INTRODUCTION
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.
ACTIVE TECTONICS OF THE EASTERN MEDITERRANEAN REGION
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).
NORTH ANATOLIAN FAULT ZONE
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).
1999 IZMIT EARTHQUAKE
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.
SUMMARY
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.
ACKNOWLEDGMENTS
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.
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Manuscript received October 25, 1999; accepted November
11, 1999. |
