Download Seismic Velocity Structure along the Western Segment of the North

 Bull. Earthq. Res. Inst.
Univ. Tokyo
Vol. 2, ,**1 pp. ,*3ῌ,,-
Seismic Velocity Structure along the Western Segment
of the North Anatolian Fault Zone Imaged by Seismic
Tomography
Mohamed K. Salah+*, S. Sahin, and M. Kaplan-
+
Earthquake Research Institute, University of Tokyo, Tokyo ++-ῌ**-,, Japan
Faculty of Engineering and Architecture, Suleyman Demirel University, Isparta, Turkey
-
General Directorate of Disaster A#airs, Earthquake Research Department, Ankara, Turkey
,
Abstract
The three-dimensional P and S wave velocity structures of the crust of the central and western
segments of the North Anatolian Fault Zone are determined by applying a tomography method to
arrival times data generated by local earthquakes that occurred beneath the study area. From the
obtained P and S wave velocity models, we further calculate Poisson’s ratio for a more reliable interpretation of the imaged seismic anomalies. With the exception of high-velocity anomalies detected at a depth of 2 km, prominent low-velocity zones are clearly visible along most parts of the
studied segment down to a depth of ,/ km. High Poisson’s ratio anomalies are widely distributed
in most parts of the studied region. Seismic activity is more intense in the high-velocity and high
Poisson’s ratio zones, although it sometimes occurs in low-velocity areas. Large crustal earthquakes
occur in areas characterized by a highly heterogeneous velocity structure, but with distinctly high
Poisson’s ratio anomalies. These results indicate that the active tectonics of this area are reflected
in the seismic velocity structure, and that large crustal earthquakes occur generally in highly heterogeneous zones revealed by seismic tomography. These inferences are of great significance for
understanding earthquake-generating processes in areas having intense seismic and tectonic activities such as the Anatolian plate.
Key words : Seismic tomography, Poisson’s ratio, P wave velocity, S wave velocity, North Anatolian
Fault Zone
+.
Introduction
southwestern boundary of the Anatolian block (Fig.
The North Anatolian Fault Zone (NAFZ) system
+). Due to the intense tectonic activity in this region
(Fig. +), about +/**-km long, delineates the northern
(e.g., Aktug and Kilicoglu, ,**0), the NAFZ has been
boundary of the Anatolian Plate, and is characterized
the locus of large seismic events in past centuries,
by a right-lateral strike slip motion (McKenzie, +31, ;
which presents a major source of risk for the popula-
Barka and Gülen, +322). In this complex tectonic en-
tion (Provost et al., ,**-). These large earthquakes
vironment, the collision of the Arabian, African, and
testify that the present-day high strain rate is accom-
Eurasian plates leads to a compressive regime to the
modated seismically (Pucci et al., ,**0), and place this
east of the Anatolian block (creating mountain ranges
fault among the most active strike-slip faults world-
such as the Zagros and the Caucasus), a right-lateral
wide (Ambraseys, +31* ; Ambraseys and Finkel, +33/ ;
strike-slip fault zone to the north (which accommo-
Ambraseys, ,**,).
dates the westward escape of the Anatolian block
Seismicity along the active segments of the
(McKenzie, +31, ; Sengör, +313), and a subduction zone
NAFZ is characterized by frequent moderate to large
associated with back arc spreading in the Aegean
earthquakes (M1) with focal mechanisms that show
Sea (McKenzie, +31, ; Barka and Gülen, +322) to the
essentially pure right-lateral strike-slip solutions
* e-mail : [email protected]
209 Mohamed K. Salah, S. Sahin and M. Kaplan
Fig. +. Simplified tectonic map of Turkey, Arabian, African, and Eurasian plates (Provost
et al., ,**-). The black arrows show the directions of relative plate velocity, and the
thick black line shows the location of the right-lateral strike-slip NAFZ. Epicenters of
+1 August +333 Gölcük earthquake and +, November +333 Düzce earthquake are shown
by star and cross, respectively. Abbreviations are : (K) Karliova triple junction ; (Er) Erzincan ; (Mu) Mudurnu Valley ; (IzF) Izmit fault ; (GF) Ganos fault.
