Seismic Tomography of the Arabian-Eurasian Collision Zone and

43 Pages·2010·1.99 MB·English

AFRL-RV-HA-TR-2010-1043

Seismic Tomography of the Arabian-Eurasian
Collision Zone and Surrounding Areas

M. Nafi Toksöz
Robert D. Van der Hilst
Youshun Sun
Haijiang Zhang

Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139

Final Report

20 May 2010

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED
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SEISMIC TOMOGRAPHY OF THE ARABIAN-EURASIAN COLLISION ZONE FA8718-07-C-0001
AND SURROUNDING AREAS 5b. GRANT NUMBER

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62601F
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M. Nafi Toksöz, Robert D. Van der Hilst, Youshun Sun, and Haijiang Zhang 1010
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Massachusetts Institute of Technology
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14. ABSTRACT

We developed P- and S- wave velocity structures in the crust and upper mantle in the Arabian-Eurasian collision zone and surrounding areas, including
Iran, Arabia, Eastern Turkey, and the Caucasus.
We first obtain Pn and Sn velocities using local and regional arrival time data. Second, we obtain the 3-D crustal P and S velocity models. Third, we
extend the model into the upper mantle and transition zone by combining local, regional and teleseismic data and crust model constraints. To extend the
model deeper into the mantle through the upper mantle transition zone, ISC/EHB data for P and PP phases are combined with the ABCE data. To
counteract the “smearing effect,” the crust and upper mantle velocity structure, derived from regional travel-times, is used. An adaptive grid method based
on ray density is used in the inversion. A P-wave velocity model extending down to a depth of 2000 km is obtained. regional-teleseismic tomography
provides a high-resolution, 3-D P-wave velocity model for the crust, upper mantle, and the transition zone. The crustal models correlate well with geologic
and tectonic features. The upper mantle tomograms show the images of the subducted Neotethys slab. The slab geometry is quite complex, reflecting the
history of the changes in the plate motions and collision processes.
Vp/Vs tomography was also determined in the study region, and an attenuation tomography was obtained in Iran.
15. SUBJECT TERMS
Seismic tomography, Attenuation, Travel-time data
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Standard Form 298 (Rev. 8-98)
i
Prescribed by ANSI Std. Z39.18
ii
Table of Contents

1. Abstract ...................................................................................................................1
2. Introduction ............................................................................................................1
3. Regional Models .....................................................................................................3
3.1. The Crust and Uppermost Mantle Models ..................................................3
3.2. Vp/Vs Tomography .....................................................................................5
3.3. Upper Mantle Tomography ..........................................................................7

4. Models for Iran ....................................................................................................10
4.1. Data Description ........................................................................................10
4.2. Seismic Tomography ......................................................................................12
4.2.1. Data for Seismic Tomography .......................................................12
4.2.2. P and S tomography of the Crust and Upper Mantle .....................13
4.3 Relocation of Earthquakes in Iran ....................................................................14
4.4 Attenuation Tomography .................................................................................15

References .........................................................................................................................18

Appendices ........................................................................................................................22

Appendix I: Deliverables .......................................................................................22

Appendix II: Pn and Sn velocities in the Mideast and Central Asia ......................23

iii
Figures

Figure 1. Topographic map of the Middle East and surrounding regions. White lines
denote the location of known faults, while the black dots represent the epicenter
locations of earthquakes in the region. ..............................................................................2

Figure 2. Ray paths for Pn (upper) and Sn (bottom) travel times. 654,999 Pn rays
were obtained form 42,984 events (black crosses) recorded by 1263 stations (red
triangles), and 121,838 Sn rays were obtained form 22,283 events recorded by 887
stations. The data were selected for epicentral distances between 2 and 12 degrees
with residuals less than 7.0s for Pn, and between 2 and 15 degree with residuals less
than 6.0s for Sn. Each event was recorded by at least 3 stations for Pn and 2 stations
for Sn. .................................................................................................................................4

Figure 3. Imaged Pn (upper) Sn velocity lateral variations. Average Pn velocity is
8.04 km/s and average Sn velocity is 4.60 km/s. Red represents slow velocities and
blue denotes high velocities. ..............................................................................................5

Figure 4. Pseudowave lateral velocity variations obtained by tomographically
inverting the difference travel-time residuals. Relative to the mean value of 10.8
km/s, the pseudowave maximum velocity perturbations is about 0.4 km/s. .......................7

Figure 5. Velocity profile at depth of 50 km. ....................................................................8

Figure 6. Velocity profile at depth of 200 km. ..................................................................8

Figure 7. Velocity profile at depth of 400 km. ..................................................................9

Figure 8. Velocity profile at depth of 600 km. .................................................................9

Figure 9. Seismic stations in Iran. INSN stations are in circles and ISTN stations are
in triangles. ........................................................................................................................10

Figure 10. Available P and S travel times from INSN (left) and ISTN (right)
networks .............................................................................................................................11

Figure 11. Seismograms recorded at short period station (PAY) and in a borehole at
SRO. ..................................................................................................................................11

Figure 12. Available ray paths from the Iranian networks (left) and EHB database
(right). ..............................................................................................................................12

Figure 13. P-wave models at depths of 21 and 51 km obtained using the double-
difference tomography method. .......................................................................................13

iv
Figure 14. Horizontal cross sections through the mantle from preliminary results of
P-wave travel-time tomography. Blue colors represent regions with faster than
average velocities, and red color show regions with slower than average velocities. ....14

