Velocity and Attenuation Structure of the Tibetan...

Velocity and Attenuation Structure of the Tibetan Lithosphere Under the Hi-CLIMB Array

From the Modeling of Pn Attributes

ALI C. BAKIR1 and ROBERT L. NOWACK

1

Abstract—Using seismic data from regional earthquakes in

Tibet recorded by the Hi-CLIMB experiment, Pn attributes are

used to constrain the velocity gradient and attenuation structure of

the Tibetan lithosphere under the Hi-CLIMB array. Numerical

modeling is performed using the spectral-element method (SEM)

for laterally varying upper-mantle velocity and attenuation, and the

seismic attributes considered include the Pn travel-time, envelope

amplitude, and pulse frequency. The results from the SEM mod-

eling provide two alternative models for the upper-mantle beneath

the Hi-CLIMB array in Tibet. The first model is derived from the

3D velocity model of GRIFFIN et al. (Bull Seism Soc Am

101:1938–1947, 2011) with a constant upper-mantle velocity gra-

dient, and laterally varying upper mantle attenuation. The second

model has a laterally varying upper-mantle velocity gradient, and

constant upper-mantle attenuation. In both cases, the Qiangtang

terrane is distinguished from the Lhasa terrane by a change in

Moho depth and upper-mantle velocities. The lower upper-mantle

velocities, as well as higher Pn attenuation, suggest hotter tem-

peratures beneath the Qiangtang terrane as compared to the Lhasa

terrane. Although the fits to the Pn amplitude and pulse frequency

data are comparable between the two models, the first model with

the constant upper-mantle velocity gradient fits the travel times

somewhat better in relation to the data errors.

Key words: Tibet lithosphere, Pn waves, Seismic attributes.

1. Introduction

The Tibetan plateau is one of the most remark-

able topographic features on Earth with an average

elevation of about 5 km above sea level. The

Himalayan convergent zone where the continental

collision between Indian and Eurasian plates occurs

is located on the south and southwest of the Tibetan

Plateau (CHEN and MOLNAR, 1981). The collision of

the northward moving Indian continent with the

southern margin of Eurasia is associated with the

formation of the Himalayas, as well as the high

elevation and thickened crust of Tibet (MCNAMARA

et al., 1997).

In order to investigate the velocity and attenuation

structure beneath Tibet, spectral-element method

(SEM) modeling of Pn wave attributes, including the

travel time, the peak envelope amplitude, and the

instantaneous pulse frequency, is performed for

regional earthquakes recorded by the Hi-CLIMB

array. Regional earthquakes are chosen to be

approximately in-line with the Hi-CLIMB array, but

also provide some azimuthal coverage. The locations

of the events are shown in Figs. 1, 2.For the SEM modeling, the P-wave velocities are

derived from GRIFFIN et al. (2011) who used 3D ray

tracing for the modeling and obtained a 3D P-wave

velocity model along with a laterally variable Moho

which was similar to those found by TSENG et al.

(2009); NABELEK et al. (2009) and NOWACK et al.

(2010) using teleseismic waves. The Moho beneath

the Lhasa terrane of southern Tibet was found to be

over 73 km thick with a Pn speed of about 8.2 km/s.

The Qiangtang terrane to the north of the Bangong-

Nujiang suture (BNS) showed a thinner crust by up to

10 km and a Pn speed of 7.8–7.9 km/s. The upper-

mantle top velocities and upper-mantle velocity gra-

dients from GRIFFIN et al. (2011) are similar to those

found by PHILLIPS et al. (2007). The upper-mantle

velocity gradients from GRIFFIN et al. (2011) are also

similar to those found from a large regional model by

MYERS et al. (2010) for the Lhasa terrane, but the

upper-mantle velocity gradients from MYERS et al.

(2010) are lower in the northern Qiangtang terrane.

However, the large regional model of MYERS et al.

(2010) has different upper-mantle velocities from

those found by GRIFFIN et al. (2011), and from earlier

1 Department of Earth and Atmospheric Sciences, Purdue

University, West Lafayette, IN 47907, USA. E-mail: alicanba-

[email protected]; [email protected]

Pure Appl. Geophys.

� 2012 Springer Basel AG

DOI 10.1007/s00024-012-0482-8 Pure and Applied Geophysics

results of LIANG and SONG (2006) and PHILLIPS et al.

(2007) in Tibet.

From the 3D velocity model of GRIFFIN et al.

(2011), 2D slices were obtained and earth-flattened

for use in the 2D SEM modeling. The locations of the

2D slices are shown in Fig. 1. For the numerical

calculations, SPECFEM2D, a parallel 2D, viscoelas-

tic SEM code (KOMATITSCH and VILOTTE, 1998 and

KOMATITSCH et al., 2005) was used, and the SEM

results corrected for 3D geometric spreading as

described by BAKIR and NOWACK (2011). For the

seismic attribute modeling, one 2D slice along the

longitude of 84� (West Line) is used for the south

regional event L1E, a 2D slice along the longitude of

85� (Central Line) is considered for the south cluster

of events L1A, L1I, L1J, and L1K, and a 2D slice

along the longitude of 86� (East Line) is used for the

north event E3B (Fig. 1). All the 2D slices derived

from GRIFFIN et al. (2011) have a laterally varying

crust and upper mantle velocity structure.

