Izmit Earthquake Research Articles

Methodology for preliminary assessment of Natech risk in urban areas

Concern for natural hazard-triggered technological disasters (Natech disasters) in densely populated and industrialized areas is growing. Residents living in urban areas subject to high natural hazard risk are often unaware of the potential for secondary disasters such as hazardous materials releases from neighboring industrial facilities, chemical storage warehouses or other establishments housing hazardous materials. Lessons from previous disasters, such as the Natech disaster during the Kocaeli earthquake in Turkey in 1999 call for the need to manage low frequency/high consequence events, particularly in today’s densely populated areas. However, there is little guidance available on how local governments and communities can assess Natech risk. To add to the problem, local governments often do not have the human or economic resources or expertise to carry out detailed risk assessments. In this article, we propose a methodology for preliminary assessment of Natech risk in urban areas. The proposed methodology is intended for use by local government officials in consultation with the public. The methodology considers possible interactions between the various systems in the urban environment: the physical infrastructure (e.g., industrial plants, lifeline systems, critical facilities), the community (e.g., population exposed), the natural environment (e.g., delicate ecosystems, river basins), and the risk and emergency management systems (e.g., structural and nonstructural measures). Factors related to vulnerability and hazard are analyzed and qualitative measures are recommended. Data from hazardous materials releases during the Kocaeli, Turkey earthquake of August 17, 1999 are used as a case study to demonstrate the applicability of the methodology. Limitations of the proposed methodology are discussed as well as future research needs.

Physical and economic losses sustained by the industry in the 1999 Kocaeli, Turkey earthquake

The aim of this article is to contribute with first-hand data on damage and failure modes at industrial facilities subject to earthquake. Physical and economic losses faced by the industry in the 1999 Kocaeli earthquake are summarized. Industrial-sector based, as well as component-based description of earthquake performance and damage are provided. The results of the post-earthquake questionnaire survey designed and executed with the aim of gathering factual data from industrial facilities that experienced physical damages, as well as suffered from business interruption losses are presented. Generalized-intensity-based mean damage ratios for industrial facilities in Turkey are given. The information provided in this article can be used for the first order estimation of building, machinery and equipment, as well as stock and business interruption losses associated with the industry during earthquakes in Turkey.

A study on the identification of liquefaction-induced failures on ground surface based on the data from the 1999 Kocaeli and Chi-Chi earthquakes

One of the major causes of earthquake damage is liquefaction. However, it doesn't result in severe harm unless it leads to ground surface damage or ground failure. Therefore, prediction of potential for ground surface damage due to liquefaction is one of the important issues in microzonation studies for liquefaction-induced damage in areas with high seismicity. In 1985, based on a database compiled from Chinese and Japanese earthquakes, Ishihara considered the influence of the non-liquefied cap soil on the occurrence or non-occurrence of ground failure (mainly sand boiling), and proposed an empirical approach to predict the potential for ground surface damage at sites susceptible to liquefaction. However, some investigators indicated that this approach is not generally valid for sites susceptible to lateral spread or ground oscillation. In this study, a contribution to improve the approach by Ishihara is made. For the purpose, an index called liquefaction severity index (LSI) and data from two devastating earthquakes, which occurred in Turkey and Taiwan in 1999, were employed. The data from liquefied and non-liquefied sites were grouped and then analysed. Based on the observations reported by reconnaissance teams who visited both earthquake sites and the results of the liquefaction potential analyses using the filed-performance data, a chart to assess the potential for ground surface disruption at liquefaction-prone areas was produced. The analyses suggest that the procedure proposed by Ishihara is quite effective particularly for the occurrence of sand boils, while the bounds suggested in this method generally may not be valid for the prediction of liquefaction-induced ground surface disruption at sites susceptible to lateral spreading. The chart proposed in this study shows an improvement over the Ishihara's approach for predicting the liquefaction-induced ground surface damage. The microzonation maps comparing the liquefaction sites observed along the southern shore of Izmit Bay and in Yuanlin, and the surface damage and non-damage zones predicted from the proposed chart can identify accurately the liquefaction (sand boiling and lateral spreading) and no-liquefaction sites.

