Synopses of Master Papers

Bulletin of IISEE, 47, 43-48, 2013

1-D VELOCITY MODEL FOR WAPDA TARBELAMICROSEISMIC NETWORK IN TARBELA ANDNORTHERN AREAS,
PAKISTAN
Zahid Shehzad*
MEE11608

Supervisor:

Toshiaki YOKOI**

ABSTRACTA new 1-D velocity structure model is proposed for Microseismic Monitoring system (MSMS) of
WAPDA Tarbela Dam Project in Northern Areas of Pakistan. It is expected to serve for routine highaccuracy earthquake hypocenter location and to work as a reference model for the tomographic studies.
We perform a series of simultaneous inversion of P and S waves’ velocity structure and hypocenter
location using VELEST and the arrival time data obtained in site during the period from March 2010
to March 2012.
The new velocity model has the additional boundaries at 23 km and 110 km and higher
velocities in the lower crust in comparison with the currently used velocity model and shows a better
consistency with the existing studies, although the analysis of hypocenter relocation using thesemodels show that they are equivalent in terms of the residual: its RMS and the distribution throughout
the available epicentral range from 0 km to 450 km.
The lack of the shallow events determined within 20 km from any station causes thepossibility of a hidden surface layer of low velocity that can be a subject for future study. The newmodel is expected to improve the accuracy of the routine hypocenter determination and to make itpossible to perform 3-D seismic tomography for the Northern Areas of Pakistan in the future.
Keywords: Simultaneous Inversion, P and S velocity structure, RMSs
1. INTRODUCTION
The Directorate of Seismology WAPDA, Tarbela Dam Project, is responsible for monitoring seismic
activity around the Tarbela Dam Project and the projects which are in the feasibility study phase in the
Northern Areas of Pakistan. The Directorate has been recording microseismic data around the Tarbela
Dam Project since 1973. Due to the recent renewal of the network, we have to improve the existing
velocity model for the determination of hypocenter parameters of earthquakes in and around theextended network of Northern Areas Microseismic Monitoring System (MSMS). Also the appropriate
velocity model is important for different tasks, such as reliable routine hypocenter determination,
seismic tomography of the area, moment tensor inversion etc.
In the present study, we determined a 1-D velocity model with station corrections by analyzing datafrom MSMS, WAPDA Tarbela. We used the program VELEST (Kissling 1988, 1995; Kissling et al.,1994).

*WAPDA Directorate of Seismology, Tarbela Dam Project, Pakistan**International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan2. TARBELA NETWORKThe new network consists of 19 field stations. Each field station consists of a short period seismometer(SS-1, Kinemetrics) and an accelerometer (Episensor, Kinemetrics): both of them have three components,
and a digital data logger (Q330S, Kinemetrics) synchronized with GPS timing system. Previously our areaof interest was about 160 km in radial distance from the SWM station near Tarbela Dam Project. Initially
Directorate of Seismology Tarbela, installed 10 field stations, and after two years we added 9 more station.
With this extension of the micro-seismic monitoring network to the Northern areas of Pakistan, the area ofseismic activity monitoring has increased
longer than 350 km (Figure 1). The fielddata are sent to the central recording andprocessing system based in Tarbela Dam
Project via V-SAT communication andthen processed and stored by Antelope
software (Kinemetrics).

Figure 1. Location of field stations andmajor tectonic structure in and around thestudy; Area. HSZ: Hindu Kush Seismic
Zone, KA: Kohistan Arc, MKT: Main
Karakorum Thrust, MMT: Main Mantle
Thrust, MBT: Main Boundary Thrust,
IKSZ: Indus Kohistan Seismic Zone, HKS:
Hazara Kashmir Syntax, Star denotes theepicenter of the 2005 Kashmir earthquake.

3. DATAThe Catalogue of earthquake parameters is generated with Antelope software by the Directorate of
Seismology Tarbela Dam Project. For the present study, we have data from March, 2010 to March,2012. The number of events which have been analyzed by seismologists in Tarbela, is in total about
600. We at selected 400 that have the RMS of the arrival time residuals less than 1sec (RMS<1sec) .
from the catalog made in Tarbela network. This number is reduced to 387 during the analysis due tothe constraints used below: the epicentral distance to the reference station less than 450 Km; more than
three arrival time data is available under this constraint.
For the analysis explained below we used arrival times of P and S waves at different stations
that belong to MSMS. Figure 4 shows the wadati diagram for the an event in March 31,2012 gives areference value of the ratio Vp/Vs 1.75. The mark of data are fitted well the line and shows theaccuracy of the timing system.
4. ANALYSIS
We have selected DARP station as a reference station in this study, because this is on hard rock andthen may not have a strong site effect, and have obtained the maximum number of phase arrivals.
The minimum focal depth is set to 0 km and the maximum epicentral distance is set 450Km from thereference station. We used upper most layers as negative, because the VELEST program takes sea
level as zero; however, the seismic stations are installed at maximum 2200 meters above sea level. We
prohibit introducing newly, so called low velocity layers in the simultaneous inversion of the
hypocenters and velocity structure.

