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3-D velocity structure of the 2003 Bam earthquake area (SE Iran):
Existence of a low-Poisson's ratio layer and its
relation to heavy damage
Hossein Sadeghi a,, S.M. Fatemi Aghda b,1, Sadaomi Suzuki c, 2, Takeshi Nakamura d,3
a Earthquake Research Center, also at Department of Geology, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iranb Department of Geology, Tarbiat Moallem University, Tehran 15614, also at Natural Disaster Research Institute, Tehran 19395-4676, Iran
c
Tono Research Institute of Earthquake Science, Mizunami 509-6132, Japand Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan
Received 1 July 2005; received in revised form 2 January 2006; accepted 31 January 2006
Available online 15 March 2006
Abstract
To understand the generation mechanism of the Bam earthquake (Mw 6.6), we studied three-dimensional VP, VS and Poisson's
ratio () structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves
and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of
the seismogenic region was well resolved to a depth of 14km with significant velocity variations of up to 5%. The general pattern
of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20kmalong a line 4.5km west of the known geological Bam fault; this source fault dips steeply westward and strikes a nearly north
south line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in
the desert. The 3-D velocity structure shows a thick layer of high VS and low (minimum: 0.20) at a depth range of 26km. The
deeper layer, with a thickness of about 2km, appears to have a low VS and high (maximum: 0.28) from 6km depth beneath Bam
to a depth of 9km south of the city. The inferred increase of Poisson's ratio from 2 to 10km in depth may be associated with a
change from rigid and SiO2-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of
aftershock distribution at the depth range of 2 to 7km coincides well with the large slip zone in the shallow thick layer of high VSand low . The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Bam area suffered
heavy damage.
2006 Elsevier B.V. All rights reserved.
Keywords: Bam earthquake; Aftershocks; Seismic velocity structure; Poisson's ratio; Shallow rigid layer; Arg-e-Bam fault
1. Introduction
On December 26, 2003, a powerful earthquake in
southeastern Iran caused catastrophic damage to the
ancient city of Bam, located on the Silk Road, and
neighboring villages with a collective population of
about 142,000. In terms of loss of life and casualties, this
Tectonophysics 417 (2006) 269283
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Corresponding author. Fax: +98 511 842 1234.
E-mail addresses: [email protected] (H. Sadeghi),
[email protected] (S.M.F. Aghda), [email protected] (S. Suzuki),
[email protected] (T. Nakamura).1 Fax: +98 21 200 9453.2 Fax: +81 572 67 3108.3 Fax: +81 92 642 2684.
0040-1951/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2006.01.005
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earthquake was the worst to occur in that year anywhere
in the world. The death toll was 26,371people, nearly
19% of the population (Statistic Center of Iran, 2004)
and tens of thousands were injured. An important
cultural loss was the almost total destruction of the well-
known historic citadel Arg-e-Bam. This monument,declared by UNESCO as a World Heritage Site, is the
biggest mud-brick structural complex in the world. Arg-
e-Bam is thought to be over 2000 years old. Since the
structure was well intact before the 2003 Bam
earthquake, this earthquake is believed to be the largest
to occur in this area in more than 2000years.
Aside from the destruction of Arg-e-Bam, the main
reason for such massive damage to Bam may be the poor
construction of the unreinforced mud-brick houses.
Even so, the damage was disproportionately and
unexpectedly large given the magnitude of the earth-quake. Maximum acceleration of 0.98g was recorded in
the vertical component at the Bam accelerograph station
in the center of Bam city by the Building and Housing
Research Center of Iran (BHRC; http://www.bhrc.gov.
ir/). The earthquake information initially provided by
the US Geological Survey, USGS (http://earthquake.
