The M_w 7.3 1995 Aqaba earthquake is the largest instrumental earthquake along the Dead Sea Fault. We complement previous seismological studies by analyzing co‐seismic ground displacement from differential interferometry computed from ERS images spanning 3 different areas. They are compared with a synthetic model derived from seismological study. Only far‐field deformation related to the main sub‐event could be revealed because the near‐field area lies within the gulf. The interferometric data imply a 56 km long and 10 km wide fault segment, connecting the Elat Deep to the Aragonese Deep, which strikes N195°E and dips 65° to the west, with 2.1 m left‐lateral slip and a 15.5° rake indicating a slight normal component. The geodetic moment compares well with the seismic momen

Avouac, Jean-Philippe

Klinger, Yann

Michel, Rémi

English

Caltech Authors - Main

GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 22, PAGES 3651-3654, NOVEMBER 15,2000 
Co-seismic deformation during the Mw 7.3 Aqaba 
earthquake (1995) from ERS-SAR interferometry 
Yann Klinger/ Remi Michel, Jean-Philippe Avouac 
Laboratoire de Detection et de Geophysique, Commisariat a l'Energie Atomique, 
Bruyeres-le-Chatel, France 
Abstract. The Mw 7.3 1995 Aqaba earthquake is the 
largest instrumental earthquake along the Dead Sea 
Fault. We complement previous seismological stud-
ies by analyzing co-seismic ground displacement from 
differential interferometry computed from ERS images 
spanning 3 different areas. They are compared with 
a synthetic model derived from seismological study. 
Only far-field deformation related to the main sub-event 
could be revealed because the near-field area lies within 
the gulf. The interferometric data imply a 56 km long 
and 10 km wide fault segment, connecting the Elat 
Deep to the Aragonese Deep, which strikes N195°E and 
dips 65°to the west, with 2.1 m left-lateral slip and a 
15.5°rake indicating a slight normal component. The 
geodetic moment compares well with the seismic mo-
ment. 
1. Introduction 
On November 22, 1995 a Mw 7.3 earthquake occurred 
in the Gulf of Aqaba, at the southern end of the Dead 
Sea fault (DSF). This event is the largest instrumen-
tal earthquake along the DSF and is comparable to 
the largest historical earthquakes documented along the 
DSF [e.g., Ambraseys et al., 1994]. In the present case, 
little is known about the exact fault location and geome-
try because the epicenter was located offshore, about 60 
km south of the tip of the gulf (Figure 1). Ground rup-
tures were reported at various places around the north-
ern part of the gulf that reflect more probably super-
ficial gravity-induced tectonics along the steep flanks 
of the gulf rather than primary ruptures [ El-Hakim, 
1996; Wurst et al., 1997; Baer et al., 1999; Klinger 
et al., 1 999]. This study complements previous seis-
mological investigations [ Fattah et al., 1997; Pinar and 
Tiirkelli, 1997; Klinger et al., 1999] by analyzing co-
seismic ground displacements determined from 3 differ-
ential interferograms computed from ERS SAR images 
(Figure 1 and Table 1). 
2. Active Tectonic Background 
The 1200 km long DSF is a major left-lateral strike-
slip fault, separating the Africa plate from the Arabia 
1 Now at SeismoLab., California Institute of Technology, 
Pasadena, California 
Copyright 2000 by the American Geophysical Union. 
Paper number 1999GL008463. 
0094-8276/00/1999GL008463$05 .00 
plate (inset in Figure 1). The Gulf of Aqaba is formed 
of three pull-apart basins, the Elat Deep, the Aragonese 
Deep and the Daka Deep from north to south. They are 
connected by en echelon strike-slip faults striking about 
N20°[Ben-Avraham, 1985] (Figure 1). The gulf is only 
25 km wide but it reaches a depth of more than -1800 m 
and the surrounding Sinai and Hedjaz mountain ranges 
peak to more than 2600 m. The tectonic significance 
of these large topography is still unclear since it might 
be related either to Neogene rifting or present tectonics 
[Ben-Avraham and Zoback, 1992]. 
2.1. Background Seismicity in the Gulf of 
Aqaba 
Two large earthquakes of magnitudes ~7 are known 
from historical records to have occurred near the north-
ern tip of the Gulf of Aqaba, in 1068 and 1212 [Abou 
Karaki, 1987; Ambraseys et al., 1994; Zilberman et al., 
1998]. Since the beginning of instrumental monitoring, 
the Gulf of Aqaba is one of the most seismically active 
zone along the DSF. In addition to the Mw 7.3 earth-
quake of 1995, three seismic swarms were recorded in 
29'00\ 
Hedjaz Mountains 
==-20km 
Figure 1. Seismotectonic map of the Gulf of Aqaba 
adapted from Ben Avraham (1985]. Dashed areas show 
extent of 1983, 1990 and 1993 seismic swarms. Stars 
represent epicenter and centroid. The trace at the bot-
tom of the gulf of the best fault segment modeled in this 
study (black segment), and associated focal mechanism 
(see Table 2) are indicated. Black boxes depict location 
of interferograms. 
3651 
3652 
Table 1. Single Look Complex SAR images used to 
compute interferograms of Figure 2a and 3. See Figure 
1 for location of the 3 zones. 
Zone Satellite Date Orbit Frame 
1 ERS2 06/09/95 705 3015 
ERS2 05/24/96 5715 3015 
2 ERS1 03/29/95 19369 585 
ERS1 11/29/95 22876 585 
3 ERS1 08/16/95 21373 567 
ERS2 08/21/97 12221 567 
1983, 1990 and 1993 which affected different parts of 