We first show that the SEM modeling, not

including attenuation, does not fit the observed Pn

amplitude and pulse frequency attributes. The SEM

results from two models which include attenuation

are then compared with the observed data. The first

model derived from GRIFFIN et al. (2011) has a con-

stant upper mantle velocity gradient, but has variable

upper mantle attenuation. The second model has a

laterally varying upper-mantle velocity gradient and

constant upper-mantle attenuation. It is found that

both models can be used to fit the observed Pn

amplitude and pulse frequency attributes. However,

the model with a constant upper mantle gradient fits

the travel time data slightly better than the variable

velocity gradient case in relation to the data errors.

Nonetheless, in both cases the Qiangtang terrane is

3535

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L1K L1IL1J

BNS

IYS

Qiangtang Terrane

Lhasa Terrane

Lat

itude

(de

g)

Longitude (deg)

West Line East LineCentral Line

Figure 1Elevation map showing selected stations of the Tibet component of the Hi-CLIMB array in Tibet (triangles) and the epicenters of the selected

local and regional earthquakes. The circles show the events used by GRIFFIN et al. (2011) to develop their velocity model where the open

circles are relocations. The dashed lines shows the locations of the 2D P-wave velocity slices obtained from the 3D velocity model of GRIFFIN

et al. (2011) with the East Line at 84�, the Central Line at 85�, and the West Line at 86� in longitude. The locations of the Bangong-Nujiang

suture (BNS) and the Indus-Yarlung suture (IYS) are shown by the dotted lines, and the Lhasa and Qiangtang terranes are also shown

A. C. Bakir, R. L. Nowack Pure Appl. Geophys.

distinguished from the Lhasa terrane by a change in

Moho depth and upper-mantle velocities. The lower

upper mantle velocities, as well as higher Pn atten-

uation, suggest hotter temperatures beneath the

Qiangtang terrane as compared to the Lhasa terrane

(NI and BARAZANGI, 1983; RAPINE et al., 1997;

TILMANN et al., 2003).

2. Regional Events

Broadband seismic stations of the Hi-CLIMB

array were used to record regional and teleseismic

earthquakes in Tibet (NABELEK et al., 2005). The

locations of the regional earthquakes used in this

study are reported from either the PDE or Engdahl

(EHB) catalogs (USGS, 2010 and ENGDAHL et al.,

1998; see GRIFFIN et al. (2011) for the specific loca-

tion information). Six regional events are utilized for

modeling of the seismic Pn attributes, one out-of-line

south event, L1E, one out-of-line north event, E3B,

and four in-line south events, L1A, L1I, L1J, and

L1K. The focal mechanism solutions of the selected

earthquakes are taken from BAUR (2007) for all events

except E3B which was obtained by T. L. Tseng

(personal communication). For each event, 2D

Focal Mechanism for E3B with Hi−CLIMB Stations

Focal Mechanisms for L1E with Hi−CLIMB Stations

Focal Mechanisms for South Events with Hi−CLIMB Stations

80 81 82 83 84 85 86 87 8827

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Longitude (degrees)

2300

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L1EL1E Revised

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(de

gree

s)

(a) (b)

Longitude (degrees)

Lat

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(de

gree

s)

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Lat

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gree

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E3B TL-1

Longitude (degrees)

(c)

Figure 2a The original and revised focal mechanism for regional event L1E. b The focal mechanisms for the cluster of south regional events L1A, L1I,

L1J, and L1K. c The focal mechanism TL-1 for event E3B. The triangles show the Hi-CLIMB stations, where the black triangles along with

the solid lines show the orientation angles used for the specification of the moment tensor for the SEM modeling. The dashed lines show the

separation of ranges used to specify the moment tensor for the SEM modeling

Velocity and Attenuation Structure of the Tibetan Lithosphere

moment tensor slices are derived from the 3D

moment tensors for different azimuth angles from the

events to the Hi-CLIMB stations (Appendix 1). These

2D moment-tensor slices were used to specify the

source in the SEM modeling. Two, three, or four

station locations are selected for the azimuthal ori-

entation of the moment tensor depending on the range

of azimuths to the stations. Three reference stations

are chosen for L1E for the specification of the moment

tensor slices. Four reference stations are chosen for

E3B since it is the most out-of-line event. Two ref-

erence stations are found to be sufficient for the cluster

of events L1A, L1I, L1J, and L1K since they are the

most in-line with the Hi-CLIMB array. In Fig. 2, the

solid lines to black triangles show the azimuths used

to specify the moment tensor slices, with the dashed

lines separating the different azimuth ranges.

Event L1E is located to the southwest of the

Hi-CLIMB array and shown in Fig. 2a. The moment

magnitude (Mw) of the event L1E is 4.0 (GRIFFIN et al.

2011). The depth of the event L1E is 16.6 km below

sea level, and 5 km is added for the SEM calculation

to allow for the average topographic elevation above

sea level. The revised focal mechanism for this event

is used in order to avoid low amplitude Pg waves

which would have source take-off angles near nodal

and are not observed in the data. However, this

revision is within the uncertainties of the focal

mechanism derived by BAUR (2007) (J. Nabelek,

personal communication).

The four south events (L1A, L1I, L1J, and L1K)

shown in Fig. 2b are located to the southeast of the

Hi-CLIMB array and are the most in-line events with

the array. All these events have similar hypocenters,

as well as similar focal mechanisms. Therefore, they

are considered as a cluster of events, even though

L1K occurred 6 months earlier than the other three

events. The moment magnitudes (Mw) of these south

events vary between 4.0 and 5.0 (GRIFFIN et al.,

2011). The depths of the south cluster events vary

between 13.1 km to 20.3 km.

