13 March, 1992 (Ms=6·8) Erzincan earthquake: A preliminary reconnaissance report

Soil Dynamics and Earthquake Engineering 11 (1992) 279- 310

13 March, 1992 (Ms-6-8) Erzincan earthquake: A preliminary reconnaissance report M. Erdik, (). Yiiziigiillii Bogazici University, Kandilli Observatory and Earthquake Research Institute, Department of Earthquake Engineering, Istanbul, Turkey

&

C.. Ylimaz, N. Akka.s Middle East Technical University, Earthquake Engineering Research Center, Ankara, Turkey

(Received 15 May 1992; accepted 18 May 1992) province is about 300000 and that of the city of Erzincan is about 80 000. The main shock and the aftershocks of 13 March 1992 Erzincan earthquake affected the Erzincan Province and the northern sections of the Tunceli province. Based on the official surveys conducted by the General Directorate of Disaster Affairs, the earthquake caused about 500 loss of lives and 2800 injuries. About 11000 households were damaged by the earthquake affecting a population of about 70 000. Most of the damage is in the province of Erzincan. In the province of Tunceli, a total of 1400 households were damaged; of these 500 either collapsed or received major structural damage. In the city of Erzincan, out of a total of 28 000 households, about 8000 (28%) were damaged by the earthquake; of these 1450 (5%) either collapsed or received major structural damage, 2880 (10%) experienced medium (repairable) damage and 3850 (14%) experienced only light damage. The earthquake caused the total collapse of about 180 buildings. These include about 35 public offices, 13 business centers, 5 hotels and 3 schools. In the surrounding provincial towns and villages: a total of 7200 households were damaged; of these 1900 either collapsed or received major structural damage. The damage suffered in the earthquake affected area is estimated to be 0.4 billion US Dollars. This estimate does not include the cost of rescue, emergency services, loss of business and employment.

1 OVERVIEW

Throughout its history the city of Erzincan has been the site of damaging earthquakes. A devastating earthquake in 1939 (Ms = 7.8) destroyed the whole town killing more than 30 000 people. Todays' Erzincan was rebuilt after the 1939 earthquake at a newer location and with a new town planning. The city was shifted from south of the railroad to the north of it and the new city plan involved wide streets, single story residential houses and other buildings limited to two stories. The building height restrictions reflected the earthquake engineering technologies in the early 1940s. This restriction proved to be a wise decision given the rather inadequate enforcement of earthquake resistant design codes. However, over the years, due to public pressure, the municipality raised the limitation to four stories and also allowed for some six story buildings along the main streets. Erzincan, being located on the North Anatolian Fault, exhibits the highest earthquake hazard in Turkey, comparable to localities on the San Andreas Fault in the USA. In deterministic terms, it belongs to the first degree hazard zone in the official Earthquake Hazard Regionalization Map of Turkey. In probabilistic terms, the peak ground acceleration that is expected to take place in Erzincan with a 10% probability of exceedance in 50 years (475 year return period) is 0.60g. l The Erzincan Province is located in North-Eastern Anatolia, roughly located between 38E-41E longitudes and 39N-40N latitudes. The capital of the province is the city of Erzincan. The total population of the Soil Dynamics and Earthquake Engineering 0267-7261/92/$05.00 © 1992 Elsevier Science Publishers Ltd.

2 TECTONICS Figure 1 provides a general neo-tectonic map of Turkey and its vicinity. 2 In the parlance of plate tectonics the Anatolian and the Northeast Anatolian Blocks are 279

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wedged out respectively to the west and the east due to the convergence of the Arabian and Eurasian Plates. The N o r t h Anatolian Fault is a dextral strike-slip fault which forms the northern boundary of the western escaping Anatolian Block. The Anatolian Block is

bounded in the south by the sinistral East Anatolian Fault. The two faults intersect at the Karhova triple Junction. The rectangular region indicated in Fig. 1 by double lines is enlarged in Fig. 2. 2 In this figure the

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13 March, 1992 (Ms : 6"8) Erzincan earthquake." A preliminary reconnaissance report

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Fig. 3. Epicentrai map of earthquakes with magnitude (Ms) larger than or equal to 5.5 for Turkey and vicinity. The data base is compiled by Kandilli Observatory and the Earthquake Research Institute and encompasses about 200 years of data until 1992. The magnitudes of events prior to the instrumental era are assigned on the basis of descriptive intensities. The rectangle to the north-east of the map encompasses the region affected by the 13 September 1992 earthquake and is enlarged in Fig. 4.

thick dashed lines indicate the ruptured segments. The Ovacik Fault is a sinistral fault involved in the opening of the Erzincan Basin. The Northeast Anatolian Fault is a sinistral fault with some thrust component. This fault can be associated with the 21 N o v e m b e r 1939 Tercan earthquake, generally regarded as a foreshock to the 26 December 1939 Erzincan earthquake. Most of the North Anatolian Fault is ruptured in the earthquake sequence starting in 1939 and ending in 1967. However, Barka & T o k s f z 2 indicate that there exists a 7 5 k m long

unruptured segment of the fault to the immediate east of Erzincan which can be classified as a siesmic gap.

