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4 SEISMIC CHARACTERISTICS, FAULTING AND STRONG MOTIONS

4.1 Earthquake Mechanism

The Kutch earthquake occurred at 8:46 AM on India Standart Time on 26 January 2001.Various seimological institutes obtained the faulting mechanism of the earthquake and itsmagnitude. Table 4.1 gives the seismic characteristics of the earthquake and the focal planesolutions are shown in Figure 4.1. Almost all of the focal plane solutions obtained by variousinstitutes implied that the earthquake was caused by thrust faulting with a slight sinistral senseand fault planes must have N40-80E strikes. The plane dipping to south should be moresteeper than the plane dipping to North. The solutions obtained by Yagi-Kikuchi (ERI-Y&K)and Kikuchi-Yamanaka (ERI-K&Y) are shown in Figure 4.2. These solutions indicated thesurface displacement should be small even though the displacement is more than 5m at theepicenter. Figure 4.3 shows the epicenters determined by different institutes. It seems that theepicenters obtained by USGS & Harvard are supported by the field observations.

Table 4.1 Seismic parameters of the Kutch earthquake given by various institutesPlane 1 Plane 2Institute ML Ms Mw LAT

(N)LON(E)

Hkm Strike Dip Rake Strike Dip Rake

Dmax(cm)

HARVARD 7.4 23.45 70.34 18 65 50 50 297 54 127USGS 7.9 7.5 23.40 70.32 22 60 66 62 292 36 136EMSC 8.1 7.4 23.35 70.83 23 64 60 65 283 33 115ERI 7.9 7.4 18 78 58 51 276 33 105 540IMD 6.9 7.6 23.60 69.80 15

Figure 4.1 Fault plane solutions of the Kutch earthquake computed by various institutes

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ERI (Kikuchi & Yamanaka)

ERI (Yagi – Kikuchi)

Figure 4.2 Computed Slip models by ERI

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Figure 4.3 Epicenters determined by various institutes

4.2 Foreshocks and Aftershocks Activity

The seismic records on the previous day (January 25, 2001) obtained by the Bhujseismological observation station during a visit on March 21, 2001 indicated that there wasnot any foreshock while there were numerous aftershocks following the mainshock. The datafrom NEIC between 1945 and 2001 shown in Figure 4.4 shows the epicenters of earthquakeswith a magnitude greater than 3. Most of the shocks are on the northern side of earthquakearea and they seem to be associated with Allahbund fault, and a few shocks are aligned withthe axis of Kathiwar uplift zone. Figure 4.5 shows the epicenters of the earthquake with amagnitude greater than 3.4 until March 19, 2001. The aftershocks are scattered around theUSGS epicenter and the distribution of aftershock data seems to confirm the fault planesolutions obtained by four different institutes. Since there was not any well-defined fault scarpon the ground surface, it is difficult to say which of the fault planes determined from the faultplane solutions corresponds to the causative fault. Nevertheless, the fault plane with the NEstrike and dipping south could be the causative fault of this earthquake in view of the spatialdistribution of the shocks.

The largest aftershock with a magnitude of 5.8 took place on January 28, 2001. The faultplane solutions of this earthquake obtained by USGS and HARVARD are shown in Figure4.6. The fault plane solutions indicate that one of the fault planes with a strike of N83-84Wdipping at an inclination of almost 45o to south or north could be the causative fault plane forthis aftershock.

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Figure 4.4 Distributions of epicenters earthquakes between 1945 and 2001 before theearthquake (the last earthquake with a magnitude of 4.8 was on January 24, 2000)

Figure 4.5 Distributions of epicenters of aftershocks together with the USGS epicenter

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Figure 4.6 Fault plane solutions for the aftershock on January 28, 2001 by various institutes

4.3 Faulting

As stated previosuly, there was not any well-defined fault scarp on the ground surface. Itseems that many earthquakes in Indian plate do not result in well-defined fault scarps. Thiswas thought to be due to the low stiffness of the earth’s crust of the Indian plate. Neverthess,some fault scarps could be observed along the roadcuts during site investigations. Figure 4.7shows a thrust type fault near Rapar, dipping south while Figure 4.8 shows a fault scarp nearthe northern embankment of the Rudramata bridge. At the same location, the bedding planesof sandstone abruptly steepens. From the striations on the fault plane and bedding planes, theinferred mechanisms of earthquakes would look like as shown in Figure 4.9. The strikes offaults and bedding planes at this location are similar to the general trend of the strike of theKutch mainland fault described in Chapter 2. The fault plane solution for this thrust faultstrikingly is similar to that of the main shock given by various institutes.

