APPLICATION OF TIME-DOMAIN MOMENT TENSOR
INVERSION TO WAVEFORM DATA FROM THE NEWSAUDI ARABIA BROADBAND SEISMIC NETWORK
Majed Matar AL-Mutairi*

Supervisor: Dr. Tatsuhiko Hara **MEE06005
ABSTRACT
Recently Saudi Geological Survey (SGS) has established a new broadband seismic network. I applied thetime domain moment tensor inversion code developed by Dreger (2003) to the waveform data from thisnew seismic network. I analyzed three earthquakes: the July 2, 2006 event (ML 4.7) in the Red Sea area,
the February 2, 2006 (ML 5.0) and April 13, 2007 (ML 4.1) events in the Gulf of Aqaba area. The obtained
focal mechanisms are consistent with the tectonic settings, i.e., extension along the spreading centers in
Red Sea and strike slip movement along the Dead Sea transform fault, respectively. Also, the focal
mechanism of the Red Sea event is partly consistent with that of the Global CMT Catalog. These results
suggest that it is possible to monitor and investigate seismic activity by moment tensor determination
using data from the new SGS broadband seismic network. I also discussed the issues to be completed
before applying the technique used in this study to routine moment tensor determination.
Keywords: Time-Domain Moment Tensor Inversion, Broadband Seismic Network, Focal Mechanism.
INTRODUCTION
Geomorphology and geology of the Saudi Arabia
The Kingdom of Saudi Arabia extends over 1800 km north-south and 1150 km east-west from the Red
Sea to the Arabian Gulf (covering an area of approximately 1, 960, 582 sq. km.). The elevations exceed
3000m in the southwest. Sand seas occupy 50% of the country; much of the remainder is rock exposure,and largely lacking soil, laterite cover, grass, and forest. The Arabian plate margins are characterized by:• extension along spreading centers in the Red Sea and Gulf of Aden in the southwest and south
where the plate is spreading from the African plate.
• strike-slip movement along the Dead Sea transform, which marks the northwest margin of theplate along which left-lateral slip is taking place.
• by strike-slip movement along the Owen fracture zone on the southeast, where right lateraldisplacement of about 300 km has occurred.
• by compression at the Zagros thrust and Bitlis suture in the north and northeast, where the plate iscolliding with the Eurasian plate.

*The author works for Saudi Geological Survey in Saudi Arabia ,** The supervisor works forInternational Institute of Seismology and Earthquake Engineering,Japan,Tsukuba.

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Seismicty of the Saudi Arabia
Saudi Arabia is subject to a range of earthquake activity because it is bounded by the Dead Sea transformon the northwest, the Red Sea spreading center on the west, the Gulf of Aden spreading centre on thesouth, and the Zagros subduction zone on the northeast.
Currently four institutions are operating seismograph networks: King Abdulaziz Center for
Science and Technology, King Saud University in Riyadh, King Abdulaziz University in Jeddah, and theSaudi Earthquake Center at the Saudi Geologic Survey in Jeddah.
Saudi National Seismic Network

Figure 1. The location of seismic network(Triangles indicate the location of broadband
stations).

The Saudi Earthquake Center has deployed broadband
seismograph in 2004 and is now running 19 broadband seismic
stations. Figure 1 shows the location of these stations. Eight
more stations will be installed in the near future. This new
network adopts satellite communication and 3 componentsbroadband digital sensors (Trillium-40).
In this study I apply the time-domain moment tensor
inversion code developed by Dreger (2003) to waveform datafrom this new broadband network. Moment tensor analyses will
be useful for monitoring seismic activity and understanding
seismotectonics in and around Saudi Arabia.

TIME-DOMAIN MOMENT TENSOR PROCEDURE
In this study, I followed the time–domain inversion procedure (Dreger and Romanowicz, 1994; Pasyanos
et al. 1996) to determine moment tensors. The software package of this technique has been used at the
University of California, Berkeley Seismological Laboratory (BSL) since 1993 to automatically analyze
events (ML>3.5) in northern California (http://seismo.berkeley.edu/~dreger/ mtindex.html).
Also, this package has been implemented at the Japan National Research Institute for Earth
Science and Disaster Prevention (NIED) to perform automated seismic moment tensor determination
using on-line broadband waveform data from “F-net” (full Range Seismograph Network of Japan)(Fukuyama et.al 1998)( http://www.fnet.bosai.go.jp/freesia/index.html).
This software package is available at International Handbook of Earthquake and Engineering
Seismology, part B attached Handbook CD, and has been widely used by a number of researchers.
In this technique, linear least squares for deviatoiric moment tensor is preformed for equation ofsynthetic seismogram for a spatial and temporal point source. Only high frequency location is used. The
assumptions for this procedure are as follows: (i) the event location is well represented by the highfrequency hypocenter location. Dreger and Helmberger (1993) showed that event mislocation on the orderof 15 km can be tolerated. (ii) the source time function may be approximated by a delta function.(iii) the
crustal structure is sufficiently well known for low frequency waveform modeling.
The source depth is determined by finding the solution that yields the largest variance reduction:⎡
VR = ⎢1 − ∑i⎣⎢( data i − synth i ) 2⎤⎥ ∗ 100 ,data ⎦⎥
2i2.......(1)Where data and synth are the data and Green’s function time series, respectively, and the summation is
performed for all the stations and components.
Another useful measure for determining source depth is given by:

RES Pdc = ∑ (datai − synthi ) 2 Pdc

……(2)

i

Where, the RES is variance modulate and Pdc is percent double-couple (the ratio of double-couple
component to total seismic moment), respectively.
DATA AND CRUSTAL MODEL
I selected three events for my study. Table 1 and Figure 2 show the origin times and hypocenters for these
events. The first event is the one that occurred on July 2 in 2006 in Red Sea; USGS determined the
hypocenter of this event and the CMT solution determined by the Global CMT project is available.
Therefore, it is possible to compare our solution to the Global
CMT solution to check whether we performed inversion
appropriately. The second event is the one that occurred on
February 2 in 2006 in Gulf of Aqaba region. USGS
determined the hypocenter of this event, and I use thishypocenter for my analysis. The third event is the one that
occurred on April 13 in 2007 in Gulf of Aqaba region The
magnitude (ML) determined by SGS is 4.1 and there is noearthquake information for USGS and Global CMT project. I
compared hypocenter determined by SGS to those determinedby USGS for some events, and found that the difference
between them is relatively small in Gulf of Aqaba region.
Therefore, it is likely to be possible to determine moment
tensor using the hypocenter determined by SGS.
Figure 2. Event locations coverage of my study in Red Sea(focal mechanism solution obtained from Global CMT catalog
for Red Sea event) and event February 2 in 2006 in Gulf of
Aqaba by SGS and USGS, Event April 13 in 2007 in Gulf of
Aqaba was recorder by SGS Network. (The green stars indicate
the locations of broadband stations, and the red circles indicate
the events location).
TABLE 1. The events information from USGS, ML from SGS and MW from the Global CMT catalog.
Date
2006/07/02

Time
23:45:05.30

Epicenter coordinates
19.330N 38.380 E

Depth
10 km

2006/02/02
2007/04/13

09:49:49.9
22:02:16

27.880N 34.410 E
28.0370N 34.5220 E

2 km
10 km

MAG
4.7 ML and 4.7 MW(Global CMT
Catalog) Red Sea.
5 ML Gulf of Aqaba.
4.13 ML Gulf of Aqaba.

In this study I used the crustal model determined by AL-Amri (1998). He used the spectral analysis of
long-period P-wave amplitude ratios to determine the thickness, seismic velocities, and density in thecrustal and the uppermost upper mantle. I set Qα and Qβ to 600 and 300, respectively. I prepared model
files for this crust model for different depth following the instruction of Dreger (2003).

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RESULTSThe July 2, 2006 Red Sea event

100
90
80
70
60
50
40
30
20
10
0

6E-11
5E-11
RES/Pdc

Varinance Red

First, I analyzed the July 2, 2006 Red Sea event. I extract the SEED file of this event to obtain the SAC
wave form data and the instrumental response files. The nearest station is SGS09 and I used the data fromthis station. I calculated the observed spectra used for moment tensor inversion. I adopted the epicenter
determined by USGS, and the epicenter distance is 201 km. Figurer 3 show the result for this event. The
variance reduction increases and the variance divided by the percent double couple decreases, respectively,
as the depth decreases (Figure 4 and Figure 5), whichsuggests that the focal depth is shallow. The determined
focal mechanism is normal fault, which is partly
consistent with the Global CMT Solution shown in
Figure 2. The normal fault is consistent with thetectonics setting explained in Introduction (i, e.,extension along spreading centers in the Red Sea andGulf of Aden). The difference between the twosolutions is likely to be due to weak constraint of thedata used in this study (from the single station) on focal
mechanism.
Figure 3. Result of the Red Sea event.

4E-11
3E-11
2E-11
1E-11
0

0

5

10

15

20

25

0

depth (km)

5

10

15

20

25

depth (km)Figure 4. Change of variance reduction withrespect to depth for the Red Sea July 2, 2006
event.

Figure 5. Change of variance divided by thepercent double-couple with respect to depth
for the Red Sea event.

