Publications Services Store,Journals,Join,Donate
Online Journals Navigation

GSA Today PDF Archive
GSA Today
 GSA Today, v. 10, no. 1, January 2000

  January Table of Contents (including full issue in pdf format)

Figure 1
Figure 2
Figure 3
Figure 4

1999 Izmit, Turkey Earthquake Was No Surprise

Robert Reilinger, Nafi Toksoz, Simon McClusky, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Aykut Barka, Istanbul Technical University, Eurasian Earth Science Institute, Ayazaga, Istanbul, Turkey


The magnitude (M) 7.4 Izmit earthquake was the largest and most deadly earthquake in Turkey in the past 60 years, and the most destructive in terms of property damage in Turkey's recorded history. It struck on a segment of the North Anatolian fault ~100 km east of Istanbul, one of the most heavily populated and industrially developed regions of the country. The earthquake caused a 120 km surface rupture (with an unmapped extension beneath Izmit Bay) with right-lateral offsets of 1.5–5 m. Apart from the loss of life and property, the Izmit earthquake is remarkable in being the latest in a series of 11 major (M >6.7) earthquakes this century that have broken more than a 1000 km length of the North Anatolian fault from near the Karliova triple junction in eastern Turkey to the Aegean Sea. The detailed record of surface offsets for these earthquakes, the tight geodetic constraints on present-day North Anatolian fault slip rates, and geologic evidence for total offset and age provide a rich data set for placing the historic earthquakes in the broader context of regional tectonic processes, and for determining the role of static stress transfer in triggering sequential earthquakes. The quantitative information on pre-, co-, and postseismic deformation being developed for the Izmit event is providing important information for evaluating the likelihood and mitigating the impact of future earthquakes in the vulnerable Istanbul region.

Manuscript received October 25, 1999; accepted November 11, 1999.

The Second 1999 Turkey Earthquake

The November 12, 1999, M = 7.1, Duzce earthquake appears to be a second event extending the Izmit break approximately 30-40 km to the east (Fig. 4). The focal mechanism and surface faulting indicate predominately right-lateral slip of 1.5–4 m on a steeply dipping fault. There is some evidence for a small component of dip-slip with the north side moving down. This earthquake highlights the importance of static stress changes from one earthquake triggering subsequent events, and further demonstrates the current increased seismic hazards in the greater Istanbul region.


The Izmit earthquake caused more than 30,000 deaths and up to $6.5 billion in direct property losses (September 14, 1999, World Bank report). The economic impact will be higher, likely exceeding $10 billion, and possibly $20 billion, including indirect and secondary losses. The psychological impact on the people of Turkey has been immense, if difficult to measure in purely economic terms.

The Izmit earthquake represents the latest in a series of major (M >6.7) earthquakes this century that collectively resulted in surface breaks along a 1000 km section of the North Anatolian fault (Ambraseys, 1970; Toksoz et al., 1979; Barka, 1996; Fig. 1 here). Because many of these earthquakes occurred after the deployment of a substantial global seismic network, significant seismic information is available. In addition, fault offsets accompanying each of these major earthquakes have been mapped in detail (Barka, 1996), providing a basis for evaluating the role of static stress transfer in triggering sequential earthquakes (Stein et al., 1997).

On the basis of the history of major earthquakes along the North Anatolian fault, Toksoz et al. (1979) identified the Marmara segment as a seismic gap. Consequently, substantial efforts have been underway to monitor seismicity and tectonic deformation in this area. Most recently, a program was begun to install continuously recording Global Positioning System (GPS) stations and a relatively dense network of GPS survey sites to monitor strain accumulation on the various branches of the fault in the Marmara region. This effort is providing information on the various phases of the earthquake cycle for the Izmit event, including pre-earthquake strain accumulation, coseismic deformation, and postseismic relaxation. Furthermore, regional GPS studies undertaken over the past 10 years provide quantitative constraints on slip rates along the North Anatolian fault and place the motions along the fault in the context of regional tectonic processes associated with the interaction of the Arabian, African, and Eurasian plates (Straub et al., 1997; Reilinger et al., 1997a; McClusky et al., 2000). As a result, rather complete seismic, geologic, and deformational records are available for the fault that produced the Izmit event. These records hold the promise of improving our understanding of the fundamental nature of earthquake processes on this and similar faults. Here, we describe the Izmit earthquake and place it in the context of prior earthquakes on the North Anatolian fault and the regional tectonic framework of the eastern Mediterranean zone of active plate interactions.


