4

Disaster Advances Vol. 7 (9) September 2014

28

Earthquake Source Parameters and their Scaling for the Uttarakhand region of North-West Himalaya

Kumar Arjun1*

, Kumar Ashwani2, Gupta S. C.

2, Mittal Himanshu

3 and Sen Arup

2

1. Department of Civil Engineering, Arni University, Kathgarh (Indora), Himachal Pradesh, INDIA

2. Department of Earthquake Engineering, Indian Institute of Technology, Roorkee-247667, Uttarakhand, INDIA

3. Centre of Excellence in Disaster Mitigation and Management, Indian Institute of Technology, Roorkee-247667, Uttarakhand, INDIA

*[email protected]

Abstract The Uttarakhand region of North-West Himalaya

exhibits high historic and instrumental seismicity as

evidenced by the occurrence of several moderate to

large-sized earthquakes in the region. Source

parameters of 16 small and moderate-sized

earthquakes (3.1Mw6.7) have been estimated and a scaling law for the region has been developed. In this

study software EQK_SRC_PARA33 has been used that

considers Brunes model that yields a fall-off of 2 beyond corner frequencyfc with high frequency

dimunition factor presented by Boore7 to estimate fmax.

The estimated seismic moments range from 5.11013

to 1.101019Nm. The source radii are confined

between 200 m to 9.7km, the stress drop ranges

between 2.59MPa to 8.34MPa respectively.

Using the data set of 16 events, a scaling law, M0 fc3 =

3.0 x 1016 Nm/s3 has been developed for the region.

From the plot between seismic moment and fmax, the

values of fmax seem to be dependent on the source size

and vary from 4 Hz to 18 Hz at various sites falling in

the Uttarakhand region of NW Himalaya. Comparing

the average stress drop of 6.0 MPa obtained in the

present study, with the global average of 3.0 MPa for

inter-plate earthquakes, it can be inferred that the

average stress drop associated with the small and

moderate earthquakes is almost double in the

Uttarakhand region of NW Himalaya. The estimates

of stress drops, fmax and scaling law will help to

simulate strong ground motion using stochastic

methods for this region.

Keywords: Source parameters, fmax, Scaling Relation,

Uttarakhand, NW Himalaya.

Introduction The Uttarakhand region of NW Himalaya lies

approximately between latitude 29.0 N to 31.5 N and

longitude 77.5 E to 81.0E. The mighty rivers like Ganga,

Yamuna, Bhagirathi, Bhilangna and Alakhananda traverse

through the region and possess immense potential for the

generation of hydroelectric power. In view of this many

hydropower projects have either been constructed or are

under investigation and planning stage. The region falls in

the seismic zones IV and V as per IS Code [IS 1893 (Part

1):2002]. Two most well studied moderate earthquakes of

the region are: the Uttarkashi earthquake of 1991 (mb 6.6

IMD) and the Chamoli earthquake of 1999 (mb 6.3 USGS).

Attempts have been made to study the seismotectonics of

this region using teleseismic data39

and local earthquake

data27

and to estimate the source parameters of the

earthquakes occurring in the region using local earthquake

data.31,34,45

These are essential requirements to estimate the

design earthquake parameters for the design of hydropower

projects as well as to estimate the peak ground motions for

the seismic hazard assessment. Fifteen earthquakes have

occurred in the Uttarakhand region of NW Himalaya and

one earthquake has occurred near Roorkee in the Ganga in

deep, south of MBT.

In this study both the strong motion data and weak motion

data have been analyzed using software

EQK_SRC_PARA28

to determine the source parameters

and scaling law for the region. Acceleration and

displacement spectrums of SH component of ground

motion generated by earthquakes have been analyzed

considering Brunes earthquake source model yielding a fall-off of 2 beyond corner frequency considered with high

frequency dimunition factor, a Butterworth high-cut filter

presented by Boore7 to estimatefmax.

Geology, tectonics and seismicity of the area The Uttarakhand region which forms part of the

northwestern Himalaya and lies between the rupture zones

of the two great earthquakes viz. the Kangra earthquake of

1905 and the Bihar-Nepal earthquake of 1934. The broad

geologic and tectonic framework of the study area that falls

in the Uttarakhand region of NW Himalaya Himalaya is

shown in figure 1. This figure depicts the surface trace of

the main boundary thrust (MBT), the main central thrust

(MCT), main frontal thrust (MFT) and regional tectonic

features such as South Almora Thrust (SAT), North

Almora Thrust (NAT), Moradabad fault (MT), Great

boundary fault (GBF) along with local tectonic features like

Alaknanda fault (AF), Ramgarh thrust (RT) and Martoli

thrust (MT)18

.

The seismicity of the Uttarakhand region of NW Himalaya

in particular is of interplate type. A significant feature of

seismicity of the Himalaya is that the distribution of

locations of earthquakes follows the trend of the mountain

range. Epicentres of most of the moderate-sized

earthquakes lie between the main boundary thrust (MBT)

and the main central thrust (MCT)44

and their occurrence is


29

due to reactivation of the parallel low angle detachment

thrust faults in the upper crust27

. These upper crustal faults

are possible slip surfaces of crustal shear zones facilitating

the uplift of the lesser and the Higher Himalaya.