usgs.gov/), was as follows: origin time 26/12/2003 at
01:56:52 (UTC) and 05:26:52 (local time); epicenter
29.004N, 58.337E; depth 10km, and moment magni-
tude 6.6. Suzuki et al. (2004) estimated the start point of
the large slip in the source fault by using the aftershock
distribution and the SP time recorded by the Bamaccelerograph station. Teleseismic focal mechanisms
from several groups (e.g., USGS; Yamanaka, 2003)
indicated a steeply dipping, right lateral strike-slip
movement on a fault with a NS trend. This agrees well
with the known tectonic pattern of the region (e.g.,
Walker and Jackson, 2002). The Bam fault, which has
long been recognized (e.g., Berberian, 1976; Hessami et
al., 2003) for its distinctive flexure scarp, extends along
the west side of the village of Baravat about 5km
southeast of Bam city. Just after the earthquake, it was
assumed that the main shock had occurred in thegeological Bam fault (e.g., Ahmadizadeh and Shakib,
2004). However, nobody could find any clear evidence
of dislocation on this existing Quaternary fault.
Comparing the Bam earthquake and the 2000 Tottori
earthquake in southwest Japan, Miyake et al. (2004)
proposed that the main shock ruptured a shallow
asperity on a fresh fault rather than on the Bam fault.
Waveform inversions using teleseismic data (e.g.,
Yamanaka, 2003; Yagi, 2003) have suggested the
existence of a shallow asperity, i.e., a large slip area,
for the Bam earthquake. Analyzing Envisat SAR
interferometry data before and after the earthquake,
Talebian et al. (2004), Binet and Bollinger (2005), and
Fielding et al. (2005) indicated that the main rupture
reached the surface some 5km west of the Bam fault.
Wang et al. (2004) used differential radar interferometry
(D-inSAR) to determine the source parameters. They
suggested that the Bam earthquake ruptured a hidden ornew fault of about 24km from (29.178N, 58.382E) to
(28.971N, 58.357E) that had an unusually strong
asperity.
The main shock was followed by a series of
aftershock events (e.g., Nakamura et al., 2005; Tatar et
al., 2005). Nakamura et al. (2005) recorded several
thousand aftershocks during the period February 6 to
March 7, 2003. They located accurate hypocenters of
2789 aftershocks by the use of a 1-D velocity model.
The main distribution of aftershocks did not correspond
to the geological Bam fault. The epicenter distribution islinearly more than 20km in parallel with a line about
3.5km west of the Bam fault. The hypocenter
distribution shows a nearly vertical trend or a slight
tendency to lie farther west with its depth increasing
from 0 to 16km. They proposed the name Arg-e-Bam
fault as the source fault to distinguish it from the Bam
fault.
Since the early 1980s, local earthquake tomography
has been successfully used to image lateral heterogene-
ities of the crust in seismogenic fault zones (see
Eberhart-Phillips, 1993 and references therein). Material
properties in the earthquake source area would certainlyhave influenced the initiation, propagation and termi-
nation of the earthquake rupture. The velocity variations
in the upper crust allow us to define the structure and
geometry of faults at depth and to identify the structures
of seismogenic zones, especially in cases of hidden and
buried faults (e.g., Eberhart-Phillips, 1990; Chiarabba
and Selvaggi, 1997; Chiarabba et al., 1997, Zhao et al.,
2004). In the present study we have applied seismic
tomography to arrival time data of the Bam earthquake
aftershocks recorded by a temporal high-sensitivity
seismic network (Suzuki et al., 2004; Nakamura et al.,2005). Arrival times of high-quality P waves and S
waves were collected. These data allowed us to
determine detailed three-dimensional (3-D) VP and VSstructures in the source area of the Bam earthquake. We
used the data to try to deduce variations in the physical
properties of rocks. Because rocks with differing
physical states can have similar seismic velocities,
seismic velocity alone is not a sensitive indicator of
variable rock properties. For this reason, it is often
useful to consider the ratios and products of seismic
parameters to differentiate 3-D variations in the
subsurface (Salah and Zhao, 2003). Poisson's ratio (or
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VP /VS) is a key parameter in studying petrologic
properties of crustal rocks (Christensen, 1996) and can
provide tighter constraints on the crustal composition
than can VP and VS alone. Poisson's ratio has proved to
be very effective for clarification of the seismogenic
behavior of crust, especially the roles of crustal fluids inthe nucleation and growth of earthquake rupture (Zhao
et al., 1996, 2002, 2004).