(e.g., Canitez and »
Uçer, +301 ; McKenzie, +31, ; Zanchi
years, which endangers the city of Istanbul, where
and Anglier, +33-).
The +333 earthquakes are the
over +* million people live (e.g., Rockwell et al., ,**+ ;
most recent to occur on the NAFZ in the ,*th century.
Papazachos et al., ,**, ; Gürer et al., ,**-). Improving
The first of these earthquakes (Gölcük earthquake)
our knowledge of the seismic velocity structure of
occurred on +1 August, striking the Izmit region,
the crust in which these large catastrophic events
west of the Marmara Sea. This Mw 1.. (USGS) earth-
take place will contribute to a better understanding
quake has a focal mechanism (Harvard CMT) that is
of the overall tectonics of this region and the forces
consistent with a right-lateral movement along an
triggering these earthquakes.
E-W strike-slip fault. Maximum surface dextral of-
Although there have been extensive studies of
fsets exceeded / m, and surface rupture extended
the tectonics and near-surface geology of this unique
along the fault for a total rupture length of ++* km
region, research on deep crustal structure and earth-
(Barka, +333 ; Barka et al., ,***, ,**,). Three months
quake activity has been hampered by the sparse
later, on +, November (Muller et al., ,**- ; Utkucu et
coverage of seismic stations. Recently, Akyol et al.
al., ,**-), the Mw 1.+ Düzce earthquake occurred.
(,**0) obtained a +-D P-wave crustal velocity model
The focal mechanism solution shows almost a pure,
for western Anatolia, which is characterized by crus-
dextral strike-slip movement on an E-W nodal plane,
tal velocities that are significantly lower than aver-
dipping /. to 0. to the north, with a rake between
age continental values. It shows four layers down to
+11 and +01 (USGS, Harvard CMT (Tibi et al., ,**+)).
a depth of ,3 km (the assumed Moho depth), with a
This earthquake produced right-lateral surface rup-
velocity of 0.,/ km/s at depths between +/ and ,+ km
tures over a total length of .* km and a maximum
and a velocity of 0..- km/s for the lowermost crust
dextral o#set of / m (Akyüz et al., ,***, ,**,). As a
(depth range ,+ῌ,3 km). The velocity at the shallow-
result of these two large events, ,*,*** people were
est layer (depth- km), however, is poorly constrain-
killed and over +**,*** people were injured and/or
ed due to the lack of head waves and refracted waves
lost their homes and property. The extent of the
at the surface in the data set they used. The low ve-
damage was mainly due to the dense population of
locities might be associated with high crustal tem-
the region. A very severe (Mw1) earthquake is ex-
peratures, a high degree of fracture, or the presence
pected in the Marmara sea region within the next -*
of fluids at a high pore pressure in the crust (Akyol et
210 Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
al., ,**0). In this study we investigate three-dimen-
and September +/, +333 and +*13 aftershocks of the
sional velocity and Poisson’s ratio structures of the
November +,, +333 Duzce earthquake (Mw1.+). These
crust of this area, and try to correlate them with
events were recorded by +. Sakarya-Bolu Micro-
other geophysical observations that will strengthen
earthquake Recording Network (Sabonet) seismic
our current knowledge of the region, thus helping to
stations and + Turkish National telemetric earth-
mitigate geoseismic hazards.
quakes network (Turknet) permanent station belonging to the Turkish General Directorate of Disaster
,.
Data
A#airs (Fig. -). The remaining +0*- events are from
In this study we used -1-- events that occurred
background seismicity in the area, and occurred be-
near or to the north of the western segment of NAFZ
tween +332 and ,**- ; and were also recorded by the
(Fig. ,a). We used +*/+ aftershocks of the August +1,
above-mentioned seismic networks.
