD EARTHQUAKES

While the analogy with a shear beam is useful for deriving the drift
spectrum for a given ground motion, other models can be utilized. One of
these possible approaches is to design a series of simple frames, and to
vary their stiffness and mass properties systematically so that
different periods are obtained. When these frames are subjected to the
ground motions considered, they can be assumed to remain elastic or be
allowed to yield, and the degree of inelastic incursion can be expressed
by means of the ductility ratio μ. The ground story displacement divided
by its height is then the drift ratio. A third way of arriving at an
equivalent expression of the drift spectrum is based on the shear beam
idealization for structural frames and the spectral displacement.

Consider the structural frame representation in Figure 17. If this frame
consists of identical in their dynamic properties, then the shear beam
expressions for mode shapes φ and periods T for mode k can be written

$\phi_{k}(y) = \text{sin}\frac{2k - 1}{2}\frac{\pi y}{H}$ (5)

$T_{k} = \frac{4n}{2k - 1}(\frac{m_{s}}{k_{s}})^{\frac{1}{2}}$ (6)

where

n = number of stories

m~s~ = story mass

k~s~ = story stiffness

H = total height of the frame = nh~s~

###### FIGURE 17. SHEAR FRAME IDEALIZATION

We confine our attention to only the first mode because the part of the
total mass mobilized in the first mode is approximately 80 percent of
the total mass of the idealized shear frame. In most frames girders
rotate at the column ends, but if the same uniform properties hold along
the height, and the periods match this generic frame and the shear frame
in Figure 17 would still have the same spectral displacements. Equation
(5) states that the greatest normalized drift in the fundamental mode
occurs at the ground story where y = h~s~

$\phi_{1}(y) = \text{sin}\frac{\pi}{2n}$ (7)

and the ground story drift ratio (GSDR) becomes

$\text{GSDR} = \frac{\text{SD}}{h_{s}}\text{sin}\frac{\pi}{2n}$ (8)

For a given period and damping ratio the spectral displacement SD can be
computed for a given ground motion. It remains to establish a functional
relationship between the remaining parameters of equation (8) and the
structural period (Gülkan and Sozen, 1999).

In principle, any equation of the type T = a H^b^ with a and b as
regression constants may be used. For consistency we used equation (1)
with c = 120 m/s and a ground story height h~s~ equal to 3 m.

In Figure 18 the comparative ground story drift ratio spectra for 5
percent damping are displayed for the FN and FP components of the ground
motions during Event 1 in İzmit and Düzce, respectively. Qualitatively,
and within the degree of accuracy expected from representing complex
structural systems by means of simple conceptual models, the three
alternative formulations are surprisingly similar. This suggests that
the use of the spectral displacements in estimating drift ratios is
quite acceptable for structural engineering use. It is also plausible to
extend this to the inelastic range because for ductility ratios less
than about 6, SD is representative also of yielding systems except for
long periods. In Figure 19 the nonlinear ground story drift ratio
spectrum is displayed for DZC during Event 1.

We immediately note that the ground motion retrieved from İzmit is not
at all consistent with the general damage there. Many neighborhoods
(such as Altmışevler) near the coastal area with recently completed 6-10
story reinforced concrete frame buildings had hundreds of collapsed or
heavily damaged structures. The ground conditions in these areas are
described as hill outwash on gently sloping topography, generally
equivalent to Z3 in the Turkish Seismic Code or the UBC97 SD. The drift
demand contained in the DZC record is compatible with the damage
reported there. For the remaining records, the relative amplitudes of
both sets of curves is in agreement with the reported housing damage in
the corresponding cities.

##### Discussion and Conclusions

Epicentral intensity for Event 1 was rated as MSK X, although there
existed grounds (changes in topography, severe structural damage even in
reasonably well constructed facilities, widespread pipe ruptures, etc.)
for rating it one-half degree higher in isolated pockets near Gölcük.
The isoseismal map in Figure 21 (Özmen, 2000) shows the strongly
elongated bias of areas that felt the most damaging ground motions. This
area has a width of only several km on either side of the fault line.
Unfortunately, only one instrument (SKR) had been located inside the MSK
X contour, and one horizontal component of this instrument
malfunctioned. Moreover, this instrument had been in located in a
facility where the soil conditions are not representative of much of the
remainder of the city. This is true also for the İZT record. No records
exist from Gölcük or Yalova where severe structural failures occurred.
The synoptic picture one has of the distribution of strong ground motion
within the epicentral area is thus shrouded, and must be interpreted
with caution.

The results of examination of records that were available to us show
that, some generally held truisms based on earlier near-field ground
motion data are not fully supported here. Our calculations seem to
indicate that the ground velocity is a better measure for structural
damage potential than the azimuth of the record with respect to fault
trace (i.e., fault-normal or fault-parallel components). Indeed, the
component with higher peak ground velocity seems to have the same
structural damage potential as the fault normal component. In a
complementary sense, the near field records that do not exhibit any
directivity effect yield the largest structural damage in the fault
parallel components with larger ground velocities, and these occur in
the direction of slip. The ground story drift ratio spectra of the
records show similarities with many of the near source records of the
1994 Northridge earthquake. Although the spectral accelerations of most
near-source records are not high as expected, the corresponding drift
demands are considerable. This is a good indication for
displacement-based design procedures for such earthquakes. It is
observed that the dominant period of the records show a good agreement
with the maximum drift demand. This may be a possible refinement for
displacement-based design requirements.

An event of this magnitude has a recurrence period of several hundreds
of years. Because of the lack of densely packed networks of strong
motion transducers, a rare opportunity was missed in understanding the
physics of the faulting and distribution of the ground motion in the
epicentral area. In general, the damage-causing potential represented by
the accelerograms we have examined in this study seem to fall short of
describing the mass destruction in structures along the south coast of
the Bay of İzmit.

![](media/image23.wmf){width="5.031102362204725in"
height="2.959055118110236in"}

![](media/image25.wmf){width="4.990277777777778in"
height="2.917361111111111in"}

**FIGURE 18. COMPARATIVE GROUND STORY DRIFT RATIO SPECTRA**

##### ![](media/image27.wmf){width="4.945669291338582in" height="3.0208661417322835in"} {#section-4}

**FIGURE 19. INELASTIC GROUND STORY DRIFT RATIO SPECTRA FOR DZC**

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[^1]: Research Assistant, Disaster Management Center, Middle East
    Technical University 06531 Ankara, Turkey

[^2]: Professor, Disaster Management Center, Middle East Technical
    University 06531 Ankara, Turkey