2. Tectonic setting
The tectonics of the region (Fig. 1) is dominated by
the convergence between the Arabian and Eurasian
plates. The convergence is trending N to NNE at
velocity ranges from 2325mm/yr as deduced from
GPS measurements (e.g., McClusky et al., 2003;
Vernant et al., 2004) to 35mm/yr according to theNUVEL-1 model (DeMets et al., 1990). To the west, the
northwest-trending Zagros fold and thrust belt, which is
an active continental collision zone, accommodates
about 10mm/yr of NNE-trending shortening (Alavi,
1994; Talebian and Jackson, 2002; Tatar et al., 2002;
Blanc et al., 2003); also, in several areas further north,
the crust is forced to accommodate the convergence by
shortening (Vernant et al., 2004). To the east, this
relative motion is accommodated by the east-trending
Makran belt. The Makran belt is the emerged portion of
an accretionary prism resulting from the subduction of
the Oman Gulf oceanic lithosphere (which forms part of
the Arabian plate) beneath the Iranian plate (Byrne et al.,
1992; McCall, 1997; Kopp et al., 2000). Earthquakes
have occurred mainly within the Zagros as a wideseismic belt (see USGS National Earthquake Informa-
tion Center catalog, available at http://neic.usgs.gov/
neis/epic/epic.html). The high seismicity in Zagros
might be due to the presence of thick layers of late
Precambrian to early Cambrian salt deposits allowing
deformation to be distributed over a wide area (Koyi et
al., 2000). To the SE of Zagros, the Gowk fault separates
the Zagros collision zone from the Lut block with
relatively low levels of seismic activity (Berberian et al.,
2000). Before the Bam earthquake, the Gowk fault was
considered the only seismically active fault in the studyarea (Ambraseys and Melville, 1982; Walker and
Jackson, 2002). Five earthquakes of Mw= 5.47.1
have occurred on this fault since 1981 (Berberian et
al., 2001; Walker and Jackson, 2002), but all have been
more than 100km from Bam. The Gowk fault to the
south, adjacent to the Bam fault, dies out in the Jebal
Barez Mountains, which themselves merge with the
active volcanic arc north of the Makran subduction zone
(Walker and Jackson, 2002). The focal mechanisms of
Fig. 1. Simplified tectonic map around the study area. The arrows show Arabian plate motion relative to Eurasia. Convergence velocities are indicated
after the NUVEL-1 model (DeMets et al., 1990) and GPS studies (Tatar et al., 2002; Vernant et al., 2004). The strike-slip motion on the Gowk fault
comes from the tectonic work (Walker and Jackson, 2002). Three active volcanoes
Bazman, Taftan and Soltan
are associated with the Makransubduction zone (Jakob and Quittmeyer, 1979). The location of the 2003 Bam earthquake and its focal mechanism ( Yamanaka, 2003) are also shown.
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the Gowk fault earthquakes (Berberian et al., 2001)
suggest that transpressional deformation is active within
the study area. The Gowk and Bam faults are both of the
right-lateral strike-slip-type, transmitting the convergent
movement accommodated north of the Iranian plateau to
the Makran and accommodating the difference inmotion due to the transition between the Zagros
collision and Makran subduction by transpressional
tectonics (see Regard et al., 2005 and references
therein).