+333 Golcuk earthquake (Mw1..) between August +1
are located between latitudes -3./ῌ.+./N and longi-
These events
tudes ,2ῌ-,E with focal depths down to about -/
km (Fig. ,a). The uneven distributions of both the
seismic stations and the seismic events in the study
area impose limitations on the resolution scale of the
anomalies obtained, as is explained in the following
sections. The error in the hypocentral locations does
not exceed ,./ km for all events. Earthquakes tend to
cluster around the two +333 main shocks, but we also
selected additional events around these clusters to
get a more uniform distribution of hypocenters in
the study area. The selected -1-- events generated
,*1*, P and +.+.2 S arrivals recorded by the +/ seismic stations shown in Fig. -. The accuracy of arrival
times is estimated to be less than *.+/ s for P wave
data and somewhat larger (*.,/ s) for the S wave
data. We examined all the residuals stepwise with
respect to the assumed initial velocity model, and
removed data with residuals beyond the limit + s.
To study the relation between the nucleation
Fig. ,. (a) The epicenteral distribution of the -1-events used in this study that occurred along the
northwestern side of the NAFZ. Star is the epicenter of +1 August +333 Gölcük earthquake and
cross is the epicenter of +, November +333 Düzce
earthquake. (b) The distribution of large earthquakes that occurred along the NAFZ (Mῌ/.*)
since +32*. Thick and thin lines denote the North
Anatolian Fault Zone (NAFZ) and the Eskisehir
Fault Zone (EFZ), respectively.
Fig. -. Locations of seismic stations used in the
present study that belong to Sabonet and Turknet
seismic networks. Thick and thin lines denote the
North Anatolian Fault Zone (NAFZ) and the Eskisehir Fault Zone (EFZ), respectively.
211 Mohamed K. Salah, S. Sahin and M. Kaplan
zones of large crustal earthquakes (Mῌ/) and the
seismic velocity and Poisson’s ratio anomalies obtained, we also collected data on ,0 events that occurred in the study area starting from January +32*
up to December ,**/, from the earthquake catalog of
the National Earthquake Information Center (NEIC),
U.S. Geological Survey (Fig. ,b). A close inspection
of the distribution of the epicenters of these large
events suggests that they occur mainly due to movements along the NAFZ as explained in the previous
section.
-.
Method
The seismic tomography method was first devel-
oped by Aki and Lee (+310). Subsequently, many researchers have successfully improved this technique
and applied it to various regions around the world
(e.g., Hirahara, +311 ; Thurber, +32- ; Spakman and
Nolet, +322 ; Zhou and Clayton, +33*, Zhao et al., +33,,
+33. ; Nakajima et al., ,**+ ; Mishra et al., ,**-).
In
Fig. .. --D configuration of the grid net adopted for
the present study in horizontal (a), and depth (b)
directions. Grid spacing is *., in the horizontal direction and 0ῌ+/ km in the depth direction. Straight
lines in (a) show the location of vertical crosssections in Figs. +-ῌ+0.
this study, we use the tomographic method of Zhao
et al. (+33,), which has been applied to many regions
having di#erent tectonic circumstances (e.g., Zhao
and Kanamori, +33/ ; Zhao et al., +330, +331, ,**+ ; Serrano et al., +332, ,**,a, b ; Kayal et al., ,**,).
