TRAVEL TIME RESIDUALS FROM THE NEW AND
OLD SEISMIC NETWORKS OF PAKISTAN AND
PRELIMINARY HYPOCENTER DETERMINATION
Nasir Mahmood∗
MEE07163

Supervisor: Tatsuhiko HARA ∗∗ABSTRACTThe Pakistan Meteorological Department has started establishing a new digital seismic network
in 2006. We evaluated data from this new network, and performed preliminary hypocenter
determination. First, we checked detectability of new stations to find that the detection capabilityof the Islamabad station seems better than the Peshawar and Quetta stations in new network.
Then, we calculated theoretical P and S wave travel times using USGS hypocenters. We
compared them to the observed travel times from the new and old Pakistan seismic networks toobtain travel time residuals. For station Quetta in the old analog network, the P and S readings
are less accurate than those from the new network. Taking these uncertainties into account, we
performed preliminary hypocenter determination of events in the Hindu Kush region using datawith relatively smaller residuals. We used two earth models: the one is iasp91 and the other has acrust structure from CRUST 2.0. Most of the events are located in the depth range consistent
with USGS focal depths when we used iasp91, while the distances between the epicenters issued
by the USGS and those determined using iasp91 are larger (on the order of 50 km ) than thosewhen we used the latter model (on the order of 25 km). Probably due to the small number of dataand not good station configuration to events in the Hindu Kush region, the dependence ofhypocenters on crust models is strong. We expect it will be possible to improve accuracy ofhypocenters by expanding our seismic network with carefully checking data.
Keywords: New digital seismic network, Travel time residuals, Hypocenter determination, Hindu
Kush Region.

INTRODUCTION
The seismic networks in Pakistan
An advanced earthquake monitoring system is necessary for countries such as Pakistan
surrounded by the active seismic zones. At the time of the October 8th 2005 Kashmir earthquake,there was a small analog seismic network of six stations including one digital station at Peshawar,
which were operated by the Pakistan Meteorological Department. The performance of thisnetwork was not sufficient, and a denser seismic network with modern communication andrecording systems is necessary to enhance the monitoring capability.
The Pakistan Meteorological Department has deployed a set of new broadband Guralpsensors (CMG 40T) at Islamabad, Peshawar, Quetta and Balakot in 2006. Scream 3.0 software
was installed for seismic data acquisitioning on a real time basis at each station. Seismic signals∗

Pakistan Meteorological Department, Ministry of Defense, Pakistan.
International Institute of Seismology and Earthquake Engineering, BRI, Tsukuba, Japan.

∗∗were digitized at 24 bits with a sampling rate 100 sps at each station. Both Islamabad andPeshawar stations are linked via telephone line to exchange the seismic data, while other stations
were under consideration to be linked. This new seismic network is expected to enhance theearthquake monitoring capability (although it is desirable to increase the number of stations
particularly in areas close to seismically active zones). In this study, we evaluated data from thisnew network, and performed preliminary hypocenter determination. First, we checked event
detectability of the new stations.
Event detection capability and data quality of new network
The Islamabad station CMG 40T sensor in new network has recorded relatively a larger numberof events, which suggests its better detection capability. The smallest event of magnitude M=3.3
detected at epicentral distance 120 km. An event (M=6.1) with epicentral distance of 728km in
the north direction was recorded with clear P and S phases. Detection capability of CMG 40T
sensor at Peshawar station was poorer than the Islamabad station. Some of the events in theHindu Kush region recorded at Islamabad but can not be recorded at the Peshawar station,although they are closer to Peshawar than to Islamabad. At Peshawar, another sensor, S-13, hasbeen deployed, and its detection capability is much better. Many events that are not detected by
CMG 40T sensor are recorded by this sensor. Due to unstable power condition, instrument
handling and some technical problems only a few events were found from the observed records ofCMG 40T sensor of Quetta station. The detection capability of this station is very poor comparedto the Peshawar and Islamabad stations. All of the events recorded by the Islamabad, Peshawarand Quetta stations between 2006 and 2007 are shown in Fig. 1.
Islamabad

Peshawar

QuettaFigure 1. Events detected by CMG 40T sensors at Islamabad, Peshawar and Quetta stations

However the WWSSN system at Quetta recorded more events, and we used these data in thefollowing part of this study. Similarly quality of seismic data from the Islamabad station wasfound comparatively better than those of the other stations in new network. Noise level of theobserved records of Islamabad (CMG 40T) was quite low. A local event of M=3.5 was recordedwith clear arrivals of P and S waves. For Peshawar station (CMG 40T), larger noise levelobserved and it was difficult to pick P arrivals for smaller events of magnitude M ≤ 4.0 only S
phase was clear. Similarly it was difficult to pick S phase for smaller events from the CMG 40T
records of Quetta station. A possible problem of noise levels may come from the local site
condition of the sensor for both these stations.