the gulf, with magnitudes up to ~6 [El-Isa et al., 1984; 
Abou I<araki et al., 1993] (Figure 1). These swarms 
probably relate to episodes of extensional faulting on 
~NS trending normal faults as indicated by the focal 
mechanism of the largest shock in 1993 [e.g. Fattah et 
al., 1997]. 
2.2. The Earthquake of Mw 7.3, November 22, 
1995 
Several seismological studies have already addressed 
the rupture process of the 1995 earthquake [Fattah et 
al., 1997; Pinar and Tiirkelli, 1997; Klinger et al., 1999; 
Baer et al., 1999]. They all point to a complex seis-
mic source with 2 or 3 sub-events on distinct fault seg-
ments. According to the inversion of broad-band body-
waves and aftershock relocation (Klinger et al., 1999], 
the largest sub-event would have released about 70% 
of the total seismic moment as left-lateral slip on a 
N200°E striking fault, dipping about 74°to the west. 
The 5°seismologically determined rake indicates only a 
slight dip-slip component. The two other sub-events 
occurred on sub-parallel fault segments with a larger 
vertical component. Although the relative position of 
the three fault segments and the timing of the rupture 
is reasonably well documented, the exact location and 
geometry of the main fault segment could not be tightly 
0 
constrained [Klinger et al., 1999]. The accurate deter-
mination of the location and geometry of the main sub-
event from seismology is indeed a non-trivial question 
due to the spatial extent of the seismic source, as il-
lustrated by the difference between the positions of the 
epicenter and centroid, respectively (Figure 1, stars). 
3. Co-Seismic Deformations from 
Differential SAR Interferometry 
Differential SAR interferometry can measure coseis-
mic surface displacements at the millimeter scale from 
the phase difference between two radar images collected 
on successive tracks over the same area (e.g. Pelzer and 
Rozen, 1995]. Using C-band radar imagery, interferom-
etry provides the satellite-to-ground component of the 
ground displacement, modulo 2.8 em, that occurred in 
the time span between the two acquisitions [e.g. Mas-
sonnet and Feigl, 1998]. The technique is particularly 
well adapted to desert areas where decorrelation factors 
are not too limiting, like the region of Aqaba. 
3.1. Data Processing 
We used ERS-1 and ERS-2 Single Look Complex data 
covering 3 frames (Table 1) around the northern tip of 
the gulf (Figure 1). A key parameter in the computa-
tion of an interferogram is the baseline (i.e. the distance 
between the orbital positions) which is generally known 
with an accuracy of several meters. An eventual arti-
fact, caused by this uncertainty, consists of a nearly con-
stant gradient of deformation, not related to any ground 
displacement, that may account for about 6 em ground 
relative displacement distributed across the image. We 
have therefore slightly adjusted the baselines in order to 
minimize the interferometric signal away from the fault 
zone where large gradients of ground displacement are 
unexpected. 
The ERS images were processed following the pro-
cedure described in Michel et al. (1999]. We used the 
30 arc-second DCW Digital Elevation Model to remove 
Figure 2. a) Synthetic interferogram computed from seismological modeling after Klinger et al., [1999]. b) 
Differential SAR interferogram over zones 2 and 3 (see Table 1). Average noise is 5 mm ( rms). Projection vector 
(-0.38; -0.061; 0.92). c) Best modeled synthetic interferogram. Black lines show location offaults. Fringe patterns 
in Figures 2a and 2b are fairly consistent. Some discrepancies remain, however, along the western coast. 
0.05 
0 
c 
~ -0.05 
i -0.1 
~ -0.15 
c 
A A' 
o measured in thys study 
- modeled from interferometry 
- modeled from seismology 
E - 0·2 L_--ctfrol2¢.o1":i3no --;;4'no---;s~o;--:6;?;;0;--7;;;o~-;eso::---c:9'oco~t~o oo-o 
Distance (km) 
0.1 
§ 
= 0 e 
~ -0.1 
-a 
,g -0.2 
, 
= 
B' 
o measured in thys study 
- modeled from interferometry 
- modeled from seismology 
E -0 .3 '-------;;;---;;-;;-~';;--7;:-::";:---;:-:,---~-.L_----'---"---
1 0 20 3 0 40 50 60 70 80 90 10 0 
Distance (km) 
Figure 3. Comparison between ground displacements 
along the radar line of sight. 1) measured from un-
wrapped interferogram, 2) computed from the seismo-
lo~ical model [Kli_nger et al. 1999] and 3) modeled in 
thts study . See Ftgure 2c for location of profiles. Gray 
box denotes location of the gulf. 
topographic fringes. The baselines used in this study 
are smaller than 30 m leading to a residual topographic 
noise (resulting from errors in the DEM) of less than 
5 mm on the measured ground displacement. Images 
from other frames were also available but were discarded 
because their baseline were too large compared to the 
quality of the DEM. The uncertainties on the measured 
ground displacements may then be modeled as a white 
noise with an amplitude of about 5 mm rms and a low 
frequency noise with an amplitude of about 6 em that 
may result from atmospheric noise or uncertainties in 
the estimates of orbital parameters. The interferogram 
covering zone 1, west of the gulf, does not show any sig-
nificant geodetic signal as expected from the theoretical 
model. By contrast, the interferograms computed from 
the eastern frames in zones 2 and 3 show fringes that 
are clearly related to ground displacement (Figure 2b) 