3 SEISMOLOGICAL ASPECTS Figure 3 provides an epicentral m a p of earthquakes with magnitude (Ms) larger than or equal to 5"5 for Turkey and the vicinity. The data base is compiled by Kandilli Observatory and the Earthquake Research Institute and

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282

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encompasses about 2000 years of data from 100 BC to 1992. The magnitudes of events prior to the instrumental era are assigned on the basis of descriptive intensities. As it can be seen, the North Anatolian Fault has been the site of numerous damaging earthquakes during several episodes of rupture in the last two millennia. During the last rupture episode, the seismic activity has migrated from east to the west starting with the 26 December 1939 earthquake and ending with the 22 July 1967 earthquake. The important events of this episode are: 3 26 December 1939 Erzincan earthquake (Ms = 7.8, Io = XI); 20 December 1922 Erbaa-Niksar earthquake

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(Ms = 7.1, Io = X); 26 November 1943 Ilgaz-Ladik earthquake ( M s = 7 . 3 , I o = X I ) ; 1 February 1944 Bolu-Gerede earthquake (Ms = 7.3, Io = X); 13 August 1951 Gerede-Ilgaz earthquake (Ms = 6.9, Io = IX); 26 May 1957 Abant earthquake (Ms = 7-0, Io=X) and 22 July 1967 Mudurnu earthquake (Ms = 7-1, Io = X). The seismological characteristics of the North Anatolian Fault can be summarized by the following conclusions: 4 Major earthquakes, usually associated with hundreds of kilometers of fault rupture, do not tend to occur on the same segment of the fault within several decades of time (such as the 26 December 1939, 26 November 1943 and 1 February 1944 earthquakes along the North Anatolian Fault and also the 9 January 1857 and 18 April 1906 earthquakes along the San Andreas Fault). Such earthquakes do not produce aftershocks of magnitude greater than 5.0. The aftershocks tend to be concentrated near the ends of the fault rupture, and these regions may continue to experience moderate earthquakes for some years following the main event. The rectangular region indicated by double lines in No

Day

2 3 4 5 6 7 8

13 13 14 15 21 29 28

Fig. 3 is enlarged in Fig. 4. In this figure the epicenters of all earthquakes in the Erzincan region are indicated, utilizing the same data base used in the preparation of Fig. 3.

More than ten earthquakes with maximum intensities between V I I I - X I took place in this millenium in the Erzincan region as indicated in Table 1.2 The 26 December 1939 Erzincan earthquake (Ms = 7.8, Io = XI) is the largest and most destructive earthquake in Turkey since 1668, causing the loss of more than 30000 lives and destruction of more than 140 000 homes. The earthquake created a 360 km surface rupture between Erzincan in the east and Amasya in the west, with lateral offsets reaching 7.5 m. The focal parameters of the 13 March 1992 Erzincan earthquake as reported by U S G S preliminary Determination of Epicenters (PDE No. 11-92) are as follows: UTC time hr: mn: sec 17:18:40-1

Lat

Lon

Depth

MB

Msz

SD

#Sta.

39.71

39.57

28D

6.2

6-8

1.1

259

The depth of the main event, D = 28km, was constrained by at least two pP phase readings. The best double coupled solution provided by USGS NEIC Quick Epicenter Determinations indicates a strike slip earthquake with a seismic moment of Mo = 1.2 x 1019. The first nodal plane (NP1) has a S t r i k e = 2 1 2 , Dip=74, Slip=-7 and the second nodal plane (NP2) has a strike= 304, Dip = 84, Slip = 163. The orientation of the NP2 coincides perfectly with the strike of the North Anatolian Fault at the epicenter. Experience with strike-slip earthquakes along the North Anatolian Fault indicates that the events with Ms > = 6.8 have always been associated with surface ruptures. For example, the 13 August 1951 GeredeIlgaz earthquake with the same magnitude of 13 March 1992 Erzincan earthquake has been associated with a verified length of rupture about 32km (60 km estimated) and with a right-lateral displacement of about 60 cm. No fault rupture has yet been found to be associated with the 13 March 1992 Erzincan earthquake. Main aftershocks as reported by USGS-PDE are as follows: UTC time hr: mn: sec 18: 37:53"6 22: 47:45.4 01:24:33.6 16:16:24.5 23:15:49.8 09: 26:19.4 09:32:42.8

Lat

Lon

Depth

MB

39"82 39:98 39'50 39-53 39-62 39-39 39"68

39-48 39:70 39.64 39'88 39.71 39-71 39"84

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4'7 4.6 4.6 5.5 4.7 4.2 4'2

Msz

4.3 5-8 3-8

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49 40 27 234 51 11 10

Turkish (Bogazici University - - Kandilli Observatory and T O B I T A K - - Marmara Research Institute), French and German teams have established short-aperture microseismic networks immediately after

13 March, 1992 (Ms = 6"8) Erzincan earthquake: A preliminary reconnaissance report Table 1. List of significant earthquakes (I>=VIII) at Erzincan

Date

Io

1011 VIII 1045 IX-X 1168-70 VIII 10April 1254 VIII 1268 IX 8May 1287 VIII 8 December 1374 VIII 1422(?) VIII 13 April 1456(?) VIII 1458 X 21 December t482 XI-X 1579 VIII 17 June 1584 IX 28 June 1667 VIII 23 June 1784 IX 1789 VIII 1939 XI

Remarks Similar to 1939 12 000 deaths 16000 deaths 15 000 deaths (Erzincan-Erzurum) Many deaths

32 000 deaths (Erzincan-Erzurum) (Erzincan-Erzurum) 15 000 deaths (Erzincan-Erzurum) 1500 deaths 10 000 deaths (Erzincan-Erzurum) 33 000 deaths

the earthquake for the assessment of the aflershock activity. Figure 5 provides the epicentral locations of the main shock, important aftershocks as well as the general aftershock area as determined by the aftershock networks.

4 INTENSITY A S S E S S M E N T S The isoseismal maps of earthquakes associated with the North Anatolian Fault have elliptical shapes with major axis parallel to the fault. 5 Based on the regression analysis of the available intensity maps, the mean endto-end distances for two different intensity contours (D(VIII) and D(VIII)) for the 13 March 1992 Erzincan earthquake of M s = 6.8 can be estimated to be D(VIII) = 47 km and D(VII) = 56 km.

