Download The October 20, 2011 Mw 5.1 Talala earthquake in the stable

Nat Hazards
DOI 10.1007/s11069-012-0226-1
ORIGINAL PAPER
The October 20, 2011 Mw 5.1 Talala earthquake
in the stable continental region of India
B. K. Rastogi • Santosh Kumar • Sandeep K. Aggrawal •
Kapil Mohan • Nagabhushan Rao • N. Purnachandra Rao
Girish Ch. Kothyari
•
Received: 9 April 2012 / Accepted: 17 May 2012
Ó Springer Science+Business Media B.V. 2012
Abstract The Talala (Sasangir) area in the Saurashtra region of Gujarat, western India, is
experiencing tremors since 2001. The swarm type of earthquake activity in 2001, 2004, and
every year from 2007 onward has occurred after the monsoon and lasted 2–3 months each
time. In 2007 some 200 shocks (largest Mw 5.0) and in 2011 about 400 shocks down to M1
are well recorded with 1–2 km location error. The focal depths are about 2–10 km and
shocks are accompanied by blast-like subterranean sounds. The epicenter (21.09 N 70.45E,
focal depth: 5 km from location program, 3 km from MTS) of the October 20, 2011
mainshock occurred about 12-km WNW of Talala town or 8-km SSW of the 2007 M w 5.0
earthquake epicenter. The epicentral trends deciphered from local earthquake data indicate
two ENE trends (Narmada trend) for about 50 km length and a conjugate 15-km-long
NNW trend (Aravali trend). The focal mechanisms by moment-tensor analysis of full wave
forms of two 2007 events of Mw 4.8 and 5.0 and the 2011 event of Mw 5.1 indicate rupture
along either of the two trends. The ENE trends follow a gravity low between the gravity
highs of Girnar mounts. Seismic reflections also indicate a fault in the area named Girnar
Fault. Most of Saurashtra region including the Talala area is covered by Deccan Trap
Basalt forming plateaus and conical ridges. There is no major fault within Saurashtra
Peninsula though it is believed to have major faults along the boundaries that are nonseismic. The intensity of the October 20, 2011 Talala earthquake is estimated to be 6.5 in
MM scale while isoseismals of 6, 5, and 4 and felt distance give Mw 5.1 based on
Johnston’s 1994 empirical regressions. The source parameters of the 2011 Talala earthquake are estimated using data from 14 broadband seismograph stations. Estimated seismic
moment, moment magnitude, stress drop, corner frequency, and source radius are found to
be 1016.6 N-m, 5.1, 1.6 MPa, 1.3 Hz, and 2,300 m, respectively. The b and p values are
obtained to be low, being 0.67 and 0.71, respectively. PGA of 35 cm/sec2 is noted and the
decay rate of acceleration has been estimated from strong motion data recorded at
5 stations with epicentral distances ranging from 32 to 200 km.
B. K. Rastogi S. Kumar (&) S. K. Aggrawal K. Mohan N. Rao G. Ch. Kothyari
Institute of Seismological Research, Raisan, Gandhinagar 382 009, India
e-mail: [email protected]
N. P. Rao
National Geophysical Research Institute, Hyderabad, India
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Keywords Intraplate seismicity Seismicity of India Earthquake intensity Moment-tensor solution Joint hypocentral determination
1 Introduction
The Talala (Sasangir) area of Junagadh district of Saurashtra region of Gujarat state in
western India experienced an earthquake of magnitude Mw 5.1 (USGS mb 5.0 and IMD ML
5.0) on October 20, 2011 at 22:48 h (IST). The epicenter (21.09 N 70.45E, focal depth
5 km, MMI 6.5) has occurred about 8-km SSW of the 2007 Mw 5.0 epicenter. It may be
noted that intensity is now generally given in Arabic numerals instead of Roman numerals
like V or VI for ease in preparing computerized shake maps as given at USGS site. And it
is given in fractions also for expressing the fine gradation of intensity. An aftershock of
magnitude 4.1 that occurred about 9 h after the mainshock and another of magnitude 4.0 on
November 12 were also strongly felt up to 100 km. In the first 2 days, 73 micro earthquakes (M [ 1.5) were recorded and a total of over 300 in about 2 months. The aftershocks reduced to a low level in 15 days but continued sporadically for more than
5 months. Epicenters are following ENE-WSW trend for a length of about 40 km.