4.2. Search for the Optimum Velocity Model
Figure 2 shows the task flow of the analysis. From Model-A0, two velocity models having thinner layers
are made: one have 28 layers and another 12 layers in order to check the influence of layer division, andthen the simultaneous inversion for velocity structure and hypocenter parameters is performed using
VELEST from these two models and Model-A0 itself. Among three inverted velocity models one inverted
from Model-A0 itself has the minimum average residual and then selected (Model-A1). From Beloussov etal. (1980) and Roecker (1982) weconstructed a Model-B0. From
Model-B0 two models having thinner
layers are made as shown in Figure 6
and the simultaneous inversion is
performed from them and Model-B0
itself. Among the inverted structures,
one inverted from Model-B0 shows
the minimum average residual andthen selected (Model-B1). While
fixing the other structural model
parameters of Model-A1 and ModelB1, we performed the search of the
optimal Moho depth. Namely, twoseries of inversion are performed bychanging only Moho depth from 57
Km to 70Km while the depths ofother boundaries are fixed. For bothmodels, Moho depth is determined
60 Km.
The inverted structures from ModelA1 and Model-B1 are named ModelA (5 layers) and Model-B (10 layers,consecutive layers having similar
velocities are merged), respectively.
Considering their different ways oflayer division, Model-A and ModelB are not very different each otherand it seems that the optimal model
may be found in between them. For
better accuracy of analysis, 267
events recorded at 6 or more stations
Figure 2. Task Flow to obtain the optimum1-D Velocity Model.
are selected and the simultaneous
inversion is performed again with these selected events.
The structure inverted from Model-A and Model-B are
Table 1. The AAUR values of the velocity
named Model-As and Model-Bs, respectively. Modelmodels used in this study
Bs differs from Model-As by the boundaries at depth
Model
Number of Events
AAUR
of 23 Km and 110 Km. then, we modify Model-As (5layerslayers)to 7 layer model and perform the inversion
Model-A0
5
All
0.867
again
to
obtain Model-Asm. We found these are similar
Model-B0
12
All
1.242
Model-A1
5
All
0.183
to each other. Model-Asm is merged with Model-Bs
Model-B1
10
All
0.188
into Model-E (9 layers) and the inversion gives the
Model-A
5
All
0.183
final 1-D structure Model-F (9 layers). Finally, we
Model-B
10
All
0.188
performthe inversion for hypocenter parameters fixing
Model-As
5
Selected
0.244
the velocity structure to Model-F in order to relocate
Model-Bs
10
Selected
0.439
all the events. The number of events becomes 387,
Model-Asm
7
Selected
0.374
Model-E
9
Selected
0.195
because the events that are recorded at less than 3
Model-F
9
Selected
0.249
stations are rejected from the analysis.
Model-F

9

All

0.174

5. RESULTS AND DISCUSSIONIn Figure 3, Model-A0: the current velocity model being used for hypocenter determination by WAPDA
also shown for comparison. Figure 3 shows that P-wave velocity of Model-F in the layers shallower thanMoho 60 Km is faster than those of Model-A0. The main newly added boundaries are at the depth of 23 km
and 110 km. They are roughly consistent withthe depth of Moho and the bottom of thelithosphere explored by Kumar et al. (2005).
Figure 4(a) shows the hypocenters relocatedusing Model-A0 whereas those relocated using
Model-F are shown in Figure 4(b). The
epicenter distribution shows not a large
difference between these two velocity models,
but there is a detectable difference between
their depth distributions. Figure 4(c) shows
Figure 3. Model-F (the final model) and Model-A0 that the number of earthquakes slightly
increased in the range between 0 to 100km.
(currently used one) and their Vp/Vs ratio.
Figure 5 shows the comparison of the RMS of
the arrival time residuals: blue bars show the hypocenters relocated using Model-F and red
a)

b)

c)
Figure 4. The hypocenters and frequency
distributions of the focal depths of 387 earthquakes
used in this study for the inversion of 1-D velocity
model. a) The hypocenters relocated using ModelA0, and b) The hypocenters relocated using
Model-F. c) the frequency distribution of focal
depths of the hypocenters relocated using Model-F(red bars) and Model-A0 (blue bars).

