1-D VELOCITY MODEL FOR SYRIAFROM LOCAL EARTHQUAKE DATA
Rami Ibrahim*
MEE06011

Supervisor: Hiroshi Takenaka**ABSTRACT
We performed inversion to estimate a 1-D velocity model of the crust and uppermost mantle
under Syria with station corrections following the procedure proposed by Kissling et al. (1994),
where simultaneous inversion for structure and hypocenters was carried out, using data of 542
selected events from the Syrian National Seismological Network during 1995-2004. wedetermined both P-wave velocity (Vp) and S-wave velocity (Vs) models. In addition to theoptimal 1-D velocity model the inversion provided a set of station corrections. Using this newmodel we relocated the 542 selected events. The comparison between the previous solutions andthe new solutions shows improvements of the total RMS values, which suggests that it is possible
to improve the accuracy of hypocenter determination by this new model.
Keywords: 1-D velocity model, Station correction, hypocenter determination

INTRODUCTION
The Syrian National Seismological Network has been in operation since 1995. It started with 20
seismic stations (short period 1 Hz). This set of stations was deployed in the western part of thecountry along the Dead Sea Fault System, which is considered the most active region in the northpart of the Arabian Plate. Later in 2003, more 7 Stations (short period 1 Hz) deployed in thenorth-eastern part of the country to monitor the seismic activity in the Euphrates graben and Abdel Aziz Sinjar uplift (Figure 1). The local network
recorded about 2762 seismic events form 1995 to 2004.
These events were located by SEISAN program(Haviskov et al., 2005). The velocity model used by
SEISAN for hypocenter determination was assembled
from refraction, reflection, well logging and gravity
data, but not according to seismic data generated fromnatural seismic sources. The appropriate velocity
model is necessary for a variety of purposes, including
the correct hypocenter determination, 3-D tomography,
moment tensor inversion and so on. Lack of such 1-D
velocity model for Syria was the main inspiration forthis research. The described 1-D velocity model will
Figure 1.The distribution of the local
be able to be used for routine and more detailednetwork.
*National Earthquake Center (NEC), Damascus, Syria.
Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University.

**1seismic data analysis for Syria. Syria is located in the north-western part of the Middle East
which is tectonically and seismically active.
The major tectonic settings in and around the country are the Anatolian-Iranian plateau andZagaros mountains, the Dead Sea fault system, the intracontinental Palmyrides fold-thrust belt,
and Abd Alaziz-Sinjar uplift.
Syria has been monitored locally for 10 years by 27 permanent digital seismic stations.
About 2762 events have been recorded by thelocal network since 1995 till 2004. Most ofrecorded earthquakes are micro-earthquakes(Mc<3.0) while the largest local recorded
earthquakes was on December 24, 1996 in
Palmyrides belt zone with a magnitude 5.4(Mc). Most of the events concentrated on themargins of the Arabian plate (Dead Sea fault
system, East Anatolian fault System) and on
the intracontinental Palmyrides belt zone,
besides some of the events from the oceanic
crust of Mediterranean sea and fewer events ofthe continental collision zone of Zagros(Figure 2).
Figure 2. The recorded earthquakes by the local
network from 1995-2004.

DATA AND A PRIORI VELOCITY MODEL
The hypocenter determination of the raw data was done by SEISAN program. P and S arrivals
were picked manually according to the quality of their onsets. P-wave arrivals have clear onsets
since they are the first arrivals but S-waves have less quality than P-waves since it is difficult
sometimes to distinguish clear onsets. For the inversion process, the well located earthquakes
were selected with a good distribution along the active regions inside and surrounding the country.
Out of 2,762 recorded earthquakes by the Syrian National Seismological Network, a first data set
resulted in 588 events with 6 (P and S) arrivals and more. Forty six events from the first set wererejected and the rest 542 events are shown in Figure 3.
The definition of a priori velocity model is an
initial reference model and a reference station. We
established the initial reference model according to
the pervious geological and geophysical studies in
Syria and Jordan. The reference station used forthe inversion is (WRDH) station (see figure 1). It
is situated to north-west of the local network. Thestation is located on good and hard limestonerocks belong to Cretaceous period. It has thelargest number of P-wave and S-wave
observations. The total P-waves and S- waves
recorded by this station are 682, P-wave arrivals
are 477 and S-wave arrivals are 235, respectively.

