Giant layers of rocks below the ground in western New Jersey have been shifting slightly — sometimes once a day, sometimes a few times a day — all because of a sizable earthquake on April 5 that rumbled in Hunterdon County and shook houses, stores and office buildings hundreds of miles away. The occasional shaking during the past two weeks has been caused by smaller earthquakes, known as aftershocks, and the number of those has now grown close to 100, according to data from the U.S. Geological Survey, the government agency that monitors earthquake activity around the world. While many of the a…