The area has experienced swarm type of earthquake activity in 2001, 2004, and every
year from 2007 after monsoons. On November 6, 2007, the Mw 5 earthquake caused damage
to about 200 adobe (kachcha) houses at Haripur, nearby Hirenvel, Chitravad, Chitrod,
Sangodra, Jalondar, and Devgam villages in an area of 13 km9 8 km. The Mw 5.0 shock
was preceded by a foreshock of Mw 4.8. During October 4–10 and Dec 10–20, 2007,
Ankolvadi area, about 20-km SE of the present activity, experienced a swarm type of
activity. Similarly, Jamnagar has experienced tremors at Khankotda, Kalavad, Lalpur, Moti
Khavdi, and Bhanvad. Surendranagar, Bhavnagar, and Porbandar districts had experienced
several earthquake sequences since 2000. Previously, a large earthquake of Mw 7.7 devastated the Bhuj area of the Kutch peninsula causing widespread damage (Gupta et al.
2001a, b). It now appears that faults in the Saurashtra region have become activated by
stress perturbation caused by the 2001 Bhuj earthquake (Rastogi et al. 2012b). The preferred
time of occurrence of these sequences is during September–November that indicates triggering effect due to rise in water table by 30 m or so after monsoons (Chopra et al. 2008a).
There is no major fault system near the epicentral zone. Two parallel ENE faults have
been indicated by trends of epicenters from 2007 onward for about 50 km length: one
through Hirenvel-Haripur, north of Talala and the other through Ankolvadi, south of Talala
both being about 20-km apart. The 2011 earthquakes have occurred at the southwestern
end of the Haripur trend. This zone corresponds to gravity low in between the two gravity
highs corresponding to the Girnar mounts. The ENE trend corresponds to Narmada trend.
2 Seismotectonics of Junagadh region
The area is covered by Deccan basalts with patches of miliolite limestone. Most of Saurashtra region is covered by Deccan Trap Basalt forming plateaus and conical ridges.
Major rock types are basalt, rhyolite, gabbro, granophyres, etc. (District Resource Map,
Junagadh District, 2002). These rocks are intruded by several basic and acidic dykes
trending NNW to NW and NNE. Laterite cover is found at the top of Deccan traps. It is
observed that trend of the major lineaments is NNW (Aravali trend) and ENE (Narmada
trend) in the epicentral region. Fracture density of the area is comparatively higher than
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Fig. 1 Seismotectonic map of Saurashtra region (Biswas 1987) and epicenters (plotted by us) during last
one hundred thirty-eight years during 1872–2010. Thick lines indicate faults identified by Geophysical
Surveys. Thin lines indicate Precambrian trends. Triangles indicate seismic stations operated since 2007
that of surrounding area. Basalt is highly fractured and jointed. The depth to basement is
1.5 km in the north that decreases in the south.
There is no major fault system near the epicentral zone. Seismotectonic Atlas of India
prepared by Geological Survey of India in 2000 traces a small N–S fault in the area. The
seismotectonic map of the area shows only a geologically old pre-Cambrian NW trend
which may not be a fault (Fig. 1). Saurashtra is bounded on all four sides by major faults.
However, only the Cambay fault in the east near Bhavnagar has shown moderate seismicity
in last 200 years. The other faults have shown small earthquake activity. The West
Cambay Fault and the North Kathiawad Fault (defining the northern boundary of Saurashtra) are over 200 km away from Talala. Two other major fault systems namely
extension of West Coast fault (NW trend), which defines the western boundary, and the
extension of Son-Narmada fault (ENE trend), which defines the southern boundary, are
around 35 and 70 km, respectively, from the epicentral area.
