CHARACTERISTICS OF NEAR-FAULT GROUND MOTION OF DHARAMSALA EARTHQUAKE OF 1986

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1 ISET GOLDEN JUBILEE SYMPOSIUM Indian Society of Earthquake Technology Department of Earthquake Engineering Building IIT Roorkee, Roorkee October 20-21, 2012 PAPER No. A013 CHARACTERISTICS OF NEAR-FAULT GROUND MOTION OF DHARAMSALA EARTHQUAKE OF 1986 Prabhat Kumar 1, Ashwani Kumar 2 and A.D. Pandey 3 1 ResearchScholar, Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, , India, prabhatkumariitr@gmail.com 2 Professor, Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, , India 3 Assistant Professor, Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, , India, adpanfeq@gmail.com ABSTRACT Near-Fault Ground Motion (NFGM) characteristics of Dharamsala earthquake of 1986 (Mw = 5.5) occurred in the Lesser Himalaya have been studied from the available strong motion recordings. Pulse detection methodology based on wavelet analysis has been adopted to extract the pulse characteristics of NFGM from the computed fault-normal component. It has been observed that velocity time history of Dharamsala earthquake at Shapur station contains the pulse type-characteristics. Out of the seven wavelets adopted for pulse detection, Daubechies wavelets (db4 and db7) provided the best results based on the pulse indicators (PI), pulse periods (Tp) and spectral pulse periods (Tv-p). For the Dharamsala earthquake, transverse component (fault-normal component) of the recorded velocity time history at Shapur station showed pulse type ground motion with pulse period (Tp) 0.52 sec and pulse indicator Pulse periods computed using available relationships based on magnitude are generally on higher side as compared to estimated pulse periods. Further, no relationship seems to be applicable for the Himalayan region to estimate the peak amplitude of the velocity pulse in near-fault regions from the magnitude of the earthquake. Response spectra at Shapur station have lower spectral amplitudes as compared to codal response spectral amplitudes (IS 1893: 2002); this is attributed to smaller magnitude of Dharamsala earthquake. Keywords: Dharamsala Earthquake; Near-Fault Ground Motion; Pulse-Characteristics; Response-spectra; Wavelet INTRODUCTION The moderate-sized shallow-focus earthquake namely, the Dharamsala earthquake of 1986 (M w = 5.5) occurred in the Himalayan region. The Dharamsala earthquake was recorded at nine stations of the Kangra array (Chandrasekekran, 1988). The strong motion recordings of this moderate-sized earthquake have been analyzed to identify near-fault pulses and study their characteristics. Near-fault pulses are attributed to directivity, fling, hanging wall and vertical effect, and are normally observed in the records obtained within 20 to 30 km from the fault. These pulses possess immense damage potential specifically for long and intermediate period structures. To allow the pulse detection, standard methodology proposed by Baker (Baker, 2007) has been adopted. Out of nine strong groundmotion recordings, only one recording showed pulse type characteristics. Methodology adopted to detect these near-fault pulses along with their characteristics and comparison of estimated pulse

2 periods and pulse amplitudes in the present study with those computed from the available relationships between magnitude-pulse periods, magnitude-pulse amplitudes and distances using world wide data forms the subject matter of this paper. PULSE DETECTION METHODOLOGY From the available nine recordings, only one record is selected based on the criteria of epicenter distance (D 30 km) and PGA ( 0.2 g) for further analyses due to its engineering significance and is listed in Table 1. The location of the earthquake and respective location of site are shown Figure 1. Figure 1 Map shows the earthquake and respective station that contain Near-Fault pulse. Table 1. Strong motion parameters of the selected recording Earthquakes (Location) Station (Location) Epi.Dst (km) PGA (g) PGV (cm/sec 2 ) Dharamsala Shapur ( N, E) ( N, E) The pulse period forms a key parameter for structural engineers because the structural response due to pulse waveform or pulse type motion depends on the ratio of fundamental period of the structure to the pulse period (Alavi and Helmut, 2001; Alavi and Krawinkler, 2000; Anderson and Bertero, 1978; Anderson et al., 1999; Baker, 2007; Bertero et al., 1978; Mahin et al., 1976). The wavelet analysis adopted by Baker (Baker, 2007) have been implemented to extract the largest velocity pulse from a given ground motion for the purpose of finding the pulse period (Tp) and location of the pulse. The pulse extracted using this procedure clearly captures the velocity pulse while filtering the high frequency ground motion contained in the original ground motion. This technique can be used to extract pulse from any type of ground motion whether a significant directivity pulse exist or not. The significance of pulse type ground motion depends on the difference between residual and original time history. Two predictor variables, i.e., PGV ratio of residual record to original record, and the energy ratio of the residual record to original record, has been adopted as Pulse Indicator (PI) to predict the likelihood that a given record is pulse like or non-pulse like (Baker, 2007). If the value of PI is above 0.85 the record contains pulse type ground motion and if the value is below 0.15 the record is non-pulse type. For computing PI the following expression has been adopted (Baker, 2007). 1 PI 1 exp( ( PGVratio ) 20.5( Energyratio ))