The tectonic framework of the eastern Mediterranean and Middle East region is dominated by the collision of the Arabian and African plates with Eurasia (e.g., McKenzie, 1970; Jackson and McKenzie, 1988). Plate tectonic models (e.g., NUVEL-1A; DeMets et al., 1994) suggest that the Arabian plate is moving in a north-northwest direction relative to Eurasia at a rate of about 18-25 mm/yr, averaged over about 3 m.y. These models also suggest that the African plate is moving in a northward direction relative to Eurasia at a rate of about 10 mm/yr. Differential motion between Africa and Arabia (–10-15 mm/yr) is thought to be taken up predominantly by left-lateral motion along the Dead Sea transform fault. The northward motion of Arabia results in continental collision along the Bitlis-Zagros fold and thrust belt, intense earthquake activity (Fig. 2), and high topography in eastern Turkey and the Caucasus Mountains. The northward motion of Arabia is also thought to contribute to westward extrusion of the Anatolian plate, which is accommodated by right-lateral slip on the North Anatolian fault and left-lateral slip on the East Anatolian fault (McKenzie, 1970). The leading edge of the African plate is being subducted along the Hellenic trench at a higher rate than the relative northward motion of the African plate, requiring that the trench moves southward relative to Eurasia proper (e.g., Sonder and England, 1989; Royden, 1993). This qualitative picture of present-day kinematics is well illustrated by the distribution and focal mechanisms of earthquakes in Figure 2. The lack of events within the Anatolian plate attests to the low level of internal deformation in this area, and the nature of strike-slip faulting along the North Anatolian (right-lateral) and East Anatolian (left-lateral) faults are consistent with westward motion and counterclockwise rotation of Anatolia relative to Eurasia. Although this qualitative description of eastern Mediterranean tectonics has proven robust and useful, quantitative estimates of plate motions, intra-plate deformation, and fault slip rates, now being provided by GPS observations, help to better constrain models for dynamic processes and lithospheric rheology (e.g., Thatcher, 1995) and provide a physical basis for effectively illuminating earthquake generation processes.

GPS results (Fig. 3 ) provide direct estimates of Arabia-Africa-Eurasia motion, the counterclockwise rotation and associated westward motion of the Anatolian (Turkish) plate, and the rapid (>30 mm/yr) southward motion of the southern Aegean region (block?) relative to Eurasia. These results also quantify strain partitioning and crustal shortening in eastern Turkey and the Caucasus, fault-slip rates on the main, active faults, and partitioning between seismic and aseismic deformation. The kinematic results in turn provide constraints on dynamic processes and the rheological character of the lithosphere in this region. For example, the increase in velocities from eastern Turkey toward the Hellenic trench requires forces other than pushing from Arabia to account for Anatolian motion. The apparently coherent motion of much of Anatolia (i.e., little internal deformation) is consistent with relatively strong continental lithosphere (e.g., Reilinger et al., 1997; Barka and Reilinger, 1997; Lundgren et al., 1998; McClusky et al., 2000).


The North Anatolian fault is a major, right-lateral, continental strike-slip fault that accommodates the westward motion and counterclockwise rotation of Anatolia and extends approximately 1200 km from the Karliova triple junction to the Aegean Sea (Figure 1). Right-lateral deformation continues east of the triple junction, but the fault has a more complex character and is not easily identified as a single surface trace (e.g., Toksoz et al., 1977; Westaway, 1994; Reilinger et al., 1997b). In the Marmara region, the fault becomes more complex, bifurcating into two or three separate branches. Right-lateral deformation extends west of the Marmara Sea into the Aegean and is thought to connect with the east-west-striking normal faults bounding the Gulf of Corinth (Armijo et al., 1996; McClusky et al., 2000).