283

The intensity scale used in Turkey is based on the MSK-64 scale. 6 This scale classifies the building types, damage ratio and the damage type and assigns intensities accordingly. For example, at intensity VI: 59% of rural structures (stone or adobe masonry) receive light damage and 5% receive medium damage; at VII: 75% of rural structures receive heavy damage and 50% of the reinforced concrete structures receive light damage; at VIII: 75% of rural structures collapse and 75% of the reinforced concrete structures receive medium damage with 5% heavy damage; at IX: 50% of the reinforced concrete structures receive heavy damage with 5% collapse. These descriptions would translate to maximum intensities of V I I I - I X in the town of Erzincan (epicentral region) and VII-VIII in the surrounding villages. No detailed intensity assessments on a township by township or village by village basis were attempted in our investigations. However, to provide an idea on the extent and the distribution of damage, the compilations done by the General Directorate of Disaster Affairs on a village basis are illustrated in Figs 6 and 7. Figure 6 portrays the regional distribution of the earthquake damage, where the villages with collapsed or heavily damaged houses are indicated with a fully shaded circle and the villages with houses receiving only medium or light damage are indicated with one-half shaded circles. In Fig 7 the damage in and around the Erzincan Basin is illustrated. The three types of shading used in Fig 7 indicate the percentage of medium damage plus heavy damage and collapse out of the total dwelling stock in the village. If this percentage is less than 25% only onefourth of the circle is shaded. If it is between 25-50% one-half of the circle is shaded and if the percentage is greater than 50%, the whole circle is shaded. One-fourth shading roughly corresponds to intensity VI, one-half shading to V I - V I I and full shading to VII-VIII.

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Fig. 5. Instrumental epicenter of 13 March 1992 main shock and the important aftershocks. The general aftershock area is shaded. See Section 3 for the source parameters of the mainshock and the important aftershocks.

284

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Table 2. Peak ground motion values \

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Fig. 6. Regional distribution of the earthquake damage. Villages with collapsed or heavily damaged houses are indicated with a fully shaded circle and the villages with houses receiving only medium damage are indicated with onehalf shaded circles. 5 STRONG GROUND MOTION Strong ground motion accelerograms of the main shock were obtained at three sites (Erzincan, Refahiye and Tercan) respectively at epicentral (instrumental) distances of 5, 70, and 70 kms (see Fig 5 for locations). The

Erzincan accelerograph station is (see Fig 24 for location) 3-5km away from the trace of the North Anatolian Fault. The strong ground motions were recorded by Kinemetrics SMA-1 analog accelerometers. The accelerographs are part of the national strong motion network owned and operated by the Ministry of Public Works and Settlement. Accelerometers are located in the basements of one or two story high small buildings used as meterological stations. The Erzincan station is sited on deep alluvium. The records were digitized by the General Directorate of Disaster Affairs and were base line corrected and processed using the Erdik & Kubin 7 procedure. The records are low-pass filtered using an Ormsby filter with cut-off frequencies of 23 and 25Hz. The high-pass filtering is applied using recursive Butterworth filtering with record-specific cut-off frequencies. Table 2 provides the peak ground motion values (PGA, peak ground acceleration), (PGV, peak ground velocity) (PGD, peak ground displacement) obtained from these records.

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Fig. 7. Distribution of damage in and around the Erzincan Basin. The shading indicates the percentage of medium damage plus heavy damage and collapse out of the total dwelling stock in the village. If this percentage is between 0-25% only one-fourth of the circle is shaded, between 25-50% one-half of the circle is shaded and if the pe_rcentage_isgreater than 50% the whole circle is shaded. One-fourth shading roughly corresponds to MSK-64 intensities of VI, one half shading VI-VII and full shading VII-VIII.

13 March, 1992 (Ms = 6.8) Erzincan earthquake." A preliminary reconnaissance report

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Fig. 9. Velocity traces of the N-S, V and E-W components of the 13 March 1992 earthquake recorded at Erzincan station. (Velocities in cm/s, times in s).

Fig. 8. Acceleration traces of the N-S, V and E-W components of the 13 March 1992 earthquake recorded at Erzincan station• (Accelerations in gals = cm/s 2, times in s). The local magnitude (ML) of the mainshock is determined from maximum trace amplitudes of calculated Wood-Anderson seismograms as described in Uhrhammer & Collinsfl Using the accelerograms obtained from the three stations, a local magnitude (ME) of 6'4 -- 6"5 has been obtained. Figures 8, 9 and 10 show the acceleration, velocity and displacement traces of the Erzincan accelerogram. Figure l l depicts the particle displacement in the horizontal plane. The Fourier amplitude transform of the accelerations and the pseudo relative velocity response spectra for the two horizontal components are plotted respectively in Figs 12 and 13. The Erzincan accelerogram is very similar to the acceleration record obtained at Station 2 of the Cholame-Shandon array in the 27 June 1966 Parkfield, California earthquake (Ms = 6.2). Station 2 was at 80m from the San Andreas Fault, which, in many respects, is identical to the North Anatolian Fault. The ground displacements of both earthquakes indicate the same impulsive form, where the ground moves around 30 cm and then returns to the original position. This displacement pulse manifested itself in the E - W and N - S shifts of about 35cm experienced by the main transformers in the

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Fig. 11. Particle displacement in the horizontal plane at Erzincan station associated with the 13 March 1992 earthquake. (Displacements in cm). Erzincan switch yard. These transformers were placed on dollies that can freely move on rails for purposes of easy maintenance. Such a simple displacement pulse may constitute the characteristic of near field strong ground motion associated with strike-slip fault mechanisms. Similar shapes were also observed in some of the aftershocks of 13 March 1992 earthquake. The original EW and NS directions of the Erzincan record can be transformed into E 34 S (parallel to the fault strike) and N 34 E (perpendicular to the fault strike) components. The peak acceleration, velocity and displacement values of the E 34 S component are respectively 0.42 g, I 10 cm/s and 26 cm. The same peaks for the N 34 E component are 0-46 g, 74 cm/s and 18 cm. The acceleration, velocity and displacement traces of the E 34 S component (strike direction) are given in Fig. 14. As it can be assessed, the ground motion in a direction parallel to the fault strike consists of a displacement pulse of about 45 cm amplitude and 2 s duration. To model the near field ground motion, Brune 9 considered an instantaneous tangential stress pulse applied to the interior of a circular fault surface which sends a shear wave in a direction perpendicular to the fault plane. The ground displacement at the fault surface is given by: u(x = O, t) = (a/3T/ff)(1 - exp ( - t / r ) ) (1) and the velocity by:

v(x = 0, t) = ( a ~ / # ) exp (-t/~-)

