Central Java ash makes Central Java highly fertile agriculture land. Paddy fields are extensive, except in the southeastern Gunung Kidul region partly due to the high concentration of limestone and its location in a rain shadow from the prevailing weather. The largest rivers are the Serayu in the west, which empties into the Indian Ocean, and the Solo which flows into East Java. On the eve of the World War II in 1942, Central Java was subdivided into 7 residencies (Dutch "residentie" or plural "residenties", Javanese "karésiḍènan" or "karésidhènan") which corresponded more or less with the main regions of this area. These
Sumatra Trench to 350 km. The width at Sumatra trench is controlled by sediment input and accretion rate and history. One distinguishing feature of the Sumatra subduction zone is that it has a relatively 120–140 km accretionary prism and a deep forearc basin (so does the rest of the whole Java subduction zone). However it does vary along the trench. The prism is broad and has a relatively shallow surface slope in the northern part, becomes steep and narrow at the central part, then becomes steep and narrow in the southern area. The inner part of the prism forms a NW-SE arc