How big are the cracks and at what rate are the cracks expanding? A crack that has been there for 90 years that started and then stopped was due to settling. Any other cracks since then can be due to heat/cold expansion. Keep your eye on any crack that may be "growing". If it gets to large you may need to get a contractor who can do concrete injection to stop or slow the continuous settling of your foundation. Concrete injection is where a contractor would inject concrete or slurry under the foundation with large amounts of pressure. Works quite well, my home was built in 1903 and is still standing strong due to slurry injection. In fact the contractor was so sure of his work guaranteed the work for 10 years against further settling, that was 4.5 years ago and I haven't had another problem.