The
method uses an e$cient --D ray tracing scheme to
compute travel times and ray paths. We adopted a
time inversions, we use the following relation :
grid spacing of *., in the horizontal direction and 0ῌ
VpῌVs,,+sῌ+, s
+/ km in the depth direction (Fig. .). Velocities at
grid nodes are taken as unknown parameters, and
to compute the Poisson’s ratio (s).
the velocity at any point in the model is calculated
The selection of the initially used velocity model
by linearly interpolating the velocities at the eight
is an important step in any tomographic inversion
grid nodes surrounding that point. For more details
because it generally a#ects the amplitude and di-
about the method, see Zhao et al. (+33,, +33.).
stribution of the velocity anomalies obtained. We
The interpretation of tomographic images is usu-
adopted a crustal velocity model that is slightly
ally nonunique, because any given velocity anom-
di#erent from the model determined by Akyol et al.
aly can be attributed to either a thermal or a chemi-
(,**0) for western Anatolia as our initial P wave
cal variation. For this reason, it is generally useful to
velocity model. Their model shows four layers down
consider some other physical parameters for a more
to a depth of ,3 km (the assumed Moho depth), with a
reliable interpretation of the anomalies obtained in
velocity of 0.,/ km/s at depths between +/ and ,+ km
terms of tectonic and geodynamic implications. Com-
and a velocity of 0..- km/s for the lowermost crust
pared to the seismic velocity itself, the Poisson’s ratio
(depth range ,+ῌ,3 km).
The velocity at the shal-
(or Vp/Vs ratio) is a better indicator of the content of
lowest layer (depth - km), however, is poorly con-
fluids and/or magma (Zhao and Negishi, +332 ; Kayal
strained due to the lack of head waves and refracted
et al., ,**, ; Takei, ,**, ; Salah and Zhao, ,**- ; Naka-
waves at the surface in the data set they used. The
jima et al., ,**+, ,**/) or serpentinization (Kamiya
model we used in our study, on the other hand, is
and Kobayashi, ,*** ; Christensen, +330). Therefore,
even simpler. We assumed velocities of /, /.3, 0.*, 0.//,
after Vp and Vs models are calculated from travel
and 1.1/ km/s at depths of /, +*, +/, ,/, and .* km,
212 Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
and vertical rays passing at this depth.
The final inversion results were obtained after
two iterations. The P and S wave root-mean-square
(RMS) travel time residuals calculated after these
iterations were *.,1 and *.-* s, respectively. In the
following paragraphs we discuss the results of Vp,
Vs, and Poisson’s ratio only in areas having a reliable
resolution.
Fig. /. Initial P wave (solid line) and S wave (dashed
line) velocity models adopted for the present study.
Figures 2 and 3 show velocity perturbations
relative to the initial velocity models of the P and S
waves at depths of ,, 2, +/, and ,/ km, respectively,
respectively (Fig. /).
Because there are almost no
together with the distribution of seismicity around
studies of S wave velocity structure in this area, we
the studied depth slice. The locations of the epicen-
used the relation : VsVp/+.1-, to derive the initial
ters of the two large +333 earthquakes are also
S wave velocity model. We checked a number of
shown. Fig. +*, on the other hand, shows perturba-
slightly di#erent initial velocity models, but finally
tions of Poisson’s ratio at the same depths. To study
selected the above-mentioned one because we found
the relation between the nucleation zones of large
that it gives the minimum RMS travel time residual.
earthquakes (Mῌ/) and the imaged anomalies, we
also plot their epicentral distribution superimposed
..
Resolution and Results
on the velocity and Poisson’s ratio structures at 2 and
Before describing the main features of the re-
+/ km depths (Figs. ++, and +,) because the focal
sults, we show the results of a checkerboard resolu-
depths of most of these events are around +* km. In
tion test (CRT), (Inoue et al., +33* ; Zhao et al., +33,,
the following paragraphs, we describe the main fea-
+33.) to demonstrate the reliability of the tomo-
tures of our results.
graphic images obtained.
The values of assumed
At the shallowest layer (, km depth), the velocity
checkerboard-type perturbations assigned to grid
structure is generally heterogeneous, having strong
nodes are -῍ ; the image of which is straightfor-
lateral variations amounting to 0῍. A low P and S
ward and easy to remember. Synthetic arrival times
wave velocity anomaly is detected at the central
are then calculated for the checkerboard model.
portion of the study area (between longitudes -* and
Numbers of stations and events with their exact
-+./E) with some portions having a high P wave
locations in the synthetic data are taken to be the
velocity. This low-velocity zone has a high Poisson’s
same as those in the real data set.
ratio, and is characterized by intense seismic activity
The CRT results for the P and S wave velocity
structures at four crustal depth slices are shown in
(Figs. 2, 3, and +*).
