2001 Bhuj, India, Earthquake Engineering Seismoscope Recordings and Eastern North America Ground-Motion Attenuation Relations

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1 Bulletin of the Seismological Society of America, Vol. 93, No. 3, pp , June Bhuj, India, Earthquake Engineering Seismoscope Recordings and Eastern North America Ground-Motion Attenuation Relations by Chris H. Cramer and Ashok Kumar Abstract Engineering seismoscope data collected at distances less than 300 km for the M 7.7 Bhuj, India, mainshock are compatible with ground-motion attenuation in eastern North America (ENA). The mainshock ground-motion data have been corrected to a common geological site condition using the factors of Joyner and Boore (2000) and a classification scheme of Quaternary or Tertiary sediments or rock. We then compare these data to ENA ground-motion attenuation relations. Despite uncertainties in recording method, geological site corrections, common tectonic setting, and the amount of regional seismic attenuation, the corrected Bhuj dataset agrees with the collective predictions by ENA ground-motion attenuation relations within a factor of 2. This level of agreement is within the dataset uncertainties and the normal variance for recorded earthquake ground motions. Introduction Strong ground motion records from large earthquakes (M 7) in eastern North America (ENA) have not yet been recorded. Ground-motion attenuation relations for ENA are based on stochastic modeling using constraints from smaller magnitude earthquakes (M 6) (Atkinson and Boore, 1997; Toro et al., 1997). Assuming conditions are analogous to those in the ENA, records from large intraplate earthquakes may serve as a guide on ground-motion predictions in ENA. The M 7.7 Bhuj, India, earthquake of 26 January 2001 was such an earthquake. Whether or not the Bhuj earthquake is an analog for M 7 ENA earthquakes, such as the New Madrid earthquakes, is not discussed here. Instead, the reader is referred to the papers presented in the Bhuj special session at the spring American Geophysical Union meeting of 2001 (EOS, 2001). The purpose of this short note is to compare the available Bhuj strong-motion data for epicentral distances less than 300 km to five ENA ground-motion attenuation relations for peak ground acceleration (PGA) and 1.0 sec spectral acceleration (Sa). The reader can evaluate the appropriateness of the two hypotheses that (1) the Bhuj earthquake can be treated as an intraplate event, and (2) the western India subcontinent has similar ground-motion attenuation characteristics to ENA. Data and Approach The structural response recorder (SRR) is an engineering seismoscope consisting of six pendulums (Fig. 1) that scratch their displacement response on a smoked watch glass mounted on each pendulum. The pendulums idealize singledegree-of-freedom systems having three periods (0.4, 0.75, and 1.25 sec) at two levels of damping (5% and 10%). Krishna and Chandrasekaran (1962) have presented the various design features of the SRR, which contains low-cost pendulum seismoscopes and is designed to represent the dynamic response of structures to strong ground motion. Each pendulum of the SRR is calibrated in the laboratory prior to installation, and the archived calibrations are used to convert scratch length to an estimate spectral motion. Although SRRs have been shake-table tested in the laboratory, revealing the influence of viscous and frictional damping and calibrating each instrument s response (Kumar and Pankaj, 1984), the best field test of the instrument took place during the magnitude 6.6 Uttarkashi earthquake of October Records were obtained from an SRR and an SMA-1 accelerograph installed side by side at Bhatwari. Using the recorded time history and calibration of each pendulum of the SRR, computer plots of SRR records were determined to be a good match (within 3%) between actual and computed SRR records (Chandra et al., 1995). (Note: All three papers cited in this paragraph are available as PDF files from either author via .) Within 300 km of the epicenter of the M Bhuj, India, earthquake, 13 SRRs and a digital strong-motion accelerograph (DSA) recorded the mainshock (Kumar et al., 2001). Figure 2 shows their locations. Table 1 presents the station information and the interpreted data. Note that the SRR data were recorded on the ground floor of one- or twostory buildings. The data of Table 1 were derived as follows. The response spectrum of the DSA accelerogram has been determined by Kumar et al. (2002). For the SRRs, Kumar et al. 1390

2 Short Notes 1391 Figure 1. Picture of a structural response recorder (uncased) showing the sixpendulum system, which records at three periods and two damping values. 25 N PAKISTAN 70 E 75 E 25 N Niruna Naliya Anjar Kandla Dwarka Khambhaliya Jamjodhpur Porbandar Amreli Junagarh INDIA Ahmedabad Anand Cambay ARABIAN SEA 20 N 70 E 75 E 20 N Figure 2. Map showing strong-motion recording sites within 300 km of the 26 January 2001 M 7.7 Bhuj, India, earthquake. The star indicates the epicenter of the mainshock, the inverted triangles show the location of the structural response recorders, and the circle shows the location of the digital strong-motion accelerograph (Ahmedabad).