The event E3B is located to the northeast of the

Hi-CLIMB array and shown in Fig. 2c. The focal

mechanism of E3B as determined by T.L. Tseng

(personal communication) is labeled as E3B TL-1.

The moment magnitude (Mw) of this event is 4.0

(GRIFFIN et al., 2011). The depth of the event E3B was

found to be 19 km by T.L. Tseng (personal commu-

nication); however, modeling the cross over distance

of the travel-times suggested a somewhat shallower

focal depth as found by GRIFFIN et al. (2011).

2.1. SEM Modeling of Regional Events

with a Constant Upper Mantle Velocity Gradient

and No Attenuation

Seismic attributes for the six regional events are

first modeled using the SEM calculations with no

attenuation. The P-wave velocity slices for the SEM

modeling are shown in Fig. 3a–c and were derived

from the 3D velocity model of GRIFFIN et al. (2011).

The S-wave model is derived from the P-wave model

by assuming a Poisson solid, and the density of the

crust is specified as 2,700 kg/m3 and in the mantle it

is specified to be 3,200 kg/m3 for the SEM modeling.

Figure 4 shows the P-wave travel time of the

SEM calculations compared with the observed

P-wave travel time for the six regional events. The

uncertainties in the observed travel-times of ±0.3 s

from GRIFFIN et al. (2011) are also shown, along with

the rms residuals of the Pn phase for each event from

the SEM modeling. It was found that with only minor

adjustments of the velocities, the SEM calculations of

the travel times along the 2D velocity slices are able

to model the observed travel times to within the data

errors for these events.

Figure 5 shows the calculated P-wave peak

envelope amplitudes of the elastic SEM calculations

compared with the observed peak envelope ampli-

tudes for the six events. A description of how the

seismic attributes for both the observed and calcu-

lated data are estimated is given in Appendix 2

(MATHENEY and NOWACK, 1995). The rms difference

between the observed and calculated Pn log-ampli-

tudes are also shown, where the calculated Pn pulse

amplitudes are generally larger than the observed

pulse amplitudes. Without changes to the velocity

model the calculated amplitudes can be decreased by

incorporating attenuation into the modeling.

Figure 6 shows the instantaneous pulse frequen-

cies of the elastic SEM calculations compared with

the observed instantaneous and centroid pulse fre-

quencies (MATHENEY and NOWACK, 1995; Appendix

2). Differences between the observed instantaneous


and centroid pules frequencies are used to provide an

approximate estimate of the noise in the data, as well

as possible interference with later arrivals. Therefore,

both estimates are shown for the observed pulse

frequency data. However, since the calculated instan-

taneous and centroid frequencies are similar, only the

calculated instantaneous pulse frequencies are shown

to avoid clutter on the plots. The calculated pulse

frequencies for the Pn branch generally increase with

distance resulting from the upper mantle velocity

gradient, and can be decreased as a function of

distance by either increasing the attenuation or

lowering the velocity gradient in the upper-mantle.

However, for the velocity model derived from GRIFFIN

et al. (2011), both the calculated envelope amplitudes

and pulse frequencies are too high compared to the

observed data when attenuation is not included, and

therefore attenuation is required to match the

Distance (km)

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th (

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p (km/sec)

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(a)

(b)

(c)

(d)

(e)

(f)

Event L1E VelocityConstant Upper Mantle Grad

Event L1E VelocityVariable Upper Mantle Grad

South Events Velocity Constant Upper Mantle Grad

South Events Velocity Variable Upper Mantle Grad

Event E3B Velocity Constant Upper Mantle Grad

Event E3B Velocity Variable Upper Mantle Grad

Vp (km

/sec)

Vp (km

/sec)

N

N

N

N

N

NS

S

S

S

S

S

Figure 3Subplots a, b, and c show the P-wave velocity slices with constant velocity gradient in the upper-mantle derived from the 3D model of GRIFFIN

et al. (2011) used for event L1E, the south cluster of events L1A, L1I, L1J, and L1K, and event E3B. Subplots d, e, and f show the P-wave

velocity slices with laterally variable velocity gradient in the upper-mantle used for event L1E, the south cluster of events L1A, L1I, L1J, and

L1K, and event E3B. Circles show the projected locations of the events along the profiles


calculated amplitudes and pulse frequencies with the

observed Pn attributes.


with a Constant Upper Mantle Velocity Gradient

and Variable Attenuation in the Upper Mantle

For the modeling here, velocity models derived

from GRIFFIN et al. (2011) are again used, but now

including seismic attenuation. In Fig. 7, the calcu-

lated pulse amplitudes from the SEM modeling with

the laterally variable attenuation and a constant upper

mantle velocity gradient are compared with the

observed pulse amplitudes, and in Fig. 8 the calcu-

lated pulse frequencies are compared with the

observed instantaneous and centroid pulse frequen-

cies. For the results shown in Figs. 7, 8, an upper-

mantle QP-1 of 0.0038 in the Lhasa terrane to the

south and an upper-mantle QP-1 of 0.0077 (a QP

-1 of

0.0083 for E3B) in the Qiangtang terrane in the north

were obtained. It was also found that modeling with

constant upper mantle attenuation did not match the

amplitude data, particular for the larger distance

ranges of event L1E to the south and for event E3B

200 300 400 500 600 700

0

5

10Observed (Cross)SEM Synthetics (Circle)

