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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 123 Nat Hazards 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 123 Nat Hazards 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 123 Nat Hazards 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 123 Nat Hazards 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 334 123 Nat Hazards 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 53 Nat Hazards 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 8.20 123 Nat Hazards 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: 123 Nat Hazards 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. 123 Nat Hazards 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 123 Nat Hazards 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 123 Nat Hazards Fig. 9 Waveform match between observed (blue) and synthetic (red) seismograms for MTS of the October 2011 mainshock 123 Nat Hazards 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 123 Nat Hazards 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, 123 Nat Hazards Fig. 13 The EW, NS, and vertical components of accelerogram and response spectra at 5 % damping at Junagadh station 123 Nat Hazards 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 123 Nat Hazards 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, 123 Nat Hazards 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 123 Nat Hazards 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. 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