It has been suggested that the earthquakes occur as a

consequence of the same under thrusting Himalayan

orogenic process in the entire region37

. The Uttarakhand

region of NW Himalaya exhibits high historic and

instrumental seismicity as evidenced by the occurrence of

several moderate to large sized earthquakes in this region.

Focal depths of the moderate earthquakes in the

Uttarakhand region lie between 12 km to 18 km. The

majority of earthquakes in the Uttarakhand region of NW

Himalaya also occur at shallow depths upto 20 km.16,27,32

The occurrence of two moderate earthquakes (Uttarkashi

and Chamoli earthquakes) at shallow depths caused

significant damage and destruction. The focal mechanisms

of Uttarkashi and Chamoli earthquakes show that the style

of faulting is low angle thrust faulting39,47

.

Local earthquake data sets used in the study Three types of data sets collected from the region of the

Uttarakhand region of NW Himalaya have been used in the

study. These data sets are briefly described as follows:

1. The first data set became available from the deployment of a strong motion array comprised of 50 strong motion

accelerographs (SMA-1 of Kinematrics) deployed in the

Uttarakhand region of NW Himalaya for the purpose of

measuring the strong ground motion due to moderate and

large-sized earthquakes occurring in the region10

. The

installation of the network started in the mid 1991 and

when about half of the network had been installed, the

Uttarkashi earthquake occurred and was recorded on 13

strong motion stations.

The locations of the strong motion stations are shown in the

figure 1 (red triangles). At each station the threshold level

(trigger level) to sense the ground motion was set about

0.01 g. The strong ground motion due to the Chamoli

earthquake of 1999 was also recorded at 11 strong motion

stations of this array. The analog recordings of these two

earthquakes were manually digitized using a semi

automatic digitizer and digital data was processed adopting

standard processing procedures48

. The data were converted

to a uniform sampling rate of 0.02 s and band-pass filtered

(0.170.20 Hz; 2527 Hz) using an Ormsby filter11

.

2. The second data set used in the study comprises 14

events in magnitude range (3.1Mw4.7) recorded on recently installed digital accelerographs in the Uttarakhand

region of NW Himalaya. These accelerograph installations

form part of the National Strong Motion Network of 300

strong motion stations deployed under Mission Mode project to cover seismic zones V, IV and some thickly

populated cities falling in seismic zone III28,38

. The digital

accelerographs are of GSR-18 type (Geosig, model GSR-

18) and data is acquired at a sampling rate of 200 Hz.

About 260 digital accelerographs, networked using

National Informatics Centre (NIC-net) allowed monitoring

the health of accelerographs as well as downloading of the

strong motion data at Department of Earthquake

Engineering, Indian Institute of Technology Roorkee. The

locations of 37 strong motion stations installed in the

Uttarakhand region of NW Himalaya are shown in figure 1

(magenta triangles).

3. The third data set comprises above 14 earthquakes

recorded by a local seismological network deployed in the

Garhwal Himalaya around Tehri region to monitor the local

seismicity. The network comprises of 12 remote stations.

Nine remote stations are radio linked to the central

receiving and recording station located at New Tehri Town

whereas the remaining three remote stations operate in an

independent mode as in figure 1 (black triangles). Each

remote station of the network houses a triaxial short-period

seismometer (Guralp: CMG 40T-1) to sense the ground

motion. The data is acquired at a sampling rate of 100 Hz.

The detailed description of the network and instrumentation

are given in report EQ: 2009-33.

Methods The time histories are first rotated to obtained SH-

component of ground motion and then spectrum is

corrected for instrument response and frequency dependent

attenuation (110f1.02

) due to path19

. Brune model8,9

that

yields a fall-off of 2 beyond corner frequency along with

high frequency dimunition factor is represented by a

Butterworth high-cut filter7 to estimate fmax above which the

spectral amplitudes decay abruptly. The mathematical

model fitted in the observed acceleration spectrum is given

as:

(1)

Similarly for displacement spectrum we have:

(2)

Based on the above expressions, software EQK_

SRC_PARA28

has been adopted for analysis. The software

estimates the spectral parameters namely; low frequency

displacement spectral level (0), corner frequency (fc) and high-cut frequency (fmax). These spectral parameters have

been used to estimate following source parameters:

The seismic moment, M025

is estimated from the value of

0 as

(3)

where is the average density (=2,670kg/m3), is shear

wave velocity in the source zone (=3.2 km/s), is the

hypocentral distancethat accounts geometrical spreading,


30

is the average radition pattern (=0.63) and is free

surface amplification (=2).

The moment magnitude21

is given by:

(4)

The source radius and stress drop8,9

are given by:

(5)

(6)

Estimation of hypocenter parameters The P-wave and S-wave arrival time data (phase-data) of

13 earthquakes has been measured from the digital

seismograms obtained from 12 remote seismological

stations. Phase-data of these earthquakes was also

measured from the strong motion records. The P-arrival

times were measured with an accuracy of 0.01 s. These

two sets of phase-data were combined and have been used

to compute the hypocenter parameters. The velocity model

given in table 1 (EQ: 2009-33) has been adopted for

computing the hypocenter parameters. The HYPOCENTER

computer program has been employed for locating these

local earthquakes36

and computations have been carried out

adopting the earthquake analysis software SEISAN22

. The

hypocenter parameters of events along with moment

magnitudes and standard errors are listed in table 2 and

their locations are plotted in figure 1.