Figure 4.7 A thrust fault near Rapar (after Biswal 2001)

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Striations on the thrust fault plane (picture towards Picture towards east north

Figure 4.8 A thrust fault with striations at the northern embankment of Rudramata bridge

Figure 4.9 Inferred fault plane solutions for a thrust fault and bedding planes with striations atthe northern embankment of Rudramata bridge

Many ground breaks were observed during site investigations. There were three groups ofground ruptures, namely, NE, NW and NS ground breaks according to the general trend oftheir strikes. While NE and NW striking ground breaks had a sinistral sense, the NS strikingground breaks did a dextral sense. The southern block associated with NE and NW strikingground breaks moved upward. As for the NS ground breaks, the western block moveddownward.

Nearby Bridge on Kankawati River (4km to Halvad and 100km East from the USGSepicenter), tension cracks in the ground with a strike of N45E was observed. At the southern

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side of the Hadakiya Creek Bridge, an en-echelon crack with a length of 200m was observedand its strike was N40E.

The second ground break was observed at the location 23o 32’ 59”N;40o 14’ 50”E (13km NWfrom the USGS epicenter). The location is 5km North of Momaymora and the orientation ofthe break is NW88/82. On the way from Chang Dam to Bachau near Manfera village, groundbreaks with a length of 2-3km was observed. The location was is 23o 24’ 00”N;40o 22’ 00”E(14km NW from the USGS epicenter). The dip direction and dip are SW40/80. The East sideof the ground break was up and the west side was down. From the sense of movement alongthe ground breaks, it was inferred that the ground breaks had a dextral sense. At this side therewas a 14cm relative horizontal movement and 10cm vertical throw. The inferred faultingmechanisms for these ground breaks are shown in Figure 4.10 with sketches of the groundbreaks.

Figure 4.10 Skecthes of NS trending ground breaks and inferred faulting mechanisms

In the Rudramata dam reservoir, some bulging of sandstone layers were observed in thedirection of NS. However, the cause of bulging was not well understood. The longitudinalaxis of the dam was EW.

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Ground breaks were observed at a location along the roadway between Anjar-Bachau. Twocrack sets were observed having strikes of N40W and N40E. The locations was 23o 09’55”N;40o 08’ 24”E (48km SW from the USGS epicenter).A ground break with a strike of N45W was observed on the roadway between Bachau andBhuj. The location is 23o 20’ 30”N;40o 18’ 31”E (9km SW from the USGS epicenter).

A set of parallel ground breaks near Chobari on the roadway between Momaymora andChobari and in the adjacent fields was observed (Figure 4.11). Their strike was N85E. Southblock is up with a vertical throw of 20cm and 10cm horizontal offset. The sense was sinistral.The inferred faulting mechanism shown in Figure 4.12 was obtained for this set of groundbreaks by assuming that fault plane is dipping towards south with an inclination of 50o. It isquite interesting that the fault plane solutions are quite similar to those obtained by variousinstitutes.

Figure 4.11 Sketches of ground breaks near Chobari village

Figure 4.12 Inferred faulting mechanisms Figure 4.13 Inferred faulting mechanism solution for Chobari ground for ground breaks near Trisuns breaks Chemical Complex

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A very long ground rupture was observed on the road and ground near the Trisuns chemicalcomplex. The upward movement of the southern block was about 10cm together with alaterally sinistral sense of movement. The site is about 10.3km far from the USGS epicenterand 14.1km from the reported fault break by Oyo-corporation. The direction of the groundbreak was N40W. The location was 23o 18’ 49”N;40o 21’ 23”E. The inferred faultingmechanism shown in Figure 4.13 was obtained for this set of ground breaks by assuming thatfault plane is dipping towards south with an inclination of 50o. It is quite interesting that thefault plane solutions are quite similar to those obtained for the aftershock on January 28, 2001by various institutes.

