Directional damage due to near-fault and site effects in the M6.4 Changureh–Avaj earthquake of 22 June 2002

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The Changureh–Avaj earthquake (M=6.4) occurred 220 km northwest of Tehran on June 22, 2002. According to the official report, 226 people were killed, 1300 injured and 33,000 houses were seriously damaged or collapsed. Two villages located in the near-fault region with a population more than 200 people were completely destroyed. The damage survey was conducted within 30 km of the epicenter. During damage investigation, an observable pattern of damage in toppled masonry and concrete block fences was recognized. The data of damaged fences were documented and analyzed to study the effect of direction and distance from the epicenter. From the 109 fences, 85 fences were built within 15° of either north or east, providing an opportunity to study the effect of orientation. The results of analysis revealed that the east–west oriented fences toppled 2.6 times more than those oriented north–south. More than 70% of toppled fences were oriented in east–west direction, with more falling towards the north than to the south. This preferred direction of damage for toppled fences was observed in all four quadrants about the epicenter and at distance up to 15 km. In the northern quadrants almost all toppled fences were oriented east–west. Two possibilities, namely, near-fault effect and 2D or 3D site response, are examined to find an explanation for the observed damage pattern at the sites. Since the preferred direction of damage inferred from toppled fences within 15 km of epicenter was found to be parallel to the direction of rupture propagation, the near-fault forwarded rupture directivity was investigated as the most possible reason for the observed phenomenon at the sites. However, there were no records of strong motions available in near-fault zone. Thus, the near-fault ground motions were theoretically simulated by using developed local source parameters and heterogeneous slip model. Next, the principal axes were calculated by using eigenvector analysis of simulated horizontal motions. The preferred direction of damage in over 90% of the sites is well predicted by the directional dependence of near-fault ground motion. Nevertheless, the preferred direction of damage in Avaj station, where the main shock was recorded, demonstrates totally different result from the one predicted by the source. The shear-wave polarization is investigated as an alternative reason at the sites. The polarization of motion is found to be the greatest at frequencies with peaks in maximized and H/V spectra, suggesting that directional motion is associated with local site and geologic conditions. The dominant direction of motion due to source mechanism and local site conditions was identified and compared with the preferred direction of damage observed at the sites. It is found that the near-fault source effects can strongly control the motion directionality and even dictate its dominant direction to the one caused by local geologic or site conditions. Finally, the outcome provides field evidence of directional damage due to near-fault effects and local site conditions. This may aid in predicting the directions of damaging earthquake motions, and serve as an important factor in the design of critical facilities.

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