Estimation of source parameters The available data of sixteen earthquakes that have been

recorded in the Uttarakhand region of NW region has been

used to estimate earthquake source parameters. This data

set includes two moderate earthquakes namely the

Uttarkashi and the Chamoli earthquakes recorded by strong

motion network (SMA) and one small earthquake occurred

at Bhagwanpur near Roorkee. This small earthquake was

recorded at seismological laboratory, IIT Roorkee by both

accelerograph and seismograph. The remaining 13

earthquakes were recorded on 12 stations of seismological

network as well as on some of the strong motion stations of

mission mode network.

Source Parameters of the Uttarkashi

Earthquake The strong motion records of the Uttarkashi earthquake of

1991 obtained from 13 strong motion stations have been

analyzed to estimate the source parameters. Different

hypocenter parameters of the Uttarkashi earthquake have

been reported by various agencies. In the present study the

hypocenter parameters estimated by India Meteorological

Department (IMD) have been considered because these parameters have been computed using the phase data

obtained from 26 seismological stations operated at local

and regional distances23

and are listed in table 3.

For the purpose of computing the source parameters of the

Uttarkashi earthquake, the processed strong motion data

available in the form of digital acceleration time histories

have been used11

. From these digital time histories, the

acceleration spectra at different stations have been

computed adopting the procedures and programs described

by Kumar et al.28

In the computed acceleration spectra it

was not possible to read the corner frequencies because

these lie below 0.1 Hz for an earthquake of this size. The

values of corner frequencies are smaller than the low cut

frequency of band pass filter used during the data

processing. The low cut frequency of the filter falls in the

range from 0.17Hz to 0.20Hz11

. However in the

acceleration spectrum, the fmax can be read accurately

although the information of corner frequency has been lost

due to filtering. In view of this the corner frequency fc8 is

approximated by the relation given below:

fc = 4.9 x 106 (/Mo)

1/3 (7)

Following Glassmoyer and Borcherdt,17

the value of fc is

related to the low frequency constant spectral levels of

displacement and high frequency constant acceleration

level is given below:

(8)

In the above expression the value of is the average

acceleration amplitude for higher frequencies above fc.

In present study, is approximated from the

average acceleration amplitude at intermediate frequencies

between fc and fmax (as data below fc is not present in the spectrum so all values below fmax are considered). The fmax

is obtained as the value of frequency at which jounce or

snap [S(f)=2A (f)], double differential of acceleration, has

peak amplitude. However, it can be picked up visually from

acceleration spectrum also.

This leads to the approximation of thorough the value

of accelerations constant spectral level as:

(9)

The value of corner frequency has been iteratively changed

to allow a best fit of the adopted Brunes model8,9

in the

acceleration spectra. For the Uttarkashi earthquakes, figure

2 show examples of the SH components of the rotated

acceleration time histories along with the acceleration and

the displacement spectra and the fitted Brunes model at Purola stations.

The computed values of spectral and source parameters for the Uttarkashi earthquake at different stations are listed in

table 4. The seismic moment for Uttarkashi earthquake is

(1.10.22) x1019

Nm. The estimated stress drop is about


31

5.260.59 MPa, radius of fault is 9.70.2 km. The fmax

values vary from 4.0 to 12.5 with average value around 7

Hz. Global CMT catalog reported seismic moment

1.77x1019

Nm and the corner frequency derived from this

and observed AIFL is 0.09 Hz and stress drop of 4.05 MPa.

Source Parameters of the Chamoli Earthquake The Chamoli earthquake of 1999 occurred in Garhwal

Himalaya region that lies in the Lesser Himalaya between

Munsiari and North Almora Thrust and was recorded on 11

strong motion stations. The hypocenter parameters as

reported by USGS, India Meteorological Department

(IMD) and Department of Earthquake Engineering (DEQ)

are given in table 5.

The estimation of source parameters of the Chamoli

earthquake has been carried out using the processed strong

motion data in the form of digital acceleration time

histories available in the Department of Earthquake

Engineering, IIT Roorkee. For the Chamoli earthquake, the

corner frequencies at different stations are smaller than the

low cut frequencies 0.17Hz to 0.20Hz11

of band pass filter

applied during the data processing. In view of this, the

same procedure of analysis as adopted for the Uttarkashi

earthquake has been applied for the Chamoli earthquake.

For the Chamoli earthquake, typical example of SH

components of the rotated acceleration time histories

obtained at Gopeshwar station along with their acceleration

and displacement spectra and the fitted Brunes model are displayed in figure 3.

Computed values of source parameters of the Chamolii

earthquake at different stations are listed in table 6. The

seismic moment for Chamoli earthquake is (4.70.72) x

1018

Nm. The estimated stress drop about 5.320.696 MPa,

radius of fault is 7.30.11 km. The fmax for Chamoli earthquake varies from 2.2 Hz to 10 Hz with average value

around 5 Hz. Global CMT catalog reported seismic

moment 7.77x1018

Nm and the corner frequency derived

from this and observed AIFL is 0.107 Hz and stress drop of

2.46 MPa.

Source Parameters of Local Events Thirteen local events that occurred in the Uttarakhand

region of NW Himalaya have been selected for the purpose

of estimating their source parameters. These events were

selected because they were recorded by both the 12-station

telemetry network deployed in the Uttarakhand region of

NW Himalaya as well as on some of the strong motion

stations deployed in the Uttarakhand region of NW

Himalaya. The digital data obtained from these two types

of instruments has been used to compute the source

parameters. The magnitudes of these events fall in the

range from 3.1 to 4.7. The locations of these events as

listed in table 3 are plotted on the map in figure 1.