Another ground break on the same roadway had the direction of N40E which has the similarstrike of the USGS fault plane solution. The location is 23o 20’ 34”N;40o 14’ 24”E (10km SWfrom the USGS epicenter).

Figure 4.14 A view and sketch of ground breaks with tilted blocks near Budarmora

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There was a huge broad rupture zone with at least 100m width and extending for about 1.5kmabout 3km from the Bodarmora village (Figure 4.14). Furthermore, many small steps of theground could be easily noticed. The southern block seems to be uplifted. The uplift movementfor a single break was about 40cm with 10-14cm horizontal slip in a sinistral sense. Theblocks bounded by ground breaks were tilted and the tilted plane has the dip direction and dipas SW4/34 while tension ground breaks had the dip direction and dip as NE4/44. Thislocation was more extensively investigated by a team of Oyo Corporation geologists and theyreported many interesting observations. They found some pressure ridges, subsidence,liquefaction and bulged zone in this location as shown in Figure 4.15. It is most likely thatthis location corresponds to the tip of the fault at ground surface. Furthermore, the groundsurface rises towards south in a step-like manner, implying similar type ground deformationduring earthquakes in the past.

Figure 4.15 Deformation ground surface at Budarmora reported by Oyo Corporation

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4.4 Strong Motion Network and Strong Motion Records

It seems that there is a strong motion network established by Earthquake EngineeringDepartment of Roorkee University in Gujarat State as shown in Figure 4.16. So far the strongmotion records observed in Ahmedabad, Delhi and Roorkee stations are only available on theweb page of Earthquake Engineering Department of Roorkee University. Figures 4.17, 4.18and 4.19 show the strong motion records from these three stations.

Figure 4.16 Strong motion stations established by Roorkee University in Gujarat State

The strong motion records obtained from the region at the Passport Office Building inAhmedabad city, indicated a peak ground acceleration of about 0.11g. The peak groundacceleration in Bombay was 50gal (personal communication Prof. Sinha of Bombay Instituteof Technology). The peak ground accelerations in Delhi and Roorkee were 4gal and 4gal,respectively. The records at Bachau and Bhuj would be very valuable to know the groundmotions in the vicinity of the epicentral area. Ahmedabad city, which was approximately240km from the earthquake epicenter, experienced very large ground acceleration. From thewell known attenuation relations (see Figure 4.20), the expected peak ground accelerationfrom attenuation relations should be less than the ones observed. The cause of high peakground acceleration may be associated with the peculiar geological structure of Ahmedabadbasin (see Figure 2.5).

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Figure 4.17 Strong Motion records at Passport Building in Ahmedabad

Figure 4.18 Strong Motion records in Delhi

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Figure 4.19 Strong Motion records at Roorkee University

Figure 4.20 Comparison of various attenuation relations

4 10 20 40 100 200 400 10004

10

20

40

100

200

400

1000

2000

HYPOCENTRAL DISTANCE (km)

MA

XIM

UM

AC

CEL

ERA

TIO

N (g

al)

Observed

Joyner and Boore (1981) Fukushima et al. (1988)