T-x

/8 (

s)

Distance (km)

-5100

200 300 400 500 600 700

0

5

10

Observed (Cross)SEM Synthetics (Circle)

T-x

/8 (

s)

Distance (km)

-5100 200 300 400 500 600 700

0

5

10


T-x

/8 (

s)

Distance (km)

-5100

200 300 400 500 600 700

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5


T-x

/8 (

s)

Distance (km)

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0

5

10


T-x

/8 (

s)

Distance (km)

-5100

200 300 400 500 600 700

0

5


T-x

/8 (

s)

Distance (km)

-5100

Event L1E Travel Times Event L1A Travel Times

Event L1I Travel Times Event L1J Travel Times

Event L1K Travel Times Event E3B TL-1 Travel Times

average pick error

average pick error

average pick error

average pick error

average pick error average pick error

Pn-rms=.24 Pn-rms=.19



Pg

Pg

Pg

Pg

Pg

Pg

Pn

Pn

Pn

Pn

Pn

Pn

Figure 4Comparison between the calculated travel times of the six regional events from the SEM modeling with models of constant upper-mantle

velocity gradient and the observed travel times. Circles are the SEM calculated travel times and crosses are observed travel times. The average

pick errors from GRIFFIN et al. (2011) and the Pn rms residuals (Pn-rms) are also shown


located to the north in the Qiangtang terrane. QP-1

values of 0.0077–0.01 were used for the crust, but

these were less well constrained from the regional

event data used here and could also result from

unmodeled crustal velocity heterogeneities. For

variable upper-mantle attenuation, a smooth transi-

tion for the SEM modeling was created around the

ramp structure on the Moho near the Bangong-

Nujiang suture (BNS). From Figs. 7, 8, it can be

seen that the SEM modeling with variable upper-

mantle attenuation and a constant upper mantle

velocity gradient gives a better fit to the seismic

attributes than the elastic case for the velocity model

of GRIFFIN et al. (2011) in terms of the Pn rms

misfits. The mismatch in Pg amplitudes for events

L1A and L1I could result from uncertainties in the

focal mechanisms since the other two events of

the cluster have better matches, and all events of the

cluster match the Pn branch. The Pn rms misfits for

the instantaneous frequencies are now also compa-

rable with the observed instantaneous frequency and

observed centroid frequency misfits.

100 200 300 400 500 600 700

1

2

3

4

5

6Observed (Cross)SEM (Circle)1900 for Area4 2100 for Area32300 for Area22500 for Area1

Distance (km)

Pg

Event E3B TL-1 Envelope Amplitudes No Q

Pn

01 2 3 4

100 200 300 400 500 600 700

1

2

3

4

5

6Observed (Cross)SEM (Circle)1900 for Area3 2100 for Area22300 for Area1

Distance (km)

Pg

Event L1E Envelope Amplitudes No Q

Pn

0 1 2 3

100 200 300 400 500 600 700

1

2

3

4

5

6Observed (Cross)SEM (Circle)1650 for Area2 1750 for Area1

Distance (km)

Pg

Event L1A Envelope Amplitudes No Q

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1I Envelope Amplitudes No Q

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1J Envelope Amplitudes No Q

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1K Envelope Amplitudes No Q

Pn

01 2

Log

10(a

mpl

itude

)L

og10

(am

plitu

de)

Log

10(a

mpl

itude

)

Log

10(a

mpl

itude

)L

og10

(am

plitu

de)

Log

10(a

mpl

itude

)




Figure 5Comparison between the Pn amplitude attributes of the six regional events from SEM modeling with no attenuation from the SEM modeling

and the observed envelope amplitudes. The circles are the SEM calculated values and the crosses are observed envelope amplitudes. The

distance ranges for the specified orientation angles for the moment tensors used in the SEM modeling indicated by vertical dashed lines. For

event L1E, the revised moment tensor is used. The Pn rms of the log-amplitudes (Pn-rms) for each event is also shown



with a Variable Velocity Gradient and Constant

Attenuation in the Upper Mantle

As discussed by BAKIR and NOWACK (2011), for

the distance ranges used here there can be trade-offs

between variable attenuation and velocity gradient

structures for the modeling of seismic attributes. To

further investigate the Pn amplitude and frequency

characteristics for this region, 2D models with

variable upper mantle velocity gradients are

constructed and used in the SEM calculations. These

models are shown in Fig. 3d–f for the different

events. For all events, the crustal structures are

chosen to be the same as in earlier calculations.

However, a single constant upper mantle QP-1 of

0.0038 is now used.