Epicentres of 6 events are located in the Lesser Himalaya

between the MBT and the MCT and the epicentres of 7

events fall either to the north of the MCT or coincide with

the trace of the MCT. The remaining one event is located in

the Ganga in deep to the south of the MBT. This event was

recorded on both the digital seismograph and digital strong

motion accelerograph placed in Seismological Observatory

of the IIT Roorkee. This event has occurred very close to

the Roorkee at an epicentral distance of about 8 km and at a

depth of about 20 km.

Some of the typical examples of the SH components of the

rotated time series of the local earthquakes (at Srikot,

Chamoli and Vinakkhal stations) along with their

acceleration spectra, displacement spectra and fitted

Brunes model are shown in figures 4, 5 and 6.

The spectral parameters and source parameters of the 14

local events are listed in table 7. The source parameters

include seismic moment, stress drop and source radius. The

values of these source parameters range from 4.001013

Nm

to 1.061018

Nm for seismic moments, 2.59 MPa to 8.34

MPa for stress drops, 200.0 m to 894.3 m for source radii.

The average values of fmax for these events vary from 6.8 Hz to 13.3 Hz. These estimated values of source parameters

by and large agrees to the reported values of source

parameters of events in the magnitude range from 3 to 5.1

Results and Discussion The estimated source parameters of the Uttarkashi and the

Chamoli earthquakes along with the source parameters of

14 local events are listed in table 7. Plots have been

prepared to study the relationships among these parameters

and are described below:

Stress Drop: The results listed in table 7 demonstrate that

the stress drops of 16 small to moderate sized earthquakes

(3.1


32

Table 2

Hypocenters parameters and magnitudes of located earthquakes

S.N. Origion

Time

( GMT)

Lat. o N

Long. o E

Depth

km

Mw RMS

(s)

ERH

(km)

ERZ

(km)

No.

of

Stations

1 20051214

07:09:48.0

30.90 79.300 25.7 4.7 0.6 4.5 8.5 13

2 20070722

23:02:12.0

31.200 78.200 12.9 4.4 0.3 3.0 1.7 9

3 20080322

14:36:44.1

29.578 80.207 16.7 3.1 0.2 9.1 2.8 11

4 20080819

10:54:26.8

29.981 80.099 17.3 4.3 0.4 3.6 2.4 16

5 20080904

12:53:21.0

30.100 80.400 39.3 4.6 0.4 4.3 13.2 19

6 20090111

10:33:42.7

29.851 80.211 16.0 3.7 0.5 7.0 9.7 10

7 20090214

16:27:49.1

29.969 80.212 12.0 3.6 0.5 7.3 7.4 12

8 20090225

4:04:21.0

30.600 79.300 39.3 3.6 0.3 3.6 3.0 12

9 20090318

11:22:42.2

30.907 78.267 16.8 3.6 0.2 4.7 2.2 13

10 20090515

18:39:21.8

30.495 79.302 18.9 4.1 0.5 6.1 3.7 13

11 20090827

16:54:15.0

30.010 79.976 15.0 3.9 0.3 2.3 2.6 13

12 20090921

09:43:52.3

30.837 78.984 53.3 4.7 0.2 2.1 2.5 24

13 20091003

05:20:56.2

29.911 79.713 25.5 4.3 0.8 16.1 8.8 13

Table 3

Hypocenter parameters of the Uttarkashi Earthquake of Oct. 20, 199123

Origion Time 21:23:16.45 GMT on Oct. 19, 1991

Latitude 30.75 N

Longitude 78.86 E

Depth of focus 12 km.

Magnitude 6.6 mb (IMD) 6.8 Mw (GCMT)


33

Table 4

Spectral parameters and source parameters of the Uttarkashi earthquake

STN fc (Hz) fmax

(Hz)

M0(Nm) Mw R (km) (MPa)

Almora 0.114 4.5 9.944x1018

6.7 9.8 4.63

Barkot 0.116 7.0 1.208x1019

6.7 9.6 5.92

Bhatwari 0.113 4.0 1.023x1019

6.7 9.9 4.64

Ghansiali 0.118 5.5 9.203x1018

6.6 9.5 4.75

Karnprayag 0.119 7.5 1.074x1019

6.7 9.4 5.68

Kosani 0.111 7.5 1.241x1019

6.7 10.1 5.33

Koteshwar 0.118 8.9 1.257x1019

6.7 9.5 6.49

Koti 0.114 5.0 1.149x1019

6.7 9.8 5.35

Purola 0.115 5.3 1.271x1019

6.7 9.7 6.07

Rudraprayag 0.115 9.5 8.254x1018

6.6 9.7 3.94

Srinagar 0.112 12.5 1.028x1019

6.7 10.0 4.54

Tehri 0.112 7.5 1.289x1019

6.7 10.0 5.69

Uttarkashi 0.117 5.5 1.071x1019

6.7 9.5 5.39

Average 0.115

0.003

6.9

2.4

(1.10.122)

x1019

6.7

0.03

9.7

0.2

5.26

0.59

Table 5

Hypocenter parameters of the Chamoli Earthquake of March 28, 1999

Source Date Origion Time

(GMT)