(2)

where ff is the Lame constant, ~ is the effective stress,/3 is the shear wave velocity and ~- is the term of decay factor of the order r//3, r being the radius of the circular fault surface. The effective stress, o, is taken as the difference between the initial stress, ao, and the minimum stress, drmin, before it levels off at the final stress, cq. With the maximum velocity equal to v - - l l 0 c m / s and assuming/3 = 3.5 k m / s , ff = 3 • 10 I1 dyne/cm 2, and taking the free-surface amplification as 2, eqn (2) yields the following effective stress value:

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= v#//3 = 46 * 10 6 dyne/cm z = 46 bars The Fourier amplitude spectrum, D(~), of this nearfield S-wave displacement is given by: 9 D(~2) = ( a / 3 / # ) ( t / ¢ ( ~ ( ~ 2 + f 2 ) ) )

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(4)

The E 34 S component of the Erzincan accelerogram recorded at only a few kilometers from the fault trace can be assumed to constitute the near field shear wave motion. The best fit of the Fourier spectrum of E 34 S component to the theoretical shape of eqn (3) or (4) can be obtained by taking (a/3/#) = 150 c m / s and.f~ = 2 Hz or ~- = 0.5. Since ~- is of the order r//3, the minimum radius of the dislocation surface, r, will be approximately 1.4 km. F r o m definition of apparent stress: l°

~l~' = #E/ Mo where rr' = (% + tyl)/2 is the apparent stress, ~/ is the seismic efficiency factor, E is energy radiated and Mo is the siesmic moment and using U h r h a m m e r & Bolt's L1 relationship to approximate the radiated energy E:

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E = 4 * 1021dyne.cm can be obtained. Using M o = 1.2 * 1019 N m the apparent stress can be obtained to be about 10 bars. In connection with the aftershock studies conducted by Bogazici University - - Kandilli Observatory and the Earthquake Research Institute, two K I N E M E T R I C S SSA-2 accelerometers were installed in Erzincan at the locations (called Erzincan-Stn.1 and Esentepe-Stn.2) as shown in Fig. 24. Table 3 provides the date, time, coordinates, epicentral distance, depth, magnitude, station number and the peak ground acceleration values for important strong motion accelerograms. Only records with the P G A of any channel greater than 10 gal is included in the table. The focal parameters of the earthquakes are determined on the basis of the micro-earthquake recording array operated by Kandilli Observatory and the Earthquake Research Institute.

6 E R Z I N C A N BASIN The prominent geological feature in the earthquake affected area is the Erzincan Basin. The Erzincan Basin is located along the N o r t h Anatolian Fault Zone between longitudes 39-25E-39-83E and latitudes 39"53N-39.87N in Northeastern Anatolia and contains the city of Erzincan as well as a number of townships and m a n y villages. A simplified geological m a p of the basin is provided in Fig. 15.12 The Erzincan Basin is described as a pull-apart basin. Its long axis strikes N W - S E parallel to the N A F Z and is about 50 km long. The basin widens to the SE to a width of about 15 km. The Euphrates River meanders in the central part of the basin. In the basin the exposed sediments belong to the Plio-Quaternary stage of deposition, characterized by fluvial facies, coarse clastics and basin margin conglomerates. The conglomerates reach up to a 200 m thickness. In the basin the alluvial fans are composed of recent

Table 3. Strong motion aftershocks Date

25 March 25 March 25 March 31 March 31 March 1 April 4 April 25 April

Time

5:57:18 8:11:56 23:06:32 1:59:51 2:08:36 19:31:30 7:34:95 10:33:14

Coordinates

Epic. dist.

Depth

Mag.

Stn.

39.59N-39.62E 39.71N-39.50E 39.73N-39.38E 39.65N-39.55E

21 6 29 12

18 12 11 1

3'5 2-6 2"8 3-2

39.86N-39.96E 39.88N-39.52E

13 25

5 5

3-3 2-3

1 1 1 2 2 2 2 2

P.G.A. (gal) N-S

V

E-W

31 12 7 19 14 10 5 16

29 I1 7 25 26 8 12 13

52 5 11 29 20 8 4 10

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Fig. 14. Acceleration, velocity and displacement components of the 13 March 1992 earthquake recorded at Erzincan station, in direction parallel to the fault strike. (Units are in gals, cm/s, cm and s). debris flows and coarse-grained braided stream deposits. The central part of the basin is filled mostly by silts, sands and gravels. The total thickness of these PlioQuaternary sediments are estimated to range between 0.5 and 3.5kms. Borehole data obtained by State Hydraulic Works (DSI ° ) in the Erzincan Basin indicate sand and gravel series with small amounts of clay and silt. The layers do not indicate any continuity between different boreholes. The deepest borehole is about 250 m deep and does not reach the bedrock. The water table to the north of the basin is at a depth of about 30m, (e.g. in Erzincan) decreasing to a few meters towards the center. A topographical map of a N-S strip of the basin around Erzincan and an approximate N - S section of the basin are provided in Fig. 16. The geological features indicated on this section can be correlated with the geologic map provided in Fig. 15. The two-dimensional resonance characteristics of