This heterogeneous structure
continues down to a depth of 2 km, but is dominated
Figs. 0 and 1, respectively. The resolution is particu-
by higher than average velocities. The amplitude of
larly good where many ray paths (event-station
the high S wave velocity anomaly is much lower
pairs) are traversing. At a depth of , km, there is
than that of the P wave, and sometimes the S wave
good resolution in the areas where the seismic sta-
velocity is lower than the average at the margins of
tions are located, and most of the input synthetic
the well-resolved zone. This is also reflected in the
anomalies are well recovered. The surrounding parts
high Poisson’s ratio anomalies seen in most parts.
have relatively poor resolution, especially where no
Seismic activity is relatively intense around this
events are located. This is due to insu$cient ray
depth slice, and occurs mostly in the zone of average
paths criss-crossing there. For deeper layers (depths
to high velocities and average to high Poisson’s ratio
2, and +/ km), the resolution is reliable only near the
with minor exceptions.
station locations and the seismically active regions
At depths of +/ and ,/ km, prominent low P and
(Figs. 0 and 1). Some parts of the study area have a
S wave velocities are widely seen in most parts of the
reasonable resolution at a depth of ,/ km because of
study area (well-resolved zone).
su$cient lengths and directions of both horizontal
higher than average at a depth of +/ km, but is gener-
213 Poisson’s ratio is
Mohamed K. Salah, S. Sahin and M. Kaplan
Fig. 0. Results of a checkerboard resolution test for P wave velocity structures at four
crustal depths. The grid spacing is ,* km. Filled and open circles show high and low velocities, respectively. Perturbation scale is shown at the lower right.
Fig. 1.
The same as Fig. 0, but for S wave velocity.
῎ 214 ῎
Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
Fig. 2. P wave velocity perturbations at depths of ,, 2, +/, and ,/ km, respectively. White circles denote
seismicity in the depth range shown between brackets below each map. Dark and light gray colors
denote high and low velocities, respectively. The perturbation scale is shown to the right. Thick and
thin lines denote the North Anatolian Fault Zone (NAFZ) and the Eskisehir Fault Zone (EFZ), respectively. Star and cross denote the epicenters of the +1 August +333 Gölcük and the +, November +333
Düzce earthquakes, respectively.
Fig. 3.
The same as Fig. 2, but for S wave velocity.
῎ 215 ῎
Mohamed K. Salah, S. Sahin and M. Kaplan
Fig. +*. Distribution of Poisson’s ratio perturbations at four depth slices. Light and dark gray colors denote high and low Poisson’s ratios, respectively. Other details are similar to those of Fig. 2.
ally low at a depth of ,/ km with the exception of the
+333 Golcuk earthquake, while cross-section DD’ pas-
zone near the epicentral area of the +1 August, +333
ses through the hypocenter of the +, November, +333
Golcuk earthquake. Seismic activity is also concen-
Duzce earthquake. The velocity and Poisson’s ratio
trated along the zones of low to average velocity/
structures are very heterogeneous at the top +* km
average to high Poisson’s ratio.
depths, but at deeper levels, low-velocity/high Pois-
In Figs. ++ and +,, we plot the seismic velocity
son’s ratio anomalies dominate. The hypocenters of
and Poisson’s ratio structures at depths of 2 and +/
the two large +333 events are located in distinctive
km along with the epicentral distribution of the large
zones characterized by average P wave velocity, low
crustal earthquakes. It is clear that the majority of
S wave velocity, and high Poisson’s ratio. The back-
the large events occur in the highly heterogeneous
ground seismic activity and the majority of the large
zones dominated by high velocities at a depth of 2
earthquakes occur also in zones dominated by aver-
km, which is underlained by lower velocity zones at
age P wave velocity, low S wave velocity, and high
a depth of +/ km. The amplitude of the low S wave
Poisson’s ratio. We discuss the implications of these
velocity zone, however, is higher, which is reflected
observations in the following paragraphs.
in the prominent higher than average Poisson’s ratios at the two depth slices. These results imply that
/.
