This is a question that I actually answered in some detail three months ago, so when it came in I thought I would just point the individual at my earlier response. However, I was struck by HOW I was asked this time. To give readers a sense of what we sometimes encounter in our email in-boxes, I share this, but I'm disguising the name and identity of the questioner. You can't make this stuff up, to paraphrase Dave Barry.
Q: Hello sir
In the defination of earthquake
Eathquake is the sudden terror or shaking of earths crust which lasts for the short time. But in 2015 the earthquake in nepal not lasted for a short time . So why we use that sentance , " which last for the short time".But Generaly in most cases it not lasted for shot time. Sir please answer this question in 24 hrs please sir i sent this question you not answered please answer . Please tel me if you want to hep ,me or not. I f you want to help, if you want to make a bright student please help me . aSK YOUR OWN SOUL AND HELP.
I cannot answer questions about your soul, nor about mine. This is not something you would ever address to a scientist.
Volunteer geoscientists in the US Geological Survey do not see questions that arrive during western hemisphere weekends. Please do not blame us for not instantly replying to your questions from <Asia>.
I have no idea what definition you are referencing, since you did not provide that information. The simple answer is the larger the moment magnitude (Mm) of an earthquake, the longer the coda. In other words, the greater the energy released, the longer the apparent shaking will last. In fact, you can get a rough idea of how big a regional earthquake is by timing the shaking.