TRAVEL TIME RESIDUALS FROM NEW AND OLD SEISMIC NETWORKSIn this study we used data from the new network of Guralp sensors (CMG 40T) at Islamabad andPeshawar stations, S-13 sensor of Peshawar station and the old WWSSN system of Quetta station
for the period 2006-2007. The main aim was to check the accuracy of data from the observed

record of the new and old networks and investigate factors causing discrepancies in the data. We
used the SEISAN version 8.1 (Havskov and Ottmoller, 2005) software to manipulate and analyze
the digital waveform data. The digital data obtained from the new network was first converted
from Guralp (GCF) to SEISAN format. Arrival times for P and S waves were picked for all
possible events from each station. Observed travel times for P and S waves were obtained bysubtracting origin times (issued by the USGS) from observed arrival times for each station.
Theoretical travel times for P and S waves were calculated using TauP toolkit software (Crotwellet al., 1999) and iasp91 model (Kennett and Engdahl, 1991). Travel time residuals for P and S
waves as well as S-P times residuals were computed using the observed and theoretical travel
times. In this way we compared observed and theoretical travel times to check the accuracy ofdata from the new and old networks and found following results for three stations Islamabad,
Peshawar and Quetta.
Data from Islamabad station
Figure 2(a) shows a comparison between observed and theoretical P wave travel times for theIslamabad station. There was a general agreement between them, while they differ considerably
for some events. Figure 2(b) shows the comparison for events whose magnitudes are equal to or
greater than 4.5. Most of the data which show large differences are removed. This suggests that
there are many misidentifications for smaller events. We made similar comparisons for S traveltimes and S-P times and find a general agreement between observed and theoretical times.
(a)

All Events(b)

Events M≥ 4.5

Figure 2. Comparison of observed and theoretical P wave travel times for Islamabad station.

Figure 3 shows the frequency distributions both for P and S travel times and S-P times
respectively for the Islamabad station. The P travel time residuals show a peak around 2 sec andmost of the observations are within the range of 0 – 5 sec. The S travel time residuals show alarger scatter, and most of the observations are within the range of -3 – 3 sec.

(a)(b)(c)Figure 3. Frequency distributions of P travel time residuals (a), S travel time residuals (b), S-Ptime residuals (c), respectively, for Islamabad.

Since there is not a large systematic deviation from 0 both for P and S wave travel time residuals,
it is possible to use absolute travel times in the following hypocenter determination.
Data from Peshawar station
Figure 4 shows the frequency distributions of the P and S travel time residuals and S-P time
residuals for Peshawar station (CMG 40T). P wave travel time residuals show a peak around 2sec ad most of the observations are within the range of -1 – 5 sec while S wave travel time
residuals show a larger scatter and most of the observations are within the range of -5 – 4 sec. S-Ptime residuals are within the range between -5 – 2 sec. Similarly we used absolute travel times
from this station in the following hypocenter determination.

(a)

(b)

(c)

Figure 4. Frequency distributions of P travel time residuals (a), S travel time residuals (b), S-P time
residuals (c), respectively, for Peshawar.

Data from Quetta stationFigure 5 show the frequency distributions of the P and S travel time residuals and residuals of theS-P times for WWSSN of Quetta station. Both P and S wave travel time residuals show larger
scatters. S-P time residuals are comparatively better. We preferred S-P times from this station.

(a)

(b)

(c)

Figure 5. Frequency distributions of P travel time residuals (a), S travel time residuals (b), S-P time
residuals (c), respectively, for Quetta.

HYPOCENTER DETERMINATION
In installation of the new digital network, the primary emphasis was to increase the number ofstations. Seismic data analysis was mostly carried out manually including earthquake location,and sometimes manual results were not satisfactory or very poor. One of the main objectives ofthis study was to improve the techniques of hypocenter determination to obtain better results.
Since data for an event from all the three stations of the new network were not available, we useddata from the analog network as well. Arrival time data from the Islamabad, Quetta and Peshawar
stations were used for hypocenter determination.