While the earlier P-wave velocity slices had an

upper mantle velocity gradient of 0.003 1/s for the

entire upper mantle before earth-flattening, the

velocity models here have an upper mantle velocity

Obs E3B TL-1 and SEM Pulse Freq No Q

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg

Pn2

6Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1900 for Area4 2100 for Area3 2300 for Area2 2500 for Area1

100

1 2 3 4

200 300 400 500 600 7000

1

3

4

5

Distance (km)

Pg Pn2

6Obs L1E and SEM Pulse Freq No Q

Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1900 for Area3 2100 for Area2 2300 for Area1

100

1 2 3

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg Pn2

6Obs L1A and SEM Pulse Freq No Q

Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1650 for Area2 1750 for Area1

100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg Pn2

6Obs L1I and SEM Pulse Freq No Q


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn

2

6Obs L1J and SEM Pulse Freq No Q


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg Pn2

6Obs L1K and SEM Pulse Freq No Q


100

1 2

Freq

uenc

y (H

z)

Pn-rms=.25IC-Pn-rms=.26






Figure 6Comparison between the calculated instantaneous pulse frequencies of the six regional events with no attenuation from the SEM modeling

with no attenuation and the observed instantaneous and pulse frequencies. The circles are the SEM calculations and the crosses and squares

are the observed instantaneous and centroid pulse frequencies. For event L1E, the modified moment tensor is used. The Pn rms between the

observed and calculated instantaneous pulse frequencies (Pn-rms) are also shown, along with the rms between the observed instantaneous and

centroid pulse frequencies (IC-Pn-rms) used to assess the errors in the data


gradient of 0.003 1/s in the Lhasa terrane to the south

and a velocity gradient of 0.001 1/s in the Qiangtang

terrane to the north (Fig. 1). This is similar to the

upper mantle velocity gradients found for this area

from the large regional model of MYERS et al. (2010).

However, again, their upper mantle top velocities

were different from those found from the more

detailed models for Tibet found by GRIFFIN et al.

(2011); LIANG and SONG (2006) and PHILLIPS et al.

(2007). Figure 9 shows the P-wave travel times of the

SEM calculations with both a constant and variable

velocity gradient upper mantle compared with the

observed travel times. It can be observed that the

P-wave travel times for the variable upper mantle

velocity gradient deviate from the observed travel

times over the distance ranges used here. In terms of

the Pn rms misfits for the travel times, the rms values

for the variable upper mantle velocity gradient case

are comparable but slightly larger than the data errors

as found by GRIFFIN et al. (2011). This is in contrast to

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event E3B TL-1 Envelope AmplitudesVar Atten/Const Grad

Pn

01 2 3 4

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1E Envelope AmplitudesVar Atten/Const Grad

Pn

0 1 2 3

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1A Envelope AmplitudesVar Atten/Const Grad

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1I Envelope AmplitudesVar Atten/Const Grad

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1J Envelope AmplitudesVar Atten/Const Grad

Pn

01 2

100 200 300 400 500 600 700

1

2

3

4

5


Log

10(a

mpl

itude

)

Distance (km)

Pg

Event L1K Envelope AmplitudesVar Atten/Const Grad

Pn

01 2

Log

10(a

mpl

itude

)

Log

10(a

mpl

itude

)L

og10

(am

plitu

de)

Log

10(a

mpl

itude

)

Log

10(a

mpl

itude

)




Figure 7Comparison between the peak envelope amplitudes of the six regional events from SEM modeling with variable upper mantle attenuation and

constant upper mantle velocity gradient models and the observed envelope amplitudes. The circles are the SEM calculations and the crosses

are observe envelope amplitudes. The Pn rms of the log-amplitudes (Pn-rms) for each event is also shown


the constant upper mantle gradient case which had

rms misfits slightly smaller than the data errors.

The calculated SEM Pn amplitude and pulse

frequency attributes with constant attenuation and

variable upper mantle velocity gradients are then

compared with the observed data. In Fig. 10 the

comparison of the constant attenuation and a variable

upper mantle velocity gradient SEM amplitudes with

the observed amplitudes is given, and in Fig. 11a

comparison of the SEM pulse frequencies with the

observed instantaneous and centroid pulse frequen-

cies is shown. The modeling results for the Pn

amplitude and pulse frequency attributes are similar

to those obtained for the variable attenuation and

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg

Pn2

6

Obs Event E3B TL-1 and SEM Pulse Freq Var Atten/Const Grad

Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1900 for Area4 2100 for Area3 2300 for Area2 2500 for Area1

100

1 2 3 4

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg Pn2

6Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1650 for Area2 1750 for Area1

100

1 2

Obs Event L1A and SEM Pulse FreqVar Atten/Const Grad

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1I and SEM Pulse FreqVar Atten/Const Grad


100

1 2

PgPn

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

2

6

100

1 2

Obs Event L1J and SEM Pulse Freq Var Atten/Const Grad

Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circles)1650 for Area2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1E and SEM Pulse Freq Var Atten/Const Grad

100

1 2 3

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg Pn2

6

Obs Event L1K and SEM Pulse FreqVar Atten/Const Grad


100

1 2








Figure 8Comparison between the instantaneous pulse frequencies of the six regional events from SEM modeling with variable attenuation and a

constant upper mantle velocity gradient models and the observed instantaneous and centroid frequencies. The circles are the SEM calculations

and the crosses and squares are observed instantaneous and centroid frequencies. The Pn rms between the observed and calculated

instantaneous pulse frequencies (Pn-rms) are also shown, along with the rms between the observed instantaneous and centroid pulse

frequencies (IC-Pn-rms) used to assess the errors in the data


constant upper-mantle velocity gradient case in terms

of rms misfits. This suggests a trade-off in model

parameters between upper-mantle attenuation and

upper mantle velocity gradients using the Pn ampli-

tude and pulse frequency attributes for this distance

range, and these trade-offs were further explored

using the modeling of synthetic data by BAKIR and

NOWACK (2011). However, the travel time data fit

slightly better for the model with a constant upper

mantle velocity gradient.

