geosciences
Article Surface Level Synthetic Ground Motions for M7.6 2001 Gujarat Earthquake
Jayaprakash Vemuri 1,*, Subramaniam Kolluru 1 and Sumer Chopra 2
1 Department of Civil Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana 502285, India; [email protected] 2 Institute of Seismological Research, Department of Science and Technology, Government of Gujarat, Raisan, Gandhinagar, Gujarat 382009, India; [email protected] * Correspondence: [email protected]; Tel.: +91-8886064222
Received: 29 September 2018; Accepted: 20 November 2018; Published: 22 November 2018
Abstract: The 2001 Gujarat earthquake was one of the most destructive intraplate earthquakes ever recorded. It had a moment magnitude of Mw 7.6 and had a maximum felt intensity of X on the Modified Mercalli Intensity scale. No strong ground motion records are available for this earthquake, barring PGA values recorded on structural response recorders at thirteen sites. In this paper, synthetic ground motions are generated at surface level using the stochastic finite-fault method. Available PGA data from thirteen stations are used to validate the synthetic ground motions. The validated methodology is extended to various sites in Gujarat. Response spectra of synthetic ground motions are compared with the prescribed spectra based on the seismic zonation given in the Indian seismic code of practice. Ground motion characteristics such as peak ground acceleration, peak ground velocity, frequency content, significant duration, and energy content of the ground motions are analyzed. Response spectra of ground motions for towns situated in the highest zone, seismic zone 5, exceeded the prescribed spectral acceleration of 0.9 g for the maximum considered earthquake. The response spectra for towns in seismic zone 5 exhibit peaks in the low period ranges, indicating high vulnerability of low rise structures designed as per the provisions of the Indian seismic code of practice. The response spectra for towns situated in seismic zone 3 were considerably lower than the prescribed maximum spectral acceleration of 0.4 g. The substantial damage reported in towns situated in seismic zone 3 is due to poor construction practices and non-compliance with provisions of seismic design standards.
Keywords: synthetic ground motions; stochastic finite-fault method; site effect; response spectra; peak ground acceleration
1. Introduction
The Mw 7.6 earthquake which struck Gujarat on 26 January, 2001 was the largest intraplate earthquake in India [1]. The maximum felt intensity was X on the Modified Mercalli Intensity (MMI) scale [2]. The earthquake mechanism was shallow oblique reverse faulting [1] with the epicenter about 9 km southwest of the village of Chobari in the Kutch region of Gujarat, India [3]. The earthquake caused widespread damage in the province of Gujarat. In the Kutch region, several towns were completely destroyed. Overall, it is estimated that the earthquake caused 13,819 deaths, damaged over a million structures, and the consequent economic loss was close to US$ 10 billion [4–6]. While the main fault rupture did not reach the surface, extensive secondary features, such as liquefaction in the form of sand blows/boils, craters and ground fissures were observed in the epicentral area [7] to a maximum environmental seismic intensity [8] degree of VIII. Figure1 shows the isoseismal map for the Bhuj earthquake of January 26, 2001, based on damage surveys [9].
Geosciences 2018, 8, 429; doi:10.3390/geosciences8120429 www.mdpi.com/journal/geosciences Geosciences 2018, 8, x FOR PEER REVIEW 2 of 17 Geosciences 2018, 8, 429 2 of 17
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45 46 FigureFigure 1. Isoseismal1. Isoseismal Map Map for for 2001 2001 Gujarat Gujarat Earthquake. Earthquake. The The intensity intensity is onis on the the MMI MMI scale. scale. (Inset: (Inset: Map 47 of India – Gujarat is highlighted).Map of India – Gujarat is highlighted).