Latitude

Longitude

Depth

(km)

Magnitude

USGS 28-03-1999 19:05:12.00 3049.20N 7928.80E 15.0 6.3 mb

IMD 28-03-1999 19:05:10.00 3017.82N 7933.84E 21.0 6.8 mb

DEQ 28-03-1999 19:05:11.25 3026.00N 7928.00E 18.0 6.7 mb

Mw=6.5 Global CMT catalog. Table 6

Spectral parameters and source parameters of Chamoli earthquake

Station fc (Hz)

fmax (Hz)

M0

(Nm)

Mw R

(km)

(MPa)

Gopeshwar 0.161 2.2 4.7x1018

6.4 7.4 5.03

Joshimath 0.165 2.7 4.3x1018

6.4 7.2 5.04

Ukhimath 0.168 4 3.2x1018

6.3 7.1 3.96

Ghansiali 0.162 7.5 5.7x1018

6.5 7.4 6.29

Almora 0.166 5 4.4x1018

6.4 7.2 5.21

Tehri 0.162 4.2 5.3x1018

6.5 7.4 5.85

Lansdowne 0.164 10 5.4x1018

6.5 7.3 6.22

Uttarkashi 0.163 5.8 5.2x1018

6.5 7.3 5.81

Chinaylisaur 0.162 5.6 4.6x1018

6.4 7.4 5.04

Barkot 0.168 7.5 4.4x1018

6.4 7.1 5.39

Roorkee 0.165 4.5 4.0x1018

6.4 7.2 4.65

Average 0.164

0.0024

5.4

2.3

(4.70.72)

x1018

6.4

0.05

7.3

0.11

5.32

0.696


34

Table 7

Source Parameters of earthquakes in Uttarakhand region of NW Himalaya

S.N. Date fc

(Hz)

fmax (Hz)

Moment

(M0)

Nm

Magnitude

(Mw)

Stress Drop

(MPa) Radius

R (m)

1 19911020 0.115 6.9 1.101019

6.7 5.26 9700

2 19990329 0.164 5.4 4.70 1018

6.4 5.32 7300

3 20051214 1.3 6.8 1.06 1018

4.7 6.35 894.3

4 20070609 4.6 8.5 1.201014

3.4 2.59 300.0

5 20070722 1.8 8.0 3.731015

4.4 4.30 723.9

6 20080322 8.0 10.0 4.001013

3.1 4.40 200.0

7 20080819 5.0 11.0 2.60 1015

4.3 7.32 250.7

8 20080904 1.6 8.3 8.471015

4.6 8.34 757.2

9 20090111 3.6 7.1 4.241014

3.7 4.30 348.6

10 20090214 4.4 11.1 2.851014

3.6 6.36 270.6

11 20090225 5.2 13.3 2.251014

3.6 8.31 227.9

12 20090318 3.8 10.4 1.61 1014

3.6 2.80 295.1

13 20090515 2.4 11.5 1.471015

4.1 5.23 498.3

14 20090827 3.5 10.2 6.841014

3.9 7.69 339.0

15 20090921 1.4 9.1 1.071016

4.7 7.63 850.0

16 20091003 2.2 8.2 2.911015

4.3 8.28 536.1

Figure 1: Map showing the locations of the instruments of three networks and epicenters of earthquakes (blue circles)

with magnitudes (Mw-this study). Tectonic features like main boundary thrust (MBT), the main central thrust

(MCT), main frontal thrust (MFT) and regional tectonic features such as South Almora Thrust (SAT), North Almora

Thrust (NAT), Moradabad fault (MT), great boundary fault (GBF) along with local tectonic features like Alaknanda

fault (AF), Ramgarh thrust (RT) and Martoli thrust (MT) are also shown18

. Black boundary refers to Uttarakhand

region of NW Himalaya.


35

Figure 2: An example of SH component of acceleration time history of Uttarkashi earthquake recorded at

Purola station, the acceleration and displament spectra along with fitted source model.

Figure 3: SH component of acceleration time history of Chamoli earthquake recorded at

Gopeshwar station, acceleration and displament spectra along with the fitted Brunes model also shown.


36

Figure 4: An example of SH-component of velocity time history of 18/03/2009 earthquake

recorded at Srikot station of Tehri network, the acceleration and displament spectra along with

fitted source model.

Figure 5: An example of SH-component acceleration time history of 15/05/2009 earthquake

recorded at Chamoli station, acceleration and displament spectra along with the fitted Brunes model.


37

Figure 6: Plot showing SH-component of velocity time history of 21/09/2009 earthquake recorded at Vinakkhal

station of Tehri network, acceleration and displament spectra along with the fitted Brunes model.

Figure 7: Plot of seismic moment versus source radius for the 16 events. The lines corresponding to static stress drops

of 1.0 MPa, 6.0 MPa and 10.0 MPa are also shown in the plot.


38

In the present study, the estimated values of stressdrops for

the Uttarkashi and the Chamoli earthquakes are 5.26 MPa

and 5.32 MPa which by and large agree with the values of

5.3 MPa and 6.5 MPa estimated from strong motion data35

.