3 Regional seismicity
The area comes under zone III of the seismic zoning map of India and is prone to moderate
seismicity of M * 6. Only in 2007, the Sasangir area of Talala taluka in Junagadh district
had experienced a damaging earthquake of magnitude Mw 5, associated damaging foreshock of Mw 4.8, and over 200 located aftershocks of Mw1.5 or greater. Earthquakes in
Saurashtra for last 200 years are listed in Table 1 and shown in Fig. 1. Cambay Fault in the
eastern boundary of Saurashtra has produced a number of moderate earthquakes, largest
being magnitude 5.7 near Bhavnagar in 1919. In the Junagadh region, Visavadar experienced a tremor of magnitude 4.3 on September 3, 1985. Una area experienced a tremor of
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Table 1 Catalog of earthquakes in Saurashtra region from earliest time to 2010
Years
Mon
DD
1872
04
1881
12
1883
1886
O.T.
Lat
Long.
14
21.75
27
22.30
10
20
04
14
1887
11
1891
1919
1922
Mag
Int
72.15
5.0
VI
Bhavnagar
CHA
70.88
4.4
V
Rajkot
USG
21.70
71.97
4.4
V
Bhavnagar
USG
22.47
70.10
4.4
V
Jamnagar
USG
11
22.30
70.88
4.4
V
Rajkot
USG
07
27
21.33
71.37
4.4
V
Amreli
USG
04
21
22.00
72.00
5.7
VII
Bhavnagar
CHA
03
13
22.00
71.00
4.3
V
Jhalavad, Rajkot
CHA
1930
Dep
Location
Ref.
22.40
71.80
4.3
V
Paliyad
CHA
22.30
71.60
5.0
VI
Botad
TAN
26
22.30
71.50
4.1
VI
Botad
TAN
07
12
22.40
71.80
5.5
VI
Paliyad
IMD
1938
07
14
22.40
71.80
5.7
VI
Paliyad
TAN
1938
07
19
22.40
71.80
5.7
VI
Paliyad
IMD
1938
07
23
22.40
71.80
5.7
VII
Paliyad
CHA
1940
10
31
22.50
70.40
5.0
VI
Jamuanathali
CHA
1968
21.60
71.25
4.3
V
Amreli
GSI
1968
21.73
70.45
4.3
V
Dhoraji
GSI
1975
22.10
71.20
4.3
V
Jasdan
GSI
GERI
1938
06
1938
06
1938
10:03
1979
02
22
22:11
21.33
72.15
3.3
Bhavnagar
1979
08
24
01:13
22.11
72.43
3.1
Khambhat
GERI
1979
09
05
10:08
21.33
72.12
3.6
Bhavnagar
GERI
1979
09
22
22:48
21.75
72.15
3.3
Bhavnagar
GERI
1980
01
06
00:42
22.23
71.78
3.2
Botad
GERI
1982
04
09
09:00
22.07
72.19
2.9
Khadi
GERI
1982
05
10
01:00
21.90
72.27
3.2
Bhavnagar
GERI
1982
06
26
18:48
22.25
71.82
3.1
Dhandhuka
GERI
1982
07
02
16:30
21.86
72.04
3.5
Bhavnagar
GERI
1985
09
03
21.03
70.88
4.3
Visavadar
GERI
1986
09
16
20.60
71.40
3.8
Rajula
GERI
1993
08
09
20.60
71.40
3.1
Rajula
GERI
1993
08
24
20.60
71.40
5.0
Rajula
IMD
1998
07
19
22.42
70.86
4.4
1998
09
21
06:23
21.81
71.93
3.0
1998
11
28
16:59
21.94
71.06
3.2
1999
09
21
11:00
21.70
72.10
2.5
III
2000
08
10
13:30
21.78
72.31
3.6
2000
08
13
13:28
21.02
70.99
7.0
2000
09
12
00 53
21.72
72.16
10.0
21.02
70.88
2001
23:18
29.0
IV
Rajkot
IMD
Bhavnagar
GERI
Gondal
GERI
Bhavnagar
GERI
V
Bhavnagar
GERI
4.6
VI
Tulsi Shyam, Junagadh
ISC
4.2
VI
Bhavnagar
IMD
2.5
Tulsi Shyam, Junagadh
GERI
2001
09
11
21.46
70.51
3.2
Haripur
2003
01
13
22.30
70.93
2.0
Rajkot
GERI
2003
01
29
21.46
70.51
3.1
Haripur, Junagadh
IMD
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Table 1 continued
Years
Mon
2003
08
DD
O.T.
2004
Lat
Long.
22.20
69.92
21.00
70.50
Dep
Mag
Location
Ref.
