Original Research Papers

Bulletin of IISEE, 46, 121-137, 2012

1-D VELOCITY MODEL FOR SYRIA FROM LOCAL
EARTHQUAKE DATA AND NEW SEISMICITY MAP
IN SYRIA
Rami Ibrahim*,**, Hiroshi Takenaka***, Mohamad Daoud*, and Tatsuhiko Hara****ABSTRACT
We performed inversion of P and S wave arrival time data from a set of 542 events recorded by the
Syrian National Seismological Network during 1995-2004 to determine 1-D velocity structure beneath
Syria. We determined both P and S wave velocity structures with a set of station corrections. The P
wave velocity in the shallow part in the new model is faster than the current model, while the P wave
velocity in the lower part of crust in the new model is slower than that in the current model. We
performed hypocenter determination using this new velocity model and compared the relocatedhypocenters to hypocenters determined for the current crustal model used by National Earthquake
Center of Syria. There is a significant difference in the distributions of their focal depths. When the
new velocity model is used, the number of the events in the depth range between 0 and 5 km increases,
while that in the depth range between 10 and 15 km decreases. The RMSs of the arrival time residuals
obtained by the new model are significantly smaller than those obtained by the current model, whichsuggests that it is possible to improve the accuracy of hypocenter determination using this new velocity
model. We find qualitative correlations between the obtained station corrections and surface geology.

INTRODUCTION
The Syrian National Seismological network recorded 2762 seismic events between 1995 and 2004. The
National Earthquake Center of Syria located these events using SEISAN program (Havskov andOttemöller, 2005). The velocity model used for hypocenter determination was constructed based onrefraction, reflection, well loggings and gravity data, but seismic data from natural earthquakes were
not used. Therefore, the current velocity model may be significantly biased by heterogeneous samplingof the crust and uppermost upper mantle both horizontally and vertically. Lack of a 1-D velocity modelfor Syria constrained by data from natural earthquakes is the motivation of this research. The
appropriate velocity model is necessary for a variety of purposes, including the reliable routine
hypocenter determination, seismic tomography, moment tensor inversion and so on.
In the present study, we determined a 1-D velocity model with station corrections byanalyzing data from the Syrian National Seismological network. We used the software VELEST(Kissling 1988; Kissling et al., 1994). The procedure to obtain a 1-D velocity model using this software
was explained by Kissling et al. (1994) and Kissling (1995).

TECTONIC SETTINGS OF SYRIA
Figure 1 shows the major tectonic units in Syria. There is the Dead Sea fault system (“DSFS” in Fig. 1)
in the west of the country; it has experienced a total slip of about 105 km since Miocene (Quennell,1958). In the middle of Syria, the intracontinental Palmyrides fold thrust belt represents an early
Mesozoic rift basin and Cenozoic convergence associated with nearby plate interaction. At its
southwestern extremity in Lebanon, the belt is cut off by the DSFS, whereas to the northeast it
stretches into the Euphrates graben system. The Palmyrides is situated between two stable tectonic
zones. The Aleppo plateau, which is located to the north-west of Palmyrides, is covered by both
National Earthquake Center, Damascus, Syria.** Earthquake Research Institute, University of Tokyo, Japan
Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Japan**** International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan****- 121 -

Paleozoic and Mesozoic sediments and is “a flat raised structure with little deformation relative to
intracontinental Palmyrides belt zone” (Best et al. 1990). To the southeast, lies the Rutbah uplift, which
Best et al. (1990) described as broad, domal basement structure. Very little deformation is found in thestrata of the uplift, except for along the northeastern edge where it trends into the Euphrates depression(Chaimov et al., 1992).
The Euphrates depression is a NNW-trending asymmetric half graben system that deepens to
the east. The Euphrates depression has been faulted down relative to the adjacent eastern platform, andthe total slip length is 3.5 km. The Abd el Aziz-Sinjar uplift lies to the northeastern part of theEuphrates depression, and is controlled by a major south dipping reverse fault. Sawaf et al. (1993)
suggested that Abd el Aziz-Sinjar uplift was a sedimentary basin in Mesozoic which inverted in the
Neogene.

