How strong are the structures can only be seen when there is an earthquake of a magnitude of just 7.5 Richter scale.
It is obviously not going to be pretty. Specially in Dhaka due to the density of high rises it will be disastrous. And if it happens during a flood, then the damage will be too much. To be honest an earthquake like this will impact every city. Even Tokyo or NY will take huge damage in such an earthquake. You remember 2011 earthquake in Japan?
The magnitude was 9.0 in Richter scale. But the epicenter was in Pacific ocean and quite far from Japan. It caused 16,000 death. While 120k buildings totally collapsed and 400k buildings half collapsed.
https://en.wikipedia.org/wiki/2011_Tōhoku_earthquake_and_tsunami
In Hanshin earthquake which was
6.9 in richter scale and epicenter was in Kobe. 400k buildings were damaged and 22% buildings in Kobe were unusable. That was in 1995, but
I do not think BD's structures are any better than Japanese structures of 1995. In 2008,
Sichuan earthquake in China which was of 8.0 richter scaledestroyed 80% buildings of that province. And also 68k people lost their lives. So, you can understand the damage? It will be as deadly as an atom bomb.
https://en.wikipedia.org/wiki/Great_Hanshin_earthquake
https://en.wikipedia.org/wiki/2008_Sichuan_earthquake
But an earthquake of this scale is unlikely in Dhaka.
At least the epicenter of such earthquake should not be near Dhaka. An Earthquake of 7.5 magnitude from anywhere in Bangladesh can be felt from Dhaka. In 2015, an
earthquake in Nepal affected Dhaka. It was of 7.8-8.1 richter scale. Epicenter was in Kathmundu. 9k people died. From which 8.8k in Nepal, 78 in India, 25 in China and 4 in BD. One building in BD was partially damaged.
https://en.wikipedia.org/wiki/April_2015_Nepal_earthquake
The earthquake threats in BD mostly comes from India or Nepal. Indian Assam, and Nepal have tons of fault lines. Sylhet and Chittagong have some fault lines that can cause high magnitude earthquakes. Richter scale magnitude increases in logarithmic scale. So the energy required to cause a 8.0 magnitude is much much higher than the energy required to cause an earthquake of 7.0. Thus an earthquake of 8.0 if much deadlier than that of 7.0. Also other than the richter scale, the damage also depends on the time period of the first shock and the after shocks. Most of the damage in Kobe earthquake was a result of the aftershocks which were around 4.5-6.2 richter scale. However the further you are away from the epicenter the less the damage.
Now take a look at the map.
The danger of Dhaka comes from Sylhet. Dauki fault line near Sylhet and Indian Shiliguri can cause an earthquake up to 7.3 scale. It will cause damage to Dhaka The bigger threat comes from Indian Assam. You can see there is a large circle in Indian border
that can cause an earthquake of greater than 8 richter scale. This one can cause huge damage to Sylhet and will also impact Dhaka but not by that much.
So, you can see the possibility of a 7.5 magnitude of earthquake hitting near Dhaka is rather low. However earthquake from other areas impacting Dhaka is quite high.
Dhaka is placed in moderate risk earthquake zone. That's because of the large population in Dhaka. Also usually major damage in earthquake is caused by other elements such as tsunami or fire or worse nuclear disaster.
However, as per your question, a 7.5 richter scale earthquake in Dhaka will pretty much destroy the whole city with it's infrastructures. An earthquake like this will likely do the same to pretty much every major city. If it happens in Tokyo, then may some very well built infrastructures might survive but most will not. In Dhaka hardly anything will survive. Lets pray such an earthquake never occurs in Bangladesh or in any other country.