48 Damage in the epicentral region of the 2001 Gujarat earthquake was not uniform and often 49 inconsistent [10]. [10]. Isoseismals Isoseismals for for the the earthquake earthquake [2] [for2] forseismic seismic zone zone 5 show 5 show that some that sometowns towns (e.g., 50 (e.g.,Bhachau, Bhachau, Rapar, Rapar, Dudhai Dudhai and andAdesar) Adesar) witnessed witnessed complete complete destruction destruction while while other other towns towns (e.g., (e.g., 51 Nakhatrana, Naliya, Khavda, and Mandvi) witnessed relatively less damage. A relationship between 52 the buildingbuilding typology typology and and ground ground motion motion characteristics characteristics can aidcan in aid understanding in understanding the unusual the unusual damage 53 patterndamage in pattern the region. in the However, region. However, ground motion ground data moti fromon this data earthquake from this is earthquake not available is andnot isavailable limited 54 andto PGA is recordingslimited to atPGA thirteen recordings stations at [11 thirteen]. The modified stations stochastic [11]. The finite-fault modified technique stochastic has finite-fault been used 55 techniqueearlier [3] tohas estimate been used ground earlier motions [3] to estimate at these grou thirteennd motions stations at and these the thirteen PGA values stations of theand syntheticthe PGA 56 valuesground of motions the synthetic were validatedground motions with recorded were validated PGAs from with 13recorded stations PGAs (marked from in 13 boxes stations in Figure(marked1). 57 inHowever, boxes in the Fig.1). ground However, motions the were ground generated motions at rock were surface, generated and may at rock not be surface, representative and may of not the 58 representativeactual motions of experienced the actual atmotions the surface experienced level, which at the may surface be obtained level, which only after may accounting be obtained for only soil 59 afteramplification. accounting Only for soil surface amplification. level ground Only motions surface can level help ground to understand motions can the help actual to damageunderstand pattern the 60 actualobserved damage in the pattern earthquake. observed in the earthquake. 61 In this paper, ground motions are estimated using the modified stochastic stochastic finite-fault finite-fault method, method, 62 at surface surface level level for for various various sites sites which which experien experiencedced damage damage in the in the2001 2001 Gujarat Gujarat earthquake. earthquake. The 63 Thevalidation validation study study performed performed at 13 stations at 13 stations where where actual actual PGA values PGA values are available are available constrains constrains the source the 64 andsource dispersion and dispersion parameters parameters used usedin the in method. the method. Essential Essential characteristics characteristics of ofthese these synthetic synthetic ground ground 65 motions are correlated withwith isoseismalsisoseismals andand reconnaissancereconnaissance reports reports of of the the 2001 2001 Gujarat Gujarat earthquake. earthquake. 1.1. Seismotectonics of Gujarat 66 Seismotectonics of Gujarat Physio-graphically, the Gujarat region comprises three distinct zones: Kutch, Saurashtra and 67 Physio-graphically, the Gujarat region comprises three distinct zones: Kutch, Saurashtra and Mainland Gujarat. The Gujarat region is a junction of three tectonic rifts: Kutch, Cambay and Narmada 68 Mainland Gujarat. The Gujarat region is a junction of three tectonic rifts: Kutch, Cambay and with many active faults [12–14]. These rifts were formed by rifting along major Precambrian trends. 69 Narmada with many active faults [12,13,14]. These rifts were formed by rifting along major The rifting occurred at successive stages during the northward movement of the Indian plate after 70 Precambrian trends. The rifting occurred at successive stages during the northward movement of the the breakup from Gondwanaland in the Late Triassic or Early Jurassic. The rifting developed around 71 Indian plate after the breakup from Gondwanaland in the Late Triassic or Early Jurassic. The rifting the Saurashtra horst. The Kutch rifting took place in the Late Triassic–Early Jurassic, Cambay rifting 72 developed around the Saurashtra horst. The Kutch rifting took place in the Late Triassic–Early 73 Jurassic, Cambay rifting in Early Cretaceous, and Narmada rifting in the Late Cretaceous [12]. The
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shear zone during the post-collision compressive regime of the Indian plate, with strike-slip movements along subparallel rift faults. The Kutch mainland fault along the rift axis became the active principal fault. The eastern part of this fault, where it overhangs the South Wagad fault, is the most strained region [13]. The epicenter of the 2001 Gujarat earthquake and its aftershocks are located in the Kutch region. The overall earthquake hazard in Gujarat ranges from low to high, and based on the estimated hazard level, the seismic zoning map of India [15] has assigned four seismic zones to Gujarat (Figure 2a), ranging from Zone II (low) to Zone V (high). As the Kutch region has the highest earthquake hazard it lies in the Zone V, with a pseudo-spectral acceleration (PSA) at Maximum Considered Earthquake (MCE) level of 0.9 g and can expect earthquake damage corresponding to Modified Mercalli Intensity (MMI) level of IX or more. A belt of 60 km–70 km width around this zone, which Geosciences 2018, 8, 429 3 of 17 covers areas of northern part of Saurashtra and areas along the eastern borders of Kutch, lies in Zone IV (PSA at MCE level is 0.625 g) of the seismic zoning map. Earthquakes arising in Kutch and local inearthquakes Early Cretaceous, along North and Narmada Kathiawar rifting fault in thenorthern Late Cretaceous Saurashtra [can12]. le Thead to rifting intensity ceased (MMI) in the of Late VIII