Discussion
/. + Large earthquakes along the NAFZ
fluids might be involved in triggering large events in
this tectonically active region, as is explained later.
The NAFZ is an active right-lateral system,
The velocity and Poisson’s ratio structures de-
which is bound to the north by the westward extrud-
scribed before are also illustrated along vertical cross-
ing Anatolian block (McKenzie, +31, ; Sengör, +313 ;
sections AA’, BB’, CC’, and DD’ (Figs. +-ῌ+0), which run
Barka, +33, ; Saroglu et al., +33,). Since the Middle/
either in the E-W or N-S directions (see Fig. . for the
Late Miocene (+-ῌ/ Ma) the NAFZ has accumulated a
location of cross-sections). Cross-sections AA’ and
geologic displacement on the order of 2/ῌ+,* km
CC’ pass through the hypocenter of the +1 August,
(Seymen, +31/ ; Sengör, +313 ; Barka, +32+ ; Barka and
῎ 216 ῎
Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
Fig. +,.
Fig. ++. P wave velocity (a), S wave velocity (b), and
Poisson’s ratio (c) perturbations at a depth of 2 km
together with the epicenteral distribution of large
earthquakes (see text for details). Other details are
similar to those of Fig. 2.
The same as Fig. ++, but at a depth of +/ km.
two main strands, the Duzce and the Mudurnu fault
segments, where GPS data indicate that the former
accommodates up to +* mm/yr (Ayhan et al., +333).
Farther west, the NAFZ splays again into three ma-
Hancock, +32. ; Westaway, +33. ; Armijo et al., +333 ;
jor strands and the northernmost one is consider-
Hubert-Ferrari et al., ,**,). This displacement trans-
ed to accommodate most of the present-day strain
lates into a long-term and short-term geologic slip
(Barka and Kadinski-Cade, +322 ; Straub et al., +331).
rate of *./ῌ*.2 cm/yr (Tokay, +31- ; Seymen, +31/ ;
The preceding paragraphs show a high rate of
Barka and Hancock, +32.) and +.2 cm/yr (Hubert-
seismic activity along the NAFZ, which is also clear
Ferrari et al., ,**,), respectively.
from the distribution of the seismic events used in
Conversely, GPS
networks have measured present-day strain rates in
this study (Fig. ,), most of which are aftershocks of
the northern part of the Anatolian block that reach
the two large +333 earthquakes (see section ,). The
῏,ῌ- cm/yr (Reilinger et al., +331, ,*** ; Straub et al.,
highly heterogeneous seismic velocity and Poisson’s
+331 ; McClusky et al., ,*** ; Kahle et al., +333, ,***).
ratio crustal structures obtained in this study are
To the east of the town of Bolu, the NAFZ is formed
consistent with this unstable seismo-tectonic set-
by a main single trace, but to the west it splays into
ting. The majority of large crustal earthquakes oc-
῎ 217 ῎
Mohamed K. Salah, S. Sahin and M. Kaplan
Fig. +-. Vertical cross-sections of P wave velocity, S
wave velocity, and Poisson’s ratio structures along
line AA’ (see Fig. . for the location of the crosssections). Low velocities and high Poisson’s ratios
are shown by light gray, whereas high velocities
and low Poisson’s ratios are shown by dark gray.
Star denotes the hypocenter of the +1 August +333
Golcuk earthquake and big circles denote large
earthquakes. Crosses show the distribution of seismicity in a +*-km-wide zone around the profile.
The perturbation scale is shown to the right of
each panel.
