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The
Earthquake and Tsunami of August 17, 1999 in the Sea of Marmara,
Turkey.
George
Pararas-Carayannis
Introduction
On August 17, 1999,
a large destructive earthquake struck northwest Turkey and generated
a local tsunami within the enclosed Sea of Marmara. This was
the strongest earthquake to strike Northern Turkey since 1967.
It occurred along the Northern Anatolian fault. Its epicenter
was in the Gulf of Izmit. Official estimates indicated that about
17,000 people lost their lives and thousands more were injured.
However, it is believed that the death toll may have been much
higher.
Earthquake
Epicenter, Origin Time, Magnitude and Aftershocks
 Epicenter: the epicenter of the earthquake was at 40.702
N, 29.987 E (USGS)
near the town of Gölcük on the western segment of the
North Anatolian Fault - which is very much like the San
Andreas fault in California.
Origin Time: The earthquake occurred at 00:01:39.80(UTC),
03:01:37 a.m local time. Depth: Focal depth was shallow 17 km.
(USGS).
Magnitude: The Izmit earthquake was measured
widely by seismic stations around the world. There were small
differences in magnitude determinations. Surface Wave Magnitude
was given as 7.8 (USGS); Moment Magnitude was given as 7.4 (USGS;
Kandilli); Duration Magnitude: 6.7 (Kandilli). Body Wave Magnitude:
6.3 (USGS), 6.8 (British Geological Survey).
Aftershocks: Numerous aftershocks with magnitude
above 4 were recorded after the main earthquake. The first of
the aftershocks (magnitude of 4.6) occurred 20minutes later.
Several others followed in subsequent days. Several days later,
on 31 August , a strong aftershock killed one person, injured
about 166 others and knocked down some of the buildings that
were already weakened by the 17 August main quake. According
to the USGS and Kandilli most of the aftershock activity was
confined to the region bounded by 40.5-40.8N and 29.8-30.0E,
which covers the area between Izmit and Adapazari to the east
of the epicenter. However there was a clusters of aftershocks
near Akyazi and near Izmit.
Tectonic
and Geological Instability of the Region
The excessive seismicity
of this particular region can be explained by current geophysical
knowledge of its structural development. The North Anatolian
fault is a major fracture that transverses the Northern part
of Asia Minor and marks the boundary between the Anatolian tectonic
plate and the larger Eurasian continental block. Because of this
unstable tectonic system the area is considered as one of the
most seismically active zones of the world. Turkey is being squeezed
sideways to the west as the Arabian plate pushes into the Eurasian
plate. The north Anatolian fault forms the edge of this Turkish
(Anatolian) crustal block so that destructive earthquakes happen
regularly along it as different sections break.
Tectonic map
of the Anatolian Plate bounded by the African, Eurasian and Arabian
plates - Modified After Nafi Toksoz of MIT/ERL
Tectonic
and Geologic Setting
The Sea of Marmara
is an inland sea separating Asia Minor from Europe. It is 280
km (175 miles) long and almost 80 km (50 miles) wide at its greatest
width. On its northeast end, it connects with the Black Sea through
the Bosphorus Strait. On its southwest end it connects with the
Aegean Sea through the Dardanelles. Although its total area is
only 11,350 square km (4,382 square miles) , its average depth
is about 494 m (1,620 feet), reaching a maximum of 1,355 m (4,446
feet) in the center. The sea was formed as a result of tectonic
movements that occurred about 2.5 million years ago, in the Late
Pliocene. The region is characterized by frequent earthquakes.
The earthquake of
August 17, 1999 occurred along the long, east-west trending,
great North Anatolian Fault Zone (NAFZ) - known to be the most
prominent active fault system in Northwestern Turkey. NAFZ passes
through Izmit Bay, traverses Marmara Sea and reaches the Saros
Gulf to the southeast. This great fault system has many similarities
to the San Andreas fault system in California. Earthquakes involve
primarily horizontal ground motions (strike-slip type of faulting).