3. Data and method
The aftershock activity of the 2003 Bam earthquake
was monitored by a seismic network consisting of nine
temporal stations (Fig. 2). Each station was equipped
with a highly sensitive three-component velocity-typeseismometer (LE-3D, Lennartz Electronics) with a
natural frequency of 1Hz, and a GPS timing system
(Suzuki et al., 2004). The waveform data were
continuously recorded at a sampling rate of 100Hz by
Fig. 2. (A) Hypocenter distribution (red dots) of the aftershocks located by 1-D velocity model (after Nakamura et al., 2005). (B) Hypocenter
distribution (red dots) relocated by using our 3-D velocity model. The lines AB, CD and EF correspond to the profiles of vertical cross-sections in Fig.
9. A NASA satellite map (http://earthobservatory.nasa.gov/) is shown. The green triangles denote stations of the temporal seismic network. Station 5
in Arg-e-Bam is also marked by a circle. The black dashed line indicates the traced line of the Bam fault inferred from the geological map supplied by
the National Geoscience Database of Iran (http://www.ngdir.ir/). Projections of the aftershock distributions on north
south (a) and east
west (c)profiles are also shown.
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a 16-bit data-logger (LS-8000SH, Hakusan). In general,
the arrival time of the P phase could be identified to
within about 0.05s (sampling rate 100 Hz), whereas the
estimation of the S-phase arrival was slightly less
accurate. Using a one-dimensional (1-D) velocity model
(Fig. 2A), we selected a useful data set of 2396
aftershocks among the 2789 aftershocks that Nakamura
et al. (2005) accurately located by the double differencemethod (Waldhauser and Ellsworth, 2000). All 2396
events were recorded by at least seven stations and by
the root mean square (rms) residual of P- and S-arrival
times
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Secondly, we researched the 3-D velocity structures
of P and S waves by using the tomography method
developed by Zhao et al. (1992). We adopted the
pseudo-bending method (Um and Thurber, 1987) for ray
tracing and a conjugate gradient algorithm (Paige and
Saunders, 1982) to invert the large and sparse system of
observation equations that relate observed travel times
to unknown parameters. A 3-D grid net was set up in the
model space of the study area to express velocity
structure. A total of 38505 phase data, consisting of
19490 P- and 19015 S-wave arrival times, were used in
this study. The unknown parameters are the hypocenters
of aftershocks and velocities at the grid nodes, both of
which are determined in an iterative inversion process.
The velocity at any point in the model is calculated by
linearly interpolating the velocities at the eight grid
nodes surrounding that point. As the dominant frequen-
cy is 810Hz for P waves and 58Hz for S waves, the
corresponding Fresnel zones do not exceed 0.8 km. This
is much smaller than the grid spacing of about 35 km
Fig. 4. Examples of three-component seismograms of a shallow aftershock (A) and a relatively deep aftershock (B) recorded at station 5 in Arg-e-Bam.
Table 2
The data of seismic stations including station corrections obtained from 1-D travel time inversion
Station Longitude (E) Latitude (N) Elevation (m) Surface geology P-wave corr. (s) S-wave corr. (s)
1 58.2771 29.1929 1280 Well-bedded ash-flow tuffs with subordinate 0.149 0.051
2 58.2893 29.0615 1202 Quaternary sediments 0.121 0.386
3 58.2864 28.9592 1235 Quaternary sediments 0.128 0.410
4 58.3969 29.1607 1080 Well-bedded ash-flow tuffs with subordinate 0.155 0.015
5 58.3690 29.1160 1071 Volcanic rock (andesite) 0.162 0.058
6 58.3949 29.0084 1015 Quaternary sediments 0.045 0.279
7 58.4614 29.0607 952 Quaternary sediments 0.109 0.243
8 58.4731 29.1261 1006 Well-bedded ash-flow tuffs with subordinate 0.118 0.011
9 58.4556 28.9466 905 Quaternary sediments 0.179 0.488
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adopted in this study, and hence would not affect the
tomographic imaging. After the VP and VS images are
determined from travel time inversion, we obtain the
Poisson's ratio () distribution by using the following
relation.