sediment filled basins have been theoretically analysed by Bard & Bouchon. 14 It has been determined that: (1) the frequency of the gravest peak of the transfer function is the same at each site and the ground motion is in phase across the whole basin; (2) the corresponding amplification is the largest in the basin center, and decays toward the edges. At the fundamental frequency, the peak value is about eight times larger at the center than that at the edges or the bedrock. At higher modes both lateral and vertical displacements are important and the largest amplifications may be located at the mid-edges. These findings are also supported by the experimental investigations of King & Tucker.15 The frequency of the gravest peak is near the one-dimensional resonance frequency for shape ratios (maximum sediment thickness/basin half-width) less than 0'2. In Fig. 17 the power spectra of the N - S and E - W acceleration components of the 13 March 1992 earthquake recorded at Erzincan station are provided. The spectra are drawn for the free vibration era of the record, (i.e. after the main displacement pulse in the displacement trace) and are smoothed using a 0.2 Hz width Parzen window. The low frequency peaks observed on the spectra can be correlated with the modal frequencies of vibration of the basin. In the N - S direction these peaks are at the frequencies of 0.30, 0.68, 1.12, 1-55 and 2.1 Hz. In the E - W direction at 0.38, 0.72, 1.10, 1.55 and 2.0Hz. A preliminary finite element model of the basin can be attempted to see if any of these frequencies can be correlated with the theoretical vibration modes of the basin. The basin is approximately 50 km long and around Erzincan City it is about 12km wide. For preliminary modeling a plane strain rectangular model is utilized with a 12 km width and a constant 0'5 km depth. Such simple models are known to yield satisfactory results for the vibration patterns and modal frequencies. I4 The basin is filled with fluvial and fan deposits consisting of sand and gravel. The density and the Poisson's ratio of the medium are assumed to be respectively 1700kg/m 3 and 0.35. The shear wave propagation velocities (Vs, in m/s) are determined on the basis of the empirical compilations of Ohta & G o t o J 6 which, for sand and gravel of alluvial origin yield: Vs = 125.02 H

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where H is the depth in meters. The results of the eigenvalue analysis of the model yield modal frequencies in the vicinity of 0.27-0.29 Hz for the first few modes of vibration. On the basis of this preliminary analysis, the fundamental frequency of vibration of the basin seems to be in the range of 0.3Hz and indicates a good correlation with the observed frequency of the first peak.

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7 GEOTECHNICAL P R O B L E M S

the slides occured along the steeper slopes of the road cuts, blocking transportation routes and damaging road surfaces. Land spreading, involving horizontal movement of liquefied deposits with extensive extension cracking

The main event and some of the large aftershocks triggered numerous small landslides, rockfalls and avalanches throughout the mesoseismal region. Most of

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be met for the liquefaction susceptibility: (1) saturated alluvial sandy layers within 20 m from ground surface; (2) ground water level within 10m from ground surface; (3) D50 values between 0.02 and 2 m m in grain size accumulation curve; and (4) standard penetration test blow count less than 30. Most of the sites in the basin cannot easily meet the third criterion on the grain size and this limitation is believed to constitute the spotty characteristics of the liquefaction observed during the Erzincan earthquake. At the eastern entrance of the basin, near G6kbay~r Railroad Station extensive response of the railroad embankment and the soil media underneath resulted in plastic deformations in the embankment with manifestations of undulations in the railroad track (Fig. 19). The trains were able to pass these undulations very slowly under controlled conditions. The railroad embankment is located on fiat terrain with marshy ground conditions. An interesting feature right after the earthquake was the formation of a grid of cracks on the snow layer covering this terrain (Fig. 20). A search conducted after the melting of snow on the same location did not reveal any evidence of these cracks in the ground. To the south of the water regulator at Mertekli a typical manifestation of liquefaction is observed with sand mounds and extensional cracking (Fig. 21). The liquefied material consists of fine sand with uniform grading. The collapse of a road embankment near the regulator (Fig. 22) is also found to be associated with the liquefaction as evidenced by the ponding of water at either side of the embankment. The liquefaction is

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occurred at a number of localities in G6kbayir, Mertekli, Eksisu and Davarli in areas with a high water table and granular soils. Figure 18 provides a regional distribution of surface cracks and liquefaction in the earthquake affected area. Fissuring and settling of sediments have caused damage to irrigation facilities. At many instances, the compaction of saturated materials was associated with the ejection of water-silt mixtures and the formation of sand mounds. The following criteria of Tokida 17 generally need to

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13 March, 1992 (Ms = 6.8) Erzincan earthquake: A preliminary reconnaissance report

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Fig. 19. Railroad undulations near Geqitk6y. The response of the soil media underneath resulted in plastic deformations in the embankment with manifestations of undulations in the railroad track (see Fig. 18 for location).

believed to be of limited extent and surficial, since the regulator structure itself with deeper foundations does not appear to be affected by the liquefaction. Soil collapse and cracking are observed near the mineral water bottling factory at Eksisu. This locality is

very near the North collapse the mineral out. Also at Eksisu road embankments observed (Fig. 23).

Anatolian Fault Zone. F r o m the water containing fine sand rushes extensive cracking and failure of at several locations have been

Fig. 20. Formation of a grid of cracks on the snow layer covering marshy terrain right after the earthquake.

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8 STRUCTURAL DAMAGE

Fig. 21. A typical manifestation of liquefaction with sand mounds and extensional cracking to the south of the water regulator at Mertekli (see Fig. 18 for location).

The distribution of the building stock according to their structural type is shown in Fig. 24. Almost all of the multistory buildings (2-6 story high) have reinforced concrete moment resisting frames with unreinforced masonry infill walls. These buildings are located along the main streets which cross the city almost at right angles; one running in the north-south direction (Ordu Street) the other in the east-west direction (Fevzi Pasa and Halit Pasa Streets) and they are utilized as commercial centers, offices, banks, schools, mosques, hospitals and public buildings. The remaining buildings consist of three-four story high reinforced concrete residential apartments, one-two story unreinforced brick masonry or single story adobe houses and single story prefabricated lightweight houses. Most of the districts have a uniform building texture. The districts on both sides of Ordu street are, however, of a mixed texture. The prefabricated houses can be classified under two groups. Group I encompasses those that were built after the 1939 Erzincan earthquake with imported material. These houses have wooden frames plastered from both sides with wire mesh and are locally called 'Kurma Ev'. Group II prefabricated buildings were built recently with light-weight panels. The small businesses, located to the south-east of the city, consist of single story reinforced concrete shops. According to the survey carried out by the General Directorate of Disaster Affairs, the distribution of damage in the city is given in Figs 25, 26 and 27. The

Fig. 22. Collapse of a road embankment near the Mertekli irrigation regulator (see Fig. 18 for location).