and that differ from the ones computed from the initial 
three sub-event sources model proposed by Klinger et 
al. [1999] (Figure 2a) using Okada's [1985] analytical 
formulation. 
3.2. Analysis of the lnterferograms 
We have modified the fault model proposed by Klinger 
et al. [1999], in order to better fit the measured differ-
ential interferograms. Tests have shown that the signal 
due to the two small sub-events is overwhelmed by the 
co-seismic deformation induced by the main sub-event, 
which accounts for all of of the fringes that are shown 
3653 
Figure 4. Measured and best fitted interferograms over 
zone 1. Projection vector (0.38; -0.08; 0.92). Average 
deformations are consistent but further interpretation 
is not reasonable due to low amplitude of observed and 
modeled deformation compared to the noise in the in-
terfcrogram. 
in Figure 2a. So, because no data are available on the 
near-field area that lies under water, only the location 
and geometry of the main sub-event might be retrieved 
from our interferograms. 
The measured and seismologically predicted interfer-
ograms (Figure 2a,b) show similarities, but the syn-
thetic fringes extend too far to the north. On the 
eastern coast, the amplitude of the synthetic deforma-
tion is lower than the measured one, suggesting that 
the main segment probably lies farther away from the 
coast. These offsets are compared on profiles across 
the unwrapped measured and synthetic interferograrns 
(Figure 3). 
Later we adjusted the parameters of the seismic source 
model by trial and error. We considered only one planar 
dislocation to get the results shown in Figure 2c and Ta-
ble 2. A fault slightly shorter and shifted to the south 
and east from to the position of the main sub-event of 
the initial model provided the best fitting model. The 
1.9 ern rrns residual error between the modeled and ob-
served ground displacement fields is comparable to the 
noise level in the measurements so that the data do 
not require any more sophisticated modeling (Figure 
3). The fringe pattern in Figure 2c is indeed similar to 
the measurements in Figure 2b. The high gradient of 
deformation, indicating a depression towards the gulf 
along the western coast, is well modelled. On the east-
ern side, the circular fringe pattern is also reasonably 
well fitted. Some discrepancies remain that might be 
partly due to the use of Okada's [1985] analytical for-
mulation, which assumes only an elastic half-space with 
a horizontal free surface and does not account for the 
large topographic variations of the area. In zone 1 the 
adjusted model predicts a low signal consistent with the 
absence of any clear residual fringes obtained in the in-
terferograrn (Figure 4). 
Table 2. ~est model adapted from the seismological model of Klinger et al., [1999] to fit the 
unwrapped mterferograms of Figure 2a. Angles are given in degrees, azimuth is given clockwise 
from North. fault reach the bottom of the Gulf. 
Latitude Longitude Length Azimuth Dip Depth Rake Slip 
(km) (km) (m) 
Klinger et al., 28.829 34.825 27.4 191.6 58.6 14 -21.2 0.42 
[1999] 29.042 34.777 61.0 199.3 74.3 14 -5.0 2.5 
29.277 34.786 25.6 24.7 67.2 1.1 -8.5 0.74 
This study 28.93 34.78 56 195.15 65 12 -15.5 2.1 
3654 KLINGER ET AL.: MW 7.3 AQABA EARTHQUAKE (1995) FROM INTERFERONlETRY 
The proposed model thus appears to provide an ac-
ceptable fit to the observed interferograms and is char-
acterized by a fault geometry relatively close to that de-
rived from seismological investigations (Table 2). The 
Mw 7.1 moment magnitude computed from this model 
(following Aki and Richards [1981], p = 3.3 1010 N/m) 
compares well with the value of Mw 7.2 assigned to 
the main sub-event from body-waves analysis [Pinar et 
Tiirkelli1 1997; Klinger et al. 1 1 999]. 
4. Conclusions and Implications 
This study brings new constraints to the location 
and geometry of the main sub-event of the 1995 Aqaba 
earthquake. It implies a dominantly strike-slip fault 
dipping about 65°to the west with a slight normal com-
ponent, connecting the Aragonese Deep to the Elat 
Deep. Thus the pull-apart basins seems to be only ac-
tivated by small earthquakes, with magnitude Mw ~< 
6 and a significant dip-slip component, that occur ei-
ther in swarms or in association with larger events on 
the connecting strike-slip faults. The strike-slip seg-
ments in the Gulf of Aqaba are also associated with 
a minor dip-slip component that can also, when accu-
mulated over time, contribute to the building of the 
gulf morphology. It is particularly striking that the lo-
cal zone of subsidence at the southern end of the Elat 
Deep closely mimics the gulf morphology in that area 
(see profiles AA' and BB' in Figure 3). The morphology 
of the Gulf of Aqaba most probably reflects present tec-
tonics and would have resulted from the accumulation 
of co-seismic deformation alike that produced by the 
1995 earthquake, together with normal faulting within 
the pull-apart basins. 
Acknowledgments. This study was completed par-
tially at Institut de Physique du Globe de Strasbourg URM 
7516, France. ERS SAR images were provided by ESA un-
der A03.253. We thank two anonymous reviewers for their 
helpful reviews. 
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Co-seismic deformation during the M_w 7.3 Aqaba earthquake (1995) from ERS-SAR interferometry