Numerous large earthquakes have occurred throughout history.
Historical
Earthquake Activity Along the Northern Anatolian Fault (Source:
Kandilli Observatory and Research Institute)
Earthquake's
Surface Rupture and Ground Displacements
The earthquake's surface
rupture extended for about 100 km east of Golcuk, but did not
continue southeast and did not join the rupture of the 1967 earthquake
- the last event in this region. Instead, the rupture turned
northeast near Akyazi, where a cluster of aftershocks subsequently
occurred. Ground displacements of about 1.5 m were measured in
this area. Subsequent field studies, indicated right lateral
ground displacements ranging from 2.5-3 m up to 4 m, with a maximum
of 4.2 m east of Lake Sapanca. Ground displacements between Lake
Sapanca and the Gulf of Izmit were about 2.60 m. Additionally,
there was evidence of about 2 meters subsidence along the north
side of the fault's block - which was particularly evident along
the coastline at Golcuk, where tsunami waves and major flooding
occurred. Such tectonic ground displacements are characteristic
of major earthquakes along the North Anatolian Fault and, possibly,
have been responsible for tsunami generation in the past.
Tectonic Map
of the Marmara Region and location of the earthquake
The August
17, 1999 Earthquake Occurred Along a Known Seismic Gap
Turkey is seismically
active. The early historic record shows that in 1754, an earthquaked
near Izmit killed about 2,000 people. In the last hundred years,
numerous large earthquakes have also occurred along the Northern
Anatolian Fault (NAF), in the western part of the country. Beginning
with an earthquake in 1939, several more quakes - with Richter
magnitudes greater than 6.7 - struck in progression along adjacent
segments of the great fault. The August 17, 1999 Izmit earthquake
was the eleventh of such a series that have broken segments of
the NAF, in both eastward and westward direction.
Historical
Earthquakes and Crustal Movements along the North Anatolian Fault
(from Stein et al., 1996)
There has been an
interesting pattern to this seismic activity. Historic seismic
records indicate that between 1939 and 1944 there was an active
westward trend in seismic activity - with a resulting surface
rupture of 600 km of adjacent fault. Subsequently, the westward trend of earthquakes
slowed down.
Earthquakes that occurred
in 1957 and 1967 ruptured an additional adjacent 100 km of the
NAF, but there was separated activity further west during 1963
and 1964. The 1963 event (Richter magnitude 6.3) in the Sea of
Marmara, to the west of Izmit, broke a section of the fault and
killed only one person. The last strong earthquake (magnitude
7.1) to strike Northern Turkey occurred in 1967. It killed 173
people.
A long seismic gap
separated the location of the 1967 quake and those of the 1963
and 1964 quakes. Quite predictably, the August 17, 1999 earthquake
occurred in this gap, where apparently seismic stress had build
up. The earthquake filled in the 100 to 150 km long gap which
existed. As early as 1979, this gap had been readily identified
by numerous scientists as a potential site for a future earthquake.
A subsequent evaluation in 1997 estimated a 12% statistical probability
of an earthquake occurring in the 30 year period, from 1996 to
2026, in this region. Obviously, the statistical probability
was underestimated as the earthquake occurred much sooner than
statistical studies had anticipated.
Elsewhere in Turkey,
a major earthquake (Richter magnitude 7.1) in 1970 near Gediz
- about 160 km (100 miles) to the south - killed over 1,000 people.
Its aftershocks continued for several years. In 1998, a less
severe earthquake in the south killed 140 people.
Was All
the Strain on the Seismic Gap Released by the August 17, 1999
Earthquake?
It appears that most
of the seismic strain along this section of the North Anatolian
fault was released by the August 17, 1999 earthquake. However,
given the measurements of 1.5 meter ground displacements in the
Akyazi area, versus the larger displacements elsewhere, it is
quite possible that not all of the seismic strain was released
by this event and that some future seismic event will release
the remaining strain. This may not happen for many years.