On the basis of the regional GPS velocity field, McClusky et al. (2000) estimated an upper bound on North Anatolian fault slip rate of 24 ± 1 mm/yr. This estimate is made by assuming that all motion of Anatolia is accommodated by slip on the North Anatolian fault, which serves as the primary boundary between Anatolia and Eurasia. Independent GPS estimates of Anatolia-Eurasia relative motion in the Marmara area by Straub et al. (1997) indicate a rate of 22 ± 3 mm/yr for Anatolia relative to a station in Istanbul (and hence a lower bound). These present-day fault slip rates are in reasonable agreement with geologic slip rates based on total fault offset and the estimated age of faulting (e.g., Sengör, 1979; Westaway, 1994; Armijo et al., 1999). This agreement suggests that Anatolia-Eurasia motion has continued in its present configuration and at approximately the same rate for the past 4–5 m.y. Such a first-order kinematic model (i.e., Anatolia moving as a coherent unit, the motion being accommodated within a narrow fault zone relative to the size of the plates) provides a physical basis for relating fault slip for specific events to the overall motion of the plates, for identifying seismic gaps (i.e., slip deficient segments), and, to the extent that the characteristic earthquake model is applicable, for estimating average earthquake repeat times (Reilinger and Barka, 1997).

A series of 11 large (M >6.7) earthquakes on the North Anatolian fault this century resulted in continuous surface breaks along more than 1000 km of the surface trace (Fig. 1). Surface offsets for many of these events have been mapped in detail (e.g., Barka, 1996), providing a basis for investigating the relationship between earthquakes and regional tectonics, as well as the interaction between successive events (e.g., Barka and Reilinger, 1997; Stein et al., 1997). Subsequent to the 1912, M = 7.4 Ganos earthquake, which broke the western segment of the northern fault branch (Fig. 1), and beginning with the 1939, M = 7.8 Erzincan rupture, four successive earthquakes (1939, 1942, 1943, 1944) migrated to the west (Dewey, 1976; Toksoz et al., 1979). Westward migration continued with the 1957 and 1967 earthquakes. Most other large earthquakes on the North Anatolian fault (1949, 1951, 1966, 1992) occurred on fault segments with low coseismic slip in prior earthquakes, or extended the break to the east (e.g., Stein et al., 1997; Fig.1). The 1999 Izmit earthquake, on a fault segment specifically identified as a seismic gap (Toksoz et al., 1979; Stein et al., 1997), appears to be a continuation of the westward migrating historic earthquake sequence.

As indicated in Figure 4, the North Anatolian fault bifurcates into several active strands in the Marmara region. While faults beneath the Marmara Sea are known to generate earthquakes (e.g., Barka, 1997), the geometry and nature of these active faults remain unclear, and the distribution of slip on specific faults within the Marmara is unknown. However, the large increase in westward velocities for GPS stations located south of the northern branch of the North Anatolian fault indicates that the majority of strain occurs on the northernmost fault segments (Straub et al., 1997; McClusky et al., 2000). In fact, preliminary modeling indicates that the pre-earthquake GPS velocity gradient across the eastern Marmara can be explained by strain accumulation along a single, approximately east-west-striking fault, including the segment that broke in the Izmit earthquake (R. Bergmann, 1999, personal communication).


The 1999, M = 7.4 Izmit earthquake epicenter was near the town of Izmit at the east end of Izmit Bay. The quake involved predominantly right-lateral, strike-slip motion on a vertical fault plane (Harvard CMT) (Fig. 4). Observed surface offsets ranged from 1.5 to 5 m along a 120 km fault break (Barka, 1999). The largest offsets were observed along the western end of the fault where it entered the Bay of Izmit. Offsets decrease to the east where the Izmit break lies north of the 1967 Mudurnu Valley earthquake fault break. The extent of faulting beneath Izmit Bay is unknown. Although significant aftershock activity reached as far west as 28.7°E, there is no evidence for right-lateral offsets in the Hersek delta (29.5°E, 40.7°N; Fig. 4). In addition, data from continuously recording GPS stations located north and south of Izmit Bay prior to the earthquake show a substantial component of north-south coseismic motion, consistent with a fault that ends (or slip decreases sharply) near or east of 29.5°E.

