Complex sentence: After receiving a warning, a person may have a few seconds or in some cases significantly longer to take action, but if the epicenter is very close there may be cases in which strong tremors come ahead of any warning.
Split into simpler sentences: After receiving a warning, a person may have few seconds or in some cases, have significantly longer time to take action. However, if the epicenter is very close, there may be cases in which strong tremors come ahead of any warning.