2.5
Lalpur Tq, Jamnagar
GERI
3.0
Talala, Junagadh
GERI
2006
09
30
00:16
22.31
70.21
4.0
2007
11
06
09:38
21.04
70.59
2008
01
25
23:36
21.79
71.76
2008
02
14
19:20
22.73
70.88
2008
10
03
23:59
21.90
2009
03
28
12:27
22.17
2010
09
17
04:30
22.02
2010
09
23
06:14
21.90
Int
V
Khankotda, Jamnagar
ISR
5
Junagadh
ISR
35.1
2.8
Bhavnagar
ISR
3.1
3.1
Morbi, Surendranagar
ISR
69.96
3.7
3.6
Bhanvad
ISR
70.75
6.2
3.0
Rajkot
ISR
70.33
2.7
Sanala
ISR
69.70
3.0
Advana
ISR
CHA Chandra (1977), GERI Guj. Engg. Res. Inst. Vadodara, TAN Tandon and Srivastava (1974), USG US
Geol. Survey, IMD India Meteorological Department
Earthquake Magnitude in Saurashtra from earliest time to 2011
6
Magnitude
5
4
3
2
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
Year
Fig. 2 The decade of 2000 shows larger no. of shocks even the M4 level and above, which is the magnitude
of completeness in the region, except the year of 1938 when a few damaging earthquakes were experienced
in Paliyad area
magnitude 4.6 on August 3, 2000. Walls collapsed in two houses and walls cracked in 75
houses in Gir Gadhada and several houses in Una. Cracks appeared in a few houses at
Pakheda as well as at Ankolwadi and Kanyasara villages of Talala. The shock was felt with
high intensity up to Jamnagar. Figure 2 indicates that the present decade has shown high
number of shocks. Prior to this, only the decade of 1930s showed somewhat large number
due to three Paliyad earthquakes of magnitude varying from 5 to 5.7 in 1938.
Prior to 2007, the Haripur village in Sasangir area of Junagadh district experienced
swarm earthquake activity in 2001 and 2004–2005 with maximum magnitude of ML3.1
(Fig. 1). Residents of Haripur reported to be experiencing shocks during August 2001.
During October–December 2001, the India Meteorological Department (Bhattacharya and
Dattarayam 2003) recorded 1,689 shocks at a local station. The activity peaked during
November 1–27, 2001 with nine shocks of magnitude 3–3.2 with maximum magnitude
earthquake on November 11, 2001. Some villages in Sasangir area experienced a series of
microearthquakes during September 5, 2004 to February 2, 2005. The Gujarat Engineering
Research Institute operated 1–3 stations during Jan 13–Feb 2, 2004 and recorded 334
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shocks (GERI 2003, 2005) out of which 10 shocks were of magnitude 2–2.7. They estimated epicenters to be about 3-km NW of Haripur and focal depth \2 km. The maximum
intensity was assigned to be IV.
Other parts of Saurashtra such as Bhavnagar (2000) (Bhattacharya et al. 2004), Lalpur
(2003, 2007), and Kalawad (2006, 2007) of Jamnagar district have witnessed similar
swarm type (burst) of seismic activity that subsided within a month or two. During
2002–2003, Rajkot experienced a few small shocks (Rastogi et al. 2012b). The August 10
and September 12, 2000 Bhavnagar tremors of magnitude 3.6 and 4.2, respectively, caused
severe damage to over hundred houses. The associated sequence of about 200 shocks
during August 9–October 26, 2000 was associated with the west Cambay fault. The shocks
were felt with subterranean sounds.
During first week of August to September 29, 2003, four villages around Sasoi Dam
(completed in 1954) in Lalpur taluka of Jamnagar district experienced shocks of magnitude
up to 2.5. A major N–S trending dyke with two offshoots in E–W and NW–SE directions is
passing through the area. The E–W portion is close to the most-affected village (GERI
Report 2003). During 2007 also the area experienced shocks, epicenters of which are
aligned in NNW direction. Khankotda area during September–October 2006 and Khankotda to Vijrakhi area of Kalavad taluka during August–September 2007 experienced
tremors (Rastogi et al. 2012b). The epicenters in Kalavad and Lalpur are associated with
NNW trending dykes.