r
2VP
VS
2
22VP
VS
2 :
4. Tomography results and resolution
Before describing the results of the 3-D tomographic
inversion, we first show the results of the checkerboard
resolution test (CRT). This test was conducted to assess
the ability of the data and the method to recover existing
velocity anomalies within the model. To make acheckerboard, we assigned positive and negative
velocity perturbations with a 5% anomaly alternately
to all the 3-D grid nodes. We set up a 3-D grid in the
study area of 58.25E58.50E and 28.9N29.2N with a
grid spacing of 0.05 (about 5 km) in the horizontal
direction. Five layers of grid nodes are set up, one at
each of five depths: 0, 3, 6, 9 and 14km. Fig. 6 shows
the grid net distribution adopted in this inversion. We
obtained the results of the CRT at these five represen-
tative layers for both VP and VS structures, as shown in
Fig. 7. The resolution test is generally good for the
layers, and synthetic anomalies are well recovered in the
study area. As the resolution naturally depends on the
ray coverage, the denser regions are expected to have
higher resolution. We chose grid nodes where more than
100 rays of P waves and more than 100 rays of S waves
Fig. 5. Simplified geological map of the Bam earthquake area based on the 1:100,000 geological map prepared by the Geological Survey of Iran,
Sheet 7648-Bam (1993). The grey triangles denote stations of the temporal seismic network. Crosses and circles on the stations show the positive and
negative station delays for the 1-D P-wave velocity model, respectively. The Bam seismic (accelerograph) station is marked by a grey diamond.
Fig. 6. Map view and cross-sectional view of the grid net adopted in
the 3-D inversion. Grid spacing in the horizontal direction is 0.05
(
5km). Five layers of grid nodes are set up at 0, 3, 6, 9 and 14kmdepths.
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passed through. The average hit counts were 3222 and
3140 for P and S rays, respectively. The starting velocity
model was the inverted 1-D model (Table 1) for the
P-wave velocity structure. The CRT showed that
5% anomalies were well reconstructed. We were
therefore able to use VP /VS=1.73 to verify that the
result is not affected by the grid configuration. After
getting the results of CRT, we solved 2369 4
Fig. 7. Results of checkerboard resolution tests (CRT) for P-wave and S-wave velocity structures. The depth of each layer is shown at the bottom of
each map. Open and solid circles denote low and high velocities, respectively.
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surrounded by low VP anomalies. The VS map shows a
maximum 5% of high velocity anomaly in the southeast
part of the study area and a strongly low velocity
anomaly in the northwest part. The Poisson's ratio map
shows a high anomaly beneath Bam and a low anomaly in the southeast. At a depth of 9km, the
significant linear trends of low VS and high appear in a
northsouth direction and along the Arg-e-Bam fault,
which Nakamura et al. (2005) proposed as the source
fault of the Bam earthquake. The maps of VP, VS and
Poisson's ratio at a depth of 14km reveal no special
anomalies exceeding 1%. By using our 3-D velocity
results, we relocated aftershocks and showed their
hypocenter distribution in Fig. 2B in comparison with
the one located by the 1-D velocity model (Fig. 2A,
Nakamura et al. (2005)). The average location errors of
the 3-D velocity results are estimated to be 0.10, 0.11
and 0.25km for NS, EW and depth, respectively. The
comparison of hypocenter distributions between Fig. 2A
and B is discussed in the next section.
5. Discussion
We first discuss the 3-D images in the vertical cross-
sections ofVP, VS and along and perpendicular to the
Bam aftershock alignment (Fig. 9). The northsouth
cross-section AB shows a general shape including the
earthquake fault plane. The eastwest cross-sections CD
and EF are perpendicular to the fault plane at 29.10N
through Bam and at 29.05N south of the city,
respectively. Those images present large variations in
seismic velocity and Poisson's ratio. The cross-section
CD along 29.10N in Fig. 9 shows a surface layer of high
(0.280.30) down to a depth of about 1km. This may
Fig. 9. Vertical cross-sections of P-wave and S-wave velocity perturbations and Poisson's ratio along the lines AB, CD and EF shown in Fig. 2B-b.
