, North Sumatra The longest axis of the island runs approximately northwest–southeast, crossing the equator near the centre. At its widest point, the island spans . The interior of the island is dominated by two geographical regions: the
Barisan Mountains in the west and swampy plains in the east. Sumatra is the closest Indonesian island to mainland Asia. To the southeast is
Java, separated by the
Sunda Strait. To the north is the
Malay Peninsula (located on the Asian mainland), separated by the
Strait of Malacca. To the east is
Borneo, across the
Karimata Strait. West of the island is the
Indian Ocean. Sumatra is a highly seismic island. The
Great Sumatran fault (a
strike-slip fault), and the
Sunda megathrust (a
subduction zone), run the entire length of the island along its west coast. Huge earthquakes have been recorded throughout history. In 1797, an 8.9 earthquake shook Western Sumatra, and in 1833, a 9.2 earthquake shook Bengkulu and Western Sumatra. Both events caused large
tsunamis. Earthquakes are very common throughout the coastal area of the west and center of the island, and tsunamis are common due to the high seismicity in the area. On 26 December 2004, the western coast and islands of Sumatra, particularly
Aceh province, were struck by a
tsunami following the
Indian Ocean earthquake. This was the longest earthquake recorded, lasting between 500 and 600 seconds (8.33–10 minutes). More than 170,000 Indonesians were killed, primarily in Aceh. Other recent earthquakes to strike Sumatra include the
2005 Nias–Simeulue earthquake and the
2010 Mentawai earthquake and tsunami.
Lake Toba is the site of a
supervolcanic eruption that occurred around 74,000 years ago, representing a climate-changing event. The most important rivers in Sumatra belong to the catchment area of the
South China Sea. Heading north to south, the
Asahan,
Rokan,
Siak,
Kampar,
Indragiri,
Batanghari flow into the
Malacca Strait, while the island's largest river, the
Musi, flows into the sea at
Bangka Strait in the south. To the east, big rivers carry silt from the mountains, forming the vast lowland interspersed by swamps. Even if mostly unsuitable for farming, the area is currently of great economic importance for Indonesia. It produces oil from both above and below the soil –
palm oil and
petroleum. Sumatra is the largest producer of
Indonesian coffee. Small-holders grow Arabica coffee (
Coffea arabica) in the highlands, while Robusta (
Coffea canephora) is found in the lowlands. Arabica coffee from the regions of Gayo, Lintong and Sidikilang is typically processed using the
Giling Basah (wet hulling) technique, which gives it a heavy body and low acidity. == Largest cities ==