Seismotectonic setting at the North Anatolian Fault Zone after the 1999 Mw=7.4 Izmit earthquake based on high-resolution aftershock locations

Abstract. The most recent devastating earthquakes that occurred along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey were the 1999 Izmit (Mw=7.4) and Düzce (Mw=7.1) events. In this study we present a catalog of Izmit aftershock hypocenters that was deduced from a network covering the entire 140 km long rupture of the mainshock. 7348 events with a location accuracy better than 5 km are analysed. Aftershocks were observed along the entire ruptured segment along a 20 km wide band of activity. Events are clustered in distinct regions and dominantly occur at 5 to 15 km depth. The eastern termination of the Izmit rupture is characterized by a sharp and steeply dipping boundary exactly where the Düzce mainshock initiated 87 days after the Izmit event. Relocation of the events using double-difference technology results in 4696 high-resolution hypocenters that allow resolving the internal structure of the seismically active areas with a resolution of 300 m (horizontal) and 400m (vertical). Below the Akyazi Plain, representing a small pull-apart structure at a triple junction of the NAFZ, we identify planes of activity that can be correlated with nodal planes of EW extensional normal faulting aftershocks. Along the easternmost Karadere-Düzce segment we identify the down-dip extension of the Karadere fault that hosted about 1 m of right-lateral coseismic slip. At the easternmost rupture we correlate a cloud-type distribution of seismic activity with the largest aftershocks in this area, a subevent of the Izmit mainshock and the Düzce mainshock that all have an almost identical focal mechanism. This part of the NAFZ is interpreted as a classical example of a seismic barrier along the fault.

Fuzzy Logic Control of a Non-linear Structural System against Earthquake Induced Vibration

In this paper, the problem of active vibration control of multi-degree-of-freedom structures is considered. Fuzzy logic and PID controllers are designed to suppress structural vibrations against earthquakes under the non-linear soil-structure interaction. The advantage of the fuzzy logic approach is the ability to handle the non-linear behavior of the system. Non-linear behavior of the soil is modeled in the dynamics of the structural system with non-linear hysteric restoring forces. The simulated system has fifteen degrees of freedom, which is modeled using spring-mass-damper subsystems. A structural system was simulated against the ground motion of the destructive Kocaeli earthquake (Mw = 7.4) in Turkey on 17 August 1999. At the end of the study the time history of the storey displacements and accelerations, the control voltages and forces, and the frequency responses of both the uncontrolled and the controlled structures are presented. The performance of designed fuzzy logic control is checked using the changing mass parameters of each storey and the results are discussed. These results show that the proposed fuzzy logic controller has great potential in active structural control.

Characterization of aftershock‐fault plane orientations of the 1999 İzmit (Turkey) earthquake using high‐resolution aftershock locations

Joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high‐resolution images of seismically active regions. In this study, we focus on the location accuracy of aftershocks of the 1999 Mw = 7.4 İzmit (NW Turkey) earthquake. We obtained a new velocity model for the region, and depicted its improvement on absolute locations in terms of uncertainty and misfit. Two well‐developed aftershock clusters located in the Akyazı area and Karadere‐Düzce region, were analyzed in detail based on a waveform cross‐correlation approach that allowed improving the location accuracy by a factor of 6. Relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of ∼300 m.

Liquefaction potential at Izmit Bay: comparison of predicted and observed soil liquefaction during the Kocaeli earthquake

The 1999 Kocaeli earthquake resulted in large-scale damage to structures and a death toll of more than 20,000. Numerous liquefaction and associated ground deformations caused by the earthquake were particularly observed along the southern coasts of Izmit Bay and Sapanca Lake, as well as the city of Adapazari. The paper discusses the soil liquefaction and lateral spreading along the southern coast of Izmit Bay. A microzonation map was created using the data from liquefaction susceptibility analyses and a liquefaction severity index. The predicted liquefiable zones showed good agreement with site observations. The assessment suggests that the amount of liquefaction-susceptible layers in the soil profile reduces away from the coastal zone.

