Could an earthquake on flat land still kill someone without any buildings/boulders or such near by?
Yes, a man in the open sitting on grass could be killed by a magnitude 8 or 9 earthquake, but in the circumstances you describe it would be very unlikely. Not all large earthquakes produce huge rifts on land, but if you were unlucky enough to have one open up beneath you then the chances are you would fall into it. Another danger would be liquefaction. In some places the moisture content of the land is such that although in normal circumstances it is perfectly solid and stable, when subjected to the violent shaking of an earthquake a phenomenon called liquefaction occurs. The ground becomes a sort of quicksand into which buildings can collapse and humans disappear. It is quite rare, however, and fatalities among people are unlikely if they are out in the open where nothing can fall on them, though I dare say a few unlucky ones might have a heart attack. On hard ground,though, even a fit and healthy young man might be severely bruised and shaken in a magnitude 9 quake.