Log checking (cracking along the length) is a resistance to shrinkage as wood at the log surface dries and shrinks more quickly than the interior of the log. This resistance creates stress that is relieved by cracking. It’s the same process that causes shrinkage cracks in concrete.
Although it may look severe, checking is not a structural concern. Checks on log exteriors may trap water, causing decay, so they should be filled with an appropriate material.