3 1392 Short Notes Table 1 Recording Sites Coordinates Epicentral Distance Sa-5% Damped Uncorrected Corrected Site (Name) N ( ) E ( ) (km) 0.4 sec 0.75 sec 1.25 sec PGA 1.0 sec Sa PGA 1.0 sec Sa Site Conditions* Comment Ahmedabad Q DSA, ground floor 10-story building Anjar T SRR, ground floor 1-story building Kandla T SRR, ground floor 2-story building Q Niruna Q SRR, ground floor 2-story building Naliya T SRR, ground floor 1-story building Cambay Q SRR, ground floor 2-story building Ahmedabad Q SRR, ground floor 2-story building Jamjodhpur R SRR, ground floor 1 story building Dwarka ** Q SRR, ground floor 1-story building Porbandar Q SRR, ground floor 2-story building Junagarh R SRR, ground floor 2-story building Khambhaliya R SRR, ground floor 2-story building Anand Q SRR, ground floor 2-story building Amreli R SRR, ground floor 1-story building *Q, Quaternary sediments (assumed NEHRP D); T, Tertiary sediments (assumed NEHRP C); and R, rock, basalt or metamorphic (no geologic correction applied or NEHRP site class assumed). Estimated from Sa /4, which uses a ratio of Sa 1.25 /Sa (see text). **Estimated from 10% damping values for Sa at 1.25 and 0.75 sec (0.110g and 0.054g, respectively) to give a ratio of Sa 1.25 /Sa The 5% damped Sa at 1.0 sec is thus estimated using 3.04 Sa 1.25 /2. (2001) estimated PGA from the 0.4 sec, 5% damped pendulum record by dividing by a standard factor of 2.5 from IS-1893 (2001). We have estimated 1.0 sec Sa values from the SRR data listed in Kumar et al. (2001) by averaging the 0.75 and 1.25 sec, 5% damped values (see Table 1). A few stations had only the 0.75 or the 1.25 sec response value at 5% damping. In these cases, the ratio Sa(1.25 sec)/sa(0.75 sec) 2.0 was assumed, based on the observations from sites that recorded both values (see footnotes to Table 1). Obviously, the SRR estimates have larger uncertainty than the DSA value, but are probably within 20% of the actual values in light of the comparison in the next paragraph. For one city, Ahmedabad, both SRR and DSA records exist. These recordings are not collocated, but were both recorded 238 km from the epicenter. The PGA and 1.0 sec Sa estimates from both records were used as a check on the SRR estimates for these ground motions. Table 1 shows this comparison. The PGA and 1.0 sec Sa values agree within 10% and 15%, respectively, which is encouraging, albeit only one example. The fact that the DSA record is from the ground floor of a 10-story building could complicate the interpretation of this SRR DSA comparison. But the response spectra from the DSA record (Kumar et al., 2002) show narrow response peaks between 0.3 and 0.8 sec that do not effect PGA and 1.0 sec Sa response. The PGA and 1.0 sec Sa values had to be corrected, as best as possible, to a common soil condition for comparison to the ENA ground-motion attenuation relations. We chose the National Earthquake Hazards Reduction Program (NEHRP) B/C boundary (V s m/sec) of Frankel et al. s (1997) U.S. national seismic hazard maps as the common site condition. The Bhuj strong-motion data were recorded on a variety of soil conditions, including rock (basalt and metamorphic rocks). The surface geology at each recording site was determined from the Geologic Map of India (1962), at a scale of 1:2,000,000, using the classification scheme of Quaternary sediments (Q) as NEHRP D, Tertiary sediments (T) asnehrp C, and rock (R). Only at one recording site, Kandla, was the geologic setting difficult to determine between Q and T. So the values at Kandla were corrected using both soil conditions. Nevertheless, there is a lot of uncertainty in deducing site condition from geologic maps. The correction factors to V s 760 m/sec site conditions were determined using the factors of Joyner and Boore (2000) except for R, which was not corrected for geology and has no assumed NEHRP site class. These geological correction factors contribute some additional uncertainty to the comparisons presented subsequently. Results Figure 3 shows the comparison of the corrected Bhuj strong-motion data for distances less than 300 km with five ENA ground-motion attenuation relations. The second closest site (Kandla) to the epicenter is represented by two soil conditions, the highest value corresponding to T and the lowest to Q. The ENA ground-motion attenuation relations used in the comparison are Atkinson and Boore (1995), Frankel et al. (1996), Toro et al. (1997), Somerville et al. (2001), and Campbell (2001). As recommended by D. Boore (personal comm., 2002), the Atkinson and Boore M 7.7 curve was computed using the same stochastic model as their original tables. The first relation uses a two-corner, point-source model, and the second and third relations use a single-corner,