All the shocks in Saurashtra are accompanied by blast sound. The shocks give shaking
and subterranean sound while only sound is heard for small shocks. The subterranean
sounds are produced when seismic waves get coupled with atmosphere. The sound indicates shallowness of the shocks. Within about 10 km distance it is explosion sound, while
at greater distance it is rumbling sound.
Saurashtra region is critically stressed as evidenced by frequent small shocks. It is
noticed that all the earthquakes in Saurashtra have occurred during August–November, that
is, soon after heavy rains which raises the water table by 30–70 m. Thus, the stress
perturbation due to rise in water table can trigger the onset of earthquakes (Table 2).
4 Earthquake monitoring
Institute of Seismological Research (ISR) is monitoring the seismic activity in the state
round the clock with 60 broadband seismographs and 50 accelerographs (Chopra et al.
2008b; Rastogi et al. 2012a). The seismic data from these stations are received via VSAT
at central station of ISR, Gandhinagar, on continuous basis. Out of these observatories,
seven are in Saurashtra located at Junagadh (Dhrafad Dam), Una, Lalpur (Sasoi Dam),
Rajkot, Amreli, Morbi, and Surendranagar. One offline station at Chitravad some 9-km east
of the mainshock epicenter is operated since 2007. Besides these, two digital seismographs
Table 2 Summary of earthquakes in Saurashtra from 1872
to 2010
123
Magn.
No. of events
2–2.9
7
3–3.9
19
4–4.9
16
5–5.7
11
Total
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at Virdi and Ghasiya and one strong motion accelerograph at Hirenvel were deployed for
precise location and focal depth of aftershocks. The locations of the permanent network
stations used in the study as well as temporary stations established for aftershock studies
are shown in Fig. 1. The list of local seismic stations is as given below:
Station Name
Date of Installation
Latitude
Longitude
Hirenvel (SMA)
October 23, 2011
21.129
70.510
Chitravad (BBS)
Since year 2007
21.110
70.529
Virdi (BBS)
October 22, 2011
21.181
70.387
Ghasiya (BBS)
October 19, 2011
21.025
70.496
5 Data analysis
5.1 Epicentral locations
The epicenters of the mainshock and aftershocks are shown in Fig. 3 and focal depths in
Fig. 4. Parameters of the mainshock and the largest aftershock are as given below:
Mainshock : October 20, 2011; 17 : 18ðGMTÞ; Mw 5:1; 21:09 N70:45 E; Depth5kmðISR)
Aftershock : October 21, 2011; 03 : 07 (GMT); Mw 4:1; 21:114 N70:5 E; Depth11:5km(ISR)
The epicenters of the mainshock and about 300 aftershocks of Mw C 1 located for 40 days
indicate NE trend for about 40 km length in Sasangir area of Maliya and Talala divisions
of Junagadh district. This trend may define a fault which may be 100–200 million years old
sympathetic to Narmada trend. The depth section in N–S direction indicates a near vertical
fault. The fault zone is about 10 km wide. The shocks are located with four or more
stations with ERH \ 1 km and ERZ \ 2 km. Moment magnitude Mw is given for shocks
of magnitude greater than 2 and Richter scale ML for smaller shocks. Magnitude-wise
distribution of aftershocks for 40 days is given in Table 3.
5.2 Velocity model
The velocity model used for location of earthquakes in Saurashtra based on Deep Seismic
Survey (Kaila et al. 1981) in a similar geological setting is as given in the following table:
Depth to the top of layer (km)
Velocity of P wave (km/sec)
0.0
4.5
3.0
5.43
6.0
6.0
10.0
6.45
15.0
6.48
20.0
6.72
25.0
7.15
30.0
7.49
35.0
7.88
42.0
8.20
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Fig. 3 Talala earthquakes of magnitude Mw about 1 or greater during October 20–November 30, 2011. It is
also showing meizoseismal area and isoseismal 6
Fig. 4 Depth section for Talala shocks for M C 2.0 from October 20 to November 10, 2011
5.3 Source parameters
We estimated source parameters for Talala earthquake of October 20, 2011 from 14
stations of ISR’s network and carrying out spectral analysis of S-wave assuming the
circular source model of Brune (1970). A sample displacement spectrum is shown in
Fig. 5. The average values are:
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Table 3 Magnitude distribution
of Talala October 20–November
30, 2011 earthquakes
Magnitude
No. of earthquakes located
0.6–1.9
288
2.0–2.9
87
3.0–3.9
9
4.0–4.9
2
5.3
1
Total
387
Fig. 5 Displacement spectra for mainshock estimated at seismic station BHV at a distance of 169 km
Seismic Moment: 10 16.6 N-m, Stress Drop: 16 bars, Corner Frequency: 1.3 Hz, Source
Radius: 2.3 km, Moment magnitude: 5.1
The interesting result is low stress drop in Saurashtra similar to that of plate boundary
regions. It is interesting as high stress drop is generally attributed to stable continental
region earthquakes.