The February 2, 2006 Gulf of Aqaba event
Then, I analyzed the February 2, 2006 Gulf of Aqaba event. The nearest station is SGS25; I used the datafrom this station. I adopted the epicenter determined by
USGS and the epicenter distance is 60 km. Figure 6 show
the result for this event. The variance reduction does notchange significantly for changes of depth (Figure 7).
Based on the variance divided by the percent double
couple, 10 km is the optimal estimate (Figure 8). These
results suggest the focal depth is shallow. The obtained
focal mechanism is oblique strike slip. The strike slip
component is consistent with the strike slip movement
along the Dead Sea transform fault.
Figure 16. Result of February 2, 2006 Gulf of
Aqaba event.

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7E-11
6E-11
RES/Pdc

Varinance Red

8E-11

100
90
80
70
60
50
40
30
20
10
0

5E-11
4E-11
3E-11
2E-11
1E-11
0

0

5

10

15

20

0

25

5

10

15

20

25

depth (km)depth (km)Figure 8. Change of variance divided by thepercent double-couple with respect to depth for
the February 2, 2006 Gulf of Aqaba event.

Figure 7. Change of variance reduction with respect
to depth for the February 2, 2006 Gulf of Aqaba
event.

The April 13, 2007 Gulf of Aqaba event
Finally, I analyzed the April 13, 2007 Gulf of Aqaba event. For this event, data from SGS25, SGS26, andSGS27 are available. I used the data from SGS27, because thenoise was lowest for this station. I used the epicenter determined
by SGS and the epicenter distance is 149km from SGS27 station.
Figure 9 show the result for this event. The slight increase of thevariance reduction for shallower depth (Figure 10), and the smallvalue of the variance divided by the percent double couple at a
depth of 5 km suggest that the focal depth is shallow (Figure 11).
The obtained focal mechanism is oblique strike slip. The strike
slip component is consistent with the strike slip movement along
the Dead Sea transform fault.
5E-12

100
90
80
70
60
50
40
30
20
10
0

4E-12
RES/Pdc

Varinance Red

Figure 9. Result of April 13, 2007 Gulf of Aqaba event .

3E-12
2E-12
1E-12
0

0

5

10

15

20

0

25

5

10

15

20

25

Depth (km)Depth (km)Figure 11. Change of variance divided by thepercent double-couple with different depths forApril 13, 2007 Gulf of Aqaba event.

Figure 10. Change of variance reduction
with different depths for Gulf of April 13,
2007 Gulf of Aqaba event.

DISCUSSION
I showed above that it is possible to determine moment tensor solutions which are consistent with tectonic
settings. Hurukawa et al. (2001) obtained a variety of focal mechanisms (normal, thrust, and strike slip) in
the Gulf of Aqaba region, where further investigation will be necessary. Also, data exchange amongneighboring countries will be effective to improve accuracy of earthquake source parameters.
Before applying the technique used in this study to routine moment tensor determination, there areseveral issues to be investigated. I compared the hypocenter determined by SGS to those determined by
USGS for same events and there are significant differences. Therefore, intensive evaluation on accuracyof SGS earthquake catalogs should be carried out; then based on that result, it will be necessary toinvestigate the ways to improve them. Data exchange with neighboring countries may be effective.

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Also, it will be necessary to study crustal structures. Although several studies have been carriedout (e.g., AL-Amri 1998), waveform modeling such as studies by Dreger and Helmberger (1991) andZhao and Helmberger (1991) have been done only for the limited path (Rodgers et al. 1999).
CONCLUSION
In this study, I applied the time domain moment tensor inversion code developed by Dreger (2003) to the
waveform data from the broadband seismic network that has been newly deployed by Saudi Geological
Survey (SGS). Considering the high seismic activities and data availability, I analyzed three events: theJuly 2, 2006 event in Red Sea area, the February 2, 2006 and April 13, 2007 events in the Gulf of Aqaba
area. I obtained the normal fault mechanism for the Red Sea event, which is partly consistent with the
Global CMT solution and consistent with the tectonic movement. The strike slip components of the focal
mechanisms determined for two events in Gulf of Aqaba were compatible with the tectonic movement.
These results suggest that it is useful to monitor and investigate seismic activity by moment tensor
analyses using broadband waveform data from the new SGS seismic network.

REFERENCES
AL-Amri, A. M. 1998, Tectonophysices. 290.
Dreger, D. S. 2003, International Handbook of Earthquake and Engineering Seismology. B.
Dreger, D. S. and D. V. Helmberger, 1991, Bull. Seism. Soc. Am. 80.
Dreger, D. and B. Romanowicz, 1994, U.S.Geol.Surv.Open-file Rept., 94-176.
Fukuyama, E., et al., 1998, Zisin. 51.
Hurukawa, N., et al., 2001, Bull. Seism. Soc. Am. 91.
Pasyanos, M., et al., 1996, Bull. Seism. Soc. Am. 86.
Rodgers, A., et al., 1999, Geophys. J. Int. 138.
Zhao, L. and D. V. Helmberger, 1991, Geophys. J. Int. 105.

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