4 Short Notes 1393 A Figure 3. Comparison of the corrected Bhuj strong-motion data with five eastern North America ground-motion attenuation relations: (a) peak ground acceleration, and (b) 1.0 sec spectral acceleration (Sa). Groundmotion attenuation relations shown are Atkinson and Boore (1995) (dotted line), Frankel et al. (1996) (solid line), Toro et al. (1997) (dot-dashed line), Somerville et al. (2001) (long-dashed line), and Campbell (2001) (short-dashed line). Circles are the structural response recorder data, the square is the Ahmedabad digital strong-motion accelerograph datum, and NEHRP B/C implies a V s30 of 760 m/sec. For the 1.0 sec Sa data, the question marks indicate values estimated from the only available values at 0.75 sec (see Table 1 and its footnotes). There are two estimates from the Kandla SRR due to uncertain geological conditions (see text). B point-source model. The fourth relation uses a finite-source model, and the fifth relation uses a hybrid modeling approach. All relations except the second have been adjusted to the NEHRP B/C site condition using the method of Frankel et al. (1996). The second relation is already for the NEHRP B/C site condition and needs no such adjustment. The measure of the distance to the source is different for various ENA ground-motion attenuation relations. Atkinson and Boore (1995) and Frankel et al. (1996) used hypocentral distance, whereas Toro et al. (1997) and Somerville et al. (2001) used the closest distance to the surface projection of the fault (Joyner Boore distance), and Campbell (2001) used the closest distance to the rupture. For the Bhuj data, Kumar et al. (2001) used epicentral distance. In Figure 3, epicentral distance is the measure used. All the groundmotion attenuation distance measures have been adjusted to epicentral distance using a Bhuj rupture, which is approximated using a circular rupture with a radius of 20 km centered on the hypocenter at a depth of 22.5 km (estimates range from 20 to 25 km) and a rupture passing through the hypocenter with a southward dip of 45. The conversion to epicentral distance for the Campbell (2001) relation still has an azimuthal variation in this case, so we used an average 2 2 correction of D e (D c 17 ), where D e is epicentral distance and D c is the closest distance to the rupture. At epicentral distances greater than 40 km, the correction to epicentral distance is less than 4 km. Figure 3 presents the comparisons for both PGA and 1.0 sec Sa. Generally, the Bhuj data fall among the ENA groundmotion attenuation relations, although this is less the case

5 1394 Short Notes for 1.0 sec Sa. For the 1.0 sec Sa estimates, the values beyond an epicentral distance of 250 km appear to be lower than the ENA relations. Overall, the Bhuj strong-motion data agree with the collective predictions of the ENA relations within the random uncertainty in ground-motion measurements of a factor of 2 or more and the ground-motion attenuation relation modeling uncertainty (Fig. 3). Certainly large uncertainties exist in the Bhuj data presented in Figure 3. We have tested to see whether radiation pattern effects are present in the Bhuj strong-motion data (Vidale, 1989), but no recording site seems to lie within a strong S-wave node or antinode. Any radiation pattern effects appear to be small compared to the geological site corrections. There may be some effect of recordings having been made in buildings, but this effect should be small given that all of the SRR recorders are on the ground floor of oneand two-story buildings. And, as indicated above, the DSA record appears unaffected at PGA and 1.0 sec Sa by the response of the 10-story building in which it is located. Thus, structural contamination in the data should be less than the geological corrections. Although one can question (1) how well the Bhuj tectonic setting serves as an analog for ENA earthquakes, (2) whether Indian and ENA seismic wave attenuations are similar, and (3) the reliability of the Bhuj SRR strong-motion estimates, the results of this comparison suggest that, from an engineering perspective, the Bhuj data broadly confirm the collective predictions of ENA ground-motion attenuation relations. The reader must decide the extent to which this may be true in detail. Acknowledgments