Structural identification for post-earthquake safety analysis of the Fat i ̇ h mosque after the 17 August 1999 Kocael i ̇ earthquake

This paper presents the results of the structural identification of a historical structure damaged during the 17 August 1999 Kocaeli earthquake. Results of the structural models and methods are cross-validated in order to identify the reliable dynamic characteristics. Spectral analysis of the eight structural points yields average peak frequencies, respectively at around 2.5, 3.5, 4.3, 5.3 Hz and so on for NW–SE direction. Similarly for the SW–NE direction, peaks are seen at around 2.6, 3.2, 4.5–5 Hz with small differences in amplitudes between different structural points. Among the stations, station 3 behaves differently than the other observation points in low and high frequency regions indicating that the data recorded at station 3 should be examined in more detail. In parametric analysis, (a) the SDOF system as an SISO-ARX model, and (b) the MDOF system as a MIMO-ARX model are adopted and modal deformations are obtained using a state-space model with the OKID technique. The SDOF model captures the important peak frequencies with sufficient accuracy, but magnitudes are arguably small. In MDOF model, responses at the crown stations 1, 2, 3, and 4 display almost the same peak magnitudes at the same frequency, 2.51 Hz for NE–SW direction, but station 3 (which possesses smaller peak amplitude at a smaller frequency, 2.2 Hz) is exceptional and indicative of an anomaly. Similar conclusion with different peak frequency for NE–SW direction is also inferred from the SISO and Spectral analyses, when compared. Station 3 among the crown stations exceptionally yields peak frequency at 1.8 Hz in spectral analysis and around 1.8 Hz in SISO analysis. Natural frequencies of the structure estimated by SDOF and MDOF systems are close to each other, and thus, they can be accepted as global properties of the structure, except station 3. Such inconsistency implies that, for indicative anomaly, there might be a practical avenue through the evaluation of the first mode estimated by spectral, SDOF and MDOF models for a structure with the members of equal importancy in global dynamic behavior and symmetrical in plan and in elevations. Calculated elastic response spectrum of the Fatih record has larger amplitudes at the period of 0.55 s (with broadband characteristic) and larger than the amplitudes of the design spectrum with 10% probability of excedance in 50 years (for new constructions) for North–South direction. These larger local peaks at 0.55 s; (1) verify the 1.8 Hz of the fundamental frequency of the local ground and (2) suggest that, as the minimum requirements for the earthquake resistant design of buildings, in accordance with the strengthening applications in current seismic design code, the 2% probability of exceedance of the 5% damped elastic design acceleration spectrum within a period of 50 years is adequate. First mode of the structure, 2.4 and 2.5 Hz for NS and EW directions respectively, falls into the range of the dominant broadband period of the seismic loading. Such a match points out the resonance possibility, which might have occurred during the earthquake. The duration of the earthquake and following shocks of many small earthquakes in addition to the resonance might be the key contributors to the damage.

Seismic risk assessment for the mega-city of Istanbul: Ductility, strength and maximum interstory drift demands

This study investigates the seismic demands due to the catastrophic 1999 M w = 7.4 Kocaeli and the M w = 7.1 Duzce earthquakes. The inelastic response spectra for the Kocaeli and Duzce earthquakes are investigated for systems with known strength and ductility. An analytical fiber element model is developed for a typical reinforced concrete building in Turkey. The interstory drifts are calculated from nonlinear dynamic analysis using 26 recorded strong-motion data from the 1999 Kocaeli and the Duzce earthquakes. In the dynamic analysis, the structural members are modeled by employing distributed plasticity fiber elements and both geometrical as well as material nonlinearities are taken into account. This study shows that the ductility and interstory drift demands due to the Kocaeli and the Duzce earthquakes were very severe (well above the code prescribed values) even for moderately inelastic structures. It is apparent from the results that the forward directivity effect is the most influential factor on the interstory drift demand. Both the distance to the fault rupture and the site conditions affect seismic demands, but the site conditions and the local topography are more influential than the distance from the fault rupture. This study shows that substantial damage should be expected in a future earthquake at all districts of Istanbul, but especially at Avcilar, Cekmece, Fatih, Bakirkoy and the Zeytinburnu districts. It is also shown that base isolation may substantially improve the performance of a structure in the inelastic domain and base isolated structures may be designed for lower minimum lateral strengths and higher strength reduction factors.

A complex seismogenic shear zone: The Düzce segment of North Anatolian Fault (Turkey)