Based on various observations of stress drops it seems that

stress drops computed using strong motion data show less

scatter as compared to those obtained from local earthquake

data. It can be inferred that for estimation of strong ground

motion for moderate sized earthquakes, the stress drop now

commonly stated as stress parameter should be around 6.0

MPa for the Uttarakhand region of NW Himalaya.

Seismic moment vs source radius: The plot in figure 7

shows the variation of source radius with seismic moment.

It is clear from the plot that for events above magnitude 4.0

the relationship between source radius and seismic moment

is linear. In this figure the lines corresponding to stress

drop of 1.0 MPa, 6.0 MPa and 10.0 MPa are also shown.

As stated above, the average stress drop for the events

occurring in the Uttarakhand region NW Himalaya is

around 6.0 MPa, however, range of the stress drop lies

between 2.6 MPaand 8.3 MPa.

Seismic moment vs corner frequency: Figure 8 depicts a

plot between the corner frequency and seismic moment. A

linear regression between these two parameters has given

the following relation:

M0 (Nm) = 3.0 x 1016

fc(Hz)-2.98

which implies M0 fc3

= 3.0 x 1016

Nm*Hz3=3.0 x 10

16

Nm/s3.

Similar scaling relations have been reported in the literature

for some of the seismically active regions. Some of the

typical relations are given below:

For Himalayan Region, India35

,

Mo fc3 =1.7 x 10

16 Nm/s

3

For Kanto Basin, Japan52

,

Mo fc3 = (2.5-3.0) x 10

16 Nm/s

3

For South-central Alaska12

,

Mo fc3 = 2.09 x 10

16 Nm/s

3

fmax: fmax is an important parameter that controls the level

of strong ground motion. There are primarily two schools

of thoughts that explain the possible reasons for the

observed fmax. First school of thought has attributed fmax to

site effects20

whereas second school of thought has ascribed

fmax to source effect.1,14,15,40-42,49,50,53

However, Aki and

Irikura3 based on several reported studies concluded that

occurrence of fmax is attributed to both site and source

effects. Earlier studies by Anderson and Hough6,

Anderson4,5

related this high-cut fall off to near surface

attenuation but a recent study by Purvance and Anderson43

supports that the high-cut fall off is primarily controlled by

source characteristics as opposed to propagation path

effects. Kumar et al29,30

inferred that fmax is controlled by

source process and seems to be independent ofepicentral

distance and focal depth.

Figure 9 is showing the variation of fmax with seismic moment, it shows decrease of fmax with increasing seismic

moment that shows its dependence to source process. A

weak dependence of fmax on source size has been reported for California earthquakes and Japanese earthquakes.

3

Figure 10 shows the variation of fmax with seismic moment for events occurring in the different ranges of magnitudes

2.

The values of fmax, obtained in the present study are plotted by magenta dots and show that the values are by and large

in agreement with those reported from various studies.

However, on close examination it appears that they follow

the linear trend obtained for San Fernando earthquakes

(1.3< M


39

Figure 8: Plot between corner frequency and seismic

moment for earthquakes occurred in the Uttarakhand

region of NW Himalaya

Figure 9: Plot between seismic moment and fmax for

earthquakes occurred in the Uttarakhand region of

NW Himalaya.

Comparing the average stress drop of 6.0 MPa obtained in

the present study, with the global average of 3.0 MPa for

inter-plate earthquakes, it can be inferred that the average

stress drop associated with the small and moderate

earthquakes is almost double in the Uttarakhand region of

NW Himalaya.The estimates of stress drops, fmax and

scaling law will help to simulate strong ground motion

using stochastic methods for the Uttarakhand region of NW

Himalayan region.

Acknowledgement The authors are thankful to Prof. M. L. Sharma and Prof.

Ashok Kumar for their support and encouragement during

this work. The authors are profusely thankful to Ministry of

Earth Sciences (MoES), Tehri Hydro Development

Corporation (THDC) and Department of Earthquake

Engineering, Indian Institute of Technology Roorkee for

funding projects under which data was collected.

Figure 10: Plot between fmax and seismic moment. The

values of fmax obtained from the Uttarakhand region of

NW Himalaya (mangenta) are overlain on results of fmax

from various studies compiled by Aki2.

References 1. Aki K., Magnitude-frequency relation for small earthquakes: A

clue to the origin of fmax of large earthquakes, J Geophys Res, 92,

1349-1355 (1987)

2. Aki K., Physical theory of earthquakes. In Bonnin J., Cara M.,

Cisternas A. and Fantechi R., eds., Sesmic Hazard in

Mediterranean Region, 3-33 (1988)

3. Aki K. and Irikura K., Characterization and mapping of

earthquake shaking for seismic zonation, Proc Fourth Int Conf

Seismic Zonation, 61110 (1991)

4. Anderson, J.G., Implication of attenuation for studies of the

earthquake source, In Earthquake Source Mechanics, Maurice

Ewing Series 6, Das S., Boatwright J. and Scholz C. H., eds.,

American Geophysical Union, Washington, D.C., 311-318 (1986)

5. Anderson J.G., A preliminary descriptive model for the

distance dependence of the spectral decay parameter in southern

California, Bull Seismol Soc Am, 81, 21862193 (1991)


40

6. Anderson J.G. and Hough S.E., A model for the shape of the

Fourier amplitude spectrum of acceleration at high frequencies,

Bull Seismol Soc Am, 74, 1969-1993 (1984)

7. Boore D.M., Stochastic simulation of high-frequency ground

motion based on seismological models of the radiated spectra,

Bull Seismol Soc Am, 73, 1865-1894 (1983)

8. Brune J.N., Tectonic stress and the spectra of seismic shear

waves from earthquake, J Geophys Res, 75, 4997-5009 (1970)

9. Brune J.N., Correction to tectonic stress and the spectra of

seismic shear waves from earthquakes, J Geophys Res, 76, 5002

(1971)

10. Chandrasekaran A.R. and Das J.D., Analysis of strong ground

motion accelerograms of Uttarkashi Earthquake of October 20.

