Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes in Northwestern India

The city of Ahmedabad of Gujarat in western India suffered severe damage during the 2001 $Mw$ 7.7 Bhuj earthquake, despite being $∼250km$ away from the epicenter. Similar damage patterns were also reported during the 1819 Allah Bund earthquake (⁠$Mw$ 7.8). To investigate the probable causes, we employed an integrated approach using multichannel analysis of surface waves (MASW), single and array microtremor measurements, broadband earthquake data, and geotechnical investigations. Significant differences in site characteristics are observed in both the damaged and undamaged areas. The investigations revealed shear‐wave velocity values in excess of $320m/s$ and less than $220m/s$ in the undamaged and damaged areas, respectively, in the top 6 m of the subsurface. The refusal (⁠$N1(60)$⁠) values are observed at 6–20 m and 20 m depth onward in the undamaged and damaged areas, respectively. The amplification factor in the damaged areas varies from 3.3 to 6.6 in the 1.5–2.0 Hz frequency range. On the other hand, the amplification factor in the undamaged areas varies from 1.0 to 3.0 in the 0.7–1.5 Hz frequency range. Nevertheless, the damages were mostly restricted to mid‐to‐high‐rise buildings, located in the western side of the Sabarmati River, where the presence of a paleochannel is reported, and in some southeastern parts of the city, along lakes and ponds. The low‐rise buildings on the eastern side of the river in the old city area remained almost intact. Our study confirms that local site effects together with the poor quality of construction contributed to the damages.