Earthquake Destruction
and Death Toll
The earthquake struck
a densely populated residential and industrial area which included
the major cities of Istanbul and Izmit. The quake caused immense
destruction to homes, apartment buildings, oil refineries and
extensive power and communication failures.
Izmit - Many apartment buildings in the
poorer section of Izmit collapsed burying thousands. Improper
building construction was primarily responsible for the high
death toll. According to official reports the earthquake killed
thousands of people and injured more than 16,000 others. The
exact death toll may never be accurately known as thousands were
reported as missing. Also, a huge fire at Turkey's largest oil
refinery outside Izmit, destroyed several storage tanks.
Golcuk - At Golcuk about 500 buildings
collapsed leaving about 20,000 people homeless. A Turkish naval
base in the port of Golcuk sustained major damage. Reportedly,
the collapse of the barracks killed 248 sailors.
The
Tsunami of August 17, 1999 in the Sea of Marmara, Turkey
Although the earthquake
involved primarily horizontal ground displacements, slumping
and landslides triggered tsunami waves which were particularly
damaging in the Gulf of Izmit, perhaps because of convergence
and a funneling effect. The long duration of the earthquake's
ground motions (45 seconds), the directivity of the surface seismic
waves, the proximity of the epicenter to the Sea of Marmara and
the Gulf of Izmit, and the overall orientation of the affected
area, strongly support that the tsunami was generated in the
Gulf of Izmit, in the eastern portion of the Sea of Marmara.
The tsunami waves
from this earthquake had an extremely short period of less than
a minute which also supports the premise that the source was
the localized subsidence of coastal areas and the underwater
slumping of unconsolidated sediments, rather than larger scale
tectonic movements which involved primarily lateral motions.
An initial recession
of the water was observed at both sides of Izmit Bay immediately
after the quake, followed by tsunami waves which had an average
runup of 2.5 m. along the coast. Maximum runup was 4 m in Golcuk
where there was considerable damage to the naval base facilities.
In fact, Golcuk and several coastal areas are now flooded permanently
as a result of the tectonic subsidence and landslides.
Also, large coastal
portions of the the town of Degirmendere remained flooded as
a result of subsidence - with sea level reaching the second floors
of apartment buildings. Similar permanent flooding, but to a
lesser extent, occurred also at Karamursel.
Lessons Learned - The lesson learned from this
event is that tsunamis can occur in any body of water since a
variety of mechanisms can generate them. Even earthquakes involving
primarily horizontal ground motions (strike-slip type of faulting)
can generate tsunamis by triggering slope failures and underwater
landslides. Obviously the tsunami risk for the Sea of Marmara
needs to be carefully evaluated. Measures must be taken to mitigate
the effects of future tsunamis in the area. Better construction
and building codes will definitely help.
(Damage from
the earthquake, tectonic subsidence, ground liquifaction and
the tsunami. Ship in the foreground thrown onshore by tsunami
wave action. Source:
Kandilli Observatory and Research Institute (modified))
Another view
of a damaged area. Source:
Kandilli Observatory and Research Institute
Assessment
of the Tsunami Potential in the Sea of Marmara and the Aegean
Archipelago
Numerous large destructive
earthquakes and tsunamis have occurred from antiquity to the
present in the Ionian Sea, Greece, the Aegean Archipelago, Turkey
and the Sea of Marmara - which separates Asia Minor from Europe.
Large earthquakes with intensity greater than VIII on the Modified
Mercalli scale have caused damaging or disastrous tsunamis -
particularly along the Southern Aegean Sea (Pararas-Carayannis,
1992). Most of the destructive tsunamis in the past originated
from a region of the Hellenic arc where normal faulting within
the southern part of the Anatolian Tectonic Plate (the Aegean
plate) is consistent with a NE-SW trending graben along which
the Santorin volcanic field has also developed. (Pararas-Carayannis,
1992).
