3-D velocity structure of the 2003 Bam earthquake area (SE Iran)

8/7/2019 3-D velocity structure of the 2003 Bam earthquake area (SE Iran)

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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
mailto:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.tecto.2006.01.005http://dx.doi.org/10.1016/j.tecto.2006.01.005http://dx.doi.org/10.1016/j.tecto.2006.01.005mailto:[email protected]:[email protected]:[email protected]:[email protected]


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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.

http://earthobservatory.nasa.gov/http://www.ngdir.ir/http://www.ngdir.ir/http://earthobservatory.nasa.gov/


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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



5 58.3690 29.1160 1071 Volcanic rock (andesite) 0.162 0.058







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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



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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.



13/15

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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3-D velocity structure of the 2003 Bam earthquake area (SE Iran)