Cretaceousin this Zone. era Due during to moderate the pre-collision local earthquakes stage of the Indianor strong plate. Kutch The earthquakes, Kutch rift basin Mainland became aGujarat shear zonewhich during lies in theZone post-collision III (PSA at MCE compressive level is 0.4 regime g) can of expect the Indian intensity plate, (MMI) with level strike-slip of VII. movements alongAmong subparallel the three rift faults. zones, The presently Kutch mainlandthe Kutch faultregion along is seismically the rift axis most became active. the The active Kutch principal region fault.of Gujarat The easternis one of part the of most this earthquake fault, where prone it overhangs intra-continental the South regions Wagad in fault, the isworld the mostand the strained mean regionreturn period [13]. The of earthquakes epicenter of in the the 2001Kutch Gujarat region ha earthquakes been estimated and its to aftershocks be 13.34 years are [16]. located This inregion the Kutchhas witnessed region. major intraplate earthquakes in the past [7,17]. Some major earthquakes which have struckThe the overall Kutch earthquake region are hazard the 1668 in Gujarat Indus delta ranges earthquake, from low to 1819 high, Kutch and based earthquake, on the estimatedand 1845 hazardLakhpat level, earthquakes. the seismic Several zoning moderate map of India earthquakes [15] has assigned (5 < M < four6) have seismic also zonesoccurred to Gujarat in the region, (Figure 2e.g.,a), ranging1956 Anjar from earthquake Zone II (low) (M to 6.0). Zone Figure V (high). 2b shows As the some Kutch of region the major has thefaults highest in the earthquake Kutch region, hazard the itKutch lies in Mainland the Zone fault V, with (KMF), a pseudo-spectral Katrol Hill Fault acceleration (KHF), Allah (PSA) Bund at Maximum Fault (ABF), Considered Island Belt Earthquake Fault (IBF), (MCE)Gedi Fault level (GF) of 0.9 and g and the canNorth expect Kathiawar earthquake Fault damage (NKF). correspondingThe epicenter of to the Modified 2001 Gujarat Mercalli earthquake Intensity (MMI)is also levelmarked of IXin orFigure more. 2b. A It beltmay of be 60 observed km–70 km that width the key around faults this in the zone, Kutch which region covers trend areas in the of northerneast-west part direction. of Saurashtra The epicentral and areas region along of the the eastern 2001 earthquake borders of Kutch,lies at a lies distance in Zone of IV400km (PSA from at MCE the levelIndian-Eurasian is 0.625 g) of plate the seismic boundary. zoning It is map. known Earthquakes that both arisingthe India–Eurasian in Kutch and plate local earthquakescollision processes along Northand the Kathiawar dilatational fault perturbations in northern Saurashtra in the ambient can lead strain to intensityfield due (MMI) to past of earthquakes VIII in this Zone.in the Due Kutch to moderateregion have local loaded earthquakes contiguous or strongregions Kutch which earthquakes, contributed to Mainland the 2001 Gujarat earthquake which lies in[18]. Zone These III (PSAobservations at MCE levelindicate is 0.4 the g) high can expectseismic intensity hazard potential (MMI) level for the of VII. Kutch region.
(a) (b)
FigureFigure 2. 2.( a(a)) Seismic Seismic Zones Zones in in Gujarat Gujarat [ 15[15]]—the - the KutchKutch regionregion isis highlighted;highlighted; ((bb)) MapMap showingshowing majormajor faultsfaults in in the the Kutch Kutch region region (Zone (Zone V). V). The The epicenter epicenter of of the the 2001 2001 Gujarat Gujarat earthquake earthquake is is shown shown as as a a star. star.
1.2. MethodologyAmong the threefor Generation zones, presently of Synthetic the Accelerograms Kutch region is seismically most active. The Kutch region of Gujarat is one of the most earthquake prone intra-continental regions in the world and the mean return period of earthquakes in the Kutch region has been estimated to be 13.34 years [16]. This region has witnessed major intraplate earthquakes in the past [7,17]. Some major earthquakes which have struck the Kutch region are the 1668 Indus delta earthquake, 1819 Kutch earthquake, and 1845 Lakhpat earthquakes. Several moderate earthquakes (5 < M < 6) have also occurred in the region, e.g., 1956 Anjar earthquake (Mw 6.0). Figure2b shows some of the major faults in the Kutch region, the Kutch Mainland fault (KMF), Katrol Hill Fault (KHF), Allah Bund Fault (ABF), Island Belt Fault (IBF), Gedi Fault (GF) and the North Kathiawar Fault (NKF). The epicenter of the 2001 Gujarat earthquake is also marked in Figure2b. It may be observed that the key faults in the Kutch region trend in the east-west direction. The epicentral region of the 2001 earthquake lies at a distance of 400km from the Indian-Eurasian plate boundary. It is known that both the India–Eurasian plate collision processes and the dilatational Geosciences 2018, 8, 429 4 of 17 perturbations in the ambient strain field due to past earthquakes in the Kutch region have loaded contiguous regions which contributed to the 2001 Gujarat earthquake [18]. These observations indicate the high seismic hazard potential for the Kutch region.
1.2. Methodology for Generation of Synthetic Accelerograms Strong motion records from major earthquakes in Gujarat are not available. In such regions where a historical database of strong ground motions is unavailable, modified records may be obtained by either generation of synthetic records or by scaling natural records obtained from other regions. In this study, the modified stochastic finite fault method [19] is used to generate synthetic ground motions. Subsequently, the synthetic ground motions are used to derive response spectra. The procedure to obtain synthetic ground motions using the modified stochastic method [19] is now described. In the stochastic point source model [20] acceleration spectrum, A( f ) of shear waves at a distance, R from a seismic source with the moment, Mo may be estimated as