This work highlights how surface geological data can integrate and complete a fault characterization including its seismogenic depth. We investigated the area struck by the November, 1999, Mw 7.1 earthquake that ruptured the Düzce segment of the North Anatolian Fault Zone (NAFZ) after the August, 1999, Mw 7.4 Izmit earthquake. To explore the persistency or evolution of the active fault setting at the surface, we compared detailed coseismic surface expressions with the long-term morphology and structural architecture of the Düzce zone. We identified a simple 1999 coseismic fault trace (CFT) and an older complex fault system (RFS) involving a wider zone of deformation. Two different parts of the Düzce segment were recognized. A western section presents a staircase trajectory of the coseismic fault trace that reactivated part of the older fault system. An eastern section presents a straight coseismic fault trace that cross-cuts the older and complex fault system. The western Düzce fault segment splays out from the Izmit counterpart of the NAFZ, forming a releasing fault junction. By comparing a model of its coseismic surface deformation field with the long-term morphology, we observed that the present topography of this western section results from the NAFZ activity by repeating coseismic deformations like those in 1999. The mechanical interaction in the release fault junction produces local transtension across the western Düzce fault and justifies its stable complexity. It does not evolve into a straight trace. The surface boundary between the Düzce fault sections also separates at depth two fault sections: one with low slip to the west, and another with a big asperity to the east. Thus, the Izmit/Düzce interaction (1) may produce an unfavorable setting for a build-up of asperities in the western Düzce segment and (2) could delay the propagation of the Izmit rupture along the Düzce segment that nucleates mainshocks only on its eastern section. In other words, the arrangement of the fault junction may permanently control how the two sections of the Düzce fault behave and rupture.

3-D pushover analysis of a collapsed reinforced concrete chimney

During the Izmit (Kocaeli) Earthquake of August 17, 1999, a 115 m high reinforced concrete chimney or heater stack, located at the Tüpras Refinery, collapsed. This stack was designed and constructed according to international standards and is representative of similar structures at refineries throughout the world, including those in earthquake-prone regions. This structure is of particular interest because several similar chimneys at the site survived the shock with only moderate damage. The particular distinction of this chimney appears to be an unusually larger rectangular opening, located about 1 3 of the height above the base, which appeared to be the region of collapse initiation. The main focus of the research is the dynamic response of the stack due to an earthquake motion recorded at a nearby site. A new 3-D pushover analysis procedure is proposed in this paper and the results will be compared with those of a nonlinear dynamic analysis. Results are presented that show the importance of the 3-D interaction effects in the dynamic response of the stack. The results also confirm that the stack could readily fail under the considered earthquake and are consistent with the debris pattern.

Structure and mechanical properties of faults in the North Anatolian Fault system from InSAR observations of coseismic deformation due to the 1999 Izmit (Turkey) earthquake

We study the structure and mechanical properties of faults in the North Anatolian Fault system by observing near‐fault deformation induced by the 1999 Mw 7.4 Izmit earthquake (Turkey). We use interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System observations to analyze the coseismic surface deformation in the near field of the Izmit rupture. The overall observed coseismic deformation is consistent with deformation predicted by a dislocation model assuming a uniform elastic crust. Previous InSAR studies revealed small‐scale changes in the radar range across the nearby faults of the North Anatolian fault system (in particular, the Mudurnu Valley and Iznik faults) (e.g., Wright et al., 2001). We demonstrate that these anomalous range changes are consistent with an elastic response of compliant fault zones to the stress perturbation induced by the Izmit earthquake. We examine the spatial variations and mechanical properties of fault zones around the Mudurnu Valley and Iznik faults using three‐dimensional finite element models. In these models, we include compliant fault zones having various geometries and elastic properties and apply stress changes deduced from a kinematic slip model of the Izmit earthquake. The best fitting models suggest that the inferred fault zones have a characteristic width of a few kilometers, depth in excess of 10 km, and reductions in the effective shear modulus of about a factor of 3 compared to the surrounding rocks. The characteristic width of the best fitting fault zone models is consistent with field observations along the North Anatolian Fault system (Ambraseys, 1970). Our results are also in agreement with InSAR observations of small‐scale deformation on faults in the Eastern California Shear Zone in response to the 1992 Landers and 1999 Hector Mine earthquakes (Fialko et al., 2002; Fialko, 2004). The inferred compliant fault zones likely represent intense damage and may be quite commonly associated with large crustal faults.