According to the following formula given by Wells and Coppersmith (1994), the rupture
length RLD for M5.1 is obtained to be 3 km:
Log RLD ¼ 0:62 M
5.4 Decay rate of aftershocks
The daily frequency of the events is shown in Fig. 6, and the time difference between the
successive events and their magnitudes is shown in Fig. 7. The decay rate of aftershocks is
defined by the constant p in the following power law given by Utsu (1969):
N ¼ K=tp
where N is the number of shocks on day one, t is no. of days elapsed after the mainshock.
The power law fit is obtained to be as follows:
N(T) ¼ 49:77 t0:71 ; where p ¼ 0:71 and K ¼ 49:77:
As shown in Fig. 8, the p value of 0.71 fits with the daily no. of aftershocks of M [ 0.6 and
it indicates that aftershocks reduced to a low level of 2 shocks/day within 2 months but
may continue for a few months more at still lower level.
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60
50
N(t) = 49.77* t-0.71
N(t)
40
30
20
10
0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
Time (t)
Fig. 6 Aftershocks decay rate of less than 1 indicates slow decay reducing M [ 0.6 frequency to less than
10/day within 15 days but continuation of felt shocks for about 2–3 months
Fig. 7 Time in hours of aftershocks since the mainshock (t0) indicates declining trend of magnitude and
larger time intervals for 13 days of the 2011 sequence shown by red. This fact was interpreted declining
trend of seismicity. For comparison, 2007 sequence is also shown by blue color
5.5 The b-value (magnitude vs. no. of shocks)
The distribution of no. of shocks for different magnitudes M is given by Gutenberg and
Richter (1956) by the following relation:
LogN ¼ a bM
where N is the cumulative number of shocks.
The plot of no. of shocks for different magnitude ranges at one unit interval (Fig. 10)
indicates the coefficient of linear fit, that is, b-value to be 0.67. The value much lower than
1 indicates lesser no. of smaller shocks as normally expected for the M5.1 mainshock. This
is typical of stable regions.
5.6 Fault-plane solution by moment-tensor analysis
Centroid moment solution (CMT) for the October 20, 2011 earthquake is shown in Fig. 8
and the waveform matching is displayed in Fig. 9. The synthetic seismograms are computed
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Fig. 8 The epicentral trends deciphered with local networks from epicenters of earthquakes of M [ 1
during 2006–2011 in the Saurashtra region indicate two ENE trends (Narmada trend) for about 50 km length
and a conjugate 15-km-long NNW trend (Aravali trend). Gravity highs exist around Mt. Girnars made up of
Deccan Traps volcanic rocks, N and S of the two trends. The two ENE trends follow a gravity low as shown
in Figs. 14 and 15. The central MTS focal mechanism corresponds to the October 20, 2011 earthquake of
Mw 5.1 as derived from wave form matching depicted in Fig. 12. The mechanism on east side corresponds
to 2007 Mw 5.0 and left one for 2007 Mw 4.8 foreshock
using the approach of Takeo (1987) that uses the wave number summation method (Bouchon et al. 1989). Inversion program of Kikuchi and Kanamori (1991) and modified by
Kosuga (1996) is used. The details of the methodology are discussed in Rao (1999); Rao
et al. (2002); and Shashidhar et al. 2011). The mechanism indicates left-lateral, strike-slip
faulting along a NE trending fault. Centroid depth is obtained to be 3 km.