Figure 3. the selected events for the inversion
process.

2VELOCITY MODEL ESTIMATION
Initial inversion run was done for the 542 selected events by using the initial reference model
which I mentioned to in the previous section. To improve the reference model many trials wereachieved. First we chose a velocity model consisting of 19 layers; each layer has a thickness of 2km up to 36 km depth. The aim of this fine model is distinguishing the smooth change from thesharp change in the velocity with depth. Another inversion was made on this model. The resulting
model showed a clear changing velocity on 4km for both P-waves and S-waves. On 10 km just Pwaves velocity increased but not S-waves velocity. Following down the velocity clearly increasedon 18 km depth and 34km depth for P and S wave-velocities. The trial resulted of a new initialreference model after combining the closing layers has the same velocity. This model consists offive layers for Vp and four layers for Vs. This improved model was used again for inversion
which produced the final velocity model.

RESULTSdepth(km)The resulting velocity model shows increasing P-waves and S-waves velocities with depth asfollows. The sedimentary layers up to 4 km depth have P-wave velocity of 5.68 km/sec and Swave velocity of 2.99 km/sec (Vp/Vs = 1.89). From 4 km to 10 km depth P-wave velocity
increased up to 5.87 km/sec and Vp/Vs=1.69, from 10 km to 18 km depth P-wave increased again
to 6.18 km/sec, S-wave velocity does not change from 4 km to 18 km and has a value of 3.48km/sec and Vp/Vs=1.78. Both velocities increase again at 18 km/depth. P-wave velocity is 6.74km/sec while S-wave velocity is 3.95 km/sec and Vp/Vs=1.70. Pn has a value of 8.0 km/sec, Snhas a value of 4.64 km/sec and Vp/Vs=1.72 (Figure 4). To check the best Moho depth which ismore suitable for the final model. We fixed the velocity. Then we applied inversion runs bychanging Moho depth. We changed the Moho depth from 30km to 42 km depth by 1km step.
Since the best value was ranging between 37 km depth and 38 km depth we did one more
inversion for 37.5 km depth. We
velocity(km/sec)
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
have obtained the best Moho0depth on 37.5 km with respect toVp-1D
less RMS value. Fixing the10Vs-1D
velocity model during thevp/vs(1D)inversion also allow the station
20
corrections and the hypocenters30
approach a local minimum. Swave time correction for the40
reference station has a value
approaching zero on 37.5 km
50depth.
60

Figure 4. The final 1-D velocity model and the Vp/Vs ratios forthe both models.

In addition to the velocity model the inversion procedure used here provides two sets of station
corrections. One for P-wave time station corrections and the other one for S-wave time station
corrections. These corrections were done according to the reference station. Only the P-wave
station correction fixed for the reference station (set =0), and the S-wave correction is freefloating. Station corrections can reduce the effect of the lateral inhomogeneity for the velocity

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model. There is no correlation between the station correction and the station elevation, because
we considered the station elevation for the inversion. The obtained station corrections just include
the effects of the subsurface geology and the Moho depth (Figures 5, 6). The negative time station
correction means that the station is located in local high velocity area with respect to the reference
station.

