After the first (main) earthquake, other smaller shocks may occur at times for: about a day
several hours
several weeks
almost a year

It's basically a smaller earthquake that happens after the main quake, in the same area. If it's stronger than the first quake, it's renamed as the main earthquake and the original main quake becomes known as a foreshock. Aftershocks can happen for up to two years after the original earthquake, losing power over time. So i'd say it's either between, Several weeks and a year.

because natural gas is colorless, odorless, and tasteless, distributors add mercaptan (a chemical that smells like sulfur) to give natural gas a distinct unpleasant odor (it smells like rotten eggs). they locate the types of rock that are likely to contain natural gas deposits.