3 As the pulse-type ground motion due to directivity effect is more dominant in the fault-normal direction (Somerville, 2003), the recorded time histories at Shapur station were oriented in faultnormal direction. For this purpose the fault plane solutions given by Molner (Molner et al., 1989) and the method suggested by Somerville (Somerville, 2002) has been used. The largest velocity pulses have been extracted and their pulse periods predicted from the fault-normal component using seven mother wavelets. Daubechies (db4 and db7), Haar (haar), Symlet (sym4), Coiflets (coif2) are orthogonal wavelets, whereas Reverse biorthogonal (rbio2.4) and Biorspline (bior1.3) are biorthogonal wavelets (Sabegh, 2010). Different mother wavelets, because of their variability, gave different values of pulse indicators and pulse periods of the extracted long period velocity pulses as tabulated in Table 2. Table 2. Pulse indicators and pulse periods using different mother wavelets for fault-normal components of strong ground motion at Shapur station. Shapur (Fault-Normal Component) Mother Wavelet Pulse Indicator (PI) Tp, Pulse Time Period db db haar sym coif rbio bior INTERPRETATION OF RESULTS The pulse periods (pseudo-period (Tp)) of the extracted pulses using seven mother wavelets are different. However, it is important to decide among the seven mother wavelets, which mother wavelet is better in extracting the parameters of the pulse type ground motion. This can be achieved on the basis of comparative study between pulse periods (Tp) obtained using different wavelets and the spectral pulse periods (Tv-p) of the ground motion and to compare the spectral shapes of extracted pulse-forms using seven wavelets with the spectral shapes of the ground motion. The pulse period of the velocity pulse has been estimated by identifying clear and global peak in the velocity response spectrum of the ground motion (Alavi and Helmut, 2000; Alavi and Krawinkler, 2001). Further, from the wavelet analysis it has been observed that the estimated pulse period (Tp) is generally larger than the pulse period (Tv-p) estimated from spectral velocity response spectra (Baker, 2007). Based on these observations, the near-fault ground motions from the Dharamsala earthquake have been interpreted as follows. Shapur Station: The orientation of recorded transverse component of ground motion at this station is in the fault-normal direction. This record has been analyzed using seven mother wavelets and the results are listed in Table 3. It is evident from this table that both Daubechies wavelets (db4 and db7) are capable of extracting the pulse-form from the velocity time history efficiently as both have pulse indicators (PI) above 0.85 and computed pulse periods (Tp) are 0.53 sec and 0.52 sec respectively. However, the other wavelets are unable to extract the pulse-form efficiently as the values of pulse indicators (PI) are below The Fault Normal Component of recorded ground motion along with their extracted pulses (db4 and db7) is shown in Figure 2. The selected time domain windows for strong ground motion is up to 5 sec because captured pulse is located in this time domain window. Figure 3 shows that pulse period (Tv-p) associated with spectral velocity of ground motion is 0.4 sec. From the shapes of the spectral velocity curves as shown in Figure 3 it is clear that the pulse obtained using db7 wavelet is close to the pulse hidden in the ground motion because spectral velocity curve of db7 wavelet is well matched and showed almost same trend as that of the spectral velocity curve of the ground motion in the intermediate-period-range as compared to the pulse shape obtained using db4 wavelet. Hence, db7 wavelet is more efficient in extracting the pulse with pulse period (Tp) 0.52