Dep Earthquake EngUniv Roorkee India Rep., EQ, 91-10 (1991)

11. Chandrasekaran A.R. and Das J.D., Strong motion arrays in

India and characteristics of recent recorded events, Memoir Geol

Soc India, 23, 81122 (1992)

12. Dutta U., Biswas N., Martirosyan A., Papageorgiou A. and

Kinoshita S., Estimation of earthquake source parameters and site

response in Anchorage, Alaska from strong motion network data

using generalized inversion method, Phys Earth Planet Interiors,

137(1-4), 1329 (2003)

13. EQ: 2009-33, Seismological Network Around Tehri Region,

Report on Processing and Interpretation of Seismological Data

Collected from January 2008 to December 2009, Earthquake

Engineering Studies, Department of Earthquake Engineering, IIT

Roorkee (2009)

14. Faccioli E., A study of spectra and peak values of strong

motion accelerograms from Italy and Yugoslavia in terms of

gross source properties, in Earthquake Source Mechanics,

Geophysical Monograph 37, Maurice Ewing Series, vol. 6, Das

S., Boatwright J. and Scholz C. H., eds., American Geophysical

Union, Washington, D.C., 297-310 (1986)

15. Fujiwara H. and Irikura K., High-frequency seismic wave

radiation from antiplane cohesive zone model and fmax as source

effect, Bull Seism Soc Am, 81(4), 1115-1128 (1991)

16. Gaur V.K., Chander R., Sarkar I., Khattri K.N. and Sinvhal

H., Seismicity and state of stress from investigations of local

earthquakes in the Kumaun Himalaya, Tectonophysics, 1l8, 243-

251 (1985)

17. Glassmoyer G. and Borcherdt R.D., Source parameters and

effects of bandwidth and local geology on high-frequency ground

motions observed for aftershocks of the northeastern Ohio

earthquake of 31 January 1986, Bull Seismol Soc Am, 80(4), 889-

912 (1990)

18. GSI, Seismotectonic Atlas of India and its Environs, Geol

Surv India Sp Pub, Narula P.L., Acharya S. K. and Banerjee J.,

eds. (2000)

19. Gupta S.C. and Kumar A., Seismic wave attenuation

characteristics of three Indian regions: A Comparative study,

Curr Sci, 82(4), 407413 (2002)

20. Hanks T.C., fmax, Bull Seismol Soc Am, 72, 1867-1879 (1982)

21. Hanks T.C. and Kanamori H., A moment magnitude scale, J

Geophys Res, 84, 23482350 (1979)

22. Havskov J. and Ottemoller L., Seisan: The Earthquake

Analysis Software for Windows, Solaris, Linux and Macosx

Version 8.2.1 (2008)

23. IMD, Instrumental record of Uttarkashi earthquake of October

20, 1991, In Uttarkashi Earthquake, October 20, 1991 (1992)

24. Kanamori H. and Anderson D.L., Theoretical basis of some

empirical relations in seismology, Bull Seismol Soc Am, 65(5),

1073-1095 (1975)

25. Keiles-Borok V., An estimation of the displacement in an

earthquake source and of source dimensions, Ann Geofis (Rome),

12, 205-214 (1959)

26. Khattri K.N., Fractal Description of Seismicity of India and

Inferences Regarding Earthquake Hazard, Curr Sci, 69, 361366 (1995)

27. Khattri K.N., Chander R., Gaur V.K., Sarkar I. and Kumar S.,

New Seismological Results on the Tectonics of the Garhwal

Himalaya, Proc Indian Acad Sci (Earth and Planet Sci), 98(1), 91-

109 (1989)

28. Kumar A., Kumar A., Mittal H., Kumar A. and Bhardwaj R.,

Software to Estimate Earthquake Spectral and Source Parameters,

International Journal of Geosciences, 3(5), 1142-1149 (2012)

29. Kumar A., Kumar A., Gupta S.C., Mittal H. and Kumar R.,

Source Parameters and fmax in Kameng Region of Arunachal

Lesser Himalaya, Journal of Asian Earth Sciences, 70-71, 35-44

(2013a)

30. Kumar A., Kumar A., Gupta S.C., Jindal A.K. and Ghangas

V., Seismicity and Source Parameters of Local Earthquakes in

Bilaspur Region of Himachal Lesser Himalaya, Arabian Journal

of Geosciences, DOI:10.1007/s12517-013-0929-y (2013b)