Figure 15. Comparison of residuals using 1D model (solid line) and 3D model (red). ....15

Figure 16. Map of an earthquake relocation in Iran, where the USGS locations are
shown as red circles, and the locations found using 3D tomographic models are
shown in blue crosses. ......................................................................................................15

Figure 17. Left: Distribution of ray paths, stations (triangles) and events (crosses).
Right: Preliminary imaged Q0 lateral variations. Blue corresponds to a high value
and red to low. The average Q0 is 330. ............................................................................16

v
vi
1. Abstract

The objectives of this study are to determine P- and S- wave velocity structures in the crust and
upper mantle, and to characterize seismic wave propagation in the Arabian-Eurasian collision
zone and surrounding areas, including Iran, Arabia, Eastern Turkey, and the Caucasus.

The travel-time tomography is carried out in three steps. First, obtain Pn and Sn velocities using
local and regional arrival time data. Second, obtain the 3-D crustal P and S velocity models.
Third, extend the model into the upper mantle and transition zone by combining local, regional
and teleseismic data and crust model constraints. We obtained the regional Pn velocity
tomograms using 160,000 arrival times (including data from Iran) from 850 stations and 18,000
earthquakes. For Sn tomography, 75,000 phase readings were used. The Pn and Sn velocity
variations agree quite well with some local differences. Pn velocities are very low under eastern
Anatolia, northwest Iran, and the Lesser Caucasus. Localized low velocity anomalies are found
under the Iranian plateau. Pn velocities are high under the Arabian Platform, the Gulf, and the
Zagros. In the north, high Pn velocities extend from the Black Sea to the Caspian and east to the
Kara Kum Basin.

To extend the model deeper into the mantle through the upper mantle transition zone, ISC/EHB
data for P and PP phases are combined with the ABCE data. To counteract the “smearing
effect,” the crust and upper mantle velocity structure, derived from regional travel-times, is used.
An adaptive grid method based on ray density is used in the inversion. A P-wave velocity model
extending down to a depth of 2000 km is obtained. The combined regional-teleseismic
tomography provides a high-resolution, 3-D P-wave velocity model for the crust, upper mantle,
and the transition zone. The crustal models correlate well with geologic and tectonic features.
The upper mantle tomograms show the images of the subducted Neotethys slab. The slab
geometry is quite complex, reflecting the history of the changes in the plate motions and
collision processes.

Vp/Vs tomography was also determined in the study region, and an attenuation tomography was
obtained in Iran.

2. Introduction

The Arabian-Eurasian plate boundary is extremely complex, and it is an ideal region to study a
young (geologically) continent-continent collision belt. The area of the study, shown in Figure
1, extends east-west from the Mediterranean to Central Asia, and north-south from the Caspian
to the Gulf of Aden and the Arabian Sea. The current tectonics of the region are controlled by
the collision and continuing convergence of the Arabian and Eurasian plates. The Arabian and
Eurasian plates collided in the early Miocene, after the Neotethys Sea was subducted beneath
Eurasia (Bird, 1978; Şengör and Yılmaz, 1981; Jackson and McKenzie, 1984; Dewey et al.,
1986). Pre-, syn-, and post-collision tectonics produced very complex structures in the region.
Over the last decade, a number of seismic studies have examined the crust and upper mantle
structure beneath the Middle East to constrain the nature of the Arabian-Eurasian collision
zone. Large-scale surface wave tomography studies have shown variable crustal thickness and
upper mantle velocities (Ritzwoller and Levshin, 1998; Pasyanos et al., 2001; Villasenor et al.,
1
2001; Pasyanos and Walter, 2002; Shapiro and Ritzwoller, 2002; Alinaghi et al., 2007; Reiter
and Rodi, 2006). Regional-scale surface wave tomographic studies further highlight the
complexity of the collision zone (Mindevalli and Mitchell, 1989; Rodgers et al., 1999; Mokhtar
et al., 2001; Maggi and Priestley, 2005) showing a thickened crust under the Caucasus and
Zagros, and low shear velocity beneath the Turkish and Iranian plateaus.

Hearn and Ni (1994), Ritzwoller et al. (1998), Al-Lazki et al. (2003; 2004), and Phillips et al.
(2007) found slow Pn velocities (≤ 8 km/s) beneath the Anatolian plateau, northwestern Iran,
the Greater Caucasus, and southwestern Arabia. The Pn velocities beneath northern Arabia
and the Caspian region are faster than average (Al Lazki et al., 2004; Ritzwoller et al., 2002).
This high degree of variability suggests that the Earth structure may be extremely complicated
in the region.

Figure 1. Topographic map of the Middle East and surrounding regions. White lines
denote the location of known faults, while the black dots represent the
epicenter locations of earthquakes in the region.
Studies of the propagation and attenuation characteristics of regional waves (e.g., Pn, Sn, and
Lg) provide additional evidence for strong heterogeneities. Surface wave studies show high
shear wave attenuation beneath Iran, Anatolia and the western part of the Arabian plate, and
relatively low attenuation in central and eastern Arabia (Seber and Mitchell, 1992; Sandvol et
al., 2001; Cong and Mitchell, 1998; Jamberie and Mitchell, 2004; Molinaro et al., 2005;
2