A vouac

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Active faulting in the gulf of Aqaba

and many others, Post-seismic analysis of failure features and seismic hazard implication,

and many others, Post-seismic analysis of failure features and seismic hazard implication, Geological Survey of Israe·z Report GSI/3/97,

Application of ERS-1 radar interferometry to the Landers Earthquake,

Centroid Moment tensor solution for October-December

Coseismic deformations associated with the November 1995, Mw=7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), de tected by synthetic aperture radar interferometry,

Decorrelation in interfero metric radar echoes,

La crise sismique du golfe d' Aqaba de

Quantitative seismology, Freeman and Comp.,

Relocalisation of the epicenter of the 1068 earthquake in the A vrona playa, southern Dead Sea rift, using paleoseismic and archeoseismic evidences. paper presented at ESC 26th (Ab stract),

SeismoLab #100-23, California Institute of Technology,

Source inversion of the 1993 and 1995 gulf of Aqaba earthquake,

Structural framework of the gulf of Elat (Aqaba),

Surface deformation due to shear and tensile faults in an half-space,

Surface displacement of the 17

Synthese et carte sismotectonique des pays de Ia bordure orientale de la Mediterranee : sismicite du systeme de failles du Jourdain-Mer Morte, Ph.D. thesis, Strasbourg I Univ.

The Aqaba 95 earthquake A quick review, Egyptian Geological Survey and Mining Authority Report,

The gulf of Aqaba earthquake swarm of

The seismicity of Egypt, Arabia and the Red Sea, an historical review,

Transform-normal exten sion and asymmetric basins: an alternative to pull-apart models,

http://authors.library.caltech.edu/33955/1/KlingerGRL1999.pdf

Co-seismic deformation during the M_w 7.3 Aqaba earthquake (1995) from ERS-SAR interferometry

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