Fig. +..
The same as Fig. +-, but along line BB’.
and are certainly of value for identifying and characterizing faults with potential for surface rupturing
earthquakes.
/. , Previous geophysical observations and obtained velocity and Poisson’s ratio models
The velocity models obtained in this study for
western Anatolia are dominated by low velocities
with the exception of a high-velocity anomaly at a
depth of 2 km and by high Poisson’s ratios (except at
cur close to zones characterized by average P wave
,/ km depth). The low Poisson’s ratios at a depth of
velocity, low S wave velocity, and high Poisson’s ra-
,/ km, which appear around the seismically active
tios (Figs. ++, and +,). In summary, a comprehensive
zone, have low or even no resolution, hence we con-
analysis of tectonic landforms and associated struc-
sider them to be unreliable features.
tures, combining the results of seismic tomography
(,**0) obtained lower crustal velocities in western
Akyol et al.
and other geophysical observations, are valuable
Anatolia and attributed them to high temperatures,
tools for defining the strain distribution pattern and
fluids, and high pore pressure, or the presence of
its evolution in the near surface. These data are the
partial melt. Their results also reflect near-surface
basis for understanding how surface deformation
geological complexities and that crustal velocities
builds up. Consequently, they provide a picture of
are significantly slower in western Anatolia, increas-
the characteristics of the principal slip zone at depth,
ing from 0.,/ km/s at a depth of ,+ km to only 0..-
῎ 218 ῎
Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
Fig. +/.
The same as Fig. +-, but along line CC’.
Fig. +0. The same as Fig. +-, but along line DD’. The
big cross denotes the hypocenter of the +, November +333 Düzce earthquake.
km/s at the Moho. The velocities obtained at the
base of the crust are within the bounds of average
crustal velocities at high temperatures (Christensen
gions are interpreted to be hot and unstable mantle
and Moony, +33/). These low velocities might indi-
lid zones, whereas the very low Pn velocity zones are
cate that fluid-filled faults and fractures in this seis-
interpreted to be regions with no mantle lid. Al-
mically active region permeate the crust (e.g., Al-
though we have no resolution at depths greater than
Shukri and Mitchell, +322 ; Mitchell et al., +331).
It
,/ km, these observations, however, support the exis-
was also found that the average Pn velocity for
tence of the low velocities and high Poisson’s ratios
the entire Aegean region is approximately 1.3 km/s
anomalies we detected at depths greater than +/ km
(Panagiotopoulos and Papazachos, +32/), which is
(Figs. +-ῌ+0). Seismic activity is concentrated mostly
lower than the worldwide average continental upper
in the top +* km depths, which might suggest the
mantle Pn velocity of 2.+ km/s (Moony and Braile,
presence of seismically active low-angle breakaway
+323).
faults within the upper crust of western Anatolia
Recently, Al-Lazki et al. (,**.) detected broad-
(Sengör, +321).
scale (/** km) zones of low (2 km/s) Pn velocity
Measurements of coda Qc by Akinci et al. (+33.)
anomalies underlying the Anatolian plate and the
in western Anatolia for the frequency range +./ῌ+* Hz
Anatolian plateau, and even smaller-scale ((,**
show a strong frequency dependence, which agrees
km), very low (1.2 km/s) Pn velocity zones beneath
with the assumption that tectonically active areas
the Isparta Angle, central Turkey and the northern
show a high attenuation due to the complex struc-
Aegean Sea region. The broad-scale low-velocity re-
ture of the region. Using a electrical resistivity sur-
219 Mohamed K. Salah, S. Sahin and M. Kaplan
vey in northwest Anatolia, Caglar (,**+) detected a
study area are dominated by low velocity anomalies.