Software
We used earthquake analysis software SEISAN 8.1 (Havskov and Ottemöller, 2005). Afterputting data into SEISAN database, we converted waveform data format from Guralp format(GCF) to SEISAN format. Mulplt command is used to plot waveform data for picking P and Sphases.
We used HYPOCENTER 3.2 program (Lienert et al., 1986) under SEISAN environment
for hypocenter determination. This program has the capabilities to locate earthquakes locally,regionally and globally. Least-squares method works behind this program, in which arrival-time
residuals are minimized to get a new set of location parameters. The procedure is repeatedthrough an iterative process till an acceptable error criterion is met. The final adjusted parameters
are then accepted as the best possible estimate of the source location.
Crustal Structure
Because there is no velocity model which has been built beneath the country, and most of theanalysis carried out manually before, it is necessary to search for a velocity model to carry out
earthquake location. We used two models: the one is iasp91 (Kennett and Engdahl, 1991) and theother is the P6 type from CRUST 2.0 (Bassin et al., 2000). This model is available at
http://mahi.ucsd.edu/Gabi/rem.dir/crust/crust2.html. Initial guess for hypocenter was set to 15 km.
Analysis and Results
We selected 12 events of the Hindu Kush region for the period 2006-2007 and performed
hypocenter determination using arrival time data for P and S waves from the new and old seismic
networks of Pakistan. As for data from the Quetta station, based on the results of travel time
residuals, we used S-P times. Figure 6 shows comparison between the hypocenters determinedusing the P6 type from CRUST2.0 and iasp91 with those determined by the USGS.

Figure 6. Comparison between the hypocenters obtained for the P6 type from CRUST2.0(left) and for iasp91 model (right) with those determined by the USGS. Larger and smaller
circles denote hypocenters of this study and the USGS, respectively.

The focal depths of the analyzed events determined by the USGS are in the range from100 to 250 km. Most of the events are located in this range when we used iasp91 model, while thedistance between the epicenters issued by the USGS and those determined in this study using
iasp91 are larger (on the order of 50 km ) than those when we used the P6 mode (on the order of25 km). Probably due to the small number of data and not good station configuration to events inthe Hindu Kush region, the dependence of hypocenters on crust models is strong. We expect itwill be possible to improve accuracy of hypocenter determined by expanding our seismic networkwith carefully checked data.

CONCLUSIONSIn this study, first, we investigated detectability of events. Our results suggest that the detection
capability of the Islamabad station seems better than those of the Peshawar and Quetta stations in
new network. Then, we calculated theoretical P and S wave travel times using hypocentral
parameters of USGS and compared them to those calculated using data from the new and old
Pakistan seismic networks to obtain the distributions of travel time residuals. For station Quetta inthe old analog network, the P and S readings are less accurate than those from the new network,
which resulted in larger travel time residuals.
Considering these uncertainties, we performed preliminary hypocenter determination ofevents in the Hindu Kush region using data with relatively smaller residuals. When we used
iasp91 as an earth model, most of the events are located in the depth range consistent with theUSGS estimates. The distance between the epicenters determined by the USGS and thosedetermined using iasp91 are larger (on the order of 50 km) than those when we used the modelfrom CRUST 2.0 (on the order of 25 km). Probably due to the small number of data and not good
station configuration to events in the Hindu Kush region, the dependence of hypocenters on crustmodels is strong.

RECOMMENDATIONS
Based on the results of this study, we recommend the followings.

・ The sites of Peshawar and Quetta stations should be examined with appropriate noise leveland further site survey while installation of new stations to enhance the quality of data.
・ Continuous evaluation of data from stations of the new network will enhance the detection
・・

capability and accuracy of hypocenter determination. Therefore, such evaluations as weredone in this study should be applied to stations which will be installed in the near future.
There is a timing problem at the Peshawar station, which should be checked and corrected.
After collecting enough data, it is desirable to construct an appropriate velocity model for
Pakistan

REFERENCES
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Havskov, J., and L. Ottemöller, 2005, Univ. Bergen, Norway.
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Lienert, B.R., E. Berg, and L.N. Frazer, 1986, Bull. Seis. Soc. Am., 76, 771-783.