Kutch Earthquake

Ms=7.9Mw=7.6

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The earthquake was felt in most parts of the country and a large area sustained damages.About 20 districts in the state of Gujarat sustained damage. The entire Kutch region ofGujarat, sustained highest damage with maximum intensity of shaking as high as X on theMSK intensity scale. Several towns and large villages, like Bhuj, Anjaar, Vondh and Bhachausustained widespread destruction. The other prominent failures in the Kutch region includeextensive liquefaction, failure of several earth dams of up to about 20m height, damage tomasonry arch and RC bridges, and failure of railroad and highway embankments. Numerousrecently-built multistorey RC frame buildings collapsed in Gandhidham and Bhuj in theKutch region, and in the more distant towns of Morbi ( ~125km east of Bhuj), Rajkot(~150km southeast of Bhuj) and Ahmedabad (~300km east of Bhuj). At least one multistoreybuilding at Surat (~345km southeast of Bhuj) collapsed killing a large number of people.Intensity Distribution was obtained from damage distribution of structures compiled fromNewspaper Accounts by Stacey Martin (Nowrosjee Wadia College in Pune, India) andSusan Hough (USGS, Pasadena). Figure 4.21 and 4.22 shows the intensity distribution for theIndia and Gujarat State, respectively. According to this map, the peak ground acceleration onrock may be greater than 124 gal.

Figure 4.21 Seismic intensity distribution obtained by Martin and Hough for India

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Figure 4.22 Seismic intensity distribution obtained by Martin and Hough for Gujarat State

Narula and Chaubey and Japanese teams conducted a macro-seismic surveys for the Kucthearthquake and their results are summarised as follows. The reconnaissance surveys wereconducted between the 3rd and 10th Feb 2001 which included an air reconnaissance followedby long traverses to assess the general damage patterns, terrain changes brought about and tolook for ground rupture. The preliminary isoseismals were obtained by utilizing the MSKscale (Medvedev-Sponheuer-Karnik). The reconnaissance surveys have indicated that intensity X has been reached in an ellipticaltract of about 2100 sq.km, which is characterized by complete destruction of adobe, brick andstone masonry buildings and many RCC buildings have suffered grade 4 to grade 5 damages;wide ground fissures and collapse of low height road embankments. This epicentral tractincludes villages of Jawaharnagar, Dudai, Adoi, Chobari, Manfara, and Rapar. The long axisof this epicentral tract is aligned in ENE-WSW to E-W direction with maximum damageconcentrated near its northern and southern boundaries. During the reconnaitory surveys only the southern boundaries of the isoseismal IX, VIII andpart of VII could be constrained and the eastern, western and northern boundaries are tentative

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because complete area could not be traversed during the time frame of investigation. Thesewill have to be verified by more detailed surveys. The ir preliminary isoseismal map is shownin Figure 4.23. The distribution of intensity VII is quite interesting and has deviated from the general E-Welongation, consistent with higher isoseismals, to almost north-south pattern along the areaoccupied by deep seated Quaternary and Cenozoic cover sediments along the Cambay Graben.The accentuation of the motions because of these thick cover have resulted in modification ofboundaries of isoseist VII. In all the isoseists there were isolated areas of lower or higherintensities depending upon the geotechnical characteristics of the ground. One suchconspicuous area is the township of Limbdi located within isoseist VII which shows damagessimilar to those of isoseist VIII.

Figure 4.23: Isoseismal map of Bhuj (India) earthquake of 26 January 2001 prepared byJapanese investigation teams led by Meguro and Hisada.

・・・Ⅹ・・・Ⅸ・・・Ⅷ・・・Ⅶ・・・Ⅵ・・・Ⅴ

・・・ⅩⅠ

MSK Scale・・・ⅩⅡ

・・・Ⅹ・・・Ⅸ・・・Ⅷ・・・Ⅶ・・・Ⅵ・・・Ⅴ

・・・ⅩⅠ

MSK Scale・・・ⅩⅡ

50km0 50km0

N

HisadaMeguro

2km0 2km0 2km0 5km0 5km0 5km0

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4.5 Estimation of Strong Motion

Taking the fault rupture process into consideration, a technique have been proposed toestimate the expected value of the peak ground motion (Sato & Kiyono, 1982) . Here, such atechnique is used in an attempt to estimate the peak accelerations based on the fault modeloutlined in 4.1 and to draw contours of the spatial distribution of the accelerations.

4.5.1 Estimation TechniqueA fault is divided into several small fault elements. Let us assume that a large earthquake canbe represented by the superposition of small earthquakes corresponding to those faultelements. Then, the ground motion gL(t) of the large earthquake can be represented usingground motions gS(t) of the small earthquakes as follows (Irikura, 1986):

? ? ?? ? ?