Fig. 1 The major tectonic units in and around Syria. “DSFS” denotes the Dead Sea Fault system.

DATA
The Syrian National Seismological Network has been in operation since 1995. Figure 2 shows thestation distribution. The network started with 20 seismic stations with short period sensors with cornerfrequencies of 1 Hz. The stations have vertical component sensors except stations ARNB, HAWK,RABH, BRBR, ZALF, and WHAB which have three components sensors. These stations were
deployed in the western part of the country along the Dead Sea Fault System, which is considered themost seismically active region in the north part of the Arabian Plate. Later in 2003, additional 7
stations (1 Hz short period sensors) were deployed in the north-eastern part of the country to monitor
the seismic activity in the Euphrates depression and Abd-el Aziz Sinjar uplift. Among these stations
just station SFNE has three components sensor and the rest contains vertical component sensors.
P and S wave arrivals recorded at the Syrian stations were picked manually, and were
processed by SEISAN at the National Earthquake Center of Syria. For the present study, from 2,762
earthquakes recorded by the Syrian National Seismological Network, we selected well located 542- 122 -

earthquakes with a good spatial distribution for the active regions inside and surrounding the country.
Figure 3 shows the distribution of the selected events. For each event, we have at least 6 P and S wave
arrivals. Figure 4 shows the numbers of P and S arrival time data for each station.

Fig. 2 The distribution of the short period seismic stations of the Syrian National Seismological
Network.

Fig. 3 The distribution of the selected events for this study.

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Fig. 4 The number of observations for each station for the 542 selected events. The stations in the rightside of the chart have smaller numbers of observations because they have been deployed since 2003.
The total numbers of P and S wave arrivals are 5236 and 2582, respectively.

PROCEDURES AND A PRIORI VELOCITY MODEL
Figure 5 shows the data analysis procedure of this study. First we constructed an initial reference
velocity model. Since such a velocity model is built based on deep seismic exploration data down to
the Moho is not available in Syria, we referred to seismic exploration experiments performed inneighboring countries under geological conditions similar to that of Syria. We found data from two
seismic refraction experiments: El-Isa et al. (1987) performed a seismic refraction experiment in thewest and middle Jordan to obtain P and S wave velocity models for the crust and uppermost upper
mantle; Mechie et al. (2005) obtained P and S wave velocity models based on refraction and reflection
explorations across the Dead Sea fault in the west of Jordan and the east of Palestine. Based on these
two studies, we constructed our initial reference velocity model shown in Table 1. We used the top
layer, for which “Depth to the top of the layer (km)” is negative, to account for station elevations. The
minimum focal depth is set to 0 km in this study.
For the velocity inversion it was necessary to choose a reference station, which should have acontinuous or nearly continuous record of events. It had to be a reliable station, preferably located near
the center of the network, and had not show extreme site effects (Kissling et al., 1994). We have chosen
WRDH as a reference station in this study. This station is located in the north-west of the network (Fig.
2) and the site conditions at this station are good (hard limestone rocks from the Cretaceous period).
This station has the largest number of P wave arrivals (447 wave arrivals) and a large number of S
wave arrival time data (235 wave arrivals).
We performed initial inversion using the initial reference velocity model for the data described in theprevious section (we set the maximal epicentral distance for phase use to 500 km). We constructed
more detailed velocity model based on the initial velocity model, consisting of 19 layers. Each layer
except for the first layer has a thickness of 2 km (Table 2). Then another inversion was carried out
using this finer layer velocity model to find the clear sharp velocity changes at depths of 4km, 10km,
18km, and 34 km. For a depth of 10km, only P wave velocity changes, while the velocity changes arefound both for P and S waves at the other depths.

