prepared by
Ender Ozkan
Ph.D. Student, The Johns Hopkins University
03/02/2000
Disclaimer: The following is a summary of a report written by the author to present a brief description to the causes and outcomes of the 1999 Izmit Earthquake.
It does not claim to be an exhaustive information source in any way. The reader is referred to other sources, such as EERI, USGS, Kandilli Observatory, etc., for detailed information on this and other earthquakes.
Abstract
Izmit earthquake was the most devastating natural hazard in the history of modern day Turkey. The outcomes raised serious concern about the construction practice in the country. This paper presents a brief description of seismology of the earthquake and examines the performance characteristics of low-rise (2-5 story) residential buildings in the Izmit area. The failure mechanisms are presented and possible considerations in design and construction of such buildings are summarized.
Introduction
On August 17, 1999 magnitude 7.4 Mw earthquake struck 8 km. southeast of
Izmit at 3:02 am. Its effects were devastating, an estimated 18,000 people died
and 300,000 left homeless. Due to its proximity to the largest city in Turkey,
Istanbul, and it is high density of industrial infrastructure, this region is considered to be one of the most populated areas in the
country.
Figure-1)
The epicenter of the 1999 Izmit earthquake. (Magellan Geographix,1992)
The earthquake occurred at 3:02 am local time, when most people were in their homes and prone to the effects of the earthquake. This fact was one of the leading factors contributing to the large number of fatalities.
The public reaction to the earthquake was swift and strong; people from many
parts of the country contributed in the help and rescue efforts. However,
contradictory scenes, as in Fig-2, raised concern among public about the
construction practices in the region.
Figure-2) A 14’th century mosque is surrounded by remains of collapsed houses. The apartment in the background was nearly undamaged. (BBC)
Turkey had experienced earthquakes with comparable magnitudes before, all of which occurred in rural areas where the population density was very low. Even though the observations made in those incidents warned about future events, none of them were as powerful and awakening as the consequences of the Izmit earthquake.
Almost all of the fatalities occurred due to the collapse of residential structures. The most common type of residential construction in Turkey is 2-5 story low-rise reinforced concrete structures, selected primarily due to its simple design and ease of construction. Usually built by private contractors, their primary structure if formed of reinforced concrete frames, square or rectangular in cross-section. Nonbearing, unreinforced masonry is the primary material used for exterior and interior walls.
The first building codes in Turkey were written in 1942, mostly adopted from the building codes in other countries. Since then numerous revisions have been made, the last one in 1997. The Turkish code is acknowledged to be modern and includes strict regulations for earthquake resistant design (EERI Special Report, 1999). However, the extent to which these codes are being followed is questionable.
The following pages will give a brief description of the seismological characteristics of the Izmit earthquake and its effects on the low-rise buildings in the area. The common failure mechanisms are described and recommendations for the design and construction of such buildings will be made.
Izmit lies on the North
Anatolian fault, one of the most active fault lines in the world. According to
investigations done by USGS
and Kandilli Observatory, the
length of the fault zone exceeds 860 km, from Yalova on the west side to Sapanca
on the east.
The North
Anatolian fault is a perfect example of a right-lateral strike slip fault
system. Lateral displacements of 2 to 3 m. were observed for the most part of
the fault, the maximum being 5.1 m. just east of Sapanca Lake. A small portion
of the fault, to the east of Golcuk, is the only exception to the strike slip
motion. Here, the predominant motion is dip slip with vertical displacements
reaching 3 m. (EERI special report, 1999)
Figure-3) Plate tectonics in the region.
A significant property of the Izmit earthquake is the notion of seismic gap, regions on the fault line where no significant earthquakes have occurred in the recent past. The Figure-4 shows the distribution of the earthquakes on this fault line in the last 60 years. A close examination of the map reveals the seismic gap in the region that was also pointed out in a 1996 USGS report by Stein, R.S. In fact, Stein estimated a 12% probability of a major earthquake on this fault in the 30-year period. Thus, seismologically, the earthquake was not surprising, nor unexpected.
Figure-4)
Time distribution of earthquakes on the North Anatolian fault (USGS, 1999)