Because the Marmara region is home to about 25% of Turkey's population and a large part of Turkey's industrial activity, and the area had been identified as a seismic gap, substantial seismic and geodetic work was underway prior to the earthquake. Part of this effort included using continuous GPS (CGPS) and survey-mode GPS (S-MGPS) to monitor the distribution of Anatolia-Eurasia motion on the various faults that compose the North Anatolian fault zone. Figure 4 shows the locations of those CGPS stations in operation prior to the earthquake (all continue to operate), and S-MGPS sites that had been observed less than two years before the main shock. In addition, the Marmara Research Center in Gebze, Turkey, installed four CGPS stations along the highest coseismic slip segment of the fault within 48 hours of the main shock (Fig. 4). The S-MGPS stations are now being reobserved and together with the CGPS stations, INSAR, seismic estimates of fault slip, and surface offsets should provide fairly detailed estimates of coseismic slip distribution on the Izmit fault. This is of more than academic interest, because the details of coseismic slip distribution are critical for estimating future earthquake hazards in the Marmara region (i.e., the extent to which the Izmit earthquake filled the seismic gap and advanced or retarded future earthquakes on other fault segments). Furthermore, some of the S-MGPS stations are being observed multiple times after the earthquake to monitor continuing postseismic motions. The resulting data, together with the data from CGPS stations, will help constrain models of postseismic after-slip and viscoelastic relaxation. Such postseismic processes can substantially increase the overall earthquake moment and can result in rapid, postseismic strain accumulation, which could affect estimates of future earthquake occurrences.


Quantitative information on pre-, co-, and postseismic deformation for the Izmit earthquake provides an important opportunity to further our understanding of basic earthquake processes, with implications for forecasting and mitigating the effects of future events on the North Anatolian fault and similar faults like the San Andreas fault in California. The remarkable series of earthquakes along virtually the entire length of the North Anatolian fault this century (excluding the Marmara Sea segments) provides an ideal data set to investigate the relationship between successive earthquakes on a major continental strike-slip fault, as well as the relationship among earthquakes, regional tectonics, and geologic deformation. Most critically, understanding the Izmit event and the nature of active faulting in the Marmara Sea is prerequisite to determining the probability and nature (location, magnitude) of future earthquakes west of the Izmit event. The vulnerability of the greater Istanbul region, as well as other large population centers in earthquake-prone areas, demands that we do our utmost to extract information from this tragic event, with the expectation that this knowledge will lead to an improved ability to mitigate future earthquake losses.


We are grateful to Francisco Gomez, Laura Serpa, and Sue Kay for constructive reviews. This study was supported in part by National Science Foundation grant EAR-9304554 and NASA grant NAG5-6145.


Ambraseys, N. N., 1970, Some characteristic features of the Anatolian fault zone: Tectonophysics, v. 9, p. 143–165.

Armijo, R., Meyer, B., King, G. C. P., Rigo, A., and Papanastassiou, D., 1996, Quaternary evolution of the Gulf of Corinth rift and its implications for the Late Cenozoic evolution of the Aegean: Royal Astronomical Society Geophysical Journal, v. 126, p. 11–53.

Armijo, R., Meyer, B., Hubert, A., and Barka, A., 1999, Propagation of the North Anatolian fault into the north Aegean: Timing and kinematics: Geology, v. 27, p. 267–270.

Barka, A. A., 1996, Slip distribution along the North Anatolian fault associated with large earthquakes of the period 1939–1967: Seismological Society of America Bulletin, v. 86, p. 1238–1254.

Barka, A., 1997, Neotectonics of the Marmara Sea, in Schindler, C., and Pfister, M., eds., Active tectonics of northwest Anatolia: The Marmara Project: Zurich, Verlag der Fachvereine, p. 55–87.

Barka, A. A., 1999, The 17 August, 1999, Izmit earthquake: Science, v. 285, p. 1858–1859.

