The first jolt, which struck off the coasts of Japan’s remote Bonin Islands, was recorded at magnitude 7.9 and up to 680 kilometers (423 miles) underground, making it one of the deepest quakes of its size. Then another oddity emerged in the cascade of aftershocks that followed: a tiny temblor that, if confirmed, would be the deepest earthquake ever detected. The ultradeep quake, described recently in the journal Geology, is estimated to have struck some 751 kilometers (467 miles) beneath the surface in the layer of our planet known as the lower mantle, where scientists have long thought earthquakes unlikely,...