West Sumatra’s 2009 Earthquake: Why a Deep Rupture Devastated Padang

On 30 September 2009, a magnitude-7.6 earthquake struck off Indonesia’s West Sumatra coast in the late afternoon. It killed 1,117 people, according to the regional disaster response coordination agency (Satkorlak PB), and heavily damaged 135,448 homes. Padang, the provincial capital, and the surrounding Pariaman area were among the places hit.

The earthquake began at 5:16 p.m. local time, according to Indonesia’s geological agency, citing the national meteorological agency’s readings. CNBC Indonesia reported an account from a survivor identified as Ismael, who said a landslide buried him up to his neck as he tried to leave his house. Padang’s disaster management agency records extensive damage across several West Sumatra districts and cities.

Daryono, an expert from the Indonesian Disaster Experts Association, told CNBC Indonesia that the earthquake originated from a break within the Indo-Australian oceanic plate as it descended beneath the Eurasian plate, rather than from a rupture along the boundary between the plates. Daryono placed the source roughly 80 to 90 kilometres deep. Indonesia’s geological agency reported a measured depth of 71 kilometres at the time; both accounts place the quake well below the seabed. The agency said the depth was why it did not generate a tsunami.

Depth offered little protection from damage on land. Daryono said the rigid oceanic rock transmitted seismic waves efficiently. He also identified soft alluvial ground in Padang as a factor that amplified shaking, while landslides worsened destruction in the Pariaman hills.

Daryono’s account of the 2009 disaster concerns a rupture inside the descending plate itself.