Barka, A., and Reilinger, R., 1997, Active tectonics of the eastern Mediterranean region deduced from GPS, neotectonic, and seismicity data: Annali Geofisica, v. 40, p. 587–610.

DeMets, C., Gordon, R. G., Argus, D. F., and Stein, S., 1994, Effects of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions: Geophysical Research Letters, v. 21, p. 2191–2194.

Dewey, J. W., 1976, Seismicity of northern Anatolia: Seismological Society of America Bulletin, v. 66, p. 843–868.

Dziewonski, A. M., Chou, T.-A., and Woodhouse, J. H., 1981, Determination of earthquake source parameters from waveform data for studies of global and regional seismicity: Journal of Geophysical Research, v. 86, p. 2825–2852.

Jackson, J., and McKenzie, D., 1988, The relationship between plate motions and seismic tremors, and the rates of active deformation in the Mediterranean and Middle East: Royal Astronomical Society Geophysical Journal, v. 93, p. 45–73.

Lundgren, P., Giardini, D., and Russo, R., 1998, A geodynamic framework for eastern Mediterranean kinematics: Geophysical Research Letters, v. 25, p. 4007–4010.

McClusky, S., et al., 2000, GPS constraints on plate motion and deformation in the eastern Mediterranean: Implications for plate dynamics: Journal of Geophysical Research (in press).

McKenzie, D. P., 1970, Plate tectonics of the Mediterranean region: Nature, v. 226, p. 239–243.

Reilinger, R., and Barka, A., 1997, GPS constraints on fault slip rates in the Arabia-Africa-Eurasia plate collision zone: Implications for earthquake recurrence times, in Giardini, D., and Balassanian, S., eds., Historical and prehistorical earthquakes in the Caucasus: Dordrecht, Netherlands, Kluwer, p. 91–108.

Reilinger, R., et al., 1997a, Global positioning system measurements of present-day crustal movements in the Arabia-Africa-Eurasia plate collision zone: Journal of Geophysical Research, v. 102, p. 9983–9999.

Reilinger, R., McClusky, S. C., Souter, B. J., Hamburger, M. W., Prilepin, M. T., Mishin, A., Guseva, T., and Balassanian, S., 1997b, Preliminary estimates of plate convergence in the Caucasus collision zone from global positioning system measurements: Geophysical Research Letters, v. 24, p. 1815–1818.

Royden, L., 1993, The tectonic expression of slab pull at continental convergent boundaries: Tectonics, v. 12, p. 303–325.

Sengör, A. M. C., 1979, The North Anatolian transform fault: Its age and tectonic significance: Geological Society [London] Journal, v. 136, p. 269–282.

Sonder, L., and England, P., 1989, Effects of temperature dependent rheology on large scale continental extension: Journal of Geophysical Research, v. 94, p. 7603–7619.

Stein, R. S., Barka, A. A., and Dieterich, J. D., 1997, Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering: Geophysical Journal International, v. 128, p. 594–604.

Straub, C., Kahle, H.-G., and Schindler, C., 1997, GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia: Journal of Geophysical Research, v. 102, p. 27,587–27,601.

Thatcher, W., 1995, Microplate versus continuum descriptions of active tectonic deformation: Journal of Geophysical Research, v. 100, p. 3885–3894.

Toksoz, M. N., Arpat, E., and Saroglu, F., 1977, East Anatolia earthquake of 24 November, 1976: Nature, v. 270, p. 423–425.

Toksoz, M. N., Shakal, A. F., and Michael, A. J., 1979, Space-time migration of earthquakes along the North Anatolian fault zone and seismic gaps: Pure and Applied Geophysics, v. 117, p. 1258–1270.

Westaway, R., 1994, Present-day kinematics of the Middle East and eastern Mediterranean: Journal of Geophysical Research, v. 99, p. 12,071–12,090.

Manuscript received October 25, 1999; accepted November 11, 1999.


  Home Page | Privacy | Contact Us

© The Geological Society of America, Inc.  

GSA Home Page Contact Us Frequently Asked Questions Search Site Map Current Issue Archive Data Repository Search Subscribe Feedback Help Submit a Manuscript GSA Store Online Journals Join GSA Donate Now!