Figure 5. Comparison of the frequency distributionof RMS of the arrival time residuals: red bars
denotes those given in the original catalog of
Tarbela network without relocation, green bars
those correspond to those in Figure 4 a) (relocatedhypocenters using Model-A0) and blue bars thosecorrespond to Figure 4 b) (relocated hypocenters
using Model-F).

a)

b)

Figure 6. Comparison of hypocenters: a) those given in the original catalog of Tarbela network
calculated using Model-A0 without relocation (black dots) vs. those relocated using Model-A0 (reddots), b) those relocated using Model-A0 (red dots) vs. those relocated using Model-F (blue dots).

a)Figure 7. Tobs (= res_s + ttime) over epicentral
distance for the events of which depth is less than
10 km, where res_s denotes residuals of each phases
and ttime the theoretical travel times calculatedusing Model-F. Namely, Tobs is equal to observed
arrival time minus determined origin time.
b)

Figure 8. Average residuals of each station that can be used as the station corrections: a) for P-wave
and b) for S-wave, respectively.

bars those relocated using Model-A0, whereas green bars those correspond to the hypocenters given in
the original catalog of Tarbela network calculated using Model-A0, however, without relocation. It isclearly shown that the first two are smaller than the last one, namely, 0.234 s, 0.236 s and 0.428 s forblue, red and green bars, respectively. This result shows that Model-F is the best fit model for theobserved arrival times among these three; however, the difference between the first two is slight.
Figure 6(a) shows the comparison of the hypocenters relocated using Model-A0 and those of the initial
location given in the original catalog of Tarbela network and a significant change by the relocation
using VELEST. In contrast, Figure 6(b) the comparison of the former model-A 0 with those relocatedusing Model-F. The epicenters are almost unchanged; however, the depth becomes shallower slightly
and systematically.

6. CONCLUSION

We perform a series of simultaneous inversion of P and S waves’ velocity structure and hypocenter
location using VELEST and the arrival time data obtained by Microseismic Monitoring System(MSMS), WAPDA, Tarbela, in order to determine P and S wave 1-D velocity structures of the crustand upper mantel beneath the Northern Areas of Pakistan with a set of station corrections. The searchstarted from two velocity models: one is the currently used model by MSMS, WAPDA Tarbela(Model-A0) and another the structure of Beloussov et al. (1980) combined with Roecker (1982), andfinally converged to a structure (Model-F) through a systematic search for the optimum Moho depth.
The Moho depth is estimated to be 60 km. This is consistent with existing studies. The finallydetermined structure model (Model-F) with 9 layers shows P-wave velocity in the upper crust is fasterthan that of the model currently used for routine hypocenter determination with 5 layers (Model-A0).
The main boundaries that are newly added are at the depth of 23 km and 110 km. Model-F has higher
velocities in comparison with Model-A0 at the lower crust that lies at the depth from 23 km to 60 km.
The relocation of hypocenters using this Model-F gives the values of the station corrections. The
quantitative correlation with the local condition of each station is remained for future study as well as
its stability or dependency on dataset.
The comparison of the residuals of the relocation using Model-F with that using Model-A0
shows that these are equivalent in terms of the residuals: its RMS and its distribution over the
epicentral distance range from 0km to 450km. Model-F, however, shows a better consistency withexisting studies (e.g., Roecker, 1982; Kumar et al., 2005). Therefore, it seems worth to test more about
this newly proposed structure model using newly obtained data, especially the phases converted at
Moho and the boundaries at the depth of 23 km and 110 km, and those excited by far events andrefracted at these boundaries. It is possible that a shallow layer is hidden at the surface due to thelimitation of dataset that does not contain any event shallower than 10 km that occurred within theepicentral distance shorter than 20 km from any station. Records due to shallow events within the
network are indispensable in order to verify whether a hidden layer exists or not.
The existing Tarbela velocity model (Model-A0), as mentioned in the first data analysis report(National Engineer, 2010), of the MSMS Tarbela Dam project was verified and a new model (ModelF) is proposed. This new model is expected to improve the accuracy of the routine hypocenter
determination and to make it possible to perform 3-D seismic tomography for the Northern Areas ofPakistan in the future.
ACKNOWLEDGEMENTS
I would like to thank to Mr. Bilal Manzoor, for his support to manage good quality of data for this study.
REFERENCES

Beloussov, V. V. et al., 1980, Tectonophysics, 70, 193-221.
Kissling, E., 1995, VELEST User’s Guide, Internal Report, Institute of Geophysics, ETH, Zurich,26.
Kissling, E., 1998, Rev. Geophys., 26, 659-698.
Kissling, E. et al., 1994, J. Geophys. Res., 99, 19635-19646.
Kumar, P. et al, 2005, Geophys. Res. Lett., 32, L07305, doi:10.1029/2004GL022291
Roecker, S. W., 1982, J. Geophys. Res., 87, 945-959.
National Engineers (NE), 2010, Report on “Preliminary 1-D Velocity model for Northern Area
Seismic Network”