5.7 Fault-plane solution from first motion directions
Fault-plane solution is also obtained from first P-motion directions noted at 16 local and
regional seismograph stations. It indicates left-lateral, strike-slip faulting along a NE
trending fault. Alternately right-lateral strike-slip faulting along a NW trending fault is
indicated (Fig. 10).
5.8 Causative fault inference from deep seismic profile and gravity
Figure 11 shows Bouguer gravity, elevation, and seismic reflections profile from Navibandar to Amreli. Figure 12 shows the gravity contours, profile location, and the interpreted causative Girnar fault. The gravity contours are from NGRI Gravity Maps series
(Mishra et al. 2004). Seismic reflections are from Rao and Tewari (2005). The causative
fault is interpreted based on seismic reflections and the linear trend of the gravity low.
5.9 Strong motion observations of magnitude Mw 5.1 Talala earthquake
Institute of Seismological Research is running and maintaining the Gujarat State Seismic
Network (GSNet). In this network, 49 strong motion equipments are installed all over
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Fig. 9 Waveform match between observed (blue) and synthetic (red) seismograms for MTS of the October
2011 mainshock
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Fig. 10 Focal mechanism obtained by first motion for the Talala event of October 20, 2011, Mw 5.1
Fig. 11 Gravity (broken line), elevation (continuous line), and seismic reflections profile from Navibandar
to Amreli (DSS profile from Rao and Tewari 2005)
Gujarat. Five strong motion stations of GSNet have recorded the strong motion data of the
2011 Talala event. These stations are present at an epicentral distance of 32–207 km.
The peak ground acceleration (PGA) and response spectra at all five stations are estimated and the component-wise PGA values (sample for one station shown in Fig. 13) are
given in Table 4. The PGA decay with distance is given in Fig. 14. It has to be mentioned
that the PGA of 35 cm/sec2 observed at Dhrafad Dam, Junagadh, at 32 km distance is very
small to damage well-built structures in the area. The response spectrum at Junagadh
shows the highest value of acceleration of 80 cm/sec2 near about 0.1 s, and therefore, we
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Fig. 12 Gravity contours in Saurashtra (NGRI Gravity Maps Series, Deep Seismic Sounding profile
location, and interpreted causative Girnar fault
can expect small damage to only one-story, non-engineered houses up to about 50 km
distance. The Rajkot station at relatively harder Deccan Trap rocks shows less acceleration
compared to Bhavnagar at softer rock. The estimated acceleration by Bolt (1978) is
0.06–0.07 g for intensity 6, that is, observed up to a distance of 8 km, 0.03–0.04 g for
intensity 5 observed up to a distance of 108 km, and 0.015–0.02 g for intensity 4 observed
up to a distance of 180 km.
6 Observation of intensity
The meizoseismal area of 15 km 9 10 km of intensity 6.5 is around the epicenter, but
elongated toward NE indicating directivity in that direction. Maximum damage was
reported from Ladudi, Abudi, and Jalondar villages where about 30 adobe houses have
totally or partially collapsed injuring 36 people. At Haripur, Sandhbeda (near Amrapur
Gir), Devgam, Chitravad in the north, and Jepur in the south, some 50 adobe houses
partially collapsed and other 100 adobe houses suffered heavy damage. A total of about
3,000 houses have suffered wide cracks. An area 55 km 9 19 km of intensity 6 is trending
NE and is passing through Maliya, Veraval, and south of Visavadar where sporadic cases
of walls collapse and wide cracks in houses are reported. This event was very strongly felt
with occasional minor cracks up to Amreli (Intensity 5 up to 135 km) while it was strongly
felt up to 200 km in most of Saurashtra with intensity 4. Many felt in Kachchh,
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Fig. 13 The EW, NS, and
vertical components of
accelerogram and response
spectra at 5 % damping at
Junagadh station
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Table 4 PGA values at five stations where strong motion of 2011 Talala earthquake is recorded
SN
Station Name
Lat, Long
1.
Junagadh
21.35,70.72
2.
Bhavnagar
21.69,72.00
3.
Rajkot
22.36,70.76
4.
Morbi
5.
Surendranagar
Dist.