4 sec and pulse indicator (PI) 0.94 as compared to other wavelets. Further, the value of pulse indicator using db7 is 0.94 which is larger than the value of pulse indicator 0.90 obtained from db4. Figure 2 Fault Normal component of recorded ground motion along with their extracted pulses (db4 and db7). Figure 3 Velocity spectra of fault-normal component and extracted pulses at Shapur station (Dharamsala earthquake) COMPARISON OF THE ESTIMATED AND COMPUTED PULSE PERIODS AND AMPLITUDES. Appliclability of various available relationships between magnitude and pulse period (Alavi and Helmut, 2001; Alavi and Krawinkler, 2000; Bray and Marek, 2004; Ghahari et al., 2010; Mavroeidis et al., 2003; Rodriguez, 2000; Somerville, 1998; Somerville et al., 1999; Somerville, 2003) has been examined and the results are presented in Table 3. The variability in the computed pulse periods may be attributed to several factors namely, the relationships are based on different type of data sets, different methodologies adopted to estimate pulse periods, types of source mechanisms and site characteristics below the recording stations to mention few. Computed pulse periods using various relationships are generally on higher side as compared to the estimated pulse period. However, the closest match seems to be with the two relationships given by Bray (Bray and Marek, 2004) and Somerville (Somerville et al., 1999). The observed pulse period is short and this might be due to compressional tectonic envoirnment and thrust type focal mechanism. This aspect needs further study when more Near-Fault strong motion data become available. From the available relationships between magnitude, distance and peak horizontal velocity developed adopting different data sets, (Somerville, 1999; Alavi and Helmut, 2001; Bray and Marek, 2004; Rodriguez, 2000) the fault-normal peak horizontal velocity for the Dharamsala earthquake has been computed. The computed values have been compared with estimated value in the present study and

5 the results are presented in Table 4. The closest match seems to be with the relationship given by Somerville et al Table 3. Comparison of estimated pulse period with computed pulse periods from available relationships Earthquakes Mw WA (Tp) Tv-p [1]* [5]* [7]* [9]* [11]* [16]* [19]* [18]* Dharamsala *References Table 4. Comparison of estimated pulse amplitude with computed pulse amplitudes from available relationships Earthquakes Station R PGV [1]* [7]* [14]* [19]* Mw (FN) (km) (cm/sec) (cm/sec) (cm/sec) (cm/sec) (cm/sec) Dharamsala Shapur *References CONCLUSIONS Daubechies wavelets of order four and seven are found to be more efficient as compared to other mother wavelets in extracting the pulse-type characteristics from the strong ground motion recordings. The fault-normal component of ground motion at Shapur station showed pulse-type ground motion with the pulse period (Tp) 0.52 sec and pulse indicator Computed pulse periods using various relationships don t agree with the estimated pulse period to estimate the NFGM characteristics namely the pulse periods and peak amplitudes of the velocity pulse. The observed pulse period is short and this might be ascribed to compressional tectonic envoirnment and thrust type focal mechanism. ACKNOWLEDGEMENTS First author acknowledges with thanks the research fellowship received from the Ministry of Human Resource Development, Government of India and Department of Earthquake Engineering, IIT Roorkee. REFERENCES 1. Alavi G and Helmut K. (2001). Effects of Near-Fault Ground Motions on Frame Structures, The John A. Blume Earthquake Engineering Center, and Stanford University., Alavi G and Krawinkler H. (2000). Consideration of Near-Fault Ground Motion Effects in Seismic Design, Proceedings 12 th World Conference on Earthquake Engineering, New Zealand. 3. Anderson J and Bertero V. (1978). Uncertainties In Establishing Design Earthquakes, ASCE Journal of Structural Engineering., 113 (8), Anderson J, Bertero V and Bertero R. (1999). Performance Improvement of Long Period Building Structures Subjected to Severe Pulse-Type Ground Motion, Pacific Earthquake Engineering Research Center, University of California, Berkeley., Baker JW. (2007). Quantitative Classification of Near-Fault Ground Motions Using Wavelet Analysis, Bulletin of Seismological Society of America., 97 (5),

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