13 March, 1992 (Ms = 6.8) Erzincan earthquake." A preliminary reconnaissance report

293

ing needed); severe (heavy structural damage: to be demolished) and collapse. In the above classification, the judgment was based mainly on the external appearance of the buildings. According to this survey, 40% of these buildings collapsed totally, 24% are severely damaged, 6% moderately damaged, 19% lightly damaged and 11% slightly damaged. It was assessed that about 60% of the severely damaged buildings, 90% of the moderately damaged buildings and 64% of the lightly damaged buildings were four stories high. It would be rewarding to be able to link the degree of damage to a particular characteristic of the building stock. Classifying the buildings according to that particular characteristic and discovering the linkage sounds simple. However, the question is: What is that characteristic? Is it the type of the materials used in the construction or the construction technology? Is it the number of stories or the function of the building? In the following we will present the possible causes of failure according to the type of the building. Reinforced concrete buildings

Fig. 23. Extensive cracking and failure of road embankments at Ekfisu (see Fig. 18 for location).

numbers marked over each district indicate the percentage of the damaged households and small businesses. Regarding the public buildings, out of a total of 256, 153 were reported to be lightly damaged, 20 moderately damaged (to be repaired), 54 heavily damaged (to be repaired and strengthened) and 29 collapsed. Out of the 43 school buildings, 31 are reported to be lightly damaged, 3 moderately damaged and 9 heavily damaged. There is no totally collapsed school building. The damage was mainly concentrated on the reinforced concrete frame buildings. An independent survey supervised by the authors was carried out on reinforced concrete buildings. The survey covered a sum of 161 buildings. Residential apartment complexes, public buildings, shops, schools, mosques, factories, etc., were included. The damage classification adopted was: slight (no structural damage); light (light structural damage: repairing needed); moderate (moderate structural damage: repairing and strengthen-

In accordance with the classification of the buildings described in the legend of Fig. 24, the possible causes of failure are summarized below. Whenever appropriate, photographs are included to illustrate the mentioned causes. It should be stressed that building damages observed are not necessarily the consequence of a single cause, but generally of a combination thereof.

Soft story. As shown in Figs 28 and 29, the columns in the first story of many buildings failed due to open space requirement for shops without infill walls. No special precaution was taken to increase ductility. This was especially evident for structures (commercial centers, hotels, public buildings, etc.), located along either side of main streets. Soft story effect also encompasses failures in middle stories of buildings due to extensive story heights and/or very slender columns. Lack of redundancy. Similar capacity columns of reinforced concrete framed structures left no redundancy in the structure. Simultaneous failure of such columns led to either collapse of heavy structural damage (Figs 30, 31). Shearwall application is very rare, almost none. The reinforced concrete building shown in Fig. 32 utilized shearwalls in the design and thus experienced no damage. Concrete quality. Poor quality concrete was evident in most of the damaged buildings. On the basis of Schmid Hammer tests, the median concrete strength was determined to be about 10 MPa.

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Fig. 24. Erzincan city layout and the distribution of the building types. The names indicate various districts. S.M.A.-1 indicates the location of accelerograph station.

13 March, 1992 (Ms : 6-8) Erzincan earthquake: A preliminary reconnaissance report

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Connections. This type of failure is very common in the reinforced concrete structures (Fig. 33). In many failure instances it was observed that the connection zone and the frame elements connected to it were not properly confined (Fig. 34). The diameter, spacing and detailing of the ties were inadequate to provide the necessary ductility. Bond and anchorage failures at the connections (Fig. 35) can be attributed to wrong detailing and poor concrete quality.

Short column. The portion of the columns over the partial infill walls, especially at the basements of the buildings behaved as short columns and failed in shear (Fig. 37).

Narrow columns. Due to the architectural requirements narrow columns flush with the walls were used and depth was increased to provide stiffness in the short plan dimension. No accommodation was made for the long plan direction. Accordingly, the increased flexural strength in that direction resulted in a shear failure (Fig. 36).

collapse) buildings in each district.

Structural alterations. Addition of stories to the existing buildings changed the behavior of the structure and caused failure. The relatively higher percentage of damage to six story buildings observed along the main streets can be attributed to this kind of alteration. Insufficient repair. Some of the structures experienced repairable damage during the 1983 (18 November, 1983, Ms = 4-8) earthquake. A high percentage of those buildings either collapsed or got heavily damaged during the 13 March 1992 earthquake because of inadequate repair. In many instances plastering over the existing cracks was considered as structural repair (Fig. 38). Torsion. The end blocks of a group of buildings were subjected to torsion around the middle block and, thus either collapsed or got heavily damaged (Fig. 39). Pounding. Due to the difference in elevations at floor levels, pounding failures occurred in some adjacent structures. Strong beam weak column. Weak columns associated with strong and rigid beam and floor assemblies caused concentration of damage in the columns.

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The causes of failure in masonry buildings are much more limited in number than those in reinforced concrete buildings. Some significant causes of failure are summarized below.

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Fig. 28. Soft story failure. Infill walls do not exist in the first story,

Insufficient wall strength. The walls had too many openings and the number of walls in a given direction was insufficient. Wrong construction practices. Heavy damage and

collapse occurred in some load bearing brick buildings due to the use of the wrong type of bricks and wrong coursing (Fig. 40), or lack of tie beams and lintels (Fig. 41). Figure 42 shows a masonry building which is properly constructed and survived the earthquake.

Fig. 29. Soft story failure. Open space requirement for shops excluded the possibility of walls.

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Fig. 30. Lack of redundancy. Simultaneous failure of similar size and capacity columns did not leave any possibility of redundancy.

Structural alterations. As observed in Fig. 43, load bearing walls were removed from the first story of a two story load bearing masonry building and this caused collapse. In addition, there were cases of added floors (Fig. 41).

Poor maintenance. The railway station had survived a much larger earthquake with slight damage in 1939. However, due to poor maintenance, the rain water penetrating through the masonry walls caused weakening of the mortar which caused cracking of the concrete block infills (Fig. 44).

Fig. 31. Lack of redundancy. Simultaneous failure of similar size and capacity columns did not leave any possibility of redundancy.