(km)
PGA
(cm/sec2)
Predominant
period
PSA
(cm/sec2)
Soil PSA
(cm/sec2)
37
34.8
0.05 s
84.51
170
169
6.6
0.07 s
23.12
46
140
3.8
0.22 s
14.32
20
22.84,70.89
195
2.6
0.15 s
7.54
13
22.73,71.58
211
3.1
0.15 s
9.63
15
The peak spectral acceleration and their periods are also given. The PGA and PSA values are in hard rock as
instruments are deployed in such ground. However, in the nearby soil-covered areas, amplification is 2 times
or so for PSA as given in the last column. These values are matching with the empirical values given by Bolt
(1978) for intensities at those distances assigned by us
Fig. 14 Decay of PGA with distance. Recorded ones are shown by filled circle while open circles indicate
inferred acceleration based on damage. The crosses represent the acceleration values at maximum distances
for intensities 6, 5, and 4 as given by Bolt and which could be on the PSA values on soil-covered ground for
the given PGA
Ahmedabad, Surat, and Vadodara up to 300 km distance (Intensity 3). The mainshock was
felt in upper floors of Mumbai (350 km).
6.1 Damage survey
Institute of Seismological Research deputed a team of Scientists during October 2011 to
the affected villages to assess the damage. Most of the adobe-type kachcha houses built
with weathered and weak limestone of assorted sizes and shapes with mud mortar in
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Fig. 15 Isoseismals 6.5, 6, 5, 4, and 3. Isoseismal areas/distances give Mw 5.0 according to Johnston’s
regression
meizoseismal area were damaged. Intensity values were assigned on Modified Mercalli
scale. The isoseismal maps are shown in Fig. 15. Photographs of two collapsed adobe
houses are shown in Fig. 16.
According to Bolt (1978), the expected PGA at 4, 5, and 6 intensity are 0.015–0.02,
0.03–0.04, and 0.06–0.07 g, respectively. Such values have possibly been experienced
there as per analysis of recorded strong motion data and assessment of inferred spectral
acceleration on soil (Table 4). The area corresponding to the farthest distance up to which
the earthquake was felt (Af) and also areas with intensity 4, 5, and 6 (Table 5, Figs. 15 and
16) give moment magnitude 5.0 as defined by Johnston (1998). According to the formula
Mw = (2/3)logMo-10.7, the log Mo for Mw 5.0, 5.1, and 5.3 are 23.55, 23.7, and 24,
respectively. From the following regressions, the logMo value is obtained to be 23.5 giving
Mw of 5.0:
log ðMoÞ ¼ 47:34 10:81 log ðAfÞ þ 1:17 log2 ðAfÞ
log ðMoÞ ¼ 30:62 4:67 log ðA4Þ þ 0:65 log2 ðA4Þ
log ðMoÞ ¼ 26:90 3:10 log ðA5Þ þ 0:52 log2 ðA5Þ
log ðMoÞ ¼ 20:94 þ 0:36 log ðA6Þ þ 0:14 log2 ðA6Þ
7 Conclusion
In Saurashtra, the structural trends of Proterozoic (NNW) and Mesozoic (ENE) ages are
activated producing small to moderate shocks of magnitude less than 6. The October 20,
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Fig. 16 Two of the collapsed houses, a A partially collapsed adobe house, b A fully collapsed adobe house
2011 shock and aftershocks define a shallow ENE 40-km-long trend, which was earlier
identified as a fault of 50 km length. The daily number of aftershocks (magnitude [ 1.5)
was 15/day for a fortnight, 5/day for 45 days, and then about one/day for more than
5 months. The Mw5.1 earthquake is a tectonic earthquake along a fault, which might have
accumulated strains due to plate-tectonic forces. The 2001 Bhuj earthquake has caused
stress increase in not only Kachchh area but Saurashtra also in the south for 200 km
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Table 5 Areas and distances of
observed intensities
Intensity
a/2 9 b/2
6.5
7.5 9 5
6
27.5 9 19
5
4
Area (km2)
Av. distance (km)
6
1,640
23
135 9 80
33,912
108
200 9 160
100,480
180
3
275
Max. Felt Dist.
350
distance. The preferred timing of earthquake in October after about 20- to 30-m rise in
water table after heavy rainfall indicates triggering effect suggesting that these earthquakes
might have been triggered by percolated rain waters through extensional fractures.
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