Figure 5. P-wave time station corrections

Figure 6. S-wave time station corrections

DISCUSSION
The reference station WRDH has a zero P-wave time station correction and S-wave time station
correction has an almost zero value (+0.009). Stations KFRA, BIDA, BTCH, DRWC, HAWK,MNKR, DRBT, and SFNV have small positive values of P and S-wave time station correction. It
may attributed to the surface geology. Since all these stations are located on a less suitablegeological conditions with respect to the reference station. Stations SLNF, BARB have bothnegative P and S-wave time station corrections, and the station sites belong to Upper Jurassic andlocated on massive homogenous limestone rocks. These station sites are better geologically than
the reference station site. Station KBSD has both negative P and S-wave time station corrections.
The station site is located on basaltic rocks which is harder than reference station site. StationMZRK has a small P-wave time correction, but a negative S-wave time correction. This station islocated on basaltic rocks which is harder than the reference station site. Stations RABH, QASN,KOOC, TOTH, BDRN, and BSHR have high positive P and S- wave time station corrections.
This is likely to be due to the deeper Moho which we will discuss later. Stations TCHB, ZALF
have both positive P and S-wave time station corrections. Station SALA has a positive P-wave
time station correction and a small negative S-wave time station correction. Probably the delay
time of these stations has been affected by a deeper Moho and these stations have been located ina wide fractured area. Station SRME has a positive P-wave time station correction and a negative
S-wave time station correction which is likely to be come from the small number of observations(four P-wave arrivals and three S-wave arrivals). Station WHAB has a small P-wave time station
correction and a small S-wave time station correction which are within the ranges of theestimated errors. Station JHLN has a small positive P-wave time station correction and negative
S-wave time station correction. The reason of which is not clear.
To get a rough estimate of the Moho depth beneath the Palmyrides region where stations
RABH, QASN, KOOC, TOTH, BDRN and BSHR are located. we calculated the average P-wave
time station correction for these stations. The average value was 0.59 sec. The difference crustal
thickness beneath the Palmyrides with respect to the crustal thickness beneath the reference
station is the product of the average P-wave time station correction and the velocity in the lower

4crust (Vp=6.74km/sec). The estimated value was around 4 km. We added this value to the Mohodepth beneath the reference station. The result (41.5 km depth) is the Moho depth beneath thePalmyrides. This value is coincident with the gravity survey along the Palmyrides region wherethe estimated Moho depth is 38-42 km (Best et al., 1990).
The inversion procedure gave new hypocenter locations for the 542 selected events. To
compare this result with the hypocenter location using the current velocity model which is being
used for the routine data analysis by the Syrian network. We performed a joint hypocenter
determination for the 542
selected events using this modelNby VELEST in single event
mode (Figure 7). The estimated
1-D velocity model showed areduction of total RMS value ofall events compared to thecurrent velocity model (Figure8). Our interesting part is inside
the network, where more precise
results are expected. The
horizontal hypocentral
variations are small especiallywestern part Syria where thenetwork distribution is denser.
The depth cross sections underSSyria show that the new
distributions reduced somespace in the data where someEW
events shifted up to the upper
crust. The average deviations inorigin time, x, y and z were 0.77
sec, 4.2 km, 3.7 km, 3.1 km
respectively.

Figure 7. Comparison between the relocated hypocentersby the new velocity model and the hypocenters by the current
velocity model.

250
current-model

number of events

200

1-D velocity-model

150

Figure 8. Difference of RMS of theresiduals between the hypocenter
locations using the current velocity
model and the estimated 1-D velocity
model

100
50
0
0.2

0.4

0.6

0.8

11.2

1.4

1.6

1.8

RMS value5CONCLUSION
The present study used an efficient method for estimating the 1-D velocity model by solving thecoupled hypocenter-velocity model problem. We inverted a set of earthquake data by using
VELEST program. The program minimized P-wave and S-wave residuals of the data set
according to the procedures outlined by Kissling et al., (1994). The inversion resulted of: (1) 1-D
velocity model including P and S-wave velocities. (2) Hypocenters have been relocated withhigher accuracy. (3) P-wave time station corrections have a good accuracy and coincident to thelocal geology. (4) S-wave time station corrections may have a less accuracy than P- time station
corrections due to lower quality and quantity of data. (5) An appropriate Moho depth has been
estimated. (6) Moho depth has a significant effect on station corrections in addition to surface
geology. Unfortunately our estimated 1-D velocity model is limited by a few numbers ofobservations for the north-eastern part of the local network.
We hope that the new model will be useful for smooth routine and detailed seismic analysis
for the Syrian National Seismological Network.

ACKNOWLEDGMENT
I am deeply indebted to my advisor Dr. Tatsuhiko Hara whose help, stimulating suggestions andencouragement helped me in all the time of researching and writing this paper. National
Earthquake Center (NEC) in Damascus for providing us the earthquake data.

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