Slow velocity and high Poisson's ratio are shown by red; fast velocity and low Poisson's ratio are shown by blue. Small black crosses denote the
relocated aftershocks within a 2km width along the profile.
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indicate a sediment layer of poorly consolidated
materials with a lot of water in the northern area of
Bam, which is an oasis city. It is plausible that a well-
developed flood plain associated with the Posht-rud
River (Fig. 5) made such water-rich sediment. On the
other hand, there is no such surface layer of highPoisson's ratio in the southern area of Bam along
29.05N, as shown in the cross-section EF. We avoid
discussing the cause of low VS and high in the shallow
layer south of 29.0N, because of the poor resolution by
CRT in Fig. 7. The most prominent anomaly is a thick
layer of low (0.200.24) at the depth range of about
26km, as is especially apparent in the cross-section EF.
A teleseismic analysis (Yamanaka, 2003) indicated that
a large slip existed in a shallow part of the fault plane.
Using radar data, Wang et al. (2004) suggested that the
maximum slip occurred at a depth of about 3km. AndFialko et al. (2005) indicated that most of the seismic
moment was released at a depth of 45km. Those depth
ranges correspond to approximately the central depth of
this layer with low VP (5.355.77km/s), high VS (3.25
3.40km/s) and low (0.200.24). The obtained VPcorresponds to the experimental value of 5.533km/s for
andesite under 200MPa in Table 2 of Christensen
(1996). But the obtained VS is slightly higher than the
experimental value of 3.034km/s for the same andesite
under 200MPa. Referring to the experimental study of
Christensen (1996), we would suggest that the observed
lower indicates rock with higher SiO2 content andgreater brittleness. If the outcrops of andesite, trachyan-
desite and dacite in the Bam area (Fig. 5) contain much
quartz, this high quartz content could explain the
obtained seismic velocities and Poisson's ratio. How-
ever, we need more detailed petrological and petrophy-
sical experimental studies of those rocks before we can
reach a conclusion.
Fig. 9-c shows a high Poisson's ratio (0.270.28) in
the depth range of about 6km beneath Bam (29.11N,
58.35E) to about 9km south of there (29.02N, 58.35E)
in the profile AB. The inferred increase in Poisson'sratio may be associated with a change from a SiO2-rich
rock to a more mafic rock. Among the mafic rocks, the
obtained values of VP (5.895.94km/s) and VS (3.24
3.33 km/s) may be related to basalt under 200 MPa, the
velocities of which are 5.914 km/s for P waves and
3.217 km/s for S waves, respectively, in Table 2 of
Christensen (1996). On the other hand, we suggest
another possible cause of the increase of Poisson's ratio:
the existence of fluids in the crust. Generally, fluid
saturation leads to an increase of Poisson's ratio (Ito et
al., 1979). Fluids can alter the rheology of rocks from
brittle to ductile behavior. The chemical influence of
fluids decreases the strength of rock through such
mechanisms as stress corrosion cracking (Atkinson
and Meredith, 1987). Fluids can also weaken a rock and
enhance creep rates and slow deformation through a
range of mechanisms (Etheridge et al., 1984; Tullis et
al., 1996). These would have enhanced stress concen-tration in the seismogenic layer, leading to mechanical
failure and thus contributing to the nucleation of the
Bam earthquake, as we discuss latter. However, we have
no exact explanation about the origin of the crustal
fluids in this region. We presented two causes, basalt
and fluids, of the high in the depth range of 69km,
but have no definitive suggestions to make about them.
Other forms of detailed prospecting, such as electro-
magnetic prospecting, are expected, especially for
researching fluids in the deep layers under the Bam area.