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Initial model based on El-Isa et al. (1987) and Mechie et al. (2005)
Inversion
Preliminary 1-D velocity model

19 layers velocity model
Inversion
Improved model (layer numbers are fixed)
Inversion
Improved model (parameters except for the Moho depth are fixed)
Search for the optimal Moho depth
Final 1-D velocity modelOver-damped inversion
Hypocenters and station corrections
Fig. 5 The flowchart of the procedure to obtain the 1-D velocity model. See the text for details.
Table 1 The initial velocity model based on data from El-ISA et al. (1987) and MECHIE et al.
(2005).
Depth to the top ofVp (km/sec)Vs (km/sec)
the layer (km)
4.60

2.65

-3

4.60

2.65

0

4.60

2.65

2

6.00

3.64

5

6.30

3.68

10.5

6.40

3.68

18.5

6.65

3.68

20

7.90

4.47

34

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Based on this result, we constructed a velocity model consisting of five layers for P wave
velocity and four layers for S wave velocity. Then we perform another inversion using this model as a
new initial model. Finally, to find the optimal Moho depth, we varied the Moho depths from 30 km to
42 km while fixing the other velocity model parameters. Figures 6 and 7 show the RMSs of theresiduals and the S wave station corrections for the reference station as a function of the Moho depths,
respectively. The minimum RMS was obtained for a depth of 38 km. The velocity model with the
Moho at a depth of 38 km is consistent with the S wave station correction of the reference station,
because it is close to zero around this depth. After obtaining this final model, we determined
hypocenters and station corrections using this model.

Table 2 The 19 layers velocity model used to search the optimal numbers of layers.
Vp (km/sec)Vs (km/sec)

Depth to the top of
the layer (km)

5.44

3.14

-3

5.44

3.14

0

5.44

3.14

2

5.44

3.14

4

5.89

3.40

6

5.89

3.40

8

6.25

3.61

10

6.25

3.61

12

6.25

3.61

14

6.25

3.61

16

6.61

3.82

18

6.61

3.82

20

6.61

3.82

22

6.61

3.82

24

6.61

3.82

26

6.61

3.82

28

6.61

3.82

30

6.61

3.82

32

7.95

4.57

34

7.95

4.57

36

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Fig. 6 The RMSs as a function of Moho depths. The depth of 38 km provides the minimum RMS(0.543).

Fig. 7 S-wave station correction for the reference station as a function of Moho depths. It is close to 0 s
for the depth of 38 km, which provides the minimum RMS as is shown in Fig. 6- 127 -RESULTS AND DISCUSSION
Velocity model, relocated hypocenters and arrival time residuals
Figure 8 and Table 3 show the estimated 1-D velocity model and corresponding Vp/Vs ratio. In Fig. 8,
we also show the current velocity model used for hypocenter determination by National Earthquake
Center of Syria for comparison. The P wave velocity in the shallow part (down to a depth of about 5km) in our model is faster than the current model. The slow P wave velocity in the current model islikely to come from seismic exploration data. Our result suggests that such low velocity is not
representative of the average 1-D velocity model for Syria. The P wave velocity in the lower part ofcrust in our model is slower than that in the current model. The constraints for this depth range arelikely to be weak considering the dataset used in the construction of this model.

Fig. 8 The final 1-D P and S wave velocity models are shown by the thick solid and dashed lines,
respectively. The corresponding Vp/Vs ratio is shown by the dotted line. The current P wave velocity
model used by the National Earthquake Center of Syria is shown by the thin solid line for comparison.
Table 3 The final velocity model and Vp/Vs ratio.
Vp (km/sec)Vs (km/sec)

Vp/Vs

Depth to the topof the of layer (km)

5.67

2.98

1.90

-3

5.68

2.99

1.90

0

5.87

3.48

1.69

4

6.18

3.48

1.78

10

6.74

3.95

1.71

18

8.00

4.64

1.72

38

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(a)(b)- 129 -

(c)Fig. 9 The hypocenters and frequency distributions of the focal depths of the 542 events used ininversion for 1-D velocity structure. (a) The hypocenters determined by the current velocity model are
shown in the map and cross sections in the N-S and E-W directions. (b) The hypocenters determinedby the model obtained in this study are shown in the map and cross sections in the N-S and E-W
directions. (c) The frequency distributions of the focal depths of the hypocenters shown in (a) (denoted
as “current model”) and (b) (denoted as “Our model”) are shown in the left and right panels,
respectively.