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Fig. 32. A building which utilized shearwalls experienced no damage in spite of incomplete infill walls.

Prefabricated buildings Only superficial damage was observed in prefabricated buildings. Figure 45 is a typical example of those buildings belonging to Group I and Fig. 46 of Group II.

Other structures

Fig. 33. Connection failure. The diameter, spacing and detailing of the ties were inadequate.

The earthquake damage observed on the other structures such as industrial buildings and bridges are individually addressed in the following paragraphs: The textile factory was built around the 1950s and the framing consists of 6m x 7.5m individual sheds (Fig. 47). The roof beams were repaired after the 1983 earthquake. The 1992 earthquake caused failure in beam column joints (Fig. 48). The possible cause of this failure was the insufficient dimensioning of the columns carrying the heavy roof. The sugar factory of about 25 m height is the only steel structure in the city. The damage observed was the failure of the brick walls. The steel frame was essentially intact but the fall of the 36m high chimney caused secondary damage in the building (Fig. 49). The flour mill, a 25 m tall reinforced concrete building experienced extensive damage due to poor joint strength (Figs 50, 51). The internal machinery was overthrown because of inadequate floor connections. One of the animal fodder mills collapsed (Fig. 52). The second one, which is a recent construction, experienced light damage (Fig. 53). The prefabricated concrete structure of the paddock

13 March, 1992 (Ms = 6-8) Erzincan earthquake." A preliminary reconnaissance report

Fig. 34. Connection failure. The diameter, spacing and detailing of the ties were inadequate.

299

Fig. 35. Connection failure. Bond and anchorage failure due to poor concrete quality and wrong detailing.

Fig. 36. Narrow columns in the short plan dimension. Increased flexural strength in that direction resulted in shear failure.

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Fig. 37. Short column failure. Portion of a column above the partial infill walls behaved as short column and failed in shear.

belonging to the abbatoire collapsed due to insufficient connection between the heavy roof beams and columns (Fig. 54). No failure was observed in two reinforced concrete arch highway bridges and one steel trussed railroad

bridge about 5 0 k m to the east of Erzincan. One reinforced concrete slab type highway overpass 10km away from the city was reported to have damage at its abutments and piers. This was due to the instability of loose and heavy material at the abutments.

Fig. 38. Insufficient repair. Plastering over existing cracks should not be considered as a structural repair.

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Fig. 39. Torsion. The end sections of a building block were subjected to torsion.

Rural structures

The majority of the rural structures in the region consisted of masonry buildings (adobe, stone, concrete block and brick), and some timber framed buildings (with adobe and stone fillers or closely spaced laths nailed to vertical members and plastered). The adobe and stone buildings were usually covered with a fiat heavy earthen roof or a galvanized corrugated metal (Figs 55, 56). Among the typical failure patterns are: partial or complete collapse of the building, separation of peripheral walls, vertical cracks at the joints or intersecting walls, out of plane collapse of peripheral walls, and vertical cracks at the junctions of walls. The timber framed houses had only plaster damage.

9 EARTHQUAKE RESISTANT DESIGN CODES ISSUE The previous editions of the Turkish Earthquake Code were published in 1942, 1953 (IAEE Is) and 1961 ( I A E E 19 ) . T h e l a s t e d i t i o n w a s p u b l i s h e d i n 1975 ( I A E E20) and it is still applicable. It was not until the last edition that the concept of ductility and detailing for confinement in the critical regions were specified and enforced. For reinforced concrete framed structures the maximum lateral force coefficient was 0.10 (for buildings up to 40m high) according to the 1961 edition of the Code.

Fig. 40. Wrong construction practice. Wrong type of bricks and wrong coursing in a masonry building caused premature failure.

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Fig. 41. Wrong construction practice. Lack of tie beams and lintels in the added story of a masonry building caused failure.

This coefficient is 0"15 (to be increased to 0"225 for important buildings such as hospitals, etc.), in the 1975 edition. There exist a number of old buildings which were designed and built according to the old codes. Even if those buildings had appropriately followed the existing codes of their times, collapse or heavy damage

would have been inevitable due to the inferiority of the codes in terms of both strength and ductility. From the strong ground motion record of this earthquake in Erzincan it is obvious that the duration of the main portion of the strong ground motion was limited to only several seconds representing a single

Fig. 42. Properly constructed additional story of a masonry building did not cause any distress during the earthquake.

13 March, 1992 (Ms -- 6"8) Erzincan earthquake: A preliminary reconnaissance report

Fig. 43. Structural alteration. Load bearing walls removed from the first story was the main cause of failure.

303

displacement pulse (see Section 5). Accordingly, the behavior of the structures was governed by strength rather than ductility. The structures which survived (including brick masonry buildings) had apparently enough strength to resist this initial impact. Although in general not included in the design, the contribution of the infill walls both to stiffness and strength was substantial. For those structures, if prevailing amplitude had continued through several cycles, heavy damage or collapse would have been inevitable. In most of the heavily damaged or collapsed reinforced concrete structures, plastic hinging of the joints took place within the first high amplitude excursion of the ground motion. The joints had neither the adequate amount of strength to take care of the first shock, nor the ductility. According to the applicable earthquake code, reinforced concrete buildings with unreinforced infill walls are to be designed for a maximum base shear coefficient of 0"15 or 0.225. Both N - S and E - W response spectra give an average value of acceleration a = 0.75g (see Fig. 13). From the ratio of the above values, a rough overall ductility requirement is estimated to be between 0.75/0.15 = 5 and 0.75/0.225 -- 3.3. Since the prevailing construction practice in the city is far from supplying these ductilities, the main reason behind the good earthquake performance of some structures is the overstrength supplied by the infill walls. If the strong ground motion had gone through several oscillations of amplitude comparable to the initial one, the overstrength provided by the infill walls would not have been observed and the structures would have collapsed.

Fig. 44. Poor maintenance. Rain water penetrating through the masonry walls caused weakening of the mortar, thus cracking occurred in the concrete block infills.