Secondly, we compare the distribution of aftershocksshown in Fig. 2B, which were located by the 3-D
velocity model, with that in Fig. 2A, which were
obtained by the 1-D model. In both distributions, the
trends are in accord with the strike and dip angles of the
focal mechanism (strike 175, dip 85 and slip 153) of
the main shock (Yamanaka, 2003). In addition, both
epicenter distributions extend for about 20km in the
strike direction. However, the main linear distribution of
the 3-D model in Fig. 2B shows a shift of about 1km to
the west in comparison with that of 1-D in Fig. 2A. This
suggests that the source fault of the main shock is about
4.5km on average to the west of the geological Bamfault. Fig. 10 shows the same seismic cross-section, EF,
as in Fig. 9, in contrast with the location of the Bam fault
on the ground surface (inverted arrow). We also show
the location of minor surface ruptures with en-echelon
patterns in the desert presented in Fig. 5b of Fielding et
al. (2005). The location of surface displacement was
nearly the same one modeled from satellite data (e.g.,
Talebian et al., 2004; Binet and Bollinger, 2005;
Fielding et al., 2005). In Fig. 10, the pattern of
aftershocks deeper than 5km is complex. But the main
linear cluster of aftershocks shallower than 5km clearlyfaces just upward from the surface ruptures and nearly
touches them, coming within 1km. Of course we have to
know that the average location errors of those
hypocenters that are shallower than 3km are estimated
to be 0.10, 0.11 and 0.32km for NS, EW and depth,
respectively. We therefore propose a simple schematic
model of the central fault plane of the Bam earthquake in
Fig. 10. As a matter of fact, the deeper part of the fault
plane is thought to be more complex. If we assume the
existence of a small second source fault (Funning et al.
(2005)) connecting to the Bam fault escarpment, we
could show that by the thick broken line labeled 2 in
279H. Sadeghi et al. / Tectonophysics 417 (2006) 269283
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Fig. 10. On the other hand, we suggest that the northern
part of the fault under Bam, including Arg-e-Bam, is nota single plane but branching planes ( Nakamura et al.,
2005), because the whole pattern of aftershocks in the
cross-section CD in Fig. 9 is very complex.
Fig. 11 shows the same seismic cross-section AB as
in Fig. 9, including the areas of low and high . Most
aftershocks are distributed between 0 and 14km in depth
and became shallower than that in Fig. 2A. The seismic
gap in the central part of the cross-section can be
distinguished at the depth range of 27km, as shown by
the dotted circle. This may correspond to the higher slip
region (asperity) of 80cm to 1m proposed by Yamanaka(2003) as a result of teleseismic analysis. In Fig. 11 we
also show the area with a slip larger than 2 m of the main
fault by the satellite data analysis of Funning et al.
(2005). This figure suggests that our seismic gap area
presents not a perfect but rather a good coincidence with
the large slip area of Funning et al. (2005). We can say
that the asperity of the Bam earthquake was very
shallow, nearly in the depth range of 27km. This
shallow asperity must be one of the reasons why the
damage was unexpectedly large given the earthquake's
magnitude. We also suggest that the asperity inferred
from the seismic gap in the depth range of 2 to 7km
nearly corresponds to the thick layer with high VS and
low , as shown in Fig. 9. On the condition of the same
density, the high VS means higher rigidity in this layer in
comparison with the surroundings. In addition, referring
to the teleseismic analyses of Yamanaka (2003) and
Yagi (2003), we suggest that the start of the higher slip isnear the bottom of the asperity. This means that the
rupture of the Bam earthquake started in or near the
deeper layer of high , even though we cannot show its
exact point or the precise hypocenter of the main shock
in Fig. 11. This allows us to assume that the nucleation
of the Bam earthquake was created in this deeper layer
filled with fluids. We surmise that the rupture of the
main shock started in or near the high layer of 69 km
in depth and then propagated with a large slip mainly in
the rigid rocks at the depth range of 26km. This
rupture with a large slip must have generated the verystrong motions on the surface and cause intense damage
in and around Bam. By using the acceleration data
observed at the Bam station and other stations (BHRC),
Miyake et al. (2004) suggested that the extremely strong
motions were localized and proposed the shallow
asperities existed on a fresh fault rather than the Bam
fault. We cannot judge between fresh and not fresh.