Evaluation of ground deformations induced by the 1999 Kocaeli earthquake (Turkey) at selected sites on shorelines

The Kocaeli earthquake (Mw = 7.4) of 17 August 1999 occurred in the Eastern Marmara Region of Turkey along the North Anadolu Fault and resulted in a very serious loss of life and property. One of the most important geotechnical issues of this event was the permanent ground deformations because of both liquefaction and faulting. These deformations occurred particularly along the southern shores of Izmit Bay and Sapanca Lake between the cities of Yalova and Adapazari in the west and east, respectively. In this study, three sites founded on delta fans, namely Degirmendere Nose, Yenikoy tea garden at Seymen on the coast of Izmit Bay, and Vakif Hotel site on the coast of Sapanca Lake were selected as typical cases. The main causes of the ground deformations at these sites were then investigated. Geotechnical characterization of the ground, derivation of displacement vectors from the pre- and post-earthquake aerial photographs, liquefaction assessments based on field performance data, and analyses carried out using the sliding body method have been fundamental in this study. The displacement vectors determined from photogrammetric evaluations conducted at Degirmendere and Seymen showed a combined movement of faulting and liquefaction. But except the movements in the close vicinity of shorelines, the dominant factor in this movement was faulting. The results obtained from the analyses suggested that the ground failure at Degirmendere was a submarine landslide mainly because of earthquake shaking rather than liquefaction. On the other hand, the ground failures at the Yenikoy tea garden on the coast of Seymen and the hotel area in Sapanca town resulted from liquefaction-induced lateral spreading. It was also obtained that the ground deformations estimated from the sliding body method were quite close to those measured by aerial photogrammetry technique.

Use of Semivariograms to Identify Earthquake Damage in an Urban Area

The 1999 Izmit earthquake (Mw 7.4) on the North Anatolian Fault Zone resulted in severe damage to the urban areas of Izmit, Adapazari (Sakarya), Golcuk, and Yalova. A semivariogram approach was used to quantify earthquake-induced spatial variation and thereby the degree of damage in the Adapazari inner city. Semivariograms were calculated for 24 transects on SPOT high resolution visible infrared (HRVIR) panchromatic images obtained before and after the earthquake. The differences between the pre- and postearthquake semivariogram shape and measures of shape, range, nugget, and sill were related to the severity of earthquake damage

Observations of tectonic microdisplacements in Europe in relation to the Iran 1997 and Turkey 1999 earthquakes

Long-term repeated measurements of microdisplacements in tectonic fault structures of various parts of Central Europe and the Balkan Mountains showed that displacement trends changed significantly at several points in the period from 1997 through 2000; afterward, long-term trends were restored. This phenomenon took place in the periods of strong (M > 7) earthquakes in Iran (1997) and in the North Anatolian fault zone in Turkey (1999). Two strong Izmit earthquakes of 1999 in Turkey at distances of 600 km from the Balkan Peninsula and 1400 km from observation points in Central Europe were the main seismic events of the period studied. Apparently, the crustal deformation due to the sources of the aforementioned earthquakes reaches the central part of the European craton. Anomalous displacements in some areas occurred due to deformation propagating for great distances in the heterogeneous block medium of the West European part of the Eurasian plate. Changes in stresses can be caused by impulsive deformations of various intensities acting on some structural units (fault segments) at various distances.

A fault zone cause of large amplification and damage in Avcılar (west of Istanbul) during 1999 Izmit earthquake

Twenty seven buildings collapsed and 2076 buildings were heavily damaged during 17 August 1999 Izmit Earthquake in Avcilar (west of Istanbul), which is nearly 100 km from the epicenter of the earthquake. The geology of Avcilar consists of Upper Miocene poorly bounded conglomerate and sandstone (Cukurcesme formation), claystone with sandstone and limestone interbedding (Gungoren member) and limestone with sandstone and claystone interbedding (Bakirkoy formation). Lithological and geotechnical parameters of these formations in Avcilar are not different from non damaged parts of the western Istanbul such as Zeytinburnu, Bakirkoy and Beylikduzu, but these formations were cut by several faults in the damaged area. Collapsed and damaged buildings are located on this fault zone. Thus, cause of large amplification and damage in Avcilar might be related with this fault zone because the fault zone behaves as a waveguide trapping seismic energy.