??L w Dn

i

n

j

n

kijkSL ttgtg

1 1 1)()( (4.5.1)

where nL, nW , and nD are respective numbers in terms of superposition with respect to faultlength, width, and dislocation, and tijk is the time lag caused by seismic waves and rupture anddislocation of the fault. In this case, the numbers in terms of superposition, n, are determinedby the following relationships.

3

0

0

S

LDWL M

Mnnnn ???? (4.5.2)

where MOL and MOS are seismic movements of large and small earthquakes, respectively. Thesuperposition formula shown in Eq. (4.5.1) is widely used. However, various improvementsand modifications have been made based on this formula. Using the source time function s(t)and source spectrum S(f), the ground motion gS(t) of a small earthquake and its Fouriertransform GS(f) become:

lms

S Rts

v

Rtg

)(4

)(3? ?

??? (4.5.3)

lms

S RfS

v

RfG

)(

4)(

3? ???? (4.5.4)

where R?? is the radiation pattern, ? is the density, vS is the shear wave velocity, and Rlm is thedistance from the observation point to the small faults Ae(l,m). The frequency-dependentradiation pattern (Kamae & Irikura, 1992) is used in this study.

The power spectrum of a large earthquake can be obtained from Eq. (4.5.1) subjected toFourier transformation and then multiplied by its complex conjugate as follows.

???

????

????

????

?? ? ? ?? ? ?

? ? ?? ? ?

?n

k

n

l

n

m

Rftin

k

n

l

n

m

Rfti

sdL

lmklmlmklm eefSfSv

R

TfP

1 1 1

/2

1 1 1

/2*23

)()()4

(2

)( ????

? ? (4.5.5)

where Td is the duration of the stationary point of ground motion, and * indicates complexconjugate. For source spectrum S(f), the ? 2-square model which is the theoretical spectrum, isused.

Once the power spectrum of ground motion can be obtained, the expected value of themaximum ground motion can be determined from the following equation (Kiureghian, 1980):

0max ?pu ? (4.5.6)

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),,,,( 210 ???? edTfp ? (4.5.7)where p is the peak coefficient and can be expressed as a function of the m-th order spectrummoment ? m(m = 0, 1, 2), the duration Td, and the zero-crossing rate ve as shown by Eq.(4.5.7).

Q value represents the effect of the propagation path with high fidelity. However, the Q valueof the ground in India is unknown. Therefore, the mean value in Japan was employed (Sato &Kiyono, 1982). According to the amplification degree, the following relationship betweengeological conditions and the amplification degree of the ground for maximum acceleration isproposed as a multiplying factor for input motions onto seismic bedrock (S wave velocity ofabout 3 km/s) (Midorikawa & Kobayashi 1980).

A = 5.5 (Quarternary)= 5.0 (Neogene to Quaternary)= 4.0 (Quarternary Extrusives)= 3.5 (Neogene)= 2.5 (Pre-neogene) (4.5.8)

By multiplying the maximum acceleration amplitude, umax, of input motions in the seismicbedrock by amplification factor A, the maximum acceleration amplitude on the groundsurface can be calculated.

4.5.2 Estimation ResultsAs for fault parameters in the calculation, the fault model by Yagi and Kikuchi (2001) is used.Magnitude was 7.6, the strike and the dip angle of the fault were set to 78 and 58 degrees, andthe number, n, in terms of superposition was 5. Figure 4.24 shows the area underconsideration and estimated peak accelerations. The attenuation curve of the estimated peakaccelerations is shown in Figure 4.25 The peak acceleration near the fault exceeds 1G. 600-800 gal is estimated for the villages and towns of Bachau, Anjar, Chobari and Gandhidam.500-600 gal is estimated for Bhuj. Although these villages and towns sustained severedamage, there are no strong ground motion records.

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Figure 4.24 Estimated spatial distribution of peak accelerations

Figure 4.25 Attenuation of estimated peak accelerations