Fig. 10 Frequency distributions of RMSs of arrival time residuals for the events used in this study forthe final model (dark gray) and the current model are shown (light gray), respectively.

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Figure 9 shows the hypocenters determined by the model obtained in this study and thosedetermined by the current velocity model with their depth distributions. While there is not a large
difference between their epicenters, there is a difference between their depth distributions. When ourmodel is used, the number of the events in the depth range between 0 and 5 km increases, while that in
the range between 10 and 15 km decreases. This difference can be found in the new seismicity map
obtained using our model, which will be presented later.
Figure 10 shows the comparison of the RMSs of the arrival time residuals obtained for ourmodel and those obtained for the current model. Clearly, the residuals of our model are smaller thanthose of the current model. The averages of the RMSs are 0.70 sec and 0.99 sec for our and current
models, respectively. This result means that our model better fits the observed arrival times.
Station corrections
Figure 11 shows the station corrections for P and S waves. In the inversion, only the P wave station
correction for the reference station is set to zero. Since we considered the station elevations in the
inversion, the obtained station corrections are expected to reflect the effects of near surface geology.
For the reference station WRDH, the P wave station correction was set to zero in the inversion, and the
S wave station correction is close to zero (0.01 sec), which was shown in the previous section.
The positive P and S wave station corrections are obtained for stations KFRA, BTCH, ARNB,
HAWK, SFNE, DRBT and MNKR. They imply lower velocities, which are qualitatively consistent
with surface geology. KFRA is on Upper Miocene basalt rocks. They are porous and also highly
weathered. BTCH is on Middle Miocene rocks, which are characterized by soft chalky and firm
nummulitic limestone. ARNB is located on Precambrian rocks. The geological structure is complicated,and consists of Serpentinized-Pridotites, Diabases, Spilities, Argillites, Radiolarites, Lava and Tuffs.
The surface rocks are highly weathered. HAWK and SFNE are on Cretaceous rocks of limestoneinterbedded by Marl. DRBT is located on Paleogene rocks characterized by limestones interbeddedwith Marl. MNKR is in the Upper Quaternary region, and the site itself is located on Tuffs rocks.
Station TCHB has positive P and S wave station corrections. The station site is located on
Paleocene and Lower Eocene chalky and nummulitic limestones. The limestones merge with Marls andClays. The geological complexity is likely to affect the observed station corrections.
Stations SLNF and BARB are located on massive homogenous limestone rocks, which
belong to Upper Jurassic. The negative P and S wave station corrections are obtained for this station,
which suggests higher velocities beneath them.
The large negative S wave station corrections are obtained for stations KBSD and MZRK.
They are located on basaltic rocks, for which higher velocities are expected.
Station SRME shows the relatively large positive P wave station correction and large negative
S wave station correction, which are likely to come from the small number of observations for thisstation (4 P wave arrivals and 3 S wave arrivals).
For Stations RABH, QASN, KOOC, TOTH, BDRN, and BSHR located in and around thePalmyrides fold thrust belt (Fig. 1), the large positive P and S wave station corrections are obtained.
Their cause should be investigated in future studies.
New seismicity map of Syria
We relocated the 2762 events recorded by the Syrian National Seismological Network between
1995-2004 using the new velocity model and station corrections obtained in this study. Figure 12
shows the seismicity obtained for the current and new velocity models with their focal depth
distributions. In Figure 12(c), we can find the same differences that were mentioned above on therelocated events used in inversion, namely, the increase and decrease in the numbers of events in thedepth range between 0 and 5 km and that between 10 and 15 km. As a result, the peak in the depth
range between 10 and 15 km disappears in the depth distribution of the relocated events.
In Figure 13, we show the seismicity in the western part of Syria considering its highseismicity and the good station coverage for this part. Again, we find the same differences, which- 131 -

implies that they are robust. These results will be useful to improve our understanding on the seismic
activity and sesimogenesis in Syria, since the focal depth distribution reflects geological andgeophysical conditions of faults such as fault develomement, temperature, and so on (e.g., Meissnerand Strehlau, 1982; Scholz, 2002), although further accumulation of data and analyses are required.
(a)

