the southern coast of honshu runs in the same direction as the nankai trough. this marks the place where the philippine sea plate moves under the eurasian plate. movement on this boundary causes many earthquakes; some of them are megathrust earthquakes. the nankai megathrust has five parts (a-e) that can break one by one. the different parts have broken either one at a time or all together many times over the last 1,300 years. this kind of quake often happens in pairs with a relatively short time between them. in addition to the two earthquakes in 1854, there were similar earthquakes in 1944 and 1946. each time, the north-eastern part broke before the southwestern part. in 1707, the earthquakes were either at the same time, or close enough together so that writing from that time do not describe them separately.