/ km thick surface layer of low resistivity character-
Poisson’s ratios are generally higher than average
izing the sedimentary sequences of this depth range,
with minor patches of low Poisson’s ratio anomalies.
and deduced that the tectonic structure is compara-
Seismic activity is concentrated near the zones char-
tively complex. Below a depth of about / km geoelec-
acterized by low to average velocities and moderate
tric models show a more resistive structure underly-
to high Poisson’s ratio. The results of a checkerboard
ing these sediments. The resistive structure is corre-
resolution test show that most of the currently ob-
lated with Precambrian crystalline rocks and gneiss
tained structures are reliable features. They are also
schist basement. This change from low to high re-
in general agreement with many other geophysical
sistivity at a depth of about / km is consistent with
observations, although the scales of such studies
the change from low (at , km depth) to high (at 2 km
should be considered. All these anomalies are associ-
depth) velocities we observed in our study.
ated with the complex and active tectonic setting of
High
average heat flow with geothermal activity (e.g., Ilk-
the region, the circulation of hot geothermal fluids,
isik, +33/ ; Pfister et al., +332 ; Gemici and Tarcan,
and the presence of sediment fillings at the surfacial
,**,), high rate of seismicity (Bozkurt, ,**+), inten-
layers.
sive faulting and extension-related Neogene and
The various pieces of evidence mentioned in the
Quaternary volcanism (e.g., Paton, +33, ; Innocenti et
previous sections suggest that the generation of a
al., ,**/) are also among the main characteristics of
large crustal earthquake is closely related to the
the region.
surrounding tectonic environment such as rifting/
Sari and Salk (,**0) found that the most pro-
spreading or the relative transform motion between
nounced structural and morphological features in
adjacent plates and the physical/chemical properties
western Turkey are graben-like structures created
of crustal materials such as magmas and fluids. Com-
by E-W normal faulting. The grabens are filled with
plex physical and chemical reactions may take place
recent sediments that give rise to relatively negative
in the source zone of a future earthquake, causing
gravity anomalies.
Caglar and Isseven (,**.) de-
heterogeneities in the material property and stress
tected three electrically conductive (.ῌ0 W m) zones
field, which may be detected by seismic tomography
at depths of /ῌ+/ km. The origins of these zones are
and other geophysical methods. These results indi-
explained by the circulation of hydrothermal fluids
cate that large earthquakes do not occur anywhere,
with low resistivity values (*.,2ῌ*./, W m) and by the
but only in anomalous areas that may be detected
e#ects of a strong hydrothermal alteration in the
with geophysical methods. Higher resolution seis-
rocks. They also detected a low magnetic anomaly
mic imaging of this area combined with other geo-
with a low intensity of about /* g in a central area
logical, geochemical, and geophysical investigations
in northwest Anatolia, and ascribed it to hydrother-
would certainly deepen our understanding of the
mal demagnetisation of the rocks due to geothermal
ongoing tectonic activity and the earthquake-gen-
activity.
erating processes, and would also contribute to the
mitigation of seismic hazards.
0.
Conclusions
In this study we collected a number of arrival
Acknowledgements
times of P and S waves from the aftershocks of the
We thank M. Uyeshima and two anonymous re-
two large +333 earthquakes, as well as local earth-
viewers for their constructive comments and discus-
quakes, and used them to map the --D crustal struc-
sions, and the National Earthquake Information Cen-
ture of P wave, S wave, and Poisson’s ratio beneath
ter (NEIC), (http : //neic.usgs.gov/neis/epic/epic_rect.
northwest Anatolia. The velocity structure is very
html) for facilitating the availability of earthquake
heterogeneous at shallow layers of the crust (, and 2
catalogs and seismological data on the web. All fig-
km depths) with amplitudes of 0ῌ. It is dominated
ures in this paper were made using GMT (Generic
mainly by low velocities at a depth of , km and high
Mapping Tools) software written by Wessel and
velocities at a depth of 2 km. At the deepest layers of
Smith (+332).
the crust (depths +/ and ,/ km) most parts of the
220 Seismic Velocity Structure along the Western Segment of the North Anatolian Fault Zone Imaged by Seismic Tomography
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