The authors wish to thank Steve Horton for information about the range of mainshock locations and focal mechanisms and for information about the velocity structure (Horton et al., 2001) beneath the State of Gujarat that was used in the radiation pattern analysis. The article was greatly improved due to reviews by Joan Gomberg, Paul Bodin, Tony Crone, and Gail Atkinson, for which the authors are greatly appreciative. References Atkinson, G. M., and D. M. Boore (1995). Ground motion relations for eastern North America, Bull. Seism. Soc. Am. 85, Atkinson, G. M., and D. M. Boore (1997). Some comparisons between recent ground motion relations, Seism. Res. Lett. 68, Campbell, K. W. (2001). Prediction of strong ground motion using hybrid empirical method: example application to eastern North America, Final report to the U.S. Geological Survey, 3 December 2001, ABS Consulting, Inc. and EQECAT, Inc., Beaverton, Oregon, 49 pp. Chandra, B., A. Kumar, and M. K. Bansal (1995). A study of strong motion data obtained during Uttarkashi Earthquake of 20 October 1991 from INSMIN network, in The Uttarkashi Earthquake, H. K. Gupta and G. D. Gupta (Editors), Geological Society of India, Memoir No. 30, EOS (2001). Continental intraplate seismicity: the M 7.7 Gujarat, India earthquake (January 26, 2001) (abstracts), EOS 82, S256 S257 and S260 S262. Frankel, A., C. Mueller, T. Barnhard, D. Perkins, E. V. Leyendecker, N. Dickman, S. Hanson, and M. Hopper (1996). National Seismic- Hazard Maps: Documentation June 1996, U.S. Geol. Surv. Open-File Rept , (last accessed 31 March 2003). Frankel, A., C. Mueller, T. Barnhard, D. Perkins, E. V. Leyendecker, N. Dickman, S. Hanson, and M. Hopper (1997). Seismic-Hazard Maps for the Conterminous United States, U.S. Geol. Surv. Open-File Rept , scale 1:7,000,000. Geologic Map of India (1962). Sixth edition, Geological Survey of India, scale 1:2,000,000. Horton, S., P. Bodin, and M. Withers (2001). Chapter 3, Aftershock Measurements, in The Bhuj Earthquake of 2001, D. Abrams, M. Aschheim, P. Bodin, S. Deaton, S. Dotson, D. Frost, S. K. Ghosh, S. Horton, A. Johnston, J. Nichols, T. Rossetto, M. Tuttle, and M. Withers (Editors), Mid-America Earthquake Center, CD Release 01-04, Urbana, Illinois, IS-1893 (2001). Criteria for Earthquake Resistant Design of Structures, Indian Standard Institution, New Delhi. Joyner, W. B., and D. M. Boore (2000). Recent developments in earthquake ground motion estimation, in Proc. of the 6th International Conf. on Seismic Zonation, November 2000, Palm Springs, California, Earthquake Engineering Research Institute, Oakland, California. Krishna, J., and A. R. Chandrasekaran (1962). Design of structural response recorder, in Proc. 2nd Symposium on Earthquake Engineering, University of Roorkee, Roorkee, India, Kumar, A., and Pankaj (1984). On SRR records considering effect of frictional damping, in Proc. Earthquake Effects on Plants and Equipment, Hyderabad, India, December 1984, Kumar, A., S. Basu, S. K. Thakkar, M. Shrikhande, P. Agarwal, J. Das, and D. K. Paul (2001). Strong motion records of the Bhuj earthquake, Presented at the International Conference on Seismic Hazard with Particular Reference to Bhuj Earthquake, 3 5 October 2001, New Delhi. Kumar, A., S. K. Thakkar, A. Bharagava, R. N. Dubey, P. Agarwal, S. Basu, and M. Shrikhande (2002). Records of instrumented buildings and study of structural response of staff quarters building of regional passport office, Ahmedabad, for Bhuj earthquake of January 26, 2001, Special volume of the Department of Science and Technology, Government of India on Scientific Works of Post 1993 Latur Earthquake. Somerville, P., N. Collins, N. Abrahamson, R. Graves, and C. Saikia (2001). Ground motion attenuation relations for the central and eastern United States, Final report to the U.S. Geological Survey, 30 June 2001, URS Group, Inc., Pasadena, California, 36 pp. Toro, G., N. Abrahamson, and J. Schneider (1997). Model of strong ground motions from earthquakes in central and eastern North America: best estimates and uncertainties, Seism. Res. Lett. 68, Vidale, J. E. (1989). Influence of focal mechanism on peak accelerations of strong motions of the Whittier Narrows, California, earthquake and an aftershock, J. Geophys. Res. 94, U.S. Geological Survey 3876 Central Ave., Suite 2 Memphis, Tennessee cramer@ceri.memphis.edu (C.H.C.) Department of Earthquake Engineering India Institute of Technology Roorkee, India akmeqfeq@iitr.ernet.in (A.K.) Manuscript received 13 September 2002.