But our shallow asperity model in the rigid rocks can be
Fig. 10. Interpretative cross-section EF in Fig. 9 showing the areas oflow Poisson's ratio (L: 0.200.24) and high Poisson's ratio (H:
0.270.28) by thin broken lines and the distribution of aftershocks
within a 2km width along the profile (black crosses). A schematic
simple model of the central fault plane of the Bam earthquake is shown
by a thick solid line marked with the number 1. A possible branching
segment of the fault plane is also shown by a broken line marked with
the number 2. The locations of minor surface ruptures ( Fielding et al.,
2005) and the Bam fault on the ground are indicated by inverted
arrows marked with SR and BF, respectively. The locations of cross-
sections AB (shown in Fig. 9) is also indicated.
Fig. 11. Interpretative cross-section AB in Fig. 9 showing the areas of
low Poisson's ratio (L: 0.200.24) and high Poisson's ratio (H:
0.270.28) by thin broken lines. The dotted circle shows the seismic
gap in the central part of the cross-section. The area with a slip larger
than 2m of the main shock (Funning et al., 2005) is shown by the thick
broken line. The locations of cross-sections AB and CD (shown in Fig.9) are indicated by inverted arrows.
280 H. Sadeghi et al. / Tectonophysics 417 (2006) 269283
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supported by the localized nature of the extremely
strong motions.
6. Conclusions
Three-dimensional VP, VS and Poisson's ratio in theepicenter area of the 2003 Bam earthquake have been
determined by using a large number of high-quality
arrival times from the aftershocks. A precise aftershock
distribution, relocated by using the 3-D structure, clearly
delineates the fault plane about 4.5km west of the
known Bam fault. The aftershock distribution in the
along-strike cross-section illuminates a rectangular fault
area of about 20km in horizontal length and 14km in
deep width from near the earth's surface. The fault plane
dips westward steeply and strikes nearly northsouth.
The main shallow cluster of aftershocks south of Bamcity is distributed just under the small ruptures found on
the ground surface. The 3-D structures of the seismo-
genic region are well resolved to a depth of 14km. A
thick layer of high VS and low anomalies (0.200.24)
is imaged at about 26km depth. Low may suggest
that the rocks have high SiO2 content. A high (0.27
0.28) zone is clearly imaged in the depth range from
about 6 km beneath Bam (29.11N, 58.35E) to about
9 km south of the city (about 29.02N, 58.35E). This zone
may suggest a change from SiO2-rich rock to a more
mafic rock, or it may suggest the existence of fluids. The
main seismic gap of aftershock distribution at the depthrange of 2 to 7km appeared in nearly good coincidence
with the large slip zone in the shallow thick layer of high
VS and low . This high VS and low may appear to
indicate high rigidity and brittleness in comparison with
the surroundings. We therefore conclude that the large
slip propagating mainly in the shallow rigid layer in and
south of Bam is one of the main reasons why the Bam
area suffered heavy damage.
Acknowledgments
We gratefully acknowledge Dr. T. Matsushima, Dr.
T. Ito, Dr. S.K. Hosseini, A.J. Gandomi, and M. Maleki
for their help with fieldwork and data analysis as well
as for our constructive discussions with them. We thank
Professor Dr. Dapeng Zhao for the tomography code
and Professor Dr. Tamao Sato for the one-dimensional
velocity inversion code. Some figures in this paper
were made using Generic Mapping Tools (GMT)
software written by Wessel and Smith (1998). The
manuscript was greatly improved by comments and
suggestions of Professor Mike Sandiford and two
anonymous reviewers.
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