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Aykut
Barka
Istanbul Technical University, Istanbul, Turkey
On 17 August 1999 at 3:02 a.m. local time, northwest Turkey [HN1] was shaken by a magnitude 7.4 earthquake [HN2], catching most people in their sleep. The earthquake's epicenter [HN3] was located at latitude 41.81ºN and longitude 30.08ºE, and the earthquake had a depth of around 10 to 16 km. Records of the maximum ground motion [HN4] varied between 0.3 and 0.4g. The earthquake lasted 37 s and consisted of two subevents, the first triggering the second. The nearest major city affected by the earthquake was Izmit at the eastern end of the Marmara Sea, but the earthquake also caused considerable damage in Istanbul, about 70 km away from the earthquake's epicenter [HN5]. The earthquake's magnitude was initially estimated as 6.7 by the Kandilli observatory [HN6], which operates a seismic network in the region. This estimate was considerably lower than the actual magnitude of 7.4 and initially gave a wrong impression of the likely damage to scientists and governmental officials.
During the first 2 days, the severity and extent of the damage and human loss were seriously underestimated. Rescue operations were delayed because of lack of communications and heavy damage on the major highway connecting Istanbul to Ankara. Alternate roads were blocked by people wanting to reach relatives and friends in the most affected areas because all telephone communication had broken down. Many apartment blocks collapsed completely, causing the death of many people in the earthquake region. A fire in the Tupras oil refineries, which refine 86% of Turkey's oil, threatened to take over other industrial sites and took 5 and a half days to contain and extinguish. The navy base and ship-building yard at Gölcük were cut by surface faulting and displaced 4 m to the right, causing considerable damage and the death of at least 400 soldiers and high ranking officers. The total death toll has now reached 15,000, but thousands more are still missing [HN7].
The surface rupture caused by the earthquake consists of four segments (see the figure). Three of them--the Gölcük, Izmit-Sapanca, and Arifiye-Akyazi segments in the west--were caused by the first subevent and extend 90 km, trending ~80ºNE; they are separated by small offsets (or stepovers) of less than 1 km. The Gölyaka segment at the eastern end, caused by the second subevent, trends ~65ºNE and is about 30 km long. The earthquake started at the western end over the first 12 s and, after a pause of 18 s, was followed by rupture in the east for 7 s. The two ends of the surface break to stop at a stepover at the Karamürsel basin in the west, where the fault is offset by about 5 km, and at Eften Lake in the east. The maximum offset along the surface break was measured near Arifiye, east of Sapanca, where the fault displaced a small country road by 5 m dextrally [HN8]. The displacement caused by the second subevent in the east was less than 1.5 m. The earthquake was almost pure right-lateral strike-slip, that is, the slip on the fault was parallel to Earth's surface, and the fault plane was almost vertical [HN9] (see the figure). The major aftershocks, with magnitudes þ4, were located in the Düzce area south of Adapazari and Izmit and the Cinarcik area.
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Fault and earthquake map. The map shows the extent of the 1999 rupture (dotted green lines) and the 1943, 1967, and 1964 ruptures (blue, lines) and the segments that received stress increase (red lines) (7). |
At Degirmendere, a small town near Gölcük at the western end of the rupture zone, the surface breaks cut the toe of the fan delta where the center of the town was located and caused a slump 100 m normal to the coast line. Part of the town slid under the water, including a hotel and several shops and restaurants. At another fan delta east of Gölcük, a 2-m-high normal fault scarp was produced along a fault, connecting two strike-slip segments. This connecting normal fault had an 8-m-high relic scarp, suggesting the occurrence of at least three previous events besides the 1999 event.
During the earthquake, local people observed a ball of flame and the sound of an explosion in the gulf area. There were also reports of an abundance of dead crabs and jellyfish 2 days before the earthquake in Degirmendere. The former observation can be explained by the release of methane gas, trapped in the gulf on the delta swamp, by the friction on the fault during the rupture. The second observation is probably related to the release of radon gas in the sea [HN10]. For several days after the earthquake, balloons of strong light coming out of the sea were seen and recorded over the gulf and the northeastern Marmara Sea. Such lights have previously been reported after earthquakes elsewhere (1). Similar phenomena have also been recognized by my students in contemporary illustrations of the 1509 and 1556 central Marmara Sea earthquakes published by Ambraseys and Finkel (2, 3). They might be related to the release of radiactive gases from the rupture zone.
The 17 August 1999 earthquake was the seventh in a sequence of earthquakes migrating westward along the North Anatolian fault [HN11] and caused a 1000-km rupture. The time interval between these westward migrating earthquakes varied from 3 months to 32 years, including the 1999 event. The first earthquake in the sequence occurred in 1939 and caused rupture along a 360-km segment of the fault, with maximum horizontal displacements of up to 7.5 m (4). The 17 August 1999 earthquake increased the earthquake risk on the Yalova segment, that is, the western continuation of the northern strand toward the Marmara Sea, and the Düzce-Bolu segment at the eastern part of the Düzce-Hendek fault. The slip at the western end of each previous rupture segment was about 4 m, triggering the buildup to the subsequent event (4). The 1999 earthquake also terminated with a 4 m offset at the western end at the Karamürsel basin.
During the 20th century, only the 1943 Hendek earthquake with a magnitude of 6.4 affected the rupture zone of the 1999 earthquake. However, earlier earthquakes in 1719, 1754, 1878, and 1894 occurred in the Gulf of Izmit (3). The 1719 and 1754 earthquakes caused the death of 6000 and 2000 people, respectively, in the Gulf of Izmit, Istanbul, and Adapazari region (3). There is little information about the 1878 earthquake, which caused considerable damage and loss of life in the Sapanca and Adapazari regions. The 1894 earthquake caused damage and loss of life (1400 people) from Istanbul to Adapazari. The rupture zone for this earthquake is believed to be located either on the Yalova segment or the Cinarcik basin. Among these earlier earthquakes, the 1719 earthquake is perhaps most similar in magnitude and location to the 1999 earthquake, although it may have occurred closer to Istanbul. The remaining two earthquakes, in 1754 and 1878, probably occurred in the area between eastern part of the Gulf of Izmit and Adapazari.
Recent measurements with the Global Positioning System [HN12] have indicated that the northern strand of the North Anatolian fault, which goes through the Gulf of Izmit and the northern Marmara Sea, has a slip rate of [HN13] about 15 mm per year (5). This suggests that the recurrence interval for a 4 to 5 m displacement is about 300 years. Modeling of the 1939 to 1967 earthquake sequence (6) illustrates that during this period, stress has increased in the Gulf of Izmit region by a few bars. Given the estimated slip rate, information from historical earthquakes, and the modeling results, the location and severity of this earthquake should not have come as a surprise.
References and Notes
The author is at the Eurasian Earth Science Institute, Istanbul Technical University, Ayazaga, 80626 Istanbul, Turkey. E-mail: barka@itu.edu.tr
