Mag 5 would be the lowest but better find one the lowest estimate of a big earthquake. I don't think anything below 6 is considered a big one.
How numbers on the Richter scale translate into human and property casualties
www.cbsnews.com
This site claims 6 and above as strong and 7 and above as major.
Well I asked bing also and the answer is the same. A big earthquake would probably refer to 7 or beyond. Here is Bing's answer:
Earthquakes are classified based on their magnitude, which is a measure of the energy released during an earthquake. Here's a general classification¹³:
- 2.5 or less: Usually not felt, but can be recorded by seismograph. These occur millions of times each year.
- 2.5 to 5.4: Often felt, but only causes minor damage. There are about 500,000 of these each year.
- 5.5 to 6.0: Causes slight damage to buildings and other structures. There are about 350 of these each year.
- 6.1 to 6.9: May cause a lot of damage in very populated areas. There are about 100 of these each year.
- 7.0 to 7.9: Considered a major earthquake, causing serious damage. There are about 10-15 of these each year.
- 8.0 or greater: Considered a great earthquake, can totally destroy communities near the epicenter. These occur once every year or two¹.
So, an earthquake with a magnitude of
7.0 or higher is generally considered "big" as it can cause serious damage³. However, the impact of an earthquake also depends on other factors such as depth, distance from the epicenter, population density, and local building standards.
There are also sources for these. No idea if you would like them also.