31. Kumar A., Pandey A.D., Sharma M.L., Gupta S.C., Verma

A.K. and Gupta B.K., Processing and Preliminary Interpretation

of Digital Data Obtained from Digital Telemetered Seismic Array

in the Garhwal Himalaya, Xth Symposium on Earthquake

Engineering, University of Roorkee (1994)

32. Kumar A., Pandey A.D., Sharma M.L., Gupta S.C., Jindal

A.K. and Jain S.K., Contemporary local seismicity of the

Garhwal Himalaya, Proc Workshop on Earthquake Disaster

preparedness, 39-47 (1997)

33. Kumar A., Mittal H., Sachdeva R. and Kumar A., Indian

National Strong Motion Network, Seism Res Lett, 83(1), 29-36

(2012)

34. Kumar D., Sarkar I., Sriram V. and Khattri K.N., Estimation

of the source parameters of the Himalaya earthquake of October

19, 1991, average effective shear wave attenuation parameter and

local site effects from accelerograms, Tectonophysics, 407, 124 (2005)


41

35. Kumar D., Sriram V., Sarkar I. and Teotia S.S., An Estimate

of a Scaling Law of Seismic Spectrum for Earthquakes in

Himalaya, Indian Minerals, 61(3-4)&62(1-4), 83-92 (2008)

36. Lienert B.R., Berg E. and Frazer L.N., Hypocenter: An

earthquake location method using centered, scaled, and adaptively

least squares, Bull Seismol Soc Am, 76, 771783 (1986)

37. Mandal P., Padhy S., Rastogi B.K., Satyanarayana H.V.S.,

Kousalya M., Vijayraghavan R. and Srinivasan A., Aftershock

activity and frequency-dependent low coda Qc in the epicentral

region of the 1999 Chamoli earthquake of Mw 6.4, Pure, Appl

Geophys, 158(9-10), 1719-1735 (2001)

38. Mittal H., Kumar A. and Ramhmachhuani R., Indian national

strong motion instrumentation network and site characterization

of its stations, International Journal of Geosciences, 3(6), 11511167 (2012)

39. Ni J. and Barazangi M., Seismotectonics of the Himalayan

Collision Geometry of the Underthrusting Indian Plate Beneath

the Himalaya, J Geo phys Res, 89, 11471163 (1984)

40. Papageorgiou A.S., On two characteristic frequencies of

acceleration spectra: patch corner frequency and fmax, Bull Seismol

Soc Am, 78, 509-529 (1988)

41. Papageorgiou A.S. and Aki K., A specific barrier model for

the quantitative description of inhomogeneous faulting and the

prediction of strong around motion. I. Description of the model,

Bull Seismol Soc Am, 73, 693-722 (1983a)

42. Papageorgiou A.S. and Aki K., A specific barrier model for

the quantitative description of inhomogeneous faulting and the

prediction of strong ground motion. I1. Application of the model,

Bull Seismol Soc Am, 73, 953-978 (1983b)

43. Purvance M.D. and Anderson J.G., A comprehensive study of

the observed spectral decay in strong-motion accelerations

recorded in Guerrero, Mexico, Bull Seism Soc Am, 93, 600611 (2003)

44. Seeber L. and Armbruster J.G., Some elements of continental

subduction along the Himalayan front, Tectonophysics, 105, 263278 (1984)

45. Sharma M.L. and Wason H.R., Occurrence of low stress drop

earthquakes in the Garwal Himalayan region, Physics of the Earth

and Plantary Interiors, 85, 265-272 (1994)

46. Sriram V. and Khattri K.N., A study of source spectrum, site

amplification functions, response spectra, Fourier spectra and

peak ground accelerations from the strong ground motion data of

the 1991 Uttarkahi earthquake, Curr Sci, 72, 728-740 (1997)

47. Thakur V.C. and Rohella R.K., Geodynamic model for

Chamoli earthquake of March 29, 1999 and seismic hazard

assessment in the Garhwal Kumaon region, Presented at the

Workshop on Chamoli earthquake and its impact, Department of

Science and Technology, India (1999)

48. Trifunac M.D. and Lee V.W., Routine Computer Processing

of Strong Motion Accelerograms, Report EERL 73-03,

Earthquake Eng. Res. Lab., Calif. Inst. of Tech., Pasadena,

California, U.S.A. (1973)

49. Tsurugi M., Tai M., Kowada A., Tatsumi Y. and Irikura K.,

Estimation of empirical site amplification effects using observed

records, 12th WCEE, 1243(1-6) (2000)

50. Tsurugi M., Kagawa T. and Irikura K., Study on high-cut

frequency characteristics of ground motions for inland crustal

earthquakes in Japan, 14th WCEE 02-0036 (2008)

51. Wason H. R. and Sharma M.L., Source parameters study of

local earthquakes in the Garhwal Himalaya based on the digital

broadband data, 12th WCEE, 1776 (1-6) (2000)

52. Watanabe K., Sato H., Kinoshita S. and Ohtake M., Source

characteristics of small to moderate earthquakes in the Kanto

region, Japan: Application of a new definition of the S-wave time

window length, Bull Seismol Soc Am, 86, 12841291 (1996)

53. Wen J. and Chen X., Variations in fmax along the ruptured fault

during the Mw 7.9Wenchaun earthquake of 12 may 2008, Bull

Seismol Soc Am, 102(3), 991-998 (2012).

(Received 10th October 2013, revised 18

th July 2014,

accepted 10th August 2014)

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