3. Discussion and Conclusions

Pn wave attributes including travel times, enve-

lope amplitudes, and instantaneous frequencies, have

been modeled using regional earthquakes recorded by

the Hi-CLIMB array in Tibet. Three 2D P-wave

velocity slices with a constant upper-mantle velocity

gradient were initially used for the SEM modeling of

the seismic attributes. A 2D P-wave velocity slice for

the south event L1E (West Line), another 2D P-wave

200 300 400 500 600 700

0

5

10Observed (Cross)SEM Synthetics Constant Gradient (Circle)SEM Synthetics Variable Gradient (Triangle)

T-x

/8 (

sec)

Distance (km)

-5

Event L1E Travel TimesConstant/Variable Gradient

100 200 300 400 500 600 700

0

5


T-x

/8 (

sec)

Distance (km)

-5

Event L1A Travel TimesConstant/Variable Gradient

100

200 300 400 500 600 700

0

5


T-x

/8 (

sec)

Distance (km)

-5

Event L1I Travel TimesConstant/Variable Gradient

100 200 300 400 500 600 700

0

5


T-x

/8 (

sec)

-5

Event L1J Travel TimesConstant/Variable Gradient

100

200 300 400 500 600 700

0

5


T-x

/8 (

sec)

Distance (km)

-5

Event L1K Travel TimesConstant/Variable Gradient

100 200 300 400 500 600 700

0

5


T-x

/8 (

sec)

Distance (km)

-5

Event E3B Travel TimesConstant/Variable Gradient

100

Distance (km)

average pick erroraverage pick error



Pn-rms=.24

Pg

Pg

Pg

Pg

Pg

Pg

Pn

Pn

Pn

Pn

Pn

Pn

Pn-rms=.19



Pn-Vgrad-rms=.38 Pn-Vgrad-rms=.25



Figure 9Comparison between the calculated travel times of the six regional events from SEM modeling compared and the observed travel times.

Circles are the constant velocity gradient SEM calculated travel times, triangles are the variable velocity gradient SEM calculated travel

times, and crosses are observed travel times. The average pick errors from GRIFFIN et al. (2011), the Pn rms residuals (Pn-rms) for the constant

upper mantle velocity case, and the Pn rms residuals (Pn-Vgrad-rms) for the variable upper mantle velocity case are also shown


velocity slice for the south cluster events L1A, L1I,

L1J, and L1K (Central Line), and a third slice for the

north event E3B (East Line) were obtained from the

3D velocity model derived by GRIFFIN et al. (2011).

The 2D velocity slices were then modified to have a

variable upper-mantle velocity gradient.

First, SEM calculations without attenuation were

performed for each event with a constant upper

mantle velocity gradient structure. The calculated

P-wave travel times match well the observed data.

However, the Pn amplitude and the instantaneous

pulse frequency attributes do not show a good fit with

the observed data, and for the velocity slices derived

from the 3D model of GRIFFIN et al. (2011) upper-

mantle attenuation is required.

Models with laterally varying upper-mantle

attenuation are then used in the SEM calculations.

Comparing the rms misfits, the Pn amplitude and

pulse frequency attributes fit the observed data much

better than for the elastic case. For a further

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1A Envelope AmplitudesConst Atten/Var Grad

Pn

0 1 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1I Envelope AmplitudesConst Atten/Var Grad

Pn

0 1 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1J Envelope AmplitudesConst Atten/Var Grad

Pn

0 1 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1K Envelope AmplitudesConst Atten/Var Grad

Pn

0 1 2

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event E3B TL-1 Envelope AmplitudesConst Atten/Var Grad

Pn

01 2 3 4

100 200 300 400 500 600 700

1

2

3

4

5


Distance (km)

Pg

Event L1E Envelope AmplitudesConst Atten/Var Grad

Pn

0 1 2 3

Log

10(a

mpl

itude

)

Log

10(a

mpl

itude

)L

og10

(am

plitu

de)

Log

10(a

mpl

itude

)L

og10

(am

plitu

de)

Log

10(a

mpl

itude

)




Figure 10Comparison between the peak envelope amplitudes of the six regional events from SEM modeling with constant attenuation and a variable

upper mantle velocity gradient models and the observed envelope amplitudes. The circles are the SEM calculations and the crosses are

observed envelope amplitudes. The Pn rms of the log-amplitudes (Pn-rms) for each event is also shown


investigation of the Pn amplitude and pulse fre-

quency attributes of these events, a variable P-wave

upper mantle velocity gradient is then constructed,

but with a constant upper-mantle attenuation struc-

ture. Although the travel times for the variable upper

mantle gradient case deviate from the observed travel

times at larger offsets, they are still comparable to the

data errors. Nonetheless, the fit to the travel times for

the case with a constant upper mantle gradient is

somewhat better and within the data errors. However,

the rms misfits for the Pn amplitude and pulse fre-

quency attributes are found to be similar to those for

the constant upper mantle velocity gradient and var-

iable attenuation SEM results. Nonetheless, from the

3D velocity model of GRIFFIN et al. (2011) the upper

mantle P-wave velocities of the Qiangtang terrane are

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1A and SEM Pulse FreqConst Atten/Var Grad


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1I and SEM Pulse FreqConst Atten/Var Grad


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1J and SEM Pulse FreqConst Atten/Var Grad


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1K and SEM Pulse FreqConst Atten/Var Grad


100

1 2

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

Pg

Pn2

6

Obs Event E3B TL-1 and SEM Pulse FreqConst Atten/Var Grad

Observed Inst Freq (Cross)Observed Centroid Freq (Square)SEM Inst Freq (Circle)1900 for Area4 2100 for Area3 2300 for Area2 2500 for Area1

