272 Die in Indonesia Quake, Tsunami; Aon Benfield Analysis

The earthquake and the subsequent tsunami, which struck the Mentawai Islands off of the Indonesian island of Sumatra has left at least 272 dead, with over 400 people still reported missing.

Indonesian authorities are also trying to deal with the eruption of Mount Merapi on the island of Java. The volcano, one of the most active in the region, is still emitting molten lava and clouds of ash, as well as incandescent heat, which has killed at least 25 people. Thousands have been evacuated, but despite the continuing danger, many residents of the area are seeking to return to their homes.

Dr. Simon Day, researcher at the Aon Benfield UCL Hazard Research Centre, analyzed the phenomenon which resulted in the disaster. He explained that the “magnitude 7.7 Mentawai Islands earthquake of 25 October 2010 was a miniature version of the giant subduction zone thrust earthquakes that have struck in the past along the plate boundary between the Indo-Australian Plate (which includes the seafloor below the Indian Ocean) and the Eurasian Plate (which includes the Island of Sumatra).”

He noted that giant earthquakes have “hit this region in 1797 and 1833, the latter being 100 times larger than this week’s.” These rupture the subduction zone plate boundary over a much wider area including areas closer to Sumatra, and therefore present a much greater threat to coastal cities such as Padang and Bengkulu through direct seismic shaking as well as tsunamis, as outlined in the Aon Benfield UCL Hazard Research Centre report published earlier this year.

“These earthquakes involve slippage of the Indian Ocean crust beneath Sumatra and also the Mentawai Islands, which owe their existence to the deformation and uplift of the overriding plate above the subduction zone. The 25 October 2010 earthquake ruptured a small section of this subduction zone at a depth of around 20 km beneath the Pagai Islands at the southern end of the Mentawai Islands group.”

Dr. Day also warned that the “potential for a mega-earthquake (and resulting tsunami) threatening the city of Padang in particular remains high. This is because the area and likely amount of slip on the rupture zone was small so will not have relieved a significant amount of the accumulated stress on the plate boundary.

“Despite the small size of the 25 October earthquake compared to the giant earthquakes, its thrust geometry and shallow depth meant that it had the potential to generate a significant tsunami. Initial reports indicate noticeable tsunami waves up to 0.4 m high in Padang and Bengkulu on the mainland of Sumatra within the first hour after the earthquake. A tsunami watch for coastal areas on Sumatra was issued by the Pacific Tsunami Warning Centre seven minutes after the earthquake but was cancelled after it became apparent that the tsunami was small at large distances from the source.

“However, major tsunami damage did occur in the Pagai Islands, much closer to the earthquake source, where incomplete reports indicate that tsunami waves were up to 10 times higher. The waves struck these islands within 15 minutes of the earthquake, underlining the extremely high vulnerability of coasts within the source regions of ‘tsunamigenic’ earthquakes that are capable of generating tsunamis.

The short time delay between the earthquake and the arrival of the tsunami in the Pagai Islands highlights the need for a high level of tsunami awareness amongst both resident and tourist populations in such areas, and a willingness to evacuate away from the shoreline in response to felt seismic shaking and initial sea level drawdowns.”

Source: Aon Benfield and News Reports