(b)

Fig. 11 The station corrections for P waves (a) and S waves (b), respectively. Black and gray circles
represent positive and negative corrections, respectively.

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(a)(b)- 133 -

(c)Fig 12. The hypocenters and frequency distributions of the focal depths of the 2762 events recorded by
the Syrian National Seismological Network between 1995-2004. (a) The hypocenters determined by
the current velocity model are shown in the map and cross sections in the N-S and E-W directions. (b)The hypocenters determined by the model obtained in this study are shown in the map and crosssections in the N-S and E-W directions. (c) The frequency distributions of the focal depths of the
hypocenters shown in (a) (denoted as “current model”) and (b) (denoted as “Our model”) are shownin the left and right panels, respectively.

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(a)

(b)

(c)

Fig 13. The hypocenters and frequency distributions of the focal depths of the events in the westernpart of Syria, Lebanon, North Jordan and South Turkey. (a) The hypocenters determined by the current
velocity model. (b) The hypocenters determined by the model obtained in this study. (c) The frequency
distributions of the focal depths of the hypocenters shown in (a) (denoted as “current model”) and (b)(denoted as “Our model”) are shown in the left and right panels, respectively.

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CONCLUDING REMARKS
We performed inversion of P and S wave arrival times from the Syrian National Seismological
Network to determine one-dimensional P and S wave velocity structures of the crust and uppermost
upper mantle beneath Syria with a set of station corrections. The Moho depth is estimated to be 38 km.
The P wave velocity in the shallow part of our model is faster than that in the current model used by
National Earthquake Center of Syria, while the P wave velocity in the lower part of the crust in ourmodel is slower. We performed hypocenter determination using this new velocity model and compared
the relocated hypocenters to those determined for the current model routinely used by National
Earthquake Center of Syria. We found a significant difference in the distributions of their focal depths.
When the new model is used, the number of the events whose focal depths are in the range between 0
and 5 km increases, while the number of events whose focal depths are in the range between 10 and 15
km decreases. Our model better explains the observed arrival times. The averages of the RMSs of thearrival time residuals are 0.70 sec and 0.99 sec for the new and current models, respectively. Thissuggests the possibility to improve the accuracy of hypocenter determination using this new velocity
model. We have found qualitative correlations between the station corrections and surface geology.
The velocity model obtained in the present study is the first model of Syria crustal structure
based on natural earthquake data recorded in Syria. This new model will be used to locate earthquakes
in and around Syria with higher accuracy. An increase of precise hypocenter data will make it possible
to perform seismic tomography for the study area in future, where our model may be used as an initialmodel for such tomographic studies. We hope that our model will be the first step for seismotectonic
studies in Syria

ACKNOWLEDGMENTS
This study was started when the first author participated in the JICA training course entitled“Seismology, Earthquake Engineering and Disaster Mitigation” during 2006-2007. We thank theanonymous reviewer for his comments, which were helpful to improve the original manuscript. We are
grateful for the National Earthquake Center of Syria for providing us the earthquake data. We thankboth Dr. Arash JafarGandomi and Dr. Genti Toyokuni from the seismological laboratory in Kyushu
University for their useful discussions. We also thank Dr. Samer Bagh and Ms. Rayan Yasamenh fromthe National Earthquake Center of Syria for their helps in making some figures using the Generic
Mapping Tools (GMT, Wessel and Smith, 1998) and GIS.

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