100

1 2 3 4

200 300 400 500 600 7000

1

3

4

5

Freq

uenc

y (H

z)

Distance (km)

PgPn2

6

Obs Event L1E and SEM Pulse Freq Const Atten/Var Grad

100

1 2 3








Figure 11Comparison between the instantaneous pulse frequencies of the six regional events from SEM modeling with constant attenuation and a

variable upper mantle velocity gradient model and the observed instantaneous and centroid frequencies. The circles are the SEM calculations

and the crosses and squares are observed instantaneous and centroid frequencies. The Pn rms between the observed and calculated

instantaneous pulse frequencies (Pn-rms) are also shown, along with the rms between the observed instantaneous and centroid pulse

frequencies (IC-Pn-rms) used to assess the errors in the data


lower than those of the Lhasa terrane, and this is

consistent with earlier studies of Pn propagation in

Tibet (e.g. LIANG et al., 2004; LIANG and SONG 2006;

SUN and TOKSOZ, 2006; and HEARN et al., 2004).

TILMANN et al. (2003) suggested this could be due to a

warmer mantle in north central Tibet.

The SEM modeling of the Pn amplitude and pulse

frequency attributes for the constant upper mantle

velocity gradient model derived from GRIFFIN et al.

(2011) suggests a variable upper mantle structure in

which the Qiangtang terrane has higher Pn attenua-

tion than the Lhasa terrane. A variable upper mantle

attenuation structure with lower Qp values to the

north is consistent with the inefficient Sn propagation

at higher frequencies beneath northern Tibet as

inferred by BARAZANGI and NI (1982), NI and

BARAZANGI (1983), MCNAMARA et al. (1995), RAPINE

et al. (1997), and BARRON and PRIESTLEY (2009). This

is also consistent with the Pn attenuation values from

XIE (2007) who found a Q0 of 183 ± 33 at 1 Hz and

g values of 0.3 ± 0.1 for paths that heavily sample

northern Tibet, and a Q0 between 250 and 270 and gvalues between 0.0 and 0.1 for paths that sample a

mixture of northern and southern Tibet. The attenu-

ation values found in this study are comparable, but

represent the individual Lhasa and Qiangtang terranes

and also are for the region of Tibet beneath the

Hi-CLIMB array about 5� in longitude to the west of

the paths studied by XIE (2007). Although the upper

mantle Qp values here are frequency independent, the

Pn pulse frequencies investigated had a limited fre-

quency range between about 1 and 2 Hz.

The variable upper mantle velocity gradient SEM

models with a constant attenuation structure in the

upper mantle can also provide a good fit to the

observed Pn amplitude and pulse frequency attributes.

The existence of such an upper mantle structure with

laterally varying velocity gradient in the region,

however, is in contrast with the first model investi-

gated here with a constant upper mantle velocity

gradient, and also in contrast with that of XIE (2007)

who assumed a simplified geometric spreading model

of D-1.3 beneath all of Tibet. Also, the model with a

constant upper mantle gradient fits the travel time data

somewhat better than the alternate model.

Possible explanations for lower Pn amplitudes, as

well as inefficient Sn propagation, beneath northern

Tibet might be widespread basaltic volcanism (NI and

BARAZANGI, 1983), a partial melting process (RAPINE

et al., 1997), a warmer mantle to the north (TILMANN

et al., 2003), and a weak or missing lithosphere

(BARRON and PRIESTLEY 2009). Temperature varia-

tions between central and northern Tibetan

lithosphere might be explained by a squeezed and

sheared upper mantle (HUANG et al., 2000), a steep

overturn of a down going slab (TILMANN et al., 2003),

or a detachment of the mantle lithosphere (CHEN and

TSENG, 2007; CHEN et al., 2010). The upper mantle of

the Qiangtang terrane can be distinguished from the

Lhasa terrane to the south by the shallow Moho,

lower Pn velocities, and lower Pn amplitudes, sug-

gesting hotter temperatures in the upper mantle

beneath the Qiangtang terrane as compared to the

Lhasa terrane.

3.1. Data and Resources

The spectral element method (SEM) was per-

formed using the SPECFEM2D code by KOMATITSCH

and VILOTTE (1998) and KOMATITSCH et al., (2005).

Acknowledgments

This work was supported by the U.S. National

Science Foundation grants EAR06-35611 (R.L.N.),

and the Air Force Research Laboratory contract

FA8718-08-C-002 (R.L.N and A.C.B.). We thank the

Editor and Reviewer for their constructive comments.

Appendix 1: Specification of the Moment Tensor

for 2D SEM Modeling

From AKI and RICHARDS (2002), Eqs. 4.96, 4.97),

the ray-theoretical far-field P-wave radiation pattern

incorporates the moment tensor using a term

c p_Mp q ð t � TP Þ cq where there are implied sums

over p and q from 1, 3, cp is the unit take-off vector at

the source, _Mp q ðtÞ is the time derivative of the

moment tensor which for a step source-time function

would be the moment tensor elements times a delta

function, and TP is the travel time. If the coordinate

system is specified with x1 - x3 in the plane of


incidence of the ray from the source, then the radiation

pattern term incorporating the moment tensor for the

far-field P-waves can be written in terms of a reduced

2 9 2 moment tensor as ð c1 c3 ÞM11 M13

M31 M33

� �

c1

c3

� �d ð t � TPÞ: The elements of this 2 9 2

moment tensor represent the strengths of the couples

and dipoles in the plane of incidence of the ray from

the source. We first make several 2D azimuthal

slices of the 3D moment tensor shown in Fig. 12a

along specific azimuths to the receivers and rotate

the moment tensor in the azimuthal directions noted

by the dashed lines. The dashed circles denote the

approximate take-off angles at the source for the Pg

and Pn waves from ray tracing using the velocity

model from GRIFFIN et al. (2011). The radiation

patterns determined from the reduced moment ten-

sor terms given above rotated along the different

azimuths are shown in Fig. 12b, and the radiation

pattern terms are consistent with the overall focal

mechanism pattern given in Fig. 12a.

