The belief that our traditional methods of construction are safe because they have stood the ‘test of time’ is both true and false. The test is in how a building performs during an earthquake and many moderate ones may have been faced successfully. It is also false because the ‘big one’ has a cycle of more than 100 years and it is all too easy to believe that an old 100 year old building is proof of the durability of a particular type of construction when it may never faced the big one. The last big one in our region was in Assam in 1897 with a magnitude of 8.7M and it pretty much demolished most of the traditional structures in eastern Bhutan.
In developed countries, masonry buildings, which includes mud buildings, are not permitted for public buildings or buildings more than 2 floors.
Like it or not, traditional structures are to stay since we have so many in our country and many more keep cropping up in rural parts. We need to learn to minimize risk.
Mud and stone masonry walls are ‘non-engineered’ in the sense that there is no common system of analyzing loads and designing a safe building the way it is possible with concrete and steel and even timber. One needs to follow rules of thumb instead. Here are a few:
1. The wall is the building’s only structural system, therefore the more of it there is the stronger. You may want large windows, but it will come at the cost of the wall area. So limit the length of the windows so that the total is no more than one-third of the total wall length. This applies to every wall.
2. For a wall to perform structurally, it must be solid from the foundation to the roof. So avoid staggering windows that leave part of the wall intact downstairs and part of it upstairs. These wall bits don’t count.
3. Avoid taking openings such as doors and windows too close to the corner of the wall. Leave at least 3-4 feet intact as that is the most important part of a load bearing wall.
4. If you can’t do good plumbing, then keep water out. Water is a mud building’s worse enemy! For that matter, ensure the roof is effective in keep rain out. The foundation of a mud building must be of solid stone that rises above the ground by a good couple of feet to keep the mud wall dry.
Here’s a brief easily comprehensible page on Designing for Earthquakes on wikipedia