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Fig. 45. A typical example of prefabricated building belonging to Group I: 'Kurma Ev'. Constructed with wooden frames plastered from both sides with wire mesh. Only superficial damage was observed.

10 LIFELINES The following is a point-by-point summary of the performance of the lifelines in the earthquake affected area. Tercan dam, located 70 km to the east of Erzincan, is

built on the Euphrates River. The information gathered reveals that the dam and the hydroelectric facilities did not experience any damage as a result of this earthquake. The railroad connecting Sivas, Erzincan and Erzurum

Fig. 46. A typical example of prefabricated building belonging to Group II. Lightweight panel construction. Only superficial damage was observed.

13 March, 1992 (Ms = 6'8) Erzincan earthquake: A preliminary reconnaissance report

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Fig. 47. General view of the reinforced concrete textile factory. Framing consists of 6 m x 7-5 m individual sheds with heavy concrete slab roofs.

transverses the Erzincan basin and the earthquake affected area from west to east. With the exception of embankment problems causing undulations in the railways at the eastern end of the basin, the railroad did not experience damage and it provided a continuous

service. The embankment problems were covered in the section on geotechnical aspects. At certain locations of the main and secondary highways the earthquake triggered embankment instabilities with extensive cracking and in a few cases with

Fig. 48. Textile factory. Connection failure possibly due to insufficient dimensioning of the columns carrying heavy roof.

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Fig. 49. Sugar factory. The only steel structure in the city. The steel frame was essentially intact but the fall of the 36 m high chimney caused secondary damage.

collapses (see Section 7). These sections were quickly repaired and the traffic moved without any difficulty. With the exception of one overpass on the ErzurumKemah road, near Erzurum, there was no damage to the bridges (see Section 9).

No damage was reported in the water mains supplying the city of Erzincan. However, a large part of the city did not receive any water for a few weeks due to the fact that the water pipe connections to the damaged buildings were not repaired and the city water

Fig. 50. Flour mill. About 25 m tall reinforced concrete building experienced extensive damage due to poor joint strength. The internal machinery was overthrown because of inadequate floor connections.

13 March, 1992 (Ms = 6"8) Erzincan earthquake: .4 preliminary reconnaissance report

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system did not have any redundancy. No substantial problems were reported with the sewage system. The main switch yard of the electricity system feeding the city of Erzincan was intact after the earthquake with the exception of shifts at the base of the main transformers. There were some failures in the secondary transformers located on steel poles throughout the city. These transformers were quickly repaired and, other than the first days when the power was cut off for safety reasons, the power was restored in a matter of a few days. Telephone communication was interrupted right after the earthquake due to the fact that the equipment inside the communication building toppled down, although the building itself was only slightly damaged. Through a portable relay truck and a satellite dish the emergency telecommunication was restored in ten hours. The telephones in the city were mostly operational within a few days. The airport, which is located 6 km to the south-east of the city of Erzincan, did not experience any damage in the earthquake and was fully operational for emergency transportation right after the earthquake.

11 DISASTER RESPONSE AND MITIGATION

Fig. 51. Flour mill (Fig. 50). Damage at the columns.

The Erzincan earthquake underscored the importance of the preparedness for such a disaster. Emergency plans and scenarios were ready at government level but they were not rehearsed and practiced. This resulted in

Fig. 52. Animal fodder mill. Old construction, collapsed totally.

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Fig. 53. Animal fodder mill. Recent construction, experienced light damage.

Fig. 54. Paddock belonging to abbatoire. Prefabricated concrete structure. Insufficient connection between the heavy roof beams and columns caused collapse.

ineffective response during the first days of the earthquake. Damage assessment and reporting were conducted by the General Directorate of Disaster Affairs. The damage assessment forms used in the surveys actually refer only to one or two story rural dwellings and describe the damage in simple categories. The terms of heavy, medium and light damage are well described for such rural structures, but become rather ambiguous and a source of disparity for multi-story reinforced concrete buildings in the urban areas. During the first 24 hours after the earthquake, most of the survivors from collapsed buildings were rescued by the military personnel stationed in Erzincan and by the unorganized volunteer action of local people and people from neighboring communities. The lack of flash-lights, oxygen torches, wire cutters and other necessary equipment hindered these activities. With the arrival of the national and international rescue teams, the rescue efforts became more professional. The Turkish rescue team was a special unit of the Civil Defense Organization under the Ministry of Interior. There were great differences among the international rescue teams. Some of them were governmental, some were non-governmental and some were totally voluntary. The need for coordination among the national and other international teams was apparent for their full utilization. Apart from the casualties and destruction, the earthquake caused significant economic damage. In the long-run the general economic life is expected to resume its normal character. However, the short-term losses encountered by the small businesses can make individual

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Fig. 55. Failure of an adobe rural house. Notice the separation and collapse of the exterior wall due to poor connection with the roof.

recoveries a ditticult task. The government will be providing houses or interest-free loans to qualified families to reconstruct or repair the damaged houses. The loan is to be paid back in 20 years. The government's tendency seems to be rebuild and repair

as fast as possible to bring the socio-economic life back to normality. The public expenditure to this end is estimated to be about 0.5 billion US Dollars. No relocation of the Erzincan City or radical changes in urban planning are being envisaged.

Fig. 56. Failure of an adobe rural house. The roof did not act as a rigid diaphragm connecting the walls.

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ACKNOWLEDGEMENTS The authors would like to thank colleagues and students who rendered assistance in earthquake investigations, data gathering and processing. This includes Professors Ahmet Isikara, Cemil Giirbiiz, Aykut Barka, Yener Ozkan, Giilay Askar, Erol Giiler, Levent Giilen, and Salih Bayraktutan, and the graduate students Eser Durukal, Kemal Beyen, Hafez Keypour, Jennifer Avci and Ugur Kadakal. The rectors of the authors' universities and the rector of Atatiirk University in Erzurum, Professor Hursit Ertugrul, provided the financial and the logistical support. The contribution of Kinemetrics, Inc. for the strong motion investigations is gratefully acknowledged.

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