We next compare the calculated P-wave ampli-

tudes for several azimuths using the reduced moment

tensor slices derived from the 3D moment tensor and

input into the 2D SEM modeling. It was found that to

be consistent with the amplitudes for the Pg and Pn

arrivals with take-off angles given by the dashed

circles given in Fig. 12a, and additional factor of

p - az in the azimuth was required for specification

in the 2D SEM code and related to how the moment

tensor is specified. The Pn and Pg amplitudes com-

puted from the SEM modeling are then shown in

Fig. 12c for several azimuths with respect to the 3D

focal mechanism in Fig. 12a. For these cases, the

SEM amplitudes of the Pg and Pn are consistent with

the radiation patterns of the 3D moment tensor.

Appendix 2: Calculation of Seismic Attributes

The amplitude and pulse frequency attributes are

obtained following the approach of MATHENEY and

Radiation Patterns for Different Azimuthal Angles

Take-off Angle (Degree)

Am

plitu

des 0.0

0.5

-0.5

-1.00 20 8040 60

Azimuthal Angles

30

210

60

240

90270

120

300

150

330

180

0E3B TL−1

2500

1900

2100

2300

Pg

Pn

100 200 300 400 500 600 7000

1

2

3

4

5

6

190 Degree210 Degree230 Degree250 Degree

SEM Envelope Amplitudes for Different Angles

Distance (km)

log1

0(E

nvel

ope

Am

plitu

de)

(a)

(b)

(c)

Pg

Pn

Azimuthal Angles190 Degree210 Degree230 Degree250 Degree

Figure 12a Shows the 3D focal mechanism for a particular double-couple moment tensor source with different azimuth angles given by dashed lines.

The regional Pg and Pn take-off angles in Tibet are shown by small circles. b Shows the amplitudes on the focal sphere from the reduced

moment tensor for the azimuthal angles given in a as a function of take-off angle derived from the 3D moment tensor. c Shows the SEM

envelope amplitudes for the azimuthal angles given in a. Different symbols represent different azimuthal angles


NOWACK (1995). The data is first bandpass filtered

with a 0.5 to 5 Hz six-pole Butterworth filter to

reduce low and high frequency noise. The Hilbert

envelope is computed from the analytic signal

(BRACEWELL, 2000). An assessment is first made if the

Hilbert envelope sufficiently drops after the first

arrival P-wave pulse. If so, a cosine tapered box-car

is chosen to window out the first arriving P-wave

pulse. If not, then it is assumed that the P-wave pulse

is contaminated with later arrivals and the trace is not

included for further processing. Once the P-wave

pulse is windowed, the peak of the Hilbert envelope

is used to measure the pulse amplitude. Following

MATHENEY and NOWACK (1995) the instantaneous

frequency at the peak amplitude is utilized to estimate

the pulse frequency. Damping and weighting are used

to stabilize the instantaneous pulse frequencies,

although this is less important at the peak of the signal

envelope. In addition to the instantaneous pulse fre-

quency estimates, the centroid of the power spectrum

weighted by the squared envelope of the windowed

pulse is used to estimate the pulse frequency. For

noise free data, the averaged instantaneous pulse fre-

quency will approach that of the weighted centroid

pulse frequency (MATHENEY and NOWACK, 1995;

NOWACK and STACY, 2002), and the consistency of the

two values can be used to assess the reliability of the

observed pulse frequencies.

An example of a data trace from the Hi-CLIMB

array, and a windowed trace and envelope amplitude,

are shown in Fig. 13. For the pulse, the amplitude is

taken at the peak of the Hilbert envelope. The

instantaneous pulse frequency is also obtained at the

time where the peak of Hilbert envelope occurs. The

centroid pulse frequency is obtained from the

weighted centroid of the power spectrum of the

windowed seismic pulse. If the envelope does not

sufficiently drop after the first arriving pulse of the

original data, the trace is not included for further

amplitude and frequency analysis (MATHENEY and

NOWACK, 1995). Also, if the estimated instantaneous

and centroid frequencies are not consistent with each

other or with adjacent trace values with distance, then

the pulse is assumed to be contaminated with later

arrivals and not included for further processing. Both

the observed and the calculated data are analyzed

using the same processing steps, although only the

instantaneous frequency values are shown for the

calculated data.

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(Received December 1, 2011, revised March 12, 2012, accepted March 14, 2012)


http://dx.doi.org/10.1785/0120100229

http://dx.doi.org/10.1126/science.1083780

http://earthquake.usgs.gov/earthquakes/eqarchives/epic/database.php

http://earthquake.usgs.gov/earthquakes/eqarchives/epic/database.php

Velocity and Attenuation Structure of the Tibetan...

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