To a lesser extent,
tsunamis have been also generated along the northeast portion
of the Aegean Sea, and the Sea of Marmara. Although most of the
earthquakes along the great North Anatolian fault involve primarily
horizontal ground displacements - and such tectonic movements
do not ordinarily generate tsunamis - some of the earthquakes
along the western segment of the fault have triggered major slumps
that have generated tsunamis. At least 9 major tsunamis have
been reported to have occurred in the Marmara Sea in the past
(Kuran and Yaciner, 1993). The most recent tsunami in the Eastern
Marmara Sea was associated with the 18 September 1963 earthquake.
Strike- slip ground
movement with a very small vertical component, can indeed generate
a tsunami in a closed body of water. A combination of disturbances
can be triggered by a large magnitude earthquake and several
secondary mechanisms for the generation of tsunami waves are
possible. Generative causes may include a combination of tectonic
movements associated with the earthquake or major subaerial or
underwater slides.
Such secondary phenomena
associated with a large earthquake can contribute to the generation
of destructive waves particularly in an enclosed body of water
like the Sea of Marmara. Tsunami generation will depend on the
earthquake's energy release, the proximity of the body of water
to the epicenter, the physical rupture along the fault, the propagation
path of surface seismic waves, and the magnitude and duration
of the dynamic, near-field, strong motions. Earthquake ground
motions of high intensity could result in strong ground accelerations
and the generation of waves in the immediate area of the earthquake
(Pararas-Carayannis, 1999). Ground liquefaction can also trigger
landslides which in turn could generate destructive waves.
Conclusions
It is believed that
earthquakes occuring along the Western portion of the Northern
Anatolian fault zone can generate destructive tsunami waves in
the Sea of Marmara. A number of grabbens, fault offsets and other
structural topomorphological features at the bottom of the Sea
of Marmara indicate that seismic activity and movements of branches
of the North Anatolian fault extend under the sea.
Even an earthquake
on land or a large aftershock could trigger a landslide in unconsolidated
deposits or sediments along the coast. The tsunami danger is
more pronounced in the eastern region of the Sea of Marmara and
particularly in the Gulf of Izmit. The tsunami risk for the Sea
of Marmara needs to be carefully evaluated. Obviously, government
authorities will have to do some serious review of what measures
must be taken to mitigate the effects of future earthquakes and
possible tsunamis in the area. Better construction and building
codes will definitely help. The potential for tsunami generation
in the Sea of Marmara is substantial.
References
and Further Reading
Ross S. Stein, Aykut
A. Barka and James H Dieterich, Progressive failure on the North Anatolian fault
since 1939 b earthquake stress triggering, Geophysical Journal International, Vol. 128,
594-604, 1996.
M. Nafi Toksoz, A.F.
Shakal, and Andrew J. Michael, (19790. Space-time migration of earthquakes along the
North Anatolian fault zone and seismic gaps by , Pageoph, Vol. 117, 1258-1270, 1979.
Earthquake Information
from National Earthquake Information Center
Earthquake Information
from Bogazici University, Kandilli Observatory and Research Institute.
Special page on the
earthquake by IRIS (Incorporated Research Institutions for Seismology)
National Earthquake
Information Center (NEIC) (USGS)
U. Kuran and A.C.Yalciner
(1993), Crack Propagations,
Earthquakes and Tsunamis in the Vicinity of Anatolia, in Tsunamis
in the World, ed.by
S.Tinti, pp.159-175
Pararas-Carayannis,
G. The Tsunami Generated
from the Eruption of the Volcano of Santorin in the Bronze Age. Natural Hazards 5::115-123,1992.
1992 Kluwer Academic Publishers. Netherlands.
Pararas-Carayannis,
G. The Earthquake
of September 7, 1999 in Athens, Greece. Article posted at http://drgeorgepc.com/Earthquake1999Greece.html
Links to other Web Sites
for More Information on the Earthquake
Additional documentation
of the August 17, 1999 Turkey Earthquake can be found at the
following web sites.
Kandilli Observatory and Research Institute (Graphics)
U.S. Geological Survey
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