Foundation–structure systems over a rupturing normal fault: Part I. Observations after the Kocaeli 1999 earthquake

Triggered by reactivation of the strike-slip North Anatolian Fault, the disastrous Mw 7.4 Kocaeli (Turkey) earthquake also produced normal faulting in the pull-apart basin of Golcuk. Surface scarps from such faulting reached almost 2.5m in height. Several structures were crossed by the surface rupture. As expected, many of them either collapsed or were severely damaged. But, surprisingly, several struc- tures survived the outcropping dislocation essentially unscathed. In fact, in some cases the surface rupture path seemed to have deviated, as if to "avoid" rupturing directly underneath a structure. In other cases damage was substantial even though the fault rupture was "masked" by the near-surface soil and did not create a scarp. The rigidity and continuity of the foundation appears to have been one of the crucial factors affecting structural performance. Interestingly, the examined structures were supported on a variety of foundation types, ranging from isolated footings, to rigid box-type foundations, to piled foundations. The paper outlines a reconnaissance of the area, providing a documented description of the observed interplay between the rupturing fault, the soil, and the structure, along with the results of soil exploration and geological trenching. In the companion paper, Part II, each system is analysed numerically to confirm the conclusions of the present paper, reveal the main aspects of Fault Rupture-Soil-Foundation-Structure Interaction (FR-SFSI), and help develop deeper insights into the mechanics of successful performance of structures built on top of such faults.

Empirical ground‐motion prediction equations for northwestern Turkey using the aftershocks of the 1999 Kocaeli earthquake

We present ground motion models for northwestern Turkey using the aftershocks of the Mw 7.4, 1999 Kocaeli earthquake. We consider 4047 velocity and acceleration records for each component of motion, from 528 earthquakes recorded by stations belonging to regional networks. The ground motion models obtained provide peak ground velocity, peak ground acceleration, and spectral accelerations for 8 different frequencies between 1 and 10 Hz. The analysis of the error distribution shows that the record‐to‐record component of variance is the largest contribution to the standard deviation of the calibrated ground‐ motion models. Furthermore, a clear dependence of inter‐event error on stress drop is observed. The empirical ground‐motion prediction equations, derived for both the larger horizontal and vertical components, are valid in the local magnitude range from 0.5 to 5.9, and for hypocentral distances up to 190 km.

Evaluation of local site effects on earthquake damages of Fatih Mosque

Fatih Mosque and its Kulliye (complex) are one of the most important historical monuments in Istanbul. Since it has been built between 1463 and 1470, Fatih Mosque had been subjected to nine strong earthquakes and suffered various degrees of structural damage at every case, including the latest August 17, 1999 Kocaeli Earthquake ( M w = 7.4, epicentral distance approximately 100 km). Recently, a project has been initiated first to study the possible causes of earthquake damage and then develop retrofitting and strengthening techniques to protect this invaluable monument from further damages in the future earthquakes. As part of this investigation, local site soil conditions had been determined and site behavior during earthquakes had been studied in detail. In this paper, the results of 1-D site response analysis, which included convolution and deconvolution analyses utilizing the strong ground motions recorded during the August 17, 1999 Kocaeli Earthquake are presented. The results of the analyses had demonstrated the considerable degree of site amplification, compatible with the recorded motions and the damage suffered. The expected site behavior during a probable future earthquake is also studied using a site specific simulated bedrock motion, and earthquake parameters to be used in dynamic structural analysis are estimated.

Anomalous seismicity preceding the 1999 Izmit event, NW Turkey

SUMMARY The 1999 Izmit earthquake (1999 August 17, Mw= 7.4) was one of the largest earthquakes to occur in northwestern Turkey during the past 100 yr. This earthquake occurred along the Izmit–Sapanca fault within the Northern Anatolian Fault Zone. Variations in the generalized fractal dimensions (Dq) of clustering in time and space, the Gutenberg–Richter b-value, and earthquake frequency (N) are evaluated in detail over an 8.5 yr time period preceding the Izmit event. Spatial and temporal comparisons of the variations in these parameters reveal anomalous intermediate term behaviour over shorter timescales than previously observed. Significant correlation is observed between changes in b-value with the spatial and temporal fractal dimension of epicentre distribution. These correlations oscillate back and forth over 2–3 yr time intervals and suggest occurrence of significant instability in the nature of intermediate-term deformation along the fault zone. Seismotectonic behaviour immediately preceding the Izmit event is represented by a significant rise in b-value to a maximum of 2.26 accompanied by relatively small increases in D. The rapid rise of b is associated with an increased frequency of low magnitude seismicity. Although D (spatial and temporal) increase slightly in the 2–3 yr period preceding rupture, they remain less than 1 indicating that seismicity remains clustered prior to rupture. In areas where earthquake frequency or station density do not permit resolution of short-term variations of fractal scaling parameters within a few months of main rupture, an accelerated period of low-magnitude seismicity (high b-